Kelliola symmetros

Kelliola symmetros (Jeffreys, 1876)

Kellia symmetros Jeffreys, 1876: 491.

Kellia (Kelliola) symmetros – Dall 1899: 890.

? Kellia symmetros – Friele & Grieg 1901: 29.

Axinodon symmetros – Warén 1980: 47.

Not Kellia symmetros – Locard 1898: 297, pl.XIII, figs 18-20.

Not Axinodon symmetros – Bouchet & Warén 1979: 216-217, figs 3A-D. — Aartsen 1996: 30, fig. 5 (is Axinodon ellipticus Verrill & Bush, 1898).

Not Montacuta (Axinodon) symmetros – Gage, Billett, Jensen & Tyler 1985: 189.

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

Material examined

Holotype

1 shell, North Atlantic, SW of Godthaab, Davis Strait, Valorous stn 9, 59˚10'N 50˚25'W, 1750 fathoms (3202 m), United States National Museum- USNM170626.

Other material

2 specimens, attached to Aeropsis rostrata, Bay of Biscay, Shackleton cruise 1977/5, stn D7, 47˚29.7'N 09˚33.3'W, 4250-4265 m, 30 Apr. 1977, leg. E. Southward, National Museum Wales, Zoology- NMW.Z. 2012.015.1; 4 specimens, attached to Aeropsis rostrata, Bay of Biscay, Shackleton cruise 1977/5, stn

D2, 47˚35.52'N 09˚44.07'W, 4120-4165 m, 29 Apr. 1977, leg. E. Southward, National Museum Wales, Zoology- NMW.Z. 2012.015.2.

Type locality

North Atlantic, Valorous St. 9, SW of Godthaab, Davis Strait, 59°10’N 50°25’W, 1750 fathoms (3202 m).

Redescription of the holotype (Fig. 2)

Shell minute, length 1.0 mm, height 0.77 mm. Thin, translucent. Equivalve. Weakly inequilateral, beaks just behind the midline. Umbos prominent, beaks orthogyrate. Outline subovate, slightly extended anteriorly; anterior dorsal margin sloping into broadly rounded anterior margin; posterior dorsal margin indistinct sloping into broadly rounded posterior margin, this slightly less expanded than anterior margin; ventral margin weakly curved. Sculpture weak, of indistinct commarginal lines most obvious on lateral margins. Prodissoconch II distinct, 373 μm across, with weak commarginal lines. Hinge plate weak, Ligament short, internal, attached to a shallow resilifer situated beneath and posterior of the beaks. Right valve with a single, projecting, cardinal peg, immediately anterior to this tooth a slight depression. Left valve with a short, weak, marginal flange in a posterior lateral position. Adductor scars oval, roughly of equal size; pallial line entire.

Description (based on material from Bay of Biscay)

SHELL. (Fig. 3) Minute, largest of length 1.3 mm, height 1.0 mm. Thin, fragile. Equivalve. Inequilateral, beaks behind midline. Umbos weakly inflated, beaks orthogyrate. Outline subovate, longer than high, length to height ratio 1.3:1, slightly extended anteriorly; anterior dorsal margin sloping, rather straight merging smoothly with rounded anterior; posterior dorsal margin shorter and sloping more steeply than anterior, merging smoothly with rounded posterior margin; anterior slightly more expanded than posterior; ventral margin gently curved. Sculpture weak almost smooth, of fine commarginal lines; radial lines apparent under transmitted light (Fig. 3A) but these very faintly raised (Fig. 3J). Prodissoconch II distinct, 380 µm in diameter sculptured with commarginal lines (Fig. 3I); Prodissoconch I weakly demarcated, 140 µm in diameter with a punctate micro-sculpture (Fig. 3I). Ligament short, internal, attached to a shallow resilifer situated beneath and posterior of the beaks. Right valve with a single, projecting, cardinal peg, immediately anterior to this tooth a slight depression (Fig. 3D). Left valve with a short, weak, marginal flange in a posterior lateral position (Fig. 3E). Adductor scars oval, roughly of equal size; pallial line entire. Ventral margin dissected by minute transverse grooves (Fig. 3K).

ANATOMY. (Fig. 4A) Mantle margin free for most of its length, joined and attached to terminal of gill axis, anterior (pedal) aperture extensive, posterior aperture very small. 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 a single demibranch, with nine non-reflected filaments in the largest specimen. Filaments rod shaped, lacking abfrontal extension or harbouring symbiotic bacteria. Labial palps small but projecting.

ASSOCIATION. Attached by byssus threads to the spines of the echinoid Aeropsis rostrata (Fig. 5).

Distribution

Kelliola symmetros is known only from the type locality and from the Bay of Biscay, at abyssal depths. The host echinoid is widely distributed in the North Atlantic (Echinoid Directory 2012).

Differential diagnosis

The hinges of K. symmetros and the Aeropsis commensal are almost identical, but K. symmetros has slightly more prominent umbos and lacks the marginal transverse grooves. Given that K. symmetros was taken in the same sample along with Aeropsis, but not attached to it, it is possible that the two are associated. This suggests an ecological affinity with the Aeropsis commensal described here. Despite the wide geographical separation of the samples considered here, Aeropsis rostrata is regarded as pan Atlantic and having an abyssal bathymetric range (WoRMS 2012). With so few specimens at hand and the poor condition of the holotype of K. symmetros, we have chosen to be conservative and regard the Aeropsis commensal from Biscay conspecific with Kelliola symmetros.

Consequently, at the family level the hinge and anatomical characters of Kelliola are entirely in keeping with the Montacutidae, consisting of a single cardinal peg in the right valve, a marginal flange in the left valve and an internal ligament. The ligament is attached to an elongate shallow depression extending below the beaks and is therefore most similar to Montacuta sensu stricto (Fig. 6). Montacuta substriata, the type species of Montacuta, has a longer cardinal tooth, has radial ridges and ovate in outline (Fig. 6). Anatomically Kelliola is similar to many montacutids, where the ctenidium is reduced to a single demibranch. However, in this genus, the filaments are very few and not reflected (Fig. 4A). This condition could be due to the small size of the specimens.

Dall (1899) noted that Jeffreys’ (1876) description of the hinge of K. symmetros was incorrect and, in re-describing it, created the new genus Kelliola for it. Dall did not make comparisons with other genera but noted that it was similar to Aligena Lea, 1846. This is difficult to understand as Aligena species have a cardinal peg in each valve (Harry 1969). Comparisons with other montacutid genera are currently complex due to a lack of compatibility in descriptions of characters and the widely varying use of generic names. Such difficulties were amply recognised by Gofas & Salas (2008) in their review of Mysella Angas, 1877 and consequent creation of the genus Kurtiella Gofas & Salas, 2008. For the purposes of this paper, comparisons are restricted to genera that have a single cardinal peg in the right valve only and left valve with varying degrees of pseudocardinal development. In hinge characters, Kelliola is most similar to Montacuta sensu stricto (Fig. 6) and Neaeromya Gabb, 1873 (Coan et al. 2000) in that the development of the posterior teeth is limited to a marginal flange in the left valve and the ligament is attached to a shallow depression beneath and posterior to the beaks. For N. rugifera (Carpenter, 1864) Narchi (1969) states that there is a tooth in each valve, although it may be reduced in the left valve (Paul Valentich-Scott pers comm); a further example of the contradictory descriptions found for the montacutid species. In Tellimya T. Brown, 1827 the resilifer is developed and the hinge plate thickened accordingly (see Ockelmann 1965, Fig. 2). In Montacutella the left valve flange is developed as a small projection (Jespersen et al. 2004) and approaches the condition seen in Aligena. The shell of Brachiomya is like that of Tellimya (Jespersen et al. 2004).

Kelliola is as different from Montacuta sensu stricto as are the other genera and a molecular study is required to evaluate the significance of the morphological characters. Kelliola is retained here until such a study is undertaken.

Species level comparisons are restricted to the few abyssal galeommatid species that have been described and none other than that described below under Syssitomya pourtalesiana sp. nov. have been found attached to echinoids. The shell of S. pourtalesiana sp. nov. is more expanded anteriorly, has a more depressed lunule and lacks marginal notches. The ctenidium is highly modified with laminar filaments whereas that of K. symmetros is not modified in this manner. Other described, Atlantic, deep-sea, galeommatids have been assigned to the genera Mysella (now Kurtiella) (Gofas & Salas 2008), Epilepton Dall, 1899 (Allen 2007) or Draculamya Oliver & Lützen, 2011 (Oliver & Lützen 2011) none having a dentition identical to Kelliola or Montacuta. Among ten undescribed galeommatoids from the deep Atlantic, Allen (2008) lists two undescribed Montacuta species that may or may not be similar to K. symmetros.