Sinepecten Schein, 2006, n. gen.
Creators: name: Schein, Elisabeth
Publisher: name: Zenodo
Year: 2006
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Bivalvia
Pectinoida
Pectinidae
Sinepecten

description: Genus Sinepecten n. gen.
Etymology: from Latin sine: without, and pecten: comb.
Diagnosis: Pectinidae deprived of radiate ornamentation, byssal notch without ctenolium, posterior auricles not delimited. Right valve showing a prismatic juvenile stage. Post­prismatic part of this valve and left valve sculptured by commarginal ridges or lamellae affecting only the external surface of the shell.
Monospecific: Sinepecten segonzaci n. sp.
Remarks: Within the subfamily Propeamussiinae, Bathypecten and Catillopecten are the genera having the strongest affinities with Sinepecten. All three genera have a juvenilelike, general shell morphology, recalling some paleozoic fossils to mind (Schein­Fatton 1988), e.g., Pterinopecten undosus Hall, 1883. The following characters are found in the post­larval shells of all Pectinidae: byssate, thin test with low convexity, no radial ornamentation, flat right valve without ctenolium, and posterior auricles poorly delimited. The arrangement of prisms of their right valve is commarginal in the three genera, as in most Propeamussiinae. Besides their resemblance, there are differences principally in the duration of the prismatic stage and in ornamentation. The outer prismatic microstructure of the right valve persists throughout ontogeny in Bathypecten and Catillopecten, whereas it is restricted to a juvenile stage in Sinepecten. The ornamentation of the adult shell, although exclusively commarginal in the three genera, is very different from one to the other. In Bathypecten, the presence of ornamentation can even be questioned, as the shell is not sculptured, but has only commarginal shallow undulations that tend to disappear through ontogeny (Schein­Fatton 1985) and can even be absent (Dijkstra & Gofas 2004). In contrast, Catillopecten and Sinepecten show a dense external ornamentation of fine ridges. In Catillopecten, the ridges of the left valve are sharp, even and regularly spaced. Moreover, the inner face is marked with grooves reflecting the ridges on the outer face. These ridges derive thus from narrow commarginal folds concerning the entire shell thickness. They become closer with growth, and sometimes the flanks of each narrow fold fuse so that the inner grooves disappear (unpublished data). In the prismatic right valve there is only a very faint and rather irregular commarginal fluting which derives from the commarginal array of prisms, and becomes more pronounced in the periphery where commarginal ridges begin to shape as in Propeamussium, Parvamussium and Cyclopecten (Schein 1989). Furthermore concentric undulations can also be present on both valves. In Sinepecten, after the prismatic stage, ornamentation and microstructure are the same on both valves. The ridges tend to turn to lamellae, and the inner face is not grooved. The ornamentation of Catillopecten and Sinepecten could be homologous if the ridges derive from narrow folds in both genera, but the absence of grooves on the inner shell face of juvenile and adult Sinepecten suggests a different origin. In the present state of knowledge (Table 1), adult Sinepecten is clearly different from Bathypecten and Catillopecten, therefore justifying the creation of a new genus. Despite the synonymy of Bathypecten and Catillopecten recently proposed without ontogenetic argumentation (Dijkstra & Gofas 2004), I consider that the present study confirms the validity of the three genera (see also the discussion below, following the description of S. segonzaci). Irregular commarginal undulations tending to disappear after the early ontogeny as in Bathypecten are typically a plesiomorphic character, common to different taxa in the Pectinidae (symplesiomorphy).
In contrast, close­set sharp ridges keeping regularly spaced throughout ontogeny constitute a derived character which separates clearly Catillopecten from Bathypecten. Moreover, ornamentation features restricted to the outer shell surface, which vary at the species level, must be distinguished from structures concerning the entire shell thickness like the typical narrow folds of the genus Catillopecten.
Etymology: species dedicated to Michel Segonzac (Ifremer, Centre of Brest, France), who trusted me with the study of the material.
Locus typicus: western Pacific, Manus back­arc Basin, (north Papua­New Guinea), hydrothermal vent area PACMANUS, Field E, Barnacle site, 03°43.62’S – 151°40.32'E, depth: 1674 m.
Type material: Type series consisting of five bivalve specimens (specimens 2 to 6) of increasing size, from 3.5 to 27 mm of UPD (Table 2), the largest one (specimen 6) being designated as the holotype. It is catalogued in the National Science Museum of Tokyo with the following registration numbers: NSMT­Mo 73697 (holotype), NSMT­Mo 73698 (specimen 5), NSMT­Mo 73699 (specimen 4), NSMT­Mo 73700 (specimen 3), NSMT­Mo 73701 (specimen 2).
Diagnosis: Shell thin, valves not gaping. Lower (right) valve flat, upper (left) valve convex. Ornamentation commarginal, affecting only the external surface, very fine ridges or lamellae covering the left valve from beak to margin in juveniles and adults, and the right valve only after the juvenile stage. Posterior auricles poorly delimited. Byssal notch without ctenolium under the anterior auricle of the right valve, widely opened in the juvenile, but gradually closing and finally overlapped by the dorsal expansion of its lower edge in the adult. Test principally calcitic, with an aragonite layer restricted to the area inside of the pallial line.
Material examined: The five specimens of the type series, which were collected alive, except specimen 5, come from the type­locality. Two of them (specimens 2 and 4) were treated for SEM observations. The left valve of specimen 5 was treated with Feigl’s solution for the identification of aragonite. A single postlarval valve in relatively poor preservation (specimen 1) was collected with sediment in a site without active venting. This material was collected by the manned submersible Shinkai 2000 during two dives: dive #1075, 22.11.1988, Field E, Barnacle site, 03°43.62’S – 151°40.32'E, 1674 m (type series); dive # 1076, 23.11.1998, Field D, slurp gun 2, 03°43.73’S – 151°40.21’E, 1623 m (specimen 1). It was fixed in formalin, then preserved in ethanol 80%.
HOLOTYPE (Figures 4–7, Tables 2–3, specimen 6): Shell irregular, distorted by adaptation to the hard substrate (Figure 4 a–b). In spite of distortion, right valve roughly flat up to a height of 11–12 mm (Figure 5 a), and left valve convex. Closed valves without any gape other than the byssal notch. Auricles dissymmetrical with a byssal notch under the anterior auricle of the right valve.
Anterior auricle (5.4 mm) slightly shorter than the posterior (5.9 mm). Posterior auricle poorly demarcated from disk, with only a shallow flexure between their surfaces, and no clear limit on the oblique posterior edge. Byssal notch without ctenolium hidden by the dorsal overlap of its ventral edge and thus observable only in dorsal view (Figure 6). Antero­dorsal edge of the disk (ventral edge of the byssal notch) sinuous, proximally leaving the depressed byssal fasciole uncovered, and distally extending slightly above the lower edge of the anterior auricle (Figure 5 b). Dorsal expansion of this edge concomitant of a peculiar growth stage marked on the disk by a strong undulation just after a very sharp growth line at a UPD of 6 mm (Figure 4 c).On both valves, disk and auricles with a dense commarginal ornamentation of ridges regularly spaced, and transforming to lamellae. On the left valve, ornamentation starts from the beak, covering disk and auricles. Interval between ridges or lamellae increasing with shell growth, from about 0.1 mm at a UPD of 12 mm, to 0.3 mm at least near the ventral edge. On the right valve, the outer prismatic layer terminates at a 10 mm UPD, and the same ornamentation as on the left valve then replaces the faint and narrow commarginal fluting.
PARATYPES (Figures 2–3, 5–12, Tables 2–3, specimens 2 to 5): The type series constitutes an ontogenetic sequence (specimens numbered by increasing size), which offers the opportunity to follow some changes in shell morphology during growth.
Shell outline: The outline of the shell in the commissure plane is the first character obviously changing through ontogeny (Figure 7), from higher than wide in the juvenile specimens 2 and 3 to as high as wide in specimens 4 and 5, and finally wider than high in the holotype. In the juvenile prismatic stage illustrated by specimens 2 and 3, when the shell is closed, the disk margin of the right valve is upturned and adpressed against the inner face of the left valve margin (Figure 2 b–c). The line along which the margin of the right valve is flexed corresponds to a line of lower mechanical resistance, so that the marginal apron detaches as soon as the valves are separated. This character, which is common in Propeamussiinae (Schein 1989; Morton & Thurston 1989), disappears after the prismatic stage (specimens 4 to 6).
Auricles: None of the specimens has both its auricles intact, but their dissymmetry can be verified by reconstruction of the complete outline. The anterior auricle is slightly longer than the posterior in specimens 2 to 4, whereas it seems to be the opposite in the holotype (Table 2). In all specimens, the posterior auricle is poorly delimited. Its posterior edge is almost perpendicular to the dorsal edge in the juvenile specimens 2 and 3, and becomes increasingly oblique in specimens 4 to 6 (Figure 7).
Byssal notch: On the anterior side of the right valve, the auricle and the byssal notch undergo a profound change (Figure 6). The byssal notch is triangular and widely open in the juvenile specimens (Figures 8, 9 a), and begins to narrow after the end of the prismatic stage (Figure 9 b). It is still quite visible on the specimen 5, but seems to have disappeared on the holotype (Figure 4 c). Though the passage for the byssus is almost invisible in the holotype, it is not closed, but has undergone a change in size, shape and orientation. In the juvenile, it lies in the plane of the right valve, but later it sinks inwards with the ventral edge of the anterior auricle (i.e. the dorsal edge of the byssal notch), and in the holotype it has shifted to a plane more or less perpendicular to the disk plane so that the opening is dorsally directed (Figures 5 b–c, 6). On the inner face of the specimen 4, there is a fold in the proximal end of the byssal notch (Figure 9 b). This is probably induced by the ventral edge of the auricle beginning to move inward. In the holotype, this edge is covered by the antero­dorsal edge of the disk (Figure 4 c). The byssal notch is not only narrowed by the overlap of its ventral edge, but also shortened by the byssal fasciole growing forward (Figures 5 b, 13). The measurements show clearly the gradual decrease of height with growth, and the sudden drop of depth in the holotype (Table 3). This morphological change starts after the prismatic stage and is achieved in the largest specimen only.
Ligament areas: On the internal face, the resilifer in which the inner ligament is attached, is prosocline (Figures 9 b, 9e). The outer ligament, which prevents shearing or rotational movements of the valves, is attached along the dorsal edge of auricles. On each auricle, the dorsal edge is coiling from the beak to the distal extremity, and this is achieved as follows: the left valve beak overhangs the right valve, but it is opposite in the anterior end, where the dorsal edge of the right valve curves over the left valve (Figure 10). This coiling is absent in the juveniles, and owing to the poor preservation of the other specimens, it was not possible to ascertain which valve curves over the other in the posterior auricle. It is probably the right valve over the left, and to a less extent than anteriorly as in most pectinids examined in comparison, i.e., Pecten maximus Linné, 1758 and Chamys varia Linné, 1758 for example. On the opposite, in the deep­sea Propeamussium lucidum (Jeffreys, 1873) described by Morton & Thurston (1989) the left valve curves over the right at the distal end of the posterior auricle, and more markedly than at the end of the anterior auricle. In all cases, the same valve curves over the other at the anterior and posterior ends of the dorsal margin. This curvature is probably necessary to make the locking device efficient.
Hinge: Immediately ventral to the outer ligament, two flat areas are separated by the resilifer on each valve: each auricle has one such area which stretches along the dorsal margin and tapers almost to nothing at its distal extremity (Figures 9 b, 9e, 10). The flat areas are edentulous, but their surface is minutely roughened, and therefore opposed to shearing. In fact, SEM observation of this toothless hinge shows that between the valves the calcite crystals clash by their points (Figure 9 f). The rod shape of the crystals and their more or less upright orientation in the hinge contrasts with their tablet shape and their arrangement parallel to the depositional surface outside the hinge where the typical foliated structure covers the inner surface of the shell (Figure 9 d).
Ornamentation of left valve: The commarginal ornamentation affects only the external surface of the shell (Figure 11 a). The interval between the ridges or lamellae covering the left valve is about 0.1 mm near the ventral margin in juvenile specimens 2 and 3 (Figures 11 a–b,e), and reaches 0.2 mm in specimens 4 and 5 (Figure 11 c). The spacing between ridges tends to increase with growth and exceeds 0.3 mm in the holotype. The increase is not regular, as measurements have shown significant variations, obviously related to frequent growth disturbances. The components of ornamentation are shaped as ridges or lamellae depending on their location. Ridges form near the beak and in the middle of the disk (Figure 11 b), both places where growth rate is high. Lamellae form in the disk periphery (Figure 11 e) and near the limits between disk and auricles, both places where growth rate decreases. Anyway, both ridges and lamellae represent a relative slowing of growth rate as evidenced in SEM views by the bunching up of growth lines on them. SEM observation also allows clarification of the details of the antimarginal microsculpture, which is superimposed on the ornamentation. In the juvenile, extremely fine and low antimarginal riblets are arranged regularly along the intervals between the commarginal ridges (Figure 11 b–e). The riblets distally cross the ridges, on which they induce a regular pattern of close­set thickenings (Figure 11 e). When the ridges turn to lamellae, these thickenings become sharper and higher distally (Figure 11 d).
Ornamentation of right valve: Juvenile specimens 2 and 3 illustrate the prismatic stage. Their right valve is externally covered with a layer of calcite prisms extending to the disk margin. On specimens 4 and 5, the prismatic stage is a little shorter than on the holotype: 8 to 9 mm for specimen 4 and 9 mm for specimen 5, versus 10 mm for specimen 6. The array of prisms forms a pattern of recurrent, commarginal sequences (Figure 9 c) corresponding to the concentric prismatic structure described by Schein (1989). As a consequence of this superficial microstructure, the right valve seems macroscopically to be unornamented in the prismatic stage. However, it displays weak and narrow commarginal flutings derived from the prismatic microstructure, and several irregular commarginal undulations (Figure 8 a). As in the holotype, the specimens 4 and 5 develop on their right valve the same ornamentation as on the left one after the prismatic stage. The right valve ornamentation is thus late when compared to the left valve. The delay is longer on the disk than on the auricles, and in the latter, the anterior auricle is the more advanced. This is illustrated by the juvenile specimen 2 with an anterior auricle displaying relatively sharp commarginal ridges (Figure 8 a). The ornamented part of the auricle lies outside the proximal prismatic area, which is very reduced (Figure 7).
Aragonitic layer: On the inner face of both valves, there is an aragonitic layer outcropping, which is limited dorsally by the distal edge of the foliated calcite inner layer and ventrally by the pallial line (Figure 12). Its radiate organisation is obvious, with narrow bands radiating from the beak. SEM observation shows that the bands are alternatively light (more emissive) and dark (less emissive).
Muscle scars: The adductor muscle scar is different in the right and the left valves (Figure 12). On the right valve, the adductor scar is bean­shaped, with two adjacent parts placed one above the other. It is clearly defined, and the size of the two parts is equivalent. On the left valve, the scar is larger and less clear, but three sub­equivalent parts seem to be outlined. Two of them are posterior and superposed, and the third scar lies anterior to the ventral one. All three correspond to the muscular sectors responsible for the three basic functions in byssally attached pectinids: rapid adduction of valves (“quick” striated muscle), slow adduction (“catch” smooth muscle), and byssal attachment (byssus and foot retractor). Unfortunately, given the poor preservation of specimens, it was not possible to precisely identify where the corresponding muscles inserted.
Larval shell: The larval shell, which was either destroyed by corrosion, or coated with ferromanganese deposits could not be observed. Some remains on the left valve of specimen 2 allow us to estimate that the antero­posterior diameter is about 215 µm as in the larval shell of Bathypecten vulcani which has a non­planktotrophic type of development (Roux et al. 1989).
In life habit, S. segonzaci is byssally attached, and its valve lower flat valve is adpressed against the substrate (Figure 5).
Hydrothermal environment: The five specimens of the type series of S. segonzaci were collected at the Barnacle vent site (PACMANUS area, Field E) on glassy basalt, among sessile barnacles, where the temperature ranged between 2.8°C at the periphery of hydrothermal fields and about 40°C in vent fluid. This site is a group of small chimneys from 6 to 8 m height, either inactive or emitting a black fluid (260 to 270°C). The vent fauna is established around these edifices, between cracks emitting a transparent fluid measured at 5 to 40°C (Hashimoto et al. 1999).
Kai Kai site is one of the two large diffuse vent sites of the Field D where chimneys are absent but where there are large diffuse vents with cracks emitting strong shimmering water at 30°C (Hashimoto et al. 1999). A single dead juvenile valve (specimen 1) was collected in a sediment area, at some tens of meters of the Kai­Kai vent site, and thus, outside of the vent influence.
The outer shell surface of the specimens shows the fingerprint of the hydrothermal environment in which they grew, i.e., an aggressive and unstable environment, with fluids rich in metal ions ready to precipitate over the shell. The specimens all had their upper (left) valve coated with a ferromanganese black deposit, thicker in umbonal area. The black deposit also affected the lower (right) valve, where it was thinner and less regularly distributed (Figure 4). Damage to the shells increases with growth and size of the shell. When the individuals are small (specimens 2 and 3), the shell is blackened but its outer surface only is affected, and the outline is regular. In contrast, the valves of the larger specimens are affected on both sides, as the outer disturbances are reflected on the inner shell face. For example, specimens 4 and 5 are more or less deformed by a great number of bumps, and moreover the periphery of the disk is densely covered with countless particles which were trapped along the growing edge and became embedded into calcification (Figure 11 f). On the left valve, the inner surface is densely scattered with marks resembling stretch marks; the inner surface of the right thinner valve is granular (Figure 12). The thinness of the growing edge explains why the inner face reflects the incorporation of foreign particles on the outer face. The high density of particles along the ventral margin of specimens 4 and 5 reflects their density in the ambient seawater. Finally, in the holotype, besides the black coatings, the shell is entirely distorted and seems even to have undergone a torsion, when compared to the younger specimens. This is obviously induced by its adaptation to the irregular substrate, but a reaction to the toxicity of fluids could also be involved.
As a result of their small size and of the black ferromanganese deposits covering the shells (as well as external surfaces of the barnacles), these pectinids are not easily visible, since they are attached to a black basaltic substrate.
description: Published as part of Schein, Elisabeth, 2006, A new deep­sea pectinid bivalve from thermal vents of Manus back­arc Basin (south­western Pacific), Sinepecten segonzaci n. gen., n. sp. (Pectinoidea: Pectinidae), and its relationships with the genera Bathypecten and Catillopecten, pp. 1-27 in Zootaxa 1135 on pages 4-16, DOI: 10.5281/zenodo.171941