Cyclochlamys perplexa (Soot-Ryen, 1960)
SOOT-RYEN, T. 1960. Pelecypods from Tristan da Cunha. Results of the Norwegian Scientific Expedition to Tristan Da Cunha 1937-1938, 49: 1-47, pls. 1-3, text-figs. 1-9. [p. 17, pl.1, figs. 8-13, text-figs. 3, 4]
1960 Cyclopecten (Cyclochlamys) perplexus Soot-Ryen, 1960
T. Soot-Ryen, 1960, plate 1.
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«Diagnosis: Shell minute, semipellucid to opaque, whitish, slightly obliquely rounded. Inequivalve, the left valve more convex and with larger umbones projecting beyond the hinge line, which is as long as the greatest length of the shell. Beaks at, or somewhat behind, the middle of the straight hinge line. Anterodorsal angle slightly rounded dorsally; posterodorsal angle blunt. Margin of anterior auricle of left valve more or less convex and set off from the main part of the valve by a small sinuosity; ventral and posteroventral margins rounded, posterior margin straight including the ear which is set off from the valve by a very slight sinuation. Prodissoconch with fine, irregular radial sculpture. Left valve with a varying number of distinct, threadlike, radial and concentric elevated lines with tooth-like, slender, hollow, projections open on the dorsal side, in the intersections. Small specimens have about 8 radial lines while larger ones develope intermediate lines upwards to 38, counted along the margin including the ears. The resulting sculpture forms a network of quadrates or rectangles. Other specimens are nearly smooth but radial elements are visible under certain light conditions. Right valve less convex and umbo scarcely projecting beyond the hinge line. Anterior ear distinctly set off from the valve by a broad, byssal sinus, and furnished with 2-6 radial lines, often imbricated by concentric lines. Sculpture of the valve and posterior ear consists of fine concentric growth lines and obscure radial elements.
Hinge plate narrow with a small narrow pit for the resilium, ligament weak. Adductor small, in the upper posterior part of the valve. Height less than 2 mm. Holotype: Z. M. O. no. 29770. Length: 1.65 mm, height: 1.6 mm (ornate form).
Type locality: St. 115, Nightingale Island, 35-55 m. Material: St. 111. 2 small specimens (smooth form) St. 113. 1 valve (sculptured form) St. 115. 5 living, 1 dead specimens (sculptured form) 16 living, 2 dead specimens (smooth form) St. 116. 2 living specimens, 1 valve (sculptured form) 24 specimens (smooth form) St. 154. 10 specimens (smooth form) Remarks: As stated under the discussion of the supraspecific groups, the material is very variable in regard to the sculpture. The heavily sculptured form is chosen as the holotype and could be named forma ornata while the more or less smooth specimens may be termed forma laevis. There seems to be no doubt about the close resemblence of forma ornata to C. transenna and perhaps the forma laevis could be compared to C. mestayerae Dell, 1956. As to the variation in the sculpture it should be mentioned that one specimen has 12 weaker radial ribs on the desk while the ribs on the auricles are obsolete. This specimen is intermediate between the two forms. All specimens are very small, less than 2 mm in height. Small specimens of about 1 mm are adults with larvae in the gill-pouch.
The small size of the specimens makes it a difficult task to study their anatomy. One specimen has been sectioned but the fixation of the material in alcohol was not sufficient for good sections and the delicacy of the soft parts did not stand the decalcination. The presence of larvae in the gills and the handling of the specimen apparently disturbed the loose connection of the demibranches. The transverse sections were also made a little obliquely and as some were destroyed during the processes, there might be some uncertainty in the interpretations. 3 other specimens were loosened from the valves and cleared in glycerol. The following notes on the anatomy are based partly on these specimens and partly on the sections. The mantle margin has as usual a velum with proximal tentacles, 2 to 3 or secondary tentacles between 2 stronger, primary ones. Neither pigment spots, nor pigmented eye-spots seem to ce present. Unpigmented eyes may perhaps be present and identical with some of the primary tentacles. The adductor is relatively weak and sitting in the upper posterior part. The foot is small, slender, but the retractor, which is fastened to the left valve above the adductor, is relatively strong. There is a byssus gland.
The gonads are for the greater part placed in the ventral visceral sac but extend to some degree dorsalwards on the sides. The irregularly ovate eggs measure about 30 μ. The gills are represented by the inner demibranch only. 1'his is fastened to the visceral mass anteriorly; the gills have an oval form with the posterior rounded apex near the margin. The gill filaments are but loosely connected with each other. Where the ascending and descending branches meet there is a distinct thicker part. Apparently both inner demibranchs are connected with each other ventrally thus forming a cage or pouch for the eggs which are retained here until the larval shell is developed. The size of newly spawned eggs is not known, but round eggs in the gills measure c. 100 μ while the length of the larval shell is about 175-180 μ. There may be varying numbers of eggs in the gills of various specimens but where they could be counted 14-16 larvae were present. The fact that larvae are retained in the gills in this species is, so far I can see, the first record of brood protection in the family Pectinidae. This is in itself an interesting observation and suggests that brood-protection perhaps is more wide spread among the different families of pelecypods than hitherto supposed.
Occurrence: Living specimens were collected at depths between 30 m and 60 m on hard bottom with stones usually heavily overgrown. Apparently the dredge-hauls were taken outside the kelp region. One valve was found in the inter-tidal zone. Distribution: Endemic. Taken at Nightingale and Inaccessible Islands but may also occur at the main island.» TRON SOOT-RYEN, 1960
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