ULTRASTRUCTURE OF EPIDERMAL CILIA AND CILIARY
ROOTLETS IN SCAPHOPODA
KENNET LUNDIN 1 , CHRISTOFFER SCHANDER 2,3 AND CHRISTIANE TODT 2
1
Go¨teborg Natural History Museum, Section of Invertebrate Zoology, PO Box 7283, SE-48235 Go¨teborg, Sweden;
2
University of Bergen, Department of Biology, PO Box 7800, 5020 Bergen, Norway;
3
Centre for Geobiology, Alle´gaten 41, 5007 Bergen, Norway
(Received 22 February 2008; accepted 24 October 2008)
ABSTRACT
Ciliary structure in Scaphopoda is hitherto unknown and may provide information useful for phylogenetic analyses. Here we describe the ultrastructure of the ciliary apparatus of multiciliated epidermal cells of four species of Scaphopoda: Antalis entalis, Antalis occidentalis, Entalina tetragona and Cadulus
propinquus, revealed by transmission electron microscopy. In all studied species the cilia have long
whip-like distal ends. The rootlet apparatus consists of a basal foot, a short anterior ciliary rootlet
and a long vertical rootlet. In other molluscan classes, the presence of an anterior rootlet has previously only been shown in species of the Neomeniomorpha, Chaetodermomorpha and
Polyplacophora, while such a rootlet is absent in Gastropoda, Bivalvia and Cephalopoda. Twin rootlets, such as present in species of lamellibranch Bivalvia and postembryonic Cephalopoda probably
represent a split vertical rootlet. The discovery of an anterior rootlet in Scaphopoda shows that the
presence of paired ciliary rootlets is not a synapomorphy of a clade comprising the aplacophoran
Neomeniomorpha and Chaetodermomorpha and the Polyplacophora, but that it represents a plesiomorphy of the Mollusca.
INTRODUCTION
The Scaphopoda are a taxon of molluscs comprising slightly
more than 500 recent species (Steiner & Kabat, 2001). Its phylogenetic position in relation to other molluscan classes has
long been a matter of discussion. Runnegar & Pojeta (1974)
introduced the so-called Diasoma concept, uniting scaphopods
and bivalves based on palaeontological data. This view seemed
to be well supported by similarities in the nervous system, the
burrowing foot and a seemingly twofold shell formation (see
Haszprunar, 2000). Based on further morphological data
Waller (1998), in contrast, placed the scaphopods as close
relatives of gastropods and cephalopods. Wanninger &
Haszprunar (2001) also showed that the assumed two-fold shell
formation in the Scaphopoda is a misinterpretation, and that
scaphopods in fact only have one larval shell. Moreover, the
Diasoma-concept has been challenged based on molecular
data (Steiner & Dreyer, 2003; Halanych, 2004; Passamaneck,
Schander & Halanych, 2004) that place Scaphopoda as the
sister group to Cephalopoda.
Ultrastructural characters of the ciliary apparatus of multiciliated epidermal (ectodermal) cells are useful in evaluations of
phylogeny, because there is a high probability of homology
when comparing ciliary structures from different taxa (cf. Tyler,
1979). Ciliary rootlets have also previously been shown to be
phylogenetically informative in molluscs (Lundin & Schander,
1999, 2001a, b, c) and related taxa (Lundin, 1997, 1998; Lundin
& Schander, 2003). There are for example clear differences
between the Conchifera, the Polyplacophora and the aplacophoran taxa Neomeniomorpha and Chaetodermomorpha.
In the scaphopod mantle cavity, anterior to the anal bulb,
there is a series of transverse ciliary bands ranging in number
from 1 to 30 depending on species and body size (for details
see Steiner, 1991). These circulate water through the mantle
cavity from the posterior opening to the anterior one. Water
Correspondence: K. Lundin; e-mail: [email protected]
can also be expelled from the mantle cavity through the posterior opening by withdrawal of the foot (Reynolds, 2006).
In this paper we describe the ultrastructure of mantle cavity
cilia of scaphopods with special focus on the ciliary rootlet
structure and discuss the phylogenetic implications of our
findings.
MATERIAL AND METHODS
Specimens of Antalis entalis (Linnaeus, 1758), Antalis occidentalis
(Stimpson, 1851), Entalina tetragona (Brocchi, 1814) and Cadulus
propinquus (G.O. Sars, 1878) were collected at Espegrend
Marine Station outside of Bergen, Norway in March 2004.
Additional specimens of A. entalis were collected at Tjärnö
Marine Biological Station outside Strömstad, Sweden, in
February 2004. The material was fixed for transmission electron microscopy (TEM) in 2.5% glutaraldehyde in sodium
cacodylate buffer, followed by postfixation in 1% osmium tetroxide in sodium cacodylate buffer and transfer to storage in
70% ethanol. The specimens were embedded in Agar 100
Epoxy resin in March 2004. Sections were given different
orientations to obtain views of the ciliary apparatus from
diverse angles. Semi-thin sections of the embedded specimens
were put on slides and examined by light microscopy to locate
ciliated areas. Standard methods for TEM were used for ultrathin microtome sectioning, mounting of sections on grids, and
contrast staining with lead citrate and uranyl acetate (see
Lundin & Schander, 1999).
For examinations we used a JEOL JEM-1230 located at the
molecular imaging centre (MIC) at the Medical Department
of the University of Bergen, Norway.
RESULTS
The epidermal ciliary structure is identical in the four examined species. The cilia have the typical 9 2 þ 2 microtubule
pattern. The cilia are long, with slender whip-like distal ends
Journal of Molluscan Studies (2009) 75: 69– 73. Advance Access Publication: 9 December 2008
# The Author 2008. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved
doi:10.1093/mollus/eyn042
K. LUNDIN ET AL.
Figure 1. A. Cilia of multiciliated epidermal cells of Antalis entalis. The cilia are long and have tapered tips (arrow). TEM. Scale bar ¼ 4.0 mm. B.
Epidermal cilia and brush border of Entalina tetragona. In the middle is a cross-section of a cilium through its slender distal part, close to the tip
(arrow). TEM. Scale bar ¼ 1.0 mm.
(Fig. 1A), which is typical for cilia whose main function is to
create water currents. Only a few microtubules continue in this
distal part to the distal tip (Fig. 1B). The cilia are proximally
anchored in the cell surface by means of a basal body and two
ciliary rootlets (Figs 2A– E, 3). There is no accessory centriole
associated with the ciliary basal body. The transition zone
between the individual cilium and the basal body is relatively
short, with a modified 9 3 microtubule pattern (i.e. a centriolar triplet derivative), a ciliary necklace with several attachment strands, a rather blurry basal disc (dense plate) and an
aggregation of glucogen granules within the basal body below
the basal plate. The lower part of the basal body is conically
tapered. There are two ciliary rootlets. These rootlets display
the typical pattern of double electron-dense cross-striation.
The anterior ciliary rootlet (also described as first rootlet),
i.e. the rootlet pointing in the direction opposite to the direction of the active beat of the ciliary axoneme, is placed on the
lower ‘anterior’ face of the basal body (Fig. 2A– D). The
anterior rootlet is only about 0.2 mm long. It is conical,
pointed and directed slightly obliquely forwards and downwards. The vertical ciliary rootlet (also described as second
rootlet or posterior rootlet) is a vertical continuation of the
basal body (Fig. 2A –D). The vertical rootlet is 2–2.5 mm long
and directed obliquely backwards and downwards. On the
‘posterior’ face of the ciliary basal body there is an hourglassshaped basal foot, with fibres radiating widely from its distal
end (Fig. 2C–E).
The microvilli among the epidermal cilia are slender, often
more or less beaded and occasionally branched. In sparsely
ciliated areas of the epidermis the microvilli often form a dense
brush border (Fig. 4A). In Entalina tetragona (Fig. 4A) and
Cadulus propinquus (Fig. 4B) the microvilli are distinctly beaded.
In the latter species, the villi are relatively short compared to
other studied species. In all studied species the glycocalyx is
relatively thin and weakly layered (Fig. 4B).
general morphology of locomotory cilia. However, the presence
of an anterior ciliary rootlet is surprising, because representatives of the Conchifera – the ‘higher’ molluscs, including scaphopods – had been thought to possess only a vertical ciliary
rootlet (see Lundin & Schander, 2001c).
Paired rootlets have previously been reported in locomotory
cilia of different organs of several bivalve taxa, such as the lips
and palps of adult Placopecten and Chlamys (Beninger et al.,
1990), the gills of Thyasira (Passos et al., 2007) and probably
also Ostrea (Bigas et al., 2001), as well as the velum of larval
Pecten (Cragg, 1989). These rootlets, however, are not perpendicular in relation to the basal body, but are both vertically
orientated and arise from the lower face of the basal body.
These twin vertical rootlets have likely been derived from a
single vertical rootlet that split into two parts, perhaps for
enhanced mechanical stability. An anterior ciliary rootlet thus
generally appears to be lacking in bivalves, including the protobranchs (Lundin & Schander, 2001a).
In Cephalopoda, locomotory cilia occur in postembryonic
stages only, and locomotory cilia with bifurcated rootlets have
been described in Loligo vulgaris and Sepia officinalis
(Sundermann, 1983). Here, the origin from a single, split, vertical rootlet is even more obvious than in bivalves. Further and
more detailed studies are still lacking and are desirable for
further evaluation of the relationships between Scaphopoda
and Cephalopoda.
In molluscs, an anterior rootlet is thus known only
from Neomeniomorpha (Lundin & Schander, 2001b),
Chaetodermomorpha
(Lundin
&
Schander,
1999),
Polyplacophora (Lundin & Schander, 2001c) and Scaphopoda
(herein). The presence of a long anterior rootlet, as in
Neomeniomorpha, has earlier been interpreted as the plesiomorphic state (Lundin & Schander, 2001c; Table 1). This
assumption is supported by the widespread occurrence of
long anterior rootlets in other lophotrochozoans (Tyler,
1979; Ax, 1995; Emschermann, 1996; Lundin & Schander,
2003; Hausen, 2005). Similar to Chaetodermomorpha and
Polyplacophora, the anterior rootlet in the investigated representatives of Scaphopoda is rather short and thus presumably
modified as compared to the state in the Neomeniomorpha.
DISCUSSION
The structure of the ciliary axoneme and basal body of the
mantle cavity cilia of scaphopods described here reflects the
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CILIARY ULTRASTRUCTURE IN SCAPHOPODA
Figure 2. Epidermal ciliary rootlets of Scaphopoda, TEM. A. Antalis entalis; note the long vertical (second) rootlet (arrows) and part of the short
anterior (first) rootlet (arrowhead). B. Antalis entalis; basal body in longitudinal section and part of the vertical rootlet of an epidermal cilium.
C. Entalina tetragona; anterior (first) rootlets are to the left and the basal feet (arrows) to the right side of the basal bodies. The vertical (second) rootlets
are obliquely directed. D. Entalina tetragona; epidermal ciliary basal bodies and rootlets in longitudinal section. E. Antalis occidentalis; cross-sections of
several epidermal ciliary basal bodies with two anterior rootlets (arrowheads) and a basal foot visible (arrow). Scale bars ¼ 0.5 mm.
The presence of an anterior rootlet in Scaphopoda shows
that this character is plesiomorphic within in the Mollusca and
not a common trait of a clade (‘Aculifera’, ‘Amphineura’)
uniting the aplacophoran groups Neomeniomorpha and
Chaetodermomorpha with the Polyplacophora. As a plesiomorphic character, the presence of an anterior rootlet cannot
be used to evaluate phylogenetic concepts within the ‘higher
molluscs’, such as the Diasoma-concept.
ACKNOWLEDGEMENTS
We are grateful to the crew of R/V Hans Brattstro¨m and the staff
of Espegrend Marine Station, Bergen, Norway and Tjärnö
Marine Biological Station, Sweden, for help in collecting the
scaphopod material. Tryggve Knag, University of Bergen, and
Gunnel Bokhede at the EM Department of the Anatomical
Institution in Gothenburg gave technical assistance. This study
was financed by grants from the Swedish Research Council,
the Meltzer foundation and from University of Bergen (to CS).
Figure 3. Schematic drawing of the ciliary rootlet apparatus of
multiciliated epidermal cells in species of the Scaphopoda.
Abbreviations: ar, anterior rootlet; bb, basal body; bf, basal foot; cs,
ciliary shaft; pr, vertical rootlet.
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K. LUNDIN ET AL.
Figure 4. A. Brush border of epidermal microvilli on Antalis occidentalis. TEM. Scale bar ¼ 1.0 mm. B. Epidermal microvilli of Cadulus propinquus.
Note the weakly layered glycocalyx. TEM. Scale bar ¼ 2.0 mm.
Table 1. Ultrastructural characters of the ciliary apparatus on multiciliated epidermal cells of studied species (adults) from major taxa of the
Mollusca and Sipuncula.
Scaphopoda
Chaetodermomorpha
Neomeniomorpha
Polyplacophora
Bivalvia
Gastropoda
Sipuncula
Distal part of cilium
Whip-like
Whip-like
Whip-like
Whip-like
Whip-like
Whip-like
Cilium set in deep pit on cell surface
No
No
Yes
Yes/No
No
No
Whip-like
No
Transition zone between cilium and
Short
Short
Short
Long
Short
Short
Short
Blurry
Thin
Blurry
Blurry
Thick
Thick/
Thick/
basal body
Basal plate (dense plate)
Ciliary necklace with connecting
Blurry
Blurry
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Present
Accessory centriole
Absent
Absent
Absent
Absent
Absent
Absent
Absent
Basal foot with continuous tubular
Present
Present
Present
Present
Present
Present
Present
Anterior ciliary rootlet; length
Short
Short
Long
Short
Absent
Absent
Long
Anterior ciliary rootlet; shape
Conical
Flat
Conical
Flat
Absent
Absent
Conical
Vertical ( posterior) ciliary rootlet;
Long
Long
Long
Long
Long
Long
Long
Conical
Conical
Conical
Conical
Conical
Conical
Conical
Brushborder of microvilli
Present
Present
Absent
Absent
Present
Absent
Absent
Microvilli on epidermal surface;
Slender/
Slender
Variable
Slender/
Slender
Slender
Slender
strands
Aggregation of granules below basal
plate
Centriolar triplet derivative in basal
body
fibres
length
Vertical ( posterior) ciliary rootlet;
shape
shape
Microvilli on epidermal surface;
Variable
Variable
Few
No
Few
Few
No
No
Yes
Layered
Layered
Homogeneous
Homogeneous
Layered
Lay./Hom.
Layered
branched
Glycocalyx
Note that the twin vertical rootlets in species of lamellibranch bivalves is regarded as a split of a single rootlet and not included in the table.
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