BATHYAUSTRIELLA THIONIPTA, A NEW LUCINID BIVALVE FROM
A HYDROTHERMAL VENT ON THE KERMADEC RIDGE,
NEW ZEALAND AND ITS RELATIONSHIP TO SHALLOW-WATER
TAXA (BIVALVIA: LUCINIDAE)
EMILY A. GLOVER 1 , JOHN D. TAYLOR 1 AND ASHLEY A. ROWDEN 2
2
1
Department of Zoology, Natural History Museum, London SW7 5BD, UK;
Marine Biodiversity Group, National Institute of Water and Atmospheric Research, PO Box 14-901, Wellington, New Zealand
(Received 20 October 2003; accepted 16 January 2004)
ABSTRACT
Bathyaustriella thionipta, a new species from water depths of 480–500 m on the Macauley Caldera,
Kermadec Ridge, is the first record of a member of the chemoautotrophic bivalve family Lucinidae
from an active hydrothermal vent. The new species and genus is morphologically similar to the shallowwater, mangrove mud-inhabiting Austriella corrugata from the central Indo-West Pacific region.
Sequences of18S and 28S rRNA genes confirm its position close to Austriella within a clade of shallowwater lucinids. Features of B. thionipta include the extremely large gill, the vacuolated foot, two coiled,
lobate structures arising from the anterior body wall and a very thick periostracum. Its probable close
relationship to shallow-water lucinids lends support to an hypothesis proposing the onshore to offshore
derivation of deep-sea and vent faunas.
INTRODUCTION
Since the discovery of chemosynthetic communities associated
with hydrothermal vents, a variety of bivalve molluscs from a
wide range of marine habitats have been identified as possessing
chemautotrophic symbiosis with sulphide- and methaneoxidizing bacteria. The symbiosis is documented in six major
groups of bivalves: Solemyidae, Mytilidae, Vesicomyidae,
Thyasiridae, Lucinidae and Teredinidae (Reid & Brand, 1986;
Fisher, 1990; Distel, 1998 ; Distel & Roberts, 1997). Of these, the
Lucinidae are by far the most diverse (~500 species), and geographically widespread. They occur in the broadest range of
habitats, ranging from intertidal mud, seagrass and mangroves
to depths of more than 2000 m (Taylor & Glover, 2000; Johnson,
Fernandez & Pergent, 2002; BMNH collections). Although
deeper-water lucinids have been reported in association with
sites of organic enrichment (R. von Cosel & P. Bouchet, personal communication), oxygen minimum zones (Cary et al., 1989;
Levin et al., 2000) and various cold seeps (Carney, 1994; Callender & Powell, 2000; Salas & Woodside, 2002), they have not
previously been recorded from hydrothermal vents.
An abundant, large lucinid recently discovered at Macauley
Caldera (Kermadec Ridge, New Zealand Exclusive Economic
Zone) (Fig. 1) is the first known record of the Lucinidae associated with a hydrothermal vent and is therefore of considerable
ecological and evolutionary interest. The aims of this paper are
to describe this new species and to establish its relationship to
other lucinids, using morphological and molecular techniques.
This is one of many new vent organisms discovered during
ongoing research on the seamounts associated with the
Kermadec Ridge (Clark & O’Shea, 2001; Cosel & Marshall,
2003; Smith et al., 2004).
The Kermadec Ridge extends north of New Zealand for about
1000 km. The active arc, lying 15–25 km west of the ridge,
comprises around 15 submarine volcanoes, some of which are
known to be hydrothermally active (de Ronde et al., 2001).
Correspondence: E.A. Glover; e-mail: [email protected]
J. Moll. Stud. (2004) 70: 283–295
Macauley Caldera is located on the northwestern submarine
flank of Macauley Island, one of the Kermadec Islands that
constitute the aerial component of the Kermadec Ridge (Fig. 1).
The caldera is 10.5 km long and 7 km wide, with water depths at
the rim of around 600 m and floor depths of over 1100 m. The
caldera rim is dotted with small cone volcanoes that emit lava
flows. A larger young, active cone (‘Macauley Cone’) lies on the
eastern caldera wall and rises to a water depth of 250 m. Rock
samples from this cone include fresh, glassy, aphyric andesite
(Wright et al., 2002a). The newly discovered ‘Giggenbach’
volcano lies 24 km to the northwest of Macauley Caldera.
Giggenbach is an irregular volcano 8.5 km wide and 13 km long
that rises from 1500 m to 65 m water depth. The crest of the
volcano comprises a shallow caldera with a central cone
(‘Giggenbach Cone’) (Wright et al., 2002b) (Fig. 1).
Most chemosymbiotic bivalves associated with hydrothermal
vents are species of Bathymodiolus (Mytilidae) and Calyptogena
(Vesicomyidae) (Tunnicliffe, McArthur & McHugh, 1998;
Cosel, 2002) with occasional reports of Acharax (Solemyidae)
(Métivier & Cosel, 1993) and Thyasira species (Thyasiridae)
(Southward et al., 2001). Bivalves are more diverse at cold-seep
sites and species of Bathymodiolus, Calyptogena, Vesicomya,
Acharax, Solemya and various Thyasiridae have been reported
along with several Lucinoma (Lucinidae) species (Sibuet & Olu,
1998; Fujikura et al., 1999, 2002; Cosel, 2002; Sahling et al., 2002;
Imhoff et al., 2003). Other chemosymbiotic bivalves, but not
lucinids, have been reported associated with whale falls and
sunken wood (Smith & Baco, 2003). The absence of Lucinidae
from vent sites is puzzling. McArthur & Tunnicliffe (1998) suggested the lack of suitable habitats for sediment-burrowing
bivalves and also the possible preference of lucinids for sites
with lower sulphide levels.
A topic of continuing interest concerns the origin of the
vent and seep bivalves and whether these represent ancient
lineages of deep-sea taxa or recent invaders from shallow water
(Newman, 1985; McArthur & Tunnicliffe, 1998; Little & Vrijenhoek, 2003). For example, a molecular analysis of Mytilidae suggests that vents and seeps have been colonized from shallow
© The Malacological Society of London 2004
E. A. GLOVER ET AL.
water and that whale falls and decomposing wood may have
acted as stepping stones to chemosynthetic habitats (Distel et al.,
2000). Molecular analysis of vent and seep Vesicomyidae indicate a Cenozoic radiation (Peek et al., 1997; Goffredi et al.,
2003), but relationships to non-vent and seep taxa (Cosel &
Salas, 2001) have not yet been established.
A first step towards determining the origin of the vent and
seep faunas is to establish the phylogenetic relationships of the
taxa to non-vent and seep congeners or outgroups. The wide
habitat range and high diversity of Lucinidae and their likely
facultative endosymbiosis (Reid, 1990, 1998; Taylor & Glover,
2000) offer an opportunity to investigate the evolutionary pathways of the vent and seep taxa. A recent molecular phylogeny of
Lucinoidea (Williams, Taylor & Glover, 2004) has established
the monophyly and identified several major clades of the
Lucinidae. Morphological and molecular data from the new
vent lucinid can now be used to establish its position in relation
to shallow-water species and other lucinids.
METHODS
Anatomy
Thin sections were made of mantle, foot and gill tissue from
formalin-fixed samples. Poor fixation of these previously frozen
samples precluded detailed histological study. For scanning
electron microscopy (SEM) of gill filaments, pieces of ctenidia
were cut with a razor blade, dehydrated through an ascending
series of acetone solutions and critical-point dried.
Shell microstructure
Shells were cut, polished and etched with 0.1 M HCl, and
acetate-peel replicas prepared and examined by optical
microscopy. The same cut and etched sections of shell were also
mounted on stubs and examined by SEM. The mineralogy of
the shell was determined using X-ray diffraction of both shell
pieces and powdered shell.
Figure 1. Map showing the position of the Kermadec Ridge with respect to New Zealand and the site locations where specimens of Bathyaustriella thionipta were
collected. Star marks type locality.
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Figure 2. Seabed photograph showing dead valves of Bathyaustriella on the sediment surface with probable living animals protruding from sediment (B). Large,
elongate shells (G) are a bathymodioline mussel, probably Gigantidas gladius Cosel & Marshall, 2003, around 200–300 mm long.
from pallial line for about three-quarters of length. Ctenidia
extremely large, cream in colour, almost filling mantle cavity.
Body wall with anterior pair of curled, lobate extensions. Foot
broad with a wide lumen terminating in a strongly muscular tip.
Posterior inhalant aperture surrounded by papillae, with long
zone of mantle fusion ventral to the aperture.
Comparison with other genera: Bathyaustriella is most similar in shell
form to the shallow water genus Austriella Tenison Woods, 1881
(type species A. sordida Tenison Woods, 1881 Lucina corrugata
Deshayes, 1843) ( Eamesiella Chavan, 1951 Pseudolucina
Chavan, 1947). Austriella corrugata burrows deeply in intertidal
mud at the fringes of mangroves at locations in the central tropical Indo-West Pacific (J.D. Taylor & E.A. Glover, personal observation). Overall shell shape, the thick periostracum, ligament
morphology and the edentulous hinge are similar in the two
genera. They differ in external shell sculpture, Austriella having
more regular commarginal lamellae (Fig. 9) and more prominent anterior and posterior sulci. Internally, the anterior adductor muscle of Bathyaustriella (Fig. 5) is longer and lies closer to
the pallial line (16–18° divergence compared with 26–28° in
Austriella). Both Austriella and Bathyaustriella have large ctenidia,
but they are extremely large in the latter, filling most of the
mantle cavity. A unique structure present in Bathyaustriella, but
absent in Austriella, is the paired, curled extension of the visceral
Institutional abbreviations
BMNH, Natural History Museum, London
NIWA, National Institute of Water and Atmospheric Research,
New Zealand
NMNZ, Museum of New Zealand Te Papa Tongarewa, Wellington
SYSTEMATIC DESCRIPTION
Family Lucinidae Fleming, 1828
Bathyaustriella new genus
(Figs 3–8)
Type species: Bathyaustriella thionipta new species
Etymology: From Greek bathys – deep: deep-water lucinid similar
to Austriella.
Diagnosis: Shells medium size, white; height (H) to 48.5 mm,
length (L) to 54.0 mm, subovate, posteriorly truncate (H/L
0.90), moderately inflated, with low, irregular commarginal
growth increments. Periostracum thick. Hinge edentulous; ligament long and broad. Anterior adductor muscle long, detached
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Figure 3. Bathyaustriella thionipta A, B. Outside and inside of right valve of holotype H-838, NIWA3277. C. Inside of left valve of holotype. D. Outside of right
valve of paratype BMNH 20030580. E. Inside of left valve of same paratype. F. Inside of right valve of paratype NMNZ M.273197. Scale bars 10 mm.
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Figure 4. Bathyaustriella thionipta. A, B. details of hinges of left and right valves of paratype NMNZ M.273197. C, D. Details of hinge of left valve of holotype to
show massive solutional erosion of the umbonal and hinge areas. Abbreviation: r ridge. Scale bars 10 mm.
Figure 5. Semi-diagrammatic images of inside of shells of Bathyaustriella thionipta and Austriella corrugata to show length and shape of the anterior adductor
muscle scar. A. Bathyaustriella thionipta. B. Austriella corrugata (Deshayes, 1843), Zamboanga, Philippines (BMNH).
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Figure 6. Anatomy of Bathyaustriella thionipta. A. Animal from left side with mantle removed. Scale bar 10 mm. B. Same animal as A but with left demibranch
removed. C. Detail of coiled, lobate structure on body wall. D. Foot of another specimen. E. Posterior apertures. Scale bar 2.0 mm. F. Detail of inhalant
aperture to show papillate margin. Abbreviations: aa, anterior adductor muscle; cle, coiled lobate extension; ea, exhalant aperture; fm, fused mantle; ft, foot;
h, heel of foot; ia, inhalant aperture; ld, left demibranch; lp, labial palps and lips; pa, posterior adductor muscle; rd, right demibranch of gill; sp, spur of foot; tm,
thickened mantle; vm, visceral mass. Scale bars: A 10 mm; C 5 mm; E 2.0 mm; F 200 m.
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Figure 7. A. Critical-point dried, transverse section of gill of Bathyaustriella thionipta showing filaments and interlamellar junctions. B. Coccoid bacteria within
the bacteriocyte zone of the gill filaments. Abbreviations: b, bacteriocyte zone; ij, interlamellar junctions. Scale bars: A 200 m; B 2 m.
central Indo-West Pacific region. At present, there is no generic
name to accommodate these species. Molecular sequences
obtained from ‘Lucina’ dalli from Hong Kong (Williams et al.,
2004) cluster with those of Austriella corrugata, suggesting a
relationship (Fig. 11).
Bathyaustriella thionipta new species
(Figs 3–8)
Types: Holotype: H-838, NIWA 3277. Figs 3A–C, 4C, D. H 42.7 mm, L 46.3 mm. Type locality: Kermadec Ridge,
Macauley Cone, 30°13.034S 178°27.112W, 480–504 m
(21.04.2002 RV Tangaroa NIWA TAN0205 station 55). Paratypes: from type locality. Figured paratypes: Fig. 3D, E, BMNH
20030580 (H 43.7 mm, L 47.5 mm); Figs 3F, 4A, B, NMNZ
M.273197 (H 26.6 mm, L 28.5 mm). Other paratypes: 10
shells BMNH 20030581; 10 shells NMNZ M.273197; 4 shells
P-1389, NIWA 3278.
Other material examined: Ten whole animals in 100% ethanol; 13
formalin-fixed whole animals (BMNH); all from type locality.
Etymology: From the Greek thio – sulphur and nipta – bath.
Shell morphology: Shell white, medium large (H to 48.4 mm; L to
54.0 mm), subovate, longer than high (H/L 0.9). Moderately
inflated, shell tumidity /length 0.3. Outline variable (Fig. 3),
posterior margin often quadrate, anterior margin rounded.
Periostracum extremely thick, fibrous, dehiscent in dried
specimens, buff-yellow in colour, often stained brown around
posterior exhalant aperture. Shell sculpture of irregular, discontinuous, low, rounded, commarginal folds and growth lines.
External shell surface chalky. Shallow posterior sulcus.
Umbonal region usually with extensive shell dissolution penetrating into lunule and hinge plate (Fig. 4D). Lunule small,
heart-shaped, symmetrical. Hinge narrow, edentulous, anteroventral edge often with irregular undulations, cardinal area
occasionally with a thin, posteriorly directed ridge or fold (Fig.
4C). Ligament long, thick, slightly sunken; nymph a prominent
calcified ridge. Narrow tongue of lamellar ligament extends to
posterior margin of shell. Anterior adductor scar long, ventrally
detached from pallial line for three-quarters of length, diverg-
Figure 8. Portion of longitudinal section through wall of mid-portion of
foot. Semi-diagrammatic tracing from digital image of thin section.
Abbreviation: lm, longitudinal muscle layer. Scale bar 250 m.
mass (Fig. 6). The extent of posterior mantle fusion is similar in
the two genera, but the posterior inhalant aperture of Austriella
lacks papillae.
Other lucinids with similar shell form, but smaller in size and
possessing lateral teeth, include ‘Lucina’ plicifera (Adams, 1856)
and ‘Lucina’ dalli Lynge, 1909. These are little known but live in
intertidal muddy sands, often within mangrove fringes, of the
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complex crossed-lamellar structure. Area beneath umbones
thickly prismatic with areas of radiating prisms with spherulitic
structure.
ing at an angle of 11–18° (Fig. 5). Posterior muscle scar small,
ovate. Anterior pedal muscle scar detached from the adductor.
Pallial line continuous but indistinct. Trace scar of pallial blood
vessel prominent in larger specimens. Shell margin outside
pallial line smooth, glossy, with indistinct radial grooves. Inner
surface of shell within pallial line often lumpy or pitted, sometimes with irregular radial ridges or pits. Interior of shell white
or yellowish.
Anatomy: Mantle relatively thick with prominent margin folds.
Within pallial line the outer mantle epithelium comprises tall
columnar cells. Area between anterior adductor muscle and
inner mantle fold is considerably thickened and folded.
Sections of this region show highly vacuolated mantle tissue that
is most probably blood space. Posterior exhalant aperture with
short inverted tube; inhalant aperture with numerous, small
stalked papillae on the middle mantle fold (Fig. 6F). Long section of mantle fusion ventral to the inhalant aperture (Fig. 6E)
involves inner and most of middle mantle folds (Mantle Fusion
Index 0.6; see Taylor & Glover, 2002).
Shell microstructure: Within a thick periostracum (~100 m), shell
comprises three layers of different aragonitic microstructures.
Outer layer (~400 m) consists of spherulitic prismatic structure, middle layer (up to 1200 m) comprises crossed-lamellar
structure and inner layer within the pallial line is almost entirely
made up of sheets of aragonitic prisms with small patches of
Figure 9. Austriella corrugata (Deshayes, 1843). A. Outside of right valve; Kisseraing Island, Mergui (BMNH 1887.3.10.435). B. Inside of right valve. C. Detail of
hinge and ligament. D, E. Outside and inside of left valve of juvenile specimen from Zamboanga, Philippines (BMNH). Scale bars 10 mm.
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junctions. Poor preservation limited detailed study of the gill
structure, but within the lateral zone of each filament bacteriocytes are densely packed with small, coccoid bacteria 1.5–1.8 m
long and 0.8–1.0 m wide (Fig. 7B).
Foot long, relatively broad, with a large, highly vacuolated
lumen with spongy-textured walls. An outer layer of longitudinal
Ctenidia composed of single, extremely large demibranchs,
cream to pale yellowish brown in colour, that blanket all of the
body and occupy most of mantle cavity (Fig. 6A). Demibranchs
thick (1500 m wide) with short, broad individual filaments
(100–120 m wide and ~500 m long) (Fig. 7A). Ascending and
descending lamellae connected by abundant interlamellar
Figure 10. Austriella corrugata (Deshayes, 1843), King Bay, Dampier, Western Australia (BMNH). A. Posterior apertures. B. General view of anatomy, with right
mantle largely removed. C. Section of ctenidia showing bacteriocytes containing rod-shaped bacteria. Abbreviations: aa, anterior adductor muscle; ex, exhalant
aperture; fm, fused mantle; ft, foot; ia, inhalant aperture; lp, labial palps and lips; mg, mantle gills; pa, posterior adductor muscle; rd, right demibranch of gill.
Scale bars: A 5.0 mm; B 10 mm; C 10 m.
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Figure 11. Molecular phylogeny of the family Lucinidae produced by Bayesian analysis of partial sequence data from 18S rRNA gene (from Williams et al., 2004:
fig. 1). Stars indicate branches with greater than 90% posterior probabilities.
Other specimens of Bathyaustriella were recovered by a subsequent sampling effort conducted by the Institute of Geological
and Nuclear Sciences (IGNS) at additional stations on
‘Macauley Cone’ (30º12.65S, 178º26.94W, 230 m, 10/5/2002,
RV Tangaroa. IGNS NZAPLUME II station 11, 5 alive;
30º12.91S, 178º27.19W, ‘Macauley Cone’, 335 m, 10/5/2002,
RV Tangaroa. IGNS NZAPLUME II station 13, 3 alive) and on
Giggenbach Cone (30º02.41S, 178º42.93W, 162 m and shallower, 10/5/2002, RV Tangaroa IGNS NZAPLUME II station
7,1 alive; 30º02.35S, 178º42.86W, 144 m and shallower,
10/5/2002, RV Tangaroa. IGNS NZAPLUME II station 8, 1 alive;
30º02.44S, 178º42.80W, 175 m and shallower, 10/5/2002, RV
Tangaroa. IGNS NZAPLUME II station 9, 8 alive) (Fig. 1). On
deck temperature measurements made of the sediment recovered with specimens from Giggenbach Cone indicated values of
~50°C (C. de Ronde, personal communication).
muscles runs length of foot, within this an inner region of
weaker circular muscles that thicken towards the tip (Fig. 8). Tip
of foot solidly muscular with a narrow lumen lacking the spongy
structure. Epithelium of distal foot highly glandular with two
types of sub-epithelial gland cells similar to those described for
other lucinids (Allen, 1958; Taylor & Glover, 1997). Heel of foot
extends posteriorly along ventral part of body wall (Fig. 6B).
Labial palps small folds at edge of thin lips. Body wall smooth
and muscular, extending anteriorly and bilaterally into a semicoiled structures ~7.5 mm in length with lobate distal terminations (Fig. 6B, C). Internally, these contain extensions of digestive diverticula.
Gene sequences: 18S and 28S rRNA sequences for Bathyaustriella
thionipta, cited as ‘Lucina’ Kermadec Ridge, GenBank numbers:
AJ581858; AJ581892 were published in Williams et al. (2004).
Distribution and habitat: Specimens of Bathyaustriella thionipta
were first recovered by rock dredge from Macauley Cone
(NIWA station TAN0205/55) where 1,259 live animals were
collected at a depth of 480–504 m. The substrate consisted of
volcanic sand, rock and rubble percolated by hydrothermal
fluids. Abundant shells of Bathyaustriella can be seen on a
number of seabed photographs taken on a camera transect conducted across the site (NIWA transect station TAN0205/79)
(Fig. 2) and numerous probable living animals protrude from
the sediment (arrow). Samples from this site include a range of
sizes, from a juvenile shell height of 5.4 mm to shells of 48 mm.
Also visible in Figure 2 are specimens of bathymodioline vent
mussels (Smith et al., 2004); these are similar in form to the
large (up to 316 mm long) Gigantidas gladius recently described
from nearby submarine volcanoes on the southern Kermadec
Ridge (Cosel & Marshall, 2003).
DISCUSSION
Bathyaustriella thionipta, the first lucinid recorded from a hydrothermal vent, is clearly an abundant, actively recruiting component of the benthic fauna at the Macauley Caldera where it
lives in association with bathymodioline mussels (Fig. 2).
Samples found at another vent site at the Giggenbach submarine volcano suggest that it may well be quite widespread at
volcanic vents on the Kermadec Ridge.
The morphology of B. thionipta is generally similar to other
described Lucinidae (Allen, 1958; Taylor & Glover, 2000).
Unusual features are the very large gill occupying most of the
mantle cavity; the foot with a large spongy lumen; the coiled
lobate structures extending laterally from the body wall and the
extremely thick periostracum. The vacuolated, spongy foot of
Bathyaustriella differs from other lucinids where usually the foot
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myidae and Bathymodiolinae, are often abundant together with
occasional Solemyidae. At hydrothermal vents, sediment accumulations suitable for burrowing animals are less common, but
several species of Thyasiridae have been reported, indicating
that occupation of the habitat by burrowing species is possible
(Southward et al., 2001). Furthermore, the high temperatures in
vent sediments might preclude occupation by more deeply
burrowing animals. In contrast, lucinids, particularly Lucinoma
species, have been recorded from cold seeps (Callender &
Powell, 2000; Fujikura et al., 2002, Salas & Woodside, 2002),
although apparently less commonly than from fossil seep
deposits (e.g. Taviani, 1994; Kelly et al., 2000) where they were
often abundant. An explanation may be that at modern seep
sites most attention has been given to the larger epifaunal animals with less sampling effort devoted to the deeper-burrowing
bivalve infauna.
Although Lucinidae have been generally considered as a
dominantly shallow-water family, recent discoveries from the
tropics (R. Cosel & Bouchet, in preparation; personal observations of museum collections) reveal an additional, unsuspected
deep-water diversity. These findings demonstrate that they have
a much greater depth range (at least to 2500 m) than previously
reported and occur in a broad range of deeper-water habitats
such as organically enriched areas, oxygen-minimum zones and
cold seeps. Little is known of the systematics and relationships of
these deep-water lucinids, but it might be assumed that because
of their facultative chemosymbiosis they are capable of colonizing a wide range of bathyal habitats including vents and seeps. In
reviewing the composition of seep faunas of the northeastern
Pacific, Levin et al. (2000b) suggested that the infauna of cold
seeps comprises shelf and slope taxa pre-adapted to organicrich sediments. The evidence presented here of a relationship
of Bathyaustriella to Austriella and other shallow-water lucinids
supports the hypothesis of the colonization of vents from
shallow waters (Little & Vrijenhoeck, 2003) and accords with
the more general pattern of onshore-offshore evolutionary
diversification of marine animals (Jablonski & Bottjer, 1991;
Jacobs & Lindberg, 1998).
is narrow and highly muscular (Allen, 1958; Taylor & Glover,
1997). In Lucinidae, the sulphide-containing interstitial water is
drawn into the mantle cavity and over the gills as a result of the
foot probing the anoxic sediment. In the vesicomyid Calyptogena
at hydrothermal vents, uptake of sulphides is hypothesized to
take place through the wall of the foot, which projects ventrally
into cracks in the rock substrate to reach the flow of hydrothermal fluids (Kennish & Lutz, 1992). This leads us to speculate that the large foot of Bathyaustriella might also have a role in
sulphide uptake, although more detailed investigation is necessary to test this idea. The coiled lobate structures appear to
contain a mass of digestive diverticula, but study of material with
improved fixation is needed to determine their possible function.
On morphological evidence, Bathyaustriella thionipta is similar
to the tropical, intertidal, mangrove-inhabiting Austriella corrugata (Fig. 9), although the symbiotic bacteria of Bathyaustriella
are small and coccoid whereas those of Austriella are large and
cylindrical (Figs 7B, 10C). Shared features include the thick
periostracum, similar structure of hinge and ligament, shell
shape and size, the form of the anterior adductor muscle, the
large ctenidia, extent of posterior mantle fusion and shell
microstructure. At the present day, the nearest known populations of Austriella to New Zealand occur in Queensland,
Australia. Their morphological similarity is corroborated by the
molecular results (Williams et al., 2004). The sequences from
Bathyaustriella clustered within a large well-supported clade (18S
rRNA; posterior probability 98%, bootstrap 95%) of shallowwater Lucinidae (Fig. 11) including Austriella, ‘Lucina’ dalli, as
well as Divaricella, Lucina, Loripes and Wallucina (Fig. 11).
Groupings within this clade were generally only weakly supported, but Bathyaustriella clustered with Austriella, ‘Lucina’ dalli
and Divaricella irpex (Williams et al., 2004). There was higher support for a relationship between Austriella corrugata and ‘Lucina’
dalli in the analysis of the combined sequences from 18S and
28S rRNA genes. What is certain is that Bathyaustriella clusters
within a clade of shallow-water lucinids, but distant from clades
containing other deeper water taxa such as the ‘Myrtea’ group
and Lucinoma (Williams et al., 2004). We conclude that Bathyaustriella is derived from a group of lucinids currently occupying
shallow-water habitats. Some genera in this shallow-water clade
date from the middle Eocene, but most date from the Oligocene or younger (Chavan, 1969). However, the fossil record is
still poorly resolved for most lucinids.
Although a relationship between an intertidal lucinid and a
vent species might seem surprising, we know that Austriella corrugata lives in one of the more extreme habitats inhabited by
shallow-water lucinids. Nothing is published on its biology, but
we have observed the species living deeply burrowed in sulphide-rich, mangrove mud around Dampier, Western Australia,
where it co-occurs with another mangrove specialist Anodontia
philippiana (Reeve, 1850). Together with Bathyaustriella, Austriella has an unusually thick, fibrous periostracum as a likely
adaptation to resist shell dissolution in highly acidic environments such as hydrothermal fluids at the sediment surface or in
the anoxic sub-surface muds in mangrove habitats. Even with
this protection both the deeply burrowed Austriella and the
semi-infaunal Bathyaustriella usually exhibit similar extensive
shell erosion of the umbonal area in living animals. We speculate that adaptations, such as a thick periostracum and tolerance
of dysoxic conditions, necessary for survival both in the mangrove muds and at the vent site, may be similar. Additionally, the
volcanic vents on the Kermadec Ridge from which Bathyaustriella have been recorded are 500 m in depth, increasing
the potential for colonization by shallow water taxa.
Despite the ubiquity of chemautotrophy in Lucinidae, they
have not been recorded previously from hydrothermal vents.
Other bivalves with chemosymbionts, the epifaunal Vesico-
ACKNOWLEDGEMENTS
The authors are grateful to Suzanne Williams (BMNH) for the
molecular sequences of Bathyaustriella, to Gordon Cressey
(BMNH) who made the X-ray determinations of shell mineralogy, to Alex Ball (BMNH) for help with Figure 6E and to
Richard Garlick (NIWA) for processing recent bathymetric data
for the Macauley area and the resulting production of Figure 1.
The present description is part of NIWA’s Seamounts: their importance to fisheries and marine ecosystems programme (Foundation
for Research Science and Technology (FRST) project
C01X0224). The collection of type specimens and seabed
images was made possible by participation in NIWA voyage
TAN0205 led by Ian Wright (FRST project CO1X0203), while
Cornel de Ronde kindly provided additional specimens resulting from IGNS voyage NZPLUME II. Bruce Marshall (NMNZ) is
acknowledged and thanked as the person who, after receipt of
molluscs for identification from the above voyages, recognized
the potential taxonomic significance of the lucinid specimens.
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