Syssitomya pourtalesiana, Oliver, 2012

Oliver, P. Graham, 2012, Taxonomy of some Galeommatoidea (Mollusca, Bivalvia) associated with deep-sea echinoids: A reassessment of the bivalve genera Axinodon Verrill & Bush, 1898 and Kelliola Dall, 1899 with descriptions of new genera Syssitomya gen. nov. and Ptilomyax gen. nov., European Journal of Taxonomy 12, pp. 1-24 : 14-18

Syssitomya pourtalesiana sp. nov.

Figs 4B, E ; 7 , 8 A-B

? Kellia symmetros – Locard 1898: 297, pl.XIII, figs 18-20.

? Kellia symmetros – Friele & Grieg 1901: 29.

Axinodon symmetros – Bouchet & Warén 1979: 216-217, fig. 3A-D.

Montacuta (Axinodon) symmetros – Gage et al. 1985: 189.

Not Axinodon symmetros – Warén 1980: 47.

Not Axinodon symmetros – Aartsen 1996: 30, fig. 5 (is Axinodon ellipticus Verrill & Bush, 1898 ).

Not Axinodon sp.1 – Olabarria 2005: 20 (is Mysella sp.).

Etymology

After Pourtalesia , the host echinoid; and the Latin termination – iana, to denote belonging with.

Material examined

Holotype

1 specimen, Norwegian Sea , Ormen Lange gas field, off Sør-Trøndelag, Central Norway, 63°47'N 03°35'E, 815-925 m, Swedish Museum of Natural History SMNH5566 .

Paratypes

Same recolt data as for the holotype: 7 specimens, Swedish Museum of Natural History SMNH 5567; 1 specimen + 2 shells, National Museum Wales, Zoology- NMW. Z.2012.014.

Other material

8 specimens, Rockall Trough, RRS Challenger, stn ES137, 54°40'N 12°19'W, 2900 m, 22 Feb. 1978, leg. I.J. Killeen NMW.Z. 2009.045.2; 6 specimens, Rockall Trough, RRS Challenger, stn ES231, 54°42'N 12°12'W, 2898 m, 17 May 1983, leg. I.J. Killeen, National Museum wales, Zoology- NMW.Z. 2009.045.3.

Type locality

Norwegian Sea, Ormen Lange gas field, off Sør-Trøndelag, Central Norway, 63°47’N 03°35’E, 815- 925 m, 2009. Swedish Museum of Natural History, det. Anders Warén.

Measurements (SMNH5566 and part of SMNH5567 paratypes)

Length (mm) Height (mm) L:H

Holotype 4.1 3.2 1.3:1

Paratype 3.0 2.3 1.3:1

Paratype 2.4 2.0 1.2:1

Paratype 1.8 1.4 1.3:1

Paratype 1.3 1.1 1.2:1

Paratype 1.3 1.1 1.2:1

Description based on specimens from the Norwegian Sea

SHELL. ( Figs 7 , 8 ) Small, largest length 4.2 mm, height 3.2 mm, breadth 2.7 mm. Thin, fragile, translucent. Equivalve. Inequilateral, beaks distinctly behind midline. Umbos moderately inflated, beaks orthogyrate or marginally prosogyrate. Outline subovate, longer than high, length to height ratio 1.3:1, distinctly expanded anteriorly becoming a little oblique; anterior dorsal margin short as a depressed but illdefined lunule( Fig. 8A 1 ); anterior broadly rounded; posterior dorsal margin sloping merging smoothly with rounded posterior margin; anterior distinctly more rounded than posterior; ventral margin gently curved. Sculpture of dense, fine commarginal ridges. Prodissoconch II distinct, 390-410 µm in diameter, sculptured with commarginal lines ( Fig. 7H ); Prodissoconch I weakly demarcated, 148-150 µm in diameter with a punctate micro-sculpture ( Fig. 7H ). Ligament short, internal, attached to a shallow resilifer situated beneath and posterior of the beaks ( Fig. 7E ). Right valve with a single, projecting, cardinal peg, immediately anterior to this tooth a prominent depression ( Fig. 7D ). Left valve with a short, marginal flange in a posterior lateral position ( Fig. 7E ). Adductor scars oval, roughly of equal size; pallial line entire. Margin entire.

ANATOMY. ( Figs 4B, E ; 8A 2 ) Mantle margin free for most of its length, joined and attached to terminal of gill axis, anterior (pedal) aperture extensive, posterior aperture very small ( Fig. 8A 2 ). Anterior mantle edge thrown into folds ( Fig. 4B ). Adductor muscles of approximately equal size. Foot with a large toe and small heel, byssus functional producing a mass of threads arising from a single stalk. Anterior pedal retractor inserted above the anterior adductor, posterior pedal retractor above the posterior adductor. Ctenidium of single demibranchs, each with up to 30 partly reflected filaments ( Fig. 4B ). Filaments laminar, extended abfrontally with an extensive bacteriocyte zone ( Fig. 4E ). Labial palps small but projecting.

Specimens from abyssal depths in the Rockall Trough do not differ significantly from those from the Norwegian Sea except for a large shell that shows greater anterior expansion. The ctenidia of this specimen show the same laminar filaments with abfrontal extension. Specimens collected in the month of February were gravid, the suprabranchial chamber holding hundreds of sub-triangular larvae, on average 112 µm in diameter.

There is no record of any association with an echinoid but these specimens originate from the same sampling programme reported upon by Gage et al. (1985) where they were attached to Pourtalesia miranda .

ASSOCIATION. Attached by byssus threads to the spines of Pourtalesia jeffreysi and P. miranda ( Fig. 9 ).

Distribution

Confirmed from the NE Atlantic, Norwegian Sea to Bay of Biscay at depths from 800-3617 m. Records ( Allen 2008) from the NW and SW Atlantic have not been confirmed although one of the host species P. miranda has been recorded in these areas.

Pourtalesia jeffreysi has two recognised subspecies with both hosting Syssitomya gen. nov.: Pourtalesia jeffreysi gibbosa Mironov, 1995 has a bathyal range while the subspecies lata Mironov, 1995 is abyssal.

The known geographic range for P. jeffreysi is the Norwegian Sea and Russian Arctic Ocean (WoRMS 2012).

Differential diagnosis

The shell characters of Syssitomya gen. nov. are montacutid in all respects, notably the anterior expansion, internal ligament and the hinge reduced to a single cardinal peg in the right valve and a marginal flange in the left valve. In shell character, similarities are greatest with Kelliola but Syssitomya gen. nov., uniquely within the Galeommatoidea , has highly modified ctenidia with abfrontally extended, laminar filaments.

At the species level, for comparison with Kelliola symmetros see above. The somewhat inflated, anteriorly expanded form of S. pourtalesiana sp. nov. is rather distinctive and not like the form of other deep-sea montacutids such as species of Kurtiella ( Gofas & Salas 2008) or Epilepton ( Allen 2007) that are more ovate, compressed and have different dentition patterns. From external appearances it more resembles some thyasirids notably Thyasira subovata (Jeffreys, 1881) (see Oliver et al. 2012) and if not found attached to its host could easily be mistaken for a thyasirid.

Figures

Fig. 4. Scanning electron micrographs of anatomy. A. K. symmetros (Jeffreys, 1876) from Biscay. B. Syssitomya pourtalesiana sp. nov. from Norwegian Sea. C. Montacuta substriata (Montagu, 1808) from North Sea. D. Excised ctenidium of M. substriata. E. Excised piece of ctenidium from S. pourtalesiana sp. nov.

Fig. 7. Syssitomya pourtalesiana sp. nov. from Norwegian Sea. A-C. Photo micrographs a external of left valve, internals of both valves. D-E. SEM of hinges of right and left valves. F-G. SEM of internal of both valves. H. SEM of prodissoconch. I. SEM of internal of right valve from NORBI cruise, abyssal, Courtesy of Anders Warén.

Fig. 8. Computer enhanced photomicrographs of whole specimens of Syssitomya pourtalesiana sp. nov. from the Norwegian Sea. A-A2. Holotype A. Exterior from right side. A1. Dorsal. A2. Ventral. B. Paratypes, size series.

Fig. 1. Holotype of Axinodon symmetros Verrill & Bush, 1898, USNM 35175. A-B. SEM of hinges of right and left valves. C-D. SEM of internal of right and left valves. E. Photo micrograph of internal of right valve. F-G. SEM of external of right and left valves. H. SEM of prodissoconch.

Fig. 2. Holotype of Kellia symmetros Jeffreys, 1876, USNM 170626. A-B. SEM of hinges of right and left valves. C-D. SEM of internal of right and left valves. E-F SEM of external of right and left valves. G-H. photo micrographs of internal and external of right valve.

Fig. 9. Pourtalesia miranda Agassiz, 1869 with Syssitomya pourtalesiana sp. nov. attached, from Biscay. Courtesy of A.J. & E.C. Southward.