NEEDLES AND PINS: ACICULAR CRYSTALLINE PERIOSTRACAL
CALCIFICATION IN VENERID BIVALVES (BIVALVIA: VENERIDAE)
EMILY A. GLOVER AND JOHN D. TAYLOR
Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
Correspondence: J.D. Taylor; e-mail: [email protected]
(Received 18 August 2009; accepted 4 November 2009)
ABSTRACT
A scanning electron microscope study of the periostracum of 50 species of venerid bivalves revealed
that periostracal calcification in the form of aragonitic needles and shorter pins is widespread within
the family. Together with organic and sediment coatings that are found in some species, these
needles form an integral part of the functional shell. Visible as a white ‘crust’ on the outside of shells,
long slender needles (up to 400 mm long and 1 mm wide) without adherent material are seen in
species of Tivela and Lioconcha and in Gomphina undulosa. Other venerids including Pitar species, Mysia
undata and Compsomyax subdiaphana have short pins, capped with a fibrous organic matrix and significant coatings of sediment. Callocardia hungerfordi and Clementia papyracea have very thick sediment coatings underlain by short pins, while Gafrarium and Circe species have short pins with a thin, robust,
organic coating and little particulate material. Finally, there are species, including Venus verrucosa,
Chione elevata and Mercenaria mercenaria, where minute, ,1 mm long pins also underpin a thin organic
coating. Details of formation were studied in Tivela lamyi and Lioconcha ornata, where the needles are
elongate hexagonal crystals of aragonite enveloped by an organic sheath, which grow at their proximal ends from within the periostracum, connected to the outer mantle epithelium via narrow channels. Growth of needles ceases following the onset of shell calcification. The distribution of the
periostracal structures was examined in relation to a published molecular phylogeny that recognized
two major clades within the family. Larger needles and pins are confined to the clade that includes
subfamilies Pitarinae, Gouldinae, Meretricinae and Petricolinae, while submicron-sized pins are
found only in the Venerinae and Chioninae of the second clade. Calcified periostracal structures
appear to be absent in Tapetinae and Dosiniinae.
INTRODUCTION
The shell microstructure of bivalve molluscs is well known
through numerous studies (e.g. Taylor, Kennedy & Hall,
1969, 1973; Carter & Clark, 1985; Shimamoto, 1986; Carter,
1990; Checa, 2000; Schneider & Carter, 2001; EstebanDelgado et al., 2008; Harper, Checa & Rodrı́guez-Navarro,
2009), but there is, additionally, a wide diversity of intra- and
extraperiostracal calcified structures that have received much
less attention, although nevertheless integral parts of the functional shell. These structures include intraperiostracal granules
(Carter & Aller, 1975; Bottjer & Carter, 1980; Carter, Lutz &
Tevesz, 1990; Taylor et al. 2004), spicules and spines ( particularly in Anomalodesmata) arising from the periostracum
(Aller, 1974; Carter, 1978; Prezant, 1979; Harper, Dreyer &
Steiner, 2006; Checa & Harper, submitted) and calcified periostracal flaps (Taylor et al., 2004). Extraperiostracal structures
include cemented encrustations (Carter, 1978; Taylor, Glover
& Braithwaite, 1999; Braithwaite, Taylor & Glover, 2000);
sediment coatings (Morton, 2000) and tubes, crypts and encasings (Savazzi, 1982; Morton, 1995, 2000; Harper & Morton,
2004).
Among the intraperiostracal spines and spicules, the most
spectacular occur within the Veneridae, where white chalky
‘encrustations’ on shells such as Lioconcha species (Fig. 1) have
been revealed to consist of dense masses of extraordinarily
slender aragonitic needles (up to 400 mm long and 1.0 mm
wide) emerging from the periostracum. Ohno (1996) briefly
described and illustrated these long needles in Lioconcha species
and documented similar, but shorter, needles (hereafter ‘pins’,
,20 mm long) in Gafrarium and Pitar species, while Morton
(2000) illustrated short pins beneath an exterior sediment
coating of Callocardia hungerfordi.
Despite these initial studies, the morphology and distribution
of these structures among the highly diverse Veneridae have
remained uninvestigated. For example, in a morphological and
molecular phylogeny (Mikkelsen et al., 2006) no periostracal or
shell microstructural characters were included in the analysis.
Nevertheless, interesting questions concern the phylogenetic
distribution of the needles and pins; are they restricted to a
particular clade of venerids or are they widely distributed
among the clades? Molecular analyses of the Veneridae
(Kappner & Bieler, 2006; Mikkelsen et al., 2006) provide a
new framework for examination of the distribution and possible
evolution of these structures.
It is unfortunate that intraperiostracal calcification of some
venerids has been mistaken for an inorganic external encrustation that has often been routinely ‘cleaned’ in shell collections.
For example, it is likely that most Lioconcha species have long
periostracal needles, but they are rarely preserved in museum
specimens and not mentioned in the most recent taxonomic
revision of the genus (Lamprell & Healy, 2002). Similarly, live
Pitar species usually retain adherent sediment around the
ventral and posterior shell margins that is routinely removed
but, as we will show, this sediment has an underpinning of calcified pins and is an integral part of the mineralized structure
of these bivalves. Our initial survey showed that these needles
and pins were both more morphologically disparate and more
widely distributed within Veneridae than previously reported,
Journal of Molluscan Studies (2010) 76: 157–179. Advance Access Publication: 12 January 2010
# The Author 2010. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved
doi:10.1093/mollus/eyp054
E. A. GLOVER AND J. D. TAYLOR
Figure 1. Shells of Veneridae illustrating periostracal needle growths and sediment/organic coatings based on underlying pins. A. Lioconcha castrensis
with needle crust around ventral margin. Panglao, Philippines (MNHN). Shell length (L) ¼ 43.9 mm. B. Lioconcha castrensis; smaller specimen
showing white needle layer covering most of shell. Panglao, Philippines (MNHN). L ¼ 24.2 mm. C. Lioconcha ornata. Mahé, Seychelles (BMNH).
L ¼ 30.9 mm. D. Tivela lamyi. Funzi, Kenya (BMNH). L ¼ 35 mm. E. Gomphina undulosa. Shark Bay, Western Australia (BMNH). L ¼ 24.4 mm.
F. Pitar trevori. Moreton Bay, Queensland (BMNH). L ¼ 17.2 mm. G. Pitar citrina. Shark Bay, Western Australia (BMNH). L ¼ 32.1 mm.
H. Clementia papyracea with sediment coating. Moreton Bay, Queensland (BMNH). L ¼ 29.8 mm. I. Callocardia hungerfordi with thick sediment
coating. Danang, Vietnam (BMNH). L ¼ 15.1 mm.
present even in such well-studied species as Mercenaria
mercenaria and Venus verrucosa. Moreover, it was realized that
pins in most species are too small to be seen by inspection with
hand lens or by routine optical microscopy and are visible only
by scanning electron microscopy (SEM). Additionally, pins are
obscured in many species, such as Circe scripta, by robust,
organic coatings.
Our objectives are to describe the intraperiostracal calcified
needles and pins of venerids in detail, with the recognition of
different morphologies, including the presence and absence of
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PERIOSTRACAL CALCIFICATION IN VENERIDAE
Table 1. List of species examined by SEM.
Species
Locality
Anomalocardia cuneimeris (Conrad, 1846)
Big Pine Key, Florida Keys, USA
Callista chione (Linnaeus, 1758)
Helford, Cornwall, UK
Callocardia hungerfordi (Sowerby, 1888)
Danang, Vietnam
Chamelea striatula (Da Costa, 1778)
Devon, UK
Chione elevata Say, 1822
Lower Matecumbe Key, Florida, USA
Circe nummulina (Lamarck, 1818)
Dampier, Western Australia
Circe scripta (Linnaeus, 1758)
Singapore
Circenita callipyga (Tomlin, 1924)
Abu Dhabi, UAE
Clementia papyracea (Gray, 1825)
Moreton Bay, Queensland, Australia
Compsomyax subdiaphana (Carpenter, 1864)
Loon Point, Santa Barbara, California, USA
Cooperella subdiaphna (Carpenter, 1864)
California, USA
Dosinia exoleta (Linnaeus, 1758)
Helford, Cornwall, UK
Gafrarium dispar (Holten, 1802)
Mahé, Seychelles
Gafrarium divaricatum (Gmelin, 1791)
Kungkrabaen Bay, Thailand
Gafrarium pectinatum (Linnaeus, 1758)
Kungkrabaen Bay, Thailand
Gafrarium tumidum Röding 1798
Kungkraben Bay, Thailand
Gemma gemma (Totten, 1834)
Provincetown, Massachusetts, USA
Gomphina undulosa (Lamarck, 1818)
Denham, Shark Bay, Western Australia
Granicorium indutum (Hedley, 1906)
Rottnest I., Western Australia
Hyphantosoma caperi Lamprell & Healy, 1997
Dampier, Western Australia
Katelysia scalarina (Lamarck, 1818)
Albany, Western Australia
Lioconcha castrensis (Linnaeus, 1758)
Lizard I., Queensland, Australia
Lioconcha castrensis
Pangalao, Philippines (MNHN)
Lioconcha ornata (Dillwyn, 1817)
Mahé, Seychelles
Lioconcha philippinarum (Hanley, 1844)
Andaman Is
Macrocallista maculata (Linnaeus, 1758)
Western Atlantic
Megapitaria aurantiaca (Sowerby, 1831)
Mexico
Megapitaria squalida (Sowerby, 1835)
Western Atlantic
Mercenaria mercenaria (Linnaeus, 1758)
Southampton, UK
Meretrix casta (Gmelin, 1791)
Madras, India
Meretrix meretrix (Linnaeus, 1758)
Singapore
Mysia undata (Pennant, 1777)
Loch Spelve, Scotland, UK
Nutricola tantilla (Gould, 1853)
Vancouver, Canada
Paphia euglypta (Philippi, 1847)
Japan
Paphia undulata (Born, 1778)
Malaysia
Periglypta listeri (Gray, 1838)
Bahamas
Petricola lapidica (Gmelin, 1791)
Port Blair, Andaman Is, India
Petricolaria pholadiformis (Lamarck, 1818)
Whitstable, UK
Pitar inflata Sowerby 1851
Moreton Bay, Queensland, Australia
Pitar citrinus (Lamarck, 1818)
Monkey Mia, Shark Bay, Western Australia
Pitar trevori Lamprell & Whitehead, 1990
Moreton Bay, Queensland
Placamen calophyllum (Philippi, 1836)
Hong Kong
Ruditapes decussatus (Linnaeus, 1758)
Devon, UK
Ruditapes philippinarum (Adams & Reeve, 1850)
Whitstable, Kent, UK
Sunetta vaginalis (Menke, 1843)
Rottnest I., Perth, Western Australia
Tivela lamyi Dautzenberg, 1929
Funzi, Kenya
Tivela mactroides (Born, 1778)
Las Cuevas Bay, Trinidad
Tivela (Pachydesma) stultorum (Mawe, 1823)
San Diego, California, USA
Turtonia minuta (Fabricius, 1780)
Anglesey, Wales, UK
Venerupis senegalensis (Gmelin, 1791)
Devon, UK
Venus verrucosa (Linnaeus, 1758)
Channel Is, UK
Outgroups
Arctica islandica (Linnaeus, 1767) (Arcticidae)
Loch Fyne, Scotland, UK
Calyptogena valdiviae (Thiele & Jaeckel, 1931) (Vesicomyidae)
Gabon (MNHN)
Corbicula fluminea (Müller, 1774) (Corbiculidae)
Lake Poyang, China
Glauconome rugosa Reeve, 1844 (Glauconomidae)
Singapore
All samples housed in The Natural History Museum, London (BMNH), except where otherwise indicated. MNHN, Muséum National d’Histoire Naturelle, Paris.
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‘mucoid’ and sediment coatings. From these observations,
together with results from a more detailed study of Tivela lamyi,
we investigate their mode of formation. Using recently published molecular analyses we trace the phylogenetic distribution of the structures across the Veneridae and discuss their
evolution. Finally, we review the possible functions of the
needles, pins and coatings.
1. Long needles without adherent coatings
These were observed among species belonging to three genera,
Tivela, Lioconcha and Gomphina.
In Tivela lamyi (Figs 1D, 3) the shells, which are up to
50 mm in length, are covered with a dense, white, chalky
coating that obscures the shell coloration, is thicker towards
the ventral margin and is usually worn away towards the
umbones. The periostracum is c. 5 mm thick with a dense
covering (Figs 1D, 3A–C) of needles each 300 mm long,
1.2 mm wide and spaced about 2.0 mm apart. The proximal
ends of the needles are rooted around 1.0 mm into the outer
layer of periostracum. The aragonitic needles are hexagonal in
cross-section with the proximal ends showing crystal faces
(Fig. 3D) and the distal ends tapering to points. Each needle is
surrounded by an organic sheath confluent with the outer
surface of the periostracum (Fig. 3D). Extending from the base
of each needle to the inner surface of the periostracum there is
short, narrow (0.75 mm) ‘channel’ filled with organic material
that is different in texture to the periostracum itself. The
pointed tips of the needles first appear on the outer periostracal
surface, emerging from minute (1.5 mm) domes with a central
perforation (Fig. 3E, F). The needles increase to their full
lengths within a short distance of about 100–150 mm from first
appearance (Fig. 3E). Subsequently, needle growth ceases
when calcification of shell layers begins on the inner periostracal surface.
In Tivela mactroides the periostracal needles are generally
similar to those of T. lamyi. The shell is covered by a dense
coating of needles 40– 50 mm long and 0.3 –0.4 mm in width,
each covered in an organic sheath and embedded about
0.5 mm into the periostracum, which is 2.5 mm thick. The
bases of the needles are deeply grooved.
Uncleaned shells of Lioconcha ornata are always coated by a
white, lustrous, powdery crust (Fig. 1C) that is often worn off
in patches and usually absent from the umbonal areas. This
species has remarkably long needles c. 430 mm in length and
1.4 mm wide and spaced about 2– 3 mm apart (c. 1.5/mm2),
arising from within the periostracum (Fig. 4A, B). Needles are
pointed at their distal ends (Fig. 4E, F), with the main shafts,
MATERIAL AND METHODS
Periostracal characters were studied in 50 species of Veneridae
(Table 1) and, as outgroups, four species from the families
Arcticidae, Vesicomyidae, Glauconomidae and Corbiculidae,
identified as close to Veneridae in recent molecular analyses
(Mikkelsen et al., 2006; Taylor et al., 2007).
Periostracal structures were examined from surface views
and fractured sections of shell pieces taken from shell margins.
The fragments were mounted on stubs, coated with gold/palladium and examined with Philips XL30 field emission and
Zeiss Ultraplus scanning electron microscopes (SEM).
For Tivela lamyi, a preserved animal was decalcified in
EDTA, and the mantle edge with intact periostracum was
sliced with a razor blade. Pieces of mantle edge were then
dehydrated through ascending acetone series, critical-point
dried, coated with gold/palladium and examined by SEM.
The mineralogy of the needles in T. lamyi and Lioconcha
ornata was investigated by X-ray diffraction from a detached
bundle of needles and a ground sample of needles.
RESULTS
The calcified intraperiostracal structures observed in our study
fall into four broad categories (Fig. 2): (1) Long, slender
needles without adherent material; (2) short pins capped with
fibrous organic material and significant sediment coatings; (3)
short pins with a thin, robust, organic coating and little particulate material; and (4) minute, ,1 mm long pins with a
thin organic coating.
Figure 2. Diagrammatic representation of the three types of needles, pins and coatings; Type 4 pins differ from Type 3 only in size and are not
illustrated. A. Long needles without coating (Type 1). B. Short pins with organic coating and embedded sediment (Type 2). C. Short pins with
thin organic coating and few particles (Type 3). Abbreviations: g, sediment grains; n, needles; oc, organic coating; p, periostracum; s, shell.
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Figure 3. Tivela lamyi. A. Fractured section showing long needles. B. Basal part of needles emerging from periostracum. C. Basal part of needles
with channels running through periostracum. D. Crystal faces at basal termination of needles and organic sheath covering the needles. E. Outer
surface of periostracum showing growth front of the needles. F. Outer surface of periostracum with emerging needles. Abbreviations: c, channels
through periostracum; os, organic outer sheath to needles; p, periostracum; s, shell.
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Figure 4. Lioconcha ornata. A. Fractured section with densely packed long needles. B. Detail of basal parts of needles. C. Crystal faces on basal
terminations of needles. D. Basal termination of needle with cup. E. Outer surface of periostracum with needles emerging from pits. F. Periostracum
from mantle edge with needle on outer surface and basal button on inner surface. G. Periostracum with emergent needles and mantle below
attached at the basal buttons. H. Inner surface of developing periostracum with basal buttons, the central cavity marks the site of mantle
attachment. Abbreviations: b, basal button; m, mantle; p, periostracum; s, shell.
hexagonal in section, terminating with crystal faces at their
proximal ends (Fig. 4C, D). Individual needles are covered by
a thin, organic sheath. The periostracum varies between 1.5
and 2.2 mm in thickness, with each needle inset to a depth of
1–2 mm (Fig. 4B– D). Short channels lead from the base of
each needle to the inner face of periostracum. Button-like
convexities with central holes mark the close connection of the
outer mantle through the channels to the base of the needles
(Fig. 4F– H).
Results from X-ray diffraction of L. ornata needles show that
they are composed of aragonite and are elongated parallel to
the c-axis.
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Figure 5. Lioconcha castrensis (A, B) and Gomphina undulosa (C– E). A. Fractured section showing long needles. B. Detail of (A) showing needles
embedded in periostracum. C. Fractured shell section with long needles. D. Detail of needles with basal channels through periostracum. E. Section
through periostracum before emergence of needles with channels and cups containing possible incipient needles. Abbreviations: c, channels through
periostracum; in, incipient needle; p, periostracum; s, shell.
Lioconcha castrensis (Figs 1A, B, 5A, B) is the largest of the
Lioconcha species, reaching 55 mm in length, and the most commonly illustrated, usually with the shell polished and stripped
of the outer layer of needles to reveal the distinctive chevron
and hieroglyphic shell pattern. In uncleaned specimens the
larger shells have a thick ventral fringe of periostracal needles
(Fig. 1A), but in smaller, younger, shells the needles cover virtually the entire shell surface (Fig. 1B). As in L. ornata the
needles are c. 400 mm long and 2.0 mm wide, hexagonal in
form and covered by an organic sheath (Fig. 5A, B). Their
distal ends are rooted 1.5 mm into periostracum, which is 2.5–
3.0 mm thick, but lacks the button-like thickening beneath
each needle. Short channels run from the bases of the needles
to the inner periostracal surface (Fig. 5B).
Lioconcha (Sulcilioconcha) philippinarum differs from the two
preceding species in possessing prominent commarginal lamellae and a glossy periostracum, but we suspect that the specimens available to us had been scrubbed clean. Nevertheless, in
the interspaces between lamellae and in the lunule we observed
remnant fine needles up to 20 mm long and 0.3 mm in width
emerging from holes in the outer periostracum, which is about
2.5 mm thick.
Dried, uncleaned shells of Gomphina undulosa (Figs 1D, 5C–
E) are covered in a patchy, white coating of needles that is
thicker near the ventral margin, but scarcely visible in wetpreserved specimens. The densely packed needles are 80 –
90 mm long and 0.6 mm wide, with pointed distal tips and
hexagonal shafts and bases (Fig. 5C, D). The periostracum is
1.8–2.0 mm thick, with the needles inset to a depth of
0.5 mm. Beneath each needle is a straight-sided channel (0.2 –
0.3 mm wide) linked to the inner face of the periostracum and
filled with granular material (Fig. 5D, E). As in Tivela and
Lioconcha species each needle is encased in a thin, organic
sheath.
2. Short pins with organic coatings and bound sediment
Species within this category all have short, calcified pins that
underlie an organic coating in which sediment is bound to
varying degrees. Callocardia hungerfordi and Clementia papyracea
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E. A. GLOVER AND J. D. TAYLOR
Figure 6. Pitar trevori. A. Outer surface of shell with attached sediment and organic layer. B. Shell surface with fibrous organic coating and
underlying calcareous pins. C. Fractured section with pins and overlying fibrous coating. D. Surface view of pins with adherent organic threads.
E. Section with pins and network of organic threads. F. Pins emerging from pits in periostracal surface. Abbreviations: f, fibrous organic coating;
g, sediment grains; p, periostracum; s, shell.
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Figure 7. Pitar citrina (A–C) and Pitar inflata (D– F). A. Surface with pins overlain by organic coating. B. Fractured section with pins and coating.
C. Basal part of pins with channels through the periostracum. D. Fractured section of shell with pins overlain by thick sediment and organic
coating. E. Pins arising from periostracum. F. Pins with basal channels, empty of content, penetrating the periostracum. Abbreviations: c, channel
through periostracum; f, fibrous organic coating; n, needles; p, periostracum; s, shell; sed, sediment and organic material.
0.5 mm into the periostracum that is 1.2 mm thick. At the base
of each pin there is a narrow channel filled with organic
material that penetrates to the inner periostracal surface
(Fig. 7C). At the shell margin the pins emerge from small pits
in the outer surface of the periostracum and reach maximum
height in a distance of about 30–40 mm.
In Pitar inflata (Fig. 7D– F) the ventral and posterior
margins of the shell are coated with a thick (c. 2 mm) encrustation of sediment in a fibrous organic matrix. Beneath this
coating the shell is covered by densely packed needles 50 –
60 mm in length and 0.6 mm in width. Each needle is encased
in an organic sheath and set 0.5 mm into the periostracum,
which is 3.0 mm thick. The periostracum is perforated by
cylindrical channels 0.4 mm wide that lead from the base of
the needles to the inner periostracal surface.
Two species we have examined have much thicker sediment coatings. In C. hungerfordi (Figs 1H, 8A – D), most of
the shell is covered by a soft, but robust and extremely thick,
organically bound, sediment coating up to 750 – 1,000 mm
thick, which is more extensive to the posterior. Beneath the
sediment coating, the shell is covered by short, closely spaced
pins c. 12 – 15 mm in length and 0.3 mm in width (Fig. 8B –
D). The pins are hexagonal in section, with their distal ends
tapering to points. Each pin is enveloped in a thin, organic
possess extremely thick sediment coatings, while in Pitar species
the sediment coating is thinner and often survives only at the
ventral shell margin.
We examined three Pitar species. In Pitar trevori (Figs 1F, 6)
the ventral and posterior margins of the shells are coated with
sandy sediment grains entrapped in a fibrous organic matrix
(Fig. 6A) that covers much of the shell, even where the sediment has disappeared. Beneath the coating are short, crystalline pins 2–4 mm long and 0.3–0.4 mm wide, spaced about 1–
2 mm apart (Fig. 6B–E). The pins are hexagonal in section,
with the distal tips bluntly tapering and the proximal ends
embedded almost to the full thickness of the extremely thin
(0.3 mm) periostracum. The fibrous coating is attached to the
distal portions of the pins and appears as a dense mesh of
threads (Fig. 6D, E).
Shells of Pitar citrina (Figs 1G, 7A –C) usually possess a
patchy, calcified crust around the ventral and posterior parts
of the shell (Fig. 1G) that consists of layers of a fibrous organic
matrix in which sediment grains are trapped. Beneath this
organic coating and supporting it, are closely packed (c. 10/
mm2) crystalline pins, around 15 mm long and 0.3 mm wide,
each encased in a thin sheath and often bunched together at
the distal ends (Fig. 7A– C). The distal tips of the pins are
bluntly rounded, while the proximal ends are rooted about
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E. A. GLOVER AND J. D. TAYLOR
Figure 8. Callocardia hungerfordi (A–D), Clementia papyracea (E, F) and Compsomyax subdiaphana (G –I). A. Fractured section of shell margin with thick
layer of adherent sediment. B. Section of shell with sheet of pins underlying sediment coating. C. Pins with distal organic coating. D. Base of pin
showing organic sheath; and thin periostracum with channels. E. Section of shell with layer of pins beneath sediment/organic coating. F. Detail of
pins with distal organic coating and emerging thin periostracum at base. G. Surface view showing tiny pins and sediment coating. H. Fractured
section with short pins in periostracum. I. Pins with organic coating, larger individual with thicker periostracum than in (E). Abbreviations: os,
organic sheath; p, periostracum, s, shell; sed, sediment and organic coating.
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Figure 9. Mysia undata (A– B) and Hyphantosoma caperi (C– D). A. Surface view with needles overlain by organic coating. B. Fractured section,
needles smeared with organic coating. C. Surface view of needles beneath outer organic coating. D. Section showing periostracum, needles and
organic coating. Abbreviations: oc, organic coating; p, periostracum, s, shell.
3. Short pins with robust organic coating and little particulate
material
sheath with a basal collar confluent with the outer periostracal layer. The distal ends of the pins are draped with
organic material that forms the matrix for the sediment
coating. The proximal ends of the pins are inserted around
0.5 – 0.75 mm into the periostracum, which is c. 1.0 mm thick.
Below each pin there is a short channel leading to the inner
surface of the periostracum (Fig. 8D). Clementia papyracea
(Figs 1I, 8E, F) is a thin-shelled (300 mm) species, largely
coated with a soft, adherent layer of fine sediment up to
450 mm thick. Beneath the sediment, the periostracum is
covered with short pins, each around 6 mm long and 0.4 mm
wide and encased in an irregular organic sheath. At their
bases, the pins are embedded to nearly the full depth of the
periostracum, which is 0.4 – 0.6 mm thick (Fig. 8F). Distally,
the pins are coated with thin sheets of organic material in
which the sediment is held.
Another species with significant sediment coating is
Compsomyax subdiaphana (Fig. 8G–I). Shells are patchily
covered with sediment held in an organic coating that is generally thicker along the ventral margin. Beneath this are extremely short, thin, blunt-ended pins 1.5 mm long and 0.1–
0.2 mm wide that protrude from the periostracum. Mucoid
threads adhere to the distal tips of the pins. The proximal ends
of the pins are shallowly set into the periostracum, which is
0.5– 2.0 mm thick.
Species in this category belong to several genera including
Hyphantosoma, Mysia, Gafrarium and Circe.
In Mysia undata (Fig. 9A, B) the surface of the shell is
covered by a thin, organic coating, generally with only fine
sediment particles attached. Beneath this coating are slender,
closely spaced pins, c. 10 mm long and 0.3–0.5 mm wide, with
their proximal ends shallowly inset 0.4 mm into the
0.75 mm-thick periostracum. Their tapering distal ends are
coated in the organic material. The ventral part of the shell of
Hyphantosoma caperi (Fig. 9C, D) is covered by a robust, densely
textured, organic coating 2–3 mm thick with few sediment particles in the outermost layer. Beneath and supporting the
coating there are closely spaced, slender pins c. 4.0 mm long
and 0.2 mm wide arising from within the 0.5 mm-thick periostracum. Short channels lead from the base of the pins to the
inner periostracal surface.
We examined four Gafrarium species, each having a rather
different form of pins, but all covered and often obscured by
an organic coating. In Gafrarium pectinatum (Fig. 10A, B) the
coarsely ribbed shell is covered with a thin organic coating
with some fine sediment attached that is often worn away on
the shell ribs. Underpinning the coating are slender, hexagonal
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E. A. GLOVER AND J. D. TAYLOR
Figure 10. Gafrarium species. A. Gafrarium pectinatum, fractured section of shell with pins and coating. B. Gafrarium pectinatum, detail of basal part of
pins with vacuolated periostracum. C. Gafrarium dispar, surface view with short pins and organic outer coating. D. Gafrarium dispar, fractured edge
with short pins and coating. E. Gafrarium divaricatum, fractured section with thick periostracum, short pins, and organic coating. F. Gafrarium
divaricatum, with short robust pins beneath coating. G. Gafrarium divaricatum, individual pin. H. Gafrarium tumidum, outer surface of shell with pins
protruding through organic coating. I. Gafrarium tumidum, section of shell with recumbent pins and vacuolated periostracum. J. Gafrarium tumidum,
individual pin with base in periostracum. Abbreviations: oc, organic coating; p, periostracum, s, shell.
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PERIOSTRACAL CALCIFICATION IN VENERIDAE
Figure 11. Circe scripta (A –C) and C. nummulina (D –E). A. Surface view of short pins and adherent coating. B. Fractured section with short pins
emerging from thick periostracum and with distal organic coating. C. Surface view of pins and underlying pits in periostracum. D. Surface view
with pins beneath outer organic coating. E. Section of periostracum with embedded pins. Abbreviations: oc, organic coating; p, periostracum,
s, shell.
pins, around 20 mm long and 0.75 mm wide in the periostracum, which is 1.5 –1.8 mm thick, insubstantial and highly
vacuolated. Each pin has an organic sheath and a basal collar
confluent with the periostracum. Similarly, Gafrarium dispar
(Fig. 10C, D) has a shell covered with a thin organic layer,
with some adherent fine sediment. This coating is underlain by
short, robust pins at a density of 1 –2 pins/mm2. The pins are
2.5 mm long and 0.7 mm wide at the base, tapering to 0.4 mm
at the blunt distal tip. Their proximal ends of the pins are set
about 1.0 mm into the periostracum, which is around 3.0 mm
thick. In contrast to the preceding species, Gafrarium divaricatum
(Fig 10E–G) seems to lack any surface encrustation or pins,
because the colour pattern of the shell is clearly visible.
However, SEM examination shows that the outer shell surface
is covered by a dense organic coating c. 2–5 mm thick. The
outer 0.5 mm is densely homogeneous with some fine particles
and bacteria, while the inner part is platy or granular in
texture. Beneath and protruding into this organic coating are
calcified pins that are widely spaced (.1/mm2) compared with
other Gafrarium species. These short, robust, hexagonal pins are
3.5–4.0 mm long and 1.8– 2.0 mm wide, with tapering distal
ends. The pins are rooted around 2.0 mm into the outer surface
of periostracum, which is 5 mm thick. In Gafrarium tumidum
(Fig. 10H–J) the pins are hexagonal in section, often curved,
10– 12 mm long and about 1.5 mm wide at the base and tapering distally. They emerge from a highly vacuolated 2–
3 mm-thick periostracum. These pins are difficult to observe,
because they are often inclined at angles, bound up and protruding through the 5 mm-thick overlying organic layer.
Two Circe species were examined. The shell of Circe scripta
(Fig. 11A– C) has a thin organic coating up to 2.0 mm thick,
which covers short, very closely packed (c. 5–8/mm2) pins 1.5–
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E. A. GLOVER AND J. D. TAYLOR
Figure 12. Petricola lapicida. A. Surface of posterior shell margin with needles. B. Section of shell margin with thin periostracum and dense needles.
C. Needles arising from vacuolated periostracum, irregular shape resulting from secondary mineralization. D. Bases of needles in periostracum.
E. Section of posterior pseudo-rib. F. Irregular bladed crystals above pitted periostracum (detail of X in E). G. Less ordered crystals from dorsal
part of pseudo-rib that develop above the true needles (detail of Y in E). Abbreviations: p, periostracum, s, shell.
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Figure 13. Petricolaria pholadiformis. A. Shell section with vacuolated periostracum, pins and overlying particulate material. B. Surface of vacuolated
periostracum with short pins. Abbreviations: p, periostracum, s, shell; sed, sediment coating.
1.75 mm long and 0.5 mm wide. These are hexagonal in form,
often curved along their lengths and with blunt distal terminations. The pins are inset 0.6 mm into periostracum, which is
2 mm thick. Beneath the thin organic coating of Circe nummulina
(Fig. 11D, E) there is a dense (c. 4/mm2) covering of short pins
4–5 mm long and around 0.2–0.3 mm wide, tapering to blunt
tips. These are inset 0.2 mm into the 1.5 mm-thick periostracum.
In the rock-boring Petricola lapicida (Fig. 12) the needles are
usually well preserved only near the margins. Shells have a
dense (4–5/mm2) covering of needles up to 85 mm long and
0.3 mm wide, which emerge from individual channels in a
highly vacuolated, thin (1 mm) periostracum (Fig. 12C).
Needles show hexagonal crystal faces near their bases, but are
more irregular along their lengths with the addition of probable secondary calcification.
In common with some other Petricola species, P. lapicida has
secondary thickening of the posterior shell outside of the periostracum, sometimes in the form of narrow, rounded, radial
pseudo-ribs. The structure of this thickened material is
complex, but appears to be superimposed on the original
needles. Sections show that resting on the periostracum there is
a zone of vertically aligned aragonitic crystals (Fig. 12E, F)
and above this there is a mass of less oriented, loosely packed,
interpenetrant crystals held within a matrix of organic material
(Fig. 12E, G) and with some sediment grains. We think that
the basal, vertically aligned crystals are periostracal needles
that have been extensively modified by secondary calcification.
This secondary calcification on the posterior shell of Petricola
warrants further investigation, but lies outside the scope of this
paper. It probably derives from secretions of the extensive
‘mucus’ glands common in the posterior mantle margin of petricolines (Yonge, 1958; Morton, 1978). By contrast, Petricolaria
pholadiformis (Fig. 13) lacks any secondary calcification and has
much smaller periostracal pins. These are thin, up to 3 mm
long and 0.1 mm wide, with densities of around 3–4/mm2,
arising from individual holes in a thin, highly vacuolated periostracum. The shell surface is usually covered by a thin, flaky
sediment coating that often obscures the pins (Fig. 13A).
listeri, Anomalocardia cuneiformis, Chione elevata and Chamelea
striatula. Although the two former species are among the most
well known of venerid bivalves, these structures have never
been recorded (e.g. Eble, 2001). Most of the pins are less than
a micron in length, often less than half a micron in width and
often very densely distributed (up to 60 pins/mm2). They
occupy a homologous position to the needles and pins
described above and, in M. mercenaria and C. striatula, we have
observed the needles emerging from channels in the periostracum, suggesting a similar mode of formation.
We examined juvenile, cultured specimens of M. mercenaria
with shell lengths of around 10 mm. The periostracal surface is
thickly covered by short (100–300 nm) tapering pins that arise
from within the thin, vacuolated periostracum (80 nm thick),
with 45–50 pins/mm2. With high-resolution SEM (Fig. 14A –
C) the pins appear to show crystal faces and are often curved
at the tip (Fig. 14B). Covering the pins is an organic coating
(4–5 mm thick) with adherent sediment particles including
diatoms. Similarly, the periostracum of V. verrucosa (Fig. 14E,
F) has short pins up to 400 nm long, at densities of around 40/
mm2, often bent or curved at the tip, arising from a thin
(250 nm) periostracum. The pins are covered by a thick,
organic coating with some entrapped particles. Chione elevata
has pins about 500 nm long, often bent at the tips or curved
over, which densely cover (50/mm2) the periostracum, which is
200 nm in thickness (Fig. 14D). The pins are also covered with
a thin organic coating. The periostracum of P. listeri is 300–
500 nm thick and covered by pins 300–500 nm long, with densities of 16– 20/mm2 (Fig. 14G, H). Superimposed on the pins
is a coating of organic material with some bound particles.
Similar short pins were also observed in A. cuneiformis, occurring at a density of about 10/mm2.
The periostracal pins of C. striatula differ from those of the
other members of the Venerinae clade described above
(Fig. 15). On juvenile shells there is a dense covering of slender
pins, with c. 60 pins/mm2, each about 5–6 mm long and 80 nm
wide. Surface views (Fig. 15A) show that the pins are often
flattened into roughly polygonal patterns, possibly resulting
from pressure of sediment grains. Each pin emerges from a
single channel through the extremely thin periostracum, which
is only 100 nm thick in a Chamelea of 5.3 mm in length but
doubles in thickness in adult shells. In larger specimens, thin
channels are visible in the periostracum beneath the pins. On
4. Minute submicron-sized pins with an organic coating
In this category we describe minute pins protruding from the
periostracum of Mercenaria mercenaria, Venus verrucosa, Periglypta
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E. A. GLOVER AND J. D. TAYLOR
Figure 14. Mercenaria mercenaria (A –C), Chione elevata (D), Venus verrucosa (E, F) and Periglypta listeri (G –H). A. Juvenile shell, surface view.
B. Surface of periostracum with short pins. C. Short pins emerging from periostracum. D. Fractured section, with short pins. E. Fractured section,
pins with organic coating. F. Fold in periostracum with short pins. G. Outer periostracum surface. H. Fold of periostracum with short pins.
Abbreviations: oc, organic coating; p, periostracum, s, shell.
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PERIOSTRACAL CALCIFICATION IN VENERIDAE
Figure 15. Chamelea striatula. A. Juvenile shell, surface view of fine needles. B. Fractured section of shell margin. C. Surface of periostracum with
pits and needles. Abbreviations: p, periostracum; s, shell.
initial observation, we concluded that pins were absent in
Chamelea, but then found that they are mostly worn away in
adult shells and usually survive only in the deep clefts of abutting commarginal lamellae.
embayments in the inner surface of the forming periostracum
(Fig. 17A). On the outer periostracal surface the positions of
the channels appear as tiny pits (Fig. 17D). Needles and the
surrounding organic sheath grow from within the cups, emerging from pits on the outer periostracal surface (Figs 17E,
18A). The needles rapidly increase in length, accreting from
their bases, as evidenced by fine growth increments (Fig. 18B).
The emerging needles are slightly tapered and possess a discrete, bulbous tip (c. 200 nm) that we interpret as the initial
nucleus (Fig. 18A). Apart from the initial tapering, the width
of the crystals is constant, probably constrained by the shape
and size of the cup and the organic sheath that envelopes each
growing crystal (Fig. 17G). Growth of needles and pins ceases
at the commencement of the principal shell calcification on the
inner periostracal surface, because connection of the needles to
Species in which needles and pins are absent
The following species were examined by SEM at the same
magnifications as the preceding species, but no needles and
pins were detected: Callista chione, Circenita callipyga, Cooperella
subdiaphana, Dosinia exoleta, Gemma gemma, Granicorium indutum,
Katelysia scalarina, Macrocallista maculata, Megapitaria aurantiaca,
M. squalida, Meretrix casta, M. meretrix, Nutricola tantilla, Paphia
euglypta, P. undulata, Placamen calophyllum, Ruditapes philippinarum,
R. decussata, Sunetta vaginalis, Tivela (Pachydesma) stultorum,
Turtonia minuta and Venerupis senegalensis.
Evidence for mode of formation of needles and pins
Obvious questions concern the mode of formation of these
remarkable periostracal needles and pins. Our observations
suggest that all are secreted in a similar manner, summarized
in Figure 16. All form at the growing margin of the bivalve
and lengthen to their full size prior to the onset of the main
shell calcification. The aragonitic crystals, hexagonal in crosssection and each surrounded by a thin, organic sheath, arise
from within the periostracum, accreting from the distal ends of
narrow channels that link to the outer mantle epithelium.
In those species studied in detail, particularly T. lamyi and
L. ornata, we have observed a narrow connection from the base
of the needles and pins to the inner surface of the periostracum
(Figs 3C, 4F, G, 17F, H). These cylindrical channels are direct
connections to the mantle epithelium and lengthen as the periostracum thickens (Fig. 17A –C). The distal ends of the channels are broader and cup-shaped. Although we have no
detailed histology, in T. lamyi the channels are filled with tissue
(Fig. 17F) that joins the epithelium of the outer mantle fold.
Sections of the mantle edge show that the position of the
needles in T. lamyi is predetermined deep within the periostracal groove, where initiation of the channels is marked by
Figure 16. Diagrammatic representation of needle and pin formation.
A. Section through shell and mantle margin. B. Detail of base of
needle with channel through periostracum to outer mantle.
Abbreviations: c, channel; if, inner mantle fold; m, mantle; mf1,
middle mantle fold 1; mf2, middle mantle fold 2; n, needle; of, outer
mantle fold; oc, organic coating; os, organic sheath; p, periostracum;
pg, periostracal groove; s, shell.
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E. A. GLOVER AND J. D. TAYLOR
Figure 17. Tivela lamyi, growth of needles. A. Section of periostracum deep in the periostracal groove with initial embayments that develop into the
cups at the base of the needles. B. Individual cup in developing periostracum with outer mantle surface. C. Underside of periostracum in
periostracal groove showing basal buttons where mantle cells are attached. D. Outer surface of developing periostracum showing pits through which
individual needles will emerge. E. Outer surface of periostracum with needles emerging from pits. F. Decalcified section of periostracum showing
channel and cup base of needle filled with mantle tissue. G. Decalcified section of needles at shell margin showing organic sheaths that covered
each needle. H. Decalcified section of periostracum with channels linking base of needles to inner surface. Abbreviations: b, basal button; bc, basal
cup to needle; c, channel through periostracum; m, mantle; of, outer mantle fold; os, organic sheath; p, periostracum; s, shell.
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PERIOSTRACAL CALCIFICATION IN VENERIDAE
Long periostracal channels as in Lioconcha and Tivela are not
observed in all species. In some cases the periostracum is so
thin that the channels would be very short. In others the periostracum is thin and vacuolated [as in G. pectinatum, G. tumidum
(Fig. 10A, B, H, I) and P. lapicida (Fig. 12B, D)] and the connection between the mantle and pins is obscure. Some other
species, such as G. divaricatum (Fig. 10E, F) have short pins and
a thick periostracum with no visible trace of channels; in this
case it appears that the mantle connection to the pins ceases
after a short time and is then followed by additional periostracal secretion. Thus, at one end of the range (e.g. Tivela), the
mantle connection to the crystals through the channels is maintained across the full thickness of the periostracum and the
connection is severed only when shell calcification begins. At
the other extreme, we suggest that the mantle connections are
severed during an earlier phase of periostracal secretion so that
no trace of channels can be seen.
As in most bivalves, the mantle edge of T. lamyi has three
main folds, outer, middle and inner, with the periostracum
arising from the periostracal groove lying between the outer
and inner folds. However, Tivela, in common with other venerids, possesses a distinct, thin, subsidiary mantle fold that is
closely pressed against the outer periostracal surface and arises
from the base of the middle fold (Fig. 16). This subsidiary fold
(often considered as the outer part of a divided middle fold)
has been described in other venerids, e.g. Chamelea striatula
(Ansell, 1961: fig. 3), Chione elevata (Morton & Knapp, 2004:
fig. 3), Callocardia hungerfordi (Morton, 2000: figs 5–7) and in
Mercenaria mercenaria by Eble (2001: figs 4.5 –4.8) who named it
the ‘periostracal fold’. The significance of this fold in the formation of periostracum, needles and pins or the overlying coatings is unknown. Detailed histological study of mantle margins
in a venerid species that secretes long needles is needed in
order to clarify the role of the different mantle folds.
Phylogenetic distribution of needles and pins
A recent molecular phylogenetic analysis of Veneridae
(Mikkelsen et al., 2006) provides a framework for examination
of the distribution of calcified intraperiostracal needles and we
have studied many of the species included in their original tree
(Fig. 19). This molecular analysis divided the Veneridae into
two major clades, one (Clade A) comprising the subfamilies
Dosiniinae, Tapetinae and Chioninae/Venerinae, and the
other (Clade B) containing Pitarinae, Callistinae, Petricolidae,
Gemminae, Turtoniidae, Meretricinae and Gouldinae. The
distribution of calcified needles and pins (Fig. 19) indicates
that long needles are restricted to Clade B, having been
recorded in T. mactroides, G. undulosa and L. ornata. Gomphina
and T. mactroides group together, but are widely separated from
Lioconcha, which aligns with Hyphantosoma, Gafrarium and Circe
species. Short pins supporting mucoid coatings with sediment
attached are also found in species of Clade B within the Pitar
group and Compsomyax. Thick sediment coatings are also
present in Callocardia subdiaphana and C. papyracea, but neither
species was included in the molecular analysis. Short pins
covered by a thick mucoid coating, but without significant
attached sediment, occur within the clade containing Circe,
Gafrarium and Hyphantosoma. The two petricoline species, P.
lapicida and P. pholadiformis, differ in that P. lapicida has
medium-length needles while P. pholadiformis has short, thin
pins. Petricola lapicida has significant rigid extraperiostracal calcification, particularly at the posterior where it forms a
pseudo-sculpture of radial ribs similar to that seen in
Samarangia (Taylor et al., 1999). Another species, Cooperella subdiaphana, often classified in the Petricolinae, has a glossy periostracum and lacks periostracal calcification, but its placement
in this group is equivocal pending molecular analysis (see
Figure 18. Tivela lamyi. A. Outer periostracal surface showing
emerging needles each with a distal ‘rounded nucleus’. B. Proximal
end of needle in periostracal cup showing microgrowth increments.
Abbreviations: al, accretion lines; bt, bulbous tip of needle.
the mantle epithelium is then severed. The details of L. ornata
are essentially similar, but the periostracum is thinner and the
channels short with a thickened base that appears on the inner
periostracal surface as a button-like structure with a central
hole (Fig. 4F– H) marking the point of mantle epithelial extension into the channel. A curious phenomenon is the curved or
bent tip that occurs in some species with very small pins
(Figs 11A, 14D); we think that in this case the pin formation is
coincident with the deposition of sticky organic coatings that
may redirect the angle of growth.
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E. A. GLOVER AND J. D. TAYLOR
Finally, no calcified pins or needles were observed in any of
the chosen outgroup species, Arctica islandica, Corbicula fluminea,
Glauconome rugosa and Calyptogena valdiviae.
DISCUSSION
Diversity and distribution
In summary, we recognize four main types of intraperiostracal
needles and pins and their accompanying extraperiostracal
organic and sediment coatings: (1) long, densely packed
needles that lack coatings; (2) short pins that underlie a thick
sediment and organic coating; (3) short pins with a robust but
thin coating with little or no adherent sediment; and (4)
submicron-sized pins, also with organic coatings.
Long needles occur in species of the three genera Lioconcha,
Tivela and Gomphina. Needles have been confirmed (Ohno,
1996; herein) in four species of Lioconcha (L. castrensis, L. ornata,
L. fastigiata and L. philippinarum) and we predict that needles
are likely present in all 19 species recognized by Lamprell &
Healy (2002). Most specimens illustrated by them have been
‘cleaned’, but needle crusts are visible on L. hieroglyphica
(Conrad, 1837) and L. melhartei Lamprell & Stanisec, 1996
(Lamprell & Healy 2002: figs 3A, G, 11A –C). One group of
Lioconcha species, often separated in the subgenus Sulcilioconcha,
have prominent commarginal lamellae and glossy shells, but
our observations on a ‘cleaned’ specimen of the type species L.
philippinarum revealed remnant needles in the lamellar interspaces and lunular area.
As well as the two species studied, T. lamyi and T. mactroides,
it is likely that many other Tivela species possess periostracal
needles. Images in published works and specimens in museum
collections show calcareous coatings, including the type species
T. tripla (Linnaeus, 1771) from West Africa (personal observation). Mikkelsen & Bieler (2007: 320), specifically mentioned
periostracal needles in Tivela abaconis (Dall, 1902) from
Florida. Bivalves from Mexico usually identified as T. byronensis
(Gray, 1838) include individuals both with and without
needles (P.V. Scott, personal communication). Classified in a
separate subgenus, the large Tivela (Pachydesma) stultorum from
California lacks periostracal calcification. Significantly, in the
molecular tree this species groups with species of Meretrix and
Macrocallista that also lack needles or pins (Mikkelsen et al.,
2006; Fig. 19).
Previously, Ohno (1996) stated that Gomphina undulosa lacked
needles, but our observations on this species demonstrate a
thick growth of needles. Earlier morphology-based classifications placed Gomphina in the subfamily Tapetinae, but molecular results for the type species, G. undulosa, place it as sister
taxon to Tivela mactroides (Mikkelsen et al., 2006: fig. 11) and
the presence of similar needles corroborates this placement.
Pitar species frequently have encrustations of sediment on
the ventral and posterior parts of the shell, suggesting that the
presence of underlying calcareous pins is probably widespread
within the group. In addition to the three species reported
here, Ohno (1996) recorded pins in Pitar striatum (Gray, 1838)
and P. subpellucidum (Sowerby, 1851) from Japan. Short pins
were illustrated on Pitar simpsoni (Dall, 1895) from Florida by
Mikkelsen & Bieler (2007), who also mentioned the presence of
periostracal pins in P. fulminatus (Menke, 1828), P. albidus
(Gmelin, 1791) and Pitarenus cordatus (Schwengel, 1951).
Superficially, Gafrarium and Circe species would seem to lack
any sediment coatings or periostracal pins. However, all species
studied have short pins of various types overlain by an organic
coating. Previously, Ohno (1996) had recognized short pins,
which he called ‘cryptic needles’, in G. dispar and G. tumidum.
Morton (2000) described the thick sediment layer and the
underlying pins of Callocardia hungerfordi and suggested that
Figure 19. Phylogenetic distribution of needles and pins plotted on
molecular tree derived from Mikkelsen et al. (2006: fig. 11). Includes
data only for species that appear in the molecular tree. Letters A and
B identify the two major clades discussed in text. Key to symbols: open
circle, no periostracal calcification; closed diamond, submicron-sized
pins; closed triangle, long needles with no coatings or sediment; closed
circle, short pins with fibrous organic coatings and sediment; closed
star, short pins with organic coating.
Mikkelsen et al., 2006: 498). Mysia undata has similar pins and
coating to those of Hyphantosoma. It was not included in the
molecular analysis, but has traditionally been classified in the
Petricolidae (but see Ansell, 1961). Among other members of
Clade B, needles and pins are absent in several major groups
including Meretrix, Callista, Megapitaria, Nutricola, Gemma and
T. (Pachydesma) stultorum. All these species have a smooth,
glossy periostracum.
Large, obvious, calcareous pins, needles and encrustations
are not recorded in venerids of Clade A. However, SEM observations revealed the presence of short pins of submicron size in
all examined species of the Venerinae/Chioninae subclade, i.e.
M. mercenaria, V. verrucosa, P. listeri, C. elevata and Anomalocardia
cuneimeris, and of slightly longer pins in C. striatula. No pins
were identified in any Tapetinae (Ruditapes, Venerupis, Paphia,
Katelysia), Dosininae or P. calophyllum.
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PERIOSTRACAL CALCIFICATION IN VENERIDAE
glands on the middle and inner mantle folds supply the mucus
that binds the sediment coating. Apart from the studies mentioned above, calcareous periostracal pins and needles have not
previously been recognized in any other venerid bivalve. It is
remarkable that the tiny pins of Mercenaria mercenaria and Venus
verrucosa have not previously been observed, despite detailed
studies of periostracal structure and formation in these species
(Neff, 1972; Eble, 2001).
With the exception of Lioconcha, Tivela and Gomphina species,
the pins and needles of all the studied venerids are overlain by
an adherent, robust, ‘mucoid’ coating in which varying
amounts of sediment particles are embedded. In Pitar species
this organic coating is distinctly fibrous in texture, forming a
dense mat in which the sediment is held. The greatest thickness
of bound sediment is seen in Callocardia and Clementia. In
Mysia, Gafrarium and Circe species there is much less sediment
and the organic coating is more homogeneous or with a flaky
texture (when dry), and through which the shell colour
pattern is clearly visible. Even in those species with submicronsized pins there is a thin overlying coating. These different
forms of organic coatings are probably produced from the
abundant gland cells of the inner and middle mantle folds, as
suggested by Morton (2000) for Callocardia and that are also
present in other venerids (Morton & Knapp, 2004). These
glands are likely similar in function to the arenophilic glands
of anomalodesmatans (Sartori, Passos & Domaneschi, 2006).
Further research is needed to characterize these various
organic coatings and their role.
As well as wide variation in the morphology of the calcified
pins and needles, we also observed considerable variation in
the thickness and structure of the periostracum. In some
species the periostracum is so thin that the needles penetrate
virtually its whole thickness. In the four examined species of
Gafrarium periostracal variation is notable. It is thin and highly
vacuolated in G. pectinatum and G. tumidum, but in G. divaricatum
and G. dispar it is thick and homogeneous in texture. The significance of the vacuolated periostracum is unclear, but it also
occurs in Petricola and Petricolaria.
layer (Gafrarium/Circe group) represent the ancestral condition.
From this can be derived longer pins with more elaborate
organic coating and attached sediment (as in the Pitar group)
with, finally, long needles without coatings (Tivela, Lioconcha)
as the most derived state. The spectacular long needles of
Tivela, Gomphina and Lioconcha species appear very similar in
morphology, but they are widely separated on the phylogenetic
tree indicating separate derivations within clades with shorter
pins. Tivela mactroides and G. undulosa are sister taxa in the molecular analysis, and separate from T. (Pachydesma) stultorum
that lacks needles, while Lioconcha ornata groups within a clade
including Gafrarium, Circe and Hyphantosoma species.
A number of bivalves in Clade B such as Meretrix,
Megapitaria, Macrocallista and Callista lack needles and pins.
Species in these genera possess thick, glossy periostraca and are
active burrowers in dynamic sandy substrata where periostracal
coatings would be easily abraded. The other species Gemma
gemma, Turtonia minuta and Nutricola are very small but also
have glossy periostraca.
Within Clade A we observed calcified pins only in the Venus/
Mercenaria clade (Fig.19), which includes species classified into
the subfamilies Venerinae and Chioninae by Kappner &
Bieler (2006) and Mikkelsen et al. (2006). Compared to the
structures in Clade B, the pins are very short, of submicron
lengths, and form a dense covering. The exception is Chamelea
striatula that has somewhat longer, more slender pins. Wider
taxonomic sampling within this clade may provide further
insights into the distribution of pins and their utility as phylogenetic characters.
The distribution of the various types of needles and pins is
also correlated with phylogenetic groupings suggested by other
morphological characters. The shell microstructure of a wide
range of Veneridae was reviewed by Shimamoto (1986), who
demonstrated the existence of two major groups, those with an
outer shell layer of composite prismatic structure and those
with an outer layer of crossed-lamellar structure. These two
groups correspond with the two major clades (A and B) of
venerids identified in molecular analyses (Mikkelsen et al.,
2006); bivalves of Clade A have an outer shell layer of composite prismatic structure, while in bivalves of Clade B the outermost layer is of crossed-lamellar structure. Significantly, the
larger periostracal needles and pins occur only in venerids
of Group B with crossed-lamellar outer shell layers.
Submicron-sized pins occur in some shells with an outer composite prismatic layer (Group A), such as Mercenaria and Venus.
We suggest that the reflected shell and mantle margins necessary for the formation of composite prismatic structure, and evidenced by the back-curved growth increments, preclude the
formation of longer needles.
Although we have sampled only a small fraction of the total
diversity of Veneridae (c. 800 living species (Mikkelsen et al.,
2006)), it is clear that periostracal calcification is widespread
throughout the family and expressed in a remarkable variety of
structures that have hitherto been poorly documented. In
addition to the intraperiostracal pins and needles of venerids
described here, thick extraperiostracal calcareous cemented
structures have been described for Granicorium and Samarangia
(Taylor et al., 1999) and others are present on various Petricola
species, but not yet investigated in detail. Cooperella subdiaphana
(Petricolinae?) sometimes agglutinates surrounding sediment to
form a hard, surrounding encasement (Morton, 1995), but we
detected no periostracal calcification.
Phylogenetic distribution in Veneridae
In phylogenetic analyses the heterodont families Arcticidae,
Corbiculidae, Glauconomidae and Vesicomyidae have usually
been regarded as sister groups to the Veneridae (Giribet &
Distel, 2004; Mikkelsen et al., 2006; Taylor et al., 2007). Our
observations on one species from each of these families revealed
an absence of any form of intraperiostracal calcified needles or
pins. This suggests that the needles and pins are novel structures for the Veneridae. Although the Petricolinae have in the
past usually been regarded as a separate family, the two
species analysed by Mikkelsen et al. (2006) in their molecular
study fall within the Veneridae and they have therefore been
included within this family, as supported by their possession of
pins shown here.
Calcified pins occur in both the major clades (Fig. 19) identified in the molecular analysis of venerids by Mikkelsen et al.
(2006). Despite differences in size and coatings, all needles and
pins appear to be formed in the same way and our evidence
indicates their likely homology. Their distribution (Fig. 19)
suggests that the ability to form intraperiostracal calcification
is ancestral within the family. However, pins or needles are
absent from several groups within the two major clades, indicating either loss of the character or lack of expression. An
alternative reconstruction of character evolution is that pins
have been derived separately in the two clades.
Within Clade B the distribution of the different morphologies of the pins and their coatings (Fig. 19) suggests an
evolutionary scenario in which short pins with a thin mucoid
Comparison with calcified periostracal structures in other bivalves
Various calcified periostracal spikes and spines have been
recorded in other bivalve families, notably among
Anomalodesmata (Aller, 1974; Prezant, 1979, 1981; Harper
177
E. A. GLOVER AND J. D. TAYLOR
et al., 2006; A. Checa & E. Harper, pers. comm.);
Gastrochaenidae (Spengleria; Carter, 1978), Mytilidae (Carter,
1990), Unionidae (E. Harper, A. Checa & A. Zeiritz, unpubl.)
and Corbulidae (Lamprell & Healy, 1998: fig. 1D; E. Glover
& J. Taylor, unpubl.). Where investigated in any detail these
differ structurally and positionally from those described here
for Veneridae and on present evidence homology is unlikely.
Similarly, intraperiostracal calcified granules seen in some
Mytilidae (Bottjer & Carter, 1980; Carter, 1990) and Lucina
pensylvanica (Taylor et al., 2004) have been separately evolved.
Present evidence therefore suggests that calcification of the
periostracum has occurred independently in different bivalve
clades. We hypothesize that slight acceleration in the onset of
the calcification front at the shell margin might result in the
secretion of periostracum simultaneously with the nucleation
and growth of aragonite crystals. Some selective advantage for
this early periostracal calcification could favour more ordered
crystallization and spine or needle morphologies. Compared
with other bivalves, Veneridae appear to effect more control
on crystal growth and morphology via the tissue-filled channels
that penetrate the periostracum beneath each needle or pin.
anchor points for the thin overlying organic crust. The only
suggestion we can offer is that the pins and coating act as an
extra layer of protection for the periostracum and outer shell
surface. A similar strategy may be seen in Lyonsia species and
some other anomalodesmatans that are often coated in sand
(Prezant, 1979; Harper, et al., 2006); in these the sediment is
attached to mucoid threads that are in turn anchored by periostracal spinules. The mucoid threads are produced by the arenophilic glands at the mantle margin (Sartori et al., 2006).
Prezant (1979) suggested that these coatings produce surface
resistance and thereby provide greater stability for the bivalve
within the sediment and also act as armour protecting the
shell.
Concluding remarks
Bivalve molluscs are remarkable for the diversity of different
shell microstructures and have been widely used as experimental animals in studies of biomineralization processes. The striking, 400 mm long, aragonitic crystalline needles of Lioconcha
and Tivela, coupled with the range of other pins and adherent
structures, add further dimensions to the biomineralization
potential of bivalves. They represent a highly organized, earlyphase mineralization that differs from normal shell secretion in
that each needle is directly associated with extensions of
mantle epithelia that occupy channels through the
periostracum.
Finally, it should be stressed that the needles and pins plus
the adherent organic layers in which sediment is embedded are
integral parts of the functional shell and it is unfortunate that
so many specimens have been routinely ‘cleaned’, removing
these spectacular structures.
Function
In Veneridae the calcified needles and pins, along with the
various sediment and organic coatings plus the periostracum,
must be considered as integral parts of the functional shell. In
most bivalves the periostracum forms the outer barrier of the
shell and is the primary substrate for shell calcification
(Harper, 1997), but venerids have supplemented the periostracum with additional calcified, organic and sediment layers.
Although we have no direct evidence, their functions might
relate to burrowing performance, act as barriers against predation, enhance abrasion resistance and protect against endolithic
boring organisms.
For the species with long needles these form the outer
barrier of the shell. They extend all over the shell in juvenile
specimens, but in larger individuals the needles are generally
worn off from dorsal areas and remain as a dense, continuous
fringe around the ventral margins of the shell. In Lioconcha
species and Tivela lamyi this effectively increases shell thickness
by about 20– 25%. The functional effectiveness is thus likely to
be highest around the marginal areas. Ohno (1996) tested the
burrowing rates of Lioconcha fastigiata with and without the
needle coating and found no significant differences, concluding
that needles do not contribute to burrowing efficiency.
However, he suggested that the needles might increase the friction of the shell surface, thus increasing stability of shell orientation in loose sandy substrates. Although we have no direct
evidence it seems more likely that the needles form a protective
barrier around the vulnerable shell margins and perhaps act as
a deterrent against predation, for example, by shell-drilling
gastropods.
In life, the shells of species of several venerid genera,
Callocardia, Clementia, Compsomyax and Pitar, are encrusted to a
greater or lesser extent with sediment coatings, the sediment
adhering to, or embedded in, a fibrous, organic ‘glue’ and all
anchored to the calcareous pins beneath. The thickest encrustations in the thin-shelled species C. hungerfordi and Clementia
papyracea are two or three times thicker than the true shell and
may serve as a physical protection against predation or as
camouflage. In Pitar species the sediment encrustations are less
extensive and are usually retained around the ventral and posterior margins, where again they may serve to protect the
thinner valve margins. Species of Circe, Gafrarium and Mysia
have thin, but robust, organic coverings resting on the intraperiostracal pins, with little adherent sediment. Finally, the
submicron-sized pins of Mercenaria, Venus and allies may act as
ACKNOWLEDGEMENTS
We thank Liz Harper (University of Cambridge) for much
interest and stimulating discussion and Gordon Cressey
(NHM) for X-ray diffraction analysis and discussion on crystallography. Philippe Bouchet (MNHN, Paris) kindly loaned
uncleaned specimens of Lioconcha castrensis, and Paul Valentich
Scott (Santa Barbara Museum of Natural History) donated
specimens of Compsomyax subdiaphana and sent images of Pacific
coast Tivela bryonensis. We are grateful to Harry Taylor (NHM)
for macrophotography and Alex Ball and Lauren Howard
(EM Unit, NHM) for frequent advice. Specimens from
Kungkrabaen Bay, Thailand, were collected as part of the
International Bivalve Workshop 2005, supported by the US
National Science Foundation PEET Program. Clementia papyracea and Pitar inflata were collected in Moreton Bay, Australia,
from the Moreton Bay Research Station as part of the Bivalve
Tree-of-Life project (http://www.bivatol.org/) supported by
the NSF AToL programme (DEB-0732854/0732903/0732860).
We thank the Department of Zoology, NHM, for continuing
support.
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