Cetoconcha spinulosa (Niele, 1912)

Pacheco, Leonel I., Teso, Valeria & Pastorino, Guido, 2024, Use of traditional tools and micro-computed tomography for the taxonomy of carnivorous bivalves from the deep waters of Southwestern Atlantic, Zoological Journal of the Linnean Society 202, pp. 1-23 : 11-13

Cetoconcha spinulosa (ƙiele, 1912)

( Figs 6, 7)

Poromya spinulosa Ŋiele 1912: 282 , pl. 18, fig. 26— Powell 1960: 184; Pouthiers and Bernard 1995: 167; Engl 2012: 64, pl. 13, fig. 4a, b.

Pholadomya antarctica Hedley 1916: 28 , pl. 3, figs 31, 32— Powell 1960: 184; Dell 1964: 228; Egorova 1982: 67, fig. 292; 1985: 21.

Pholadomya adelaidis Hedley 1916: 28 , pl. 3 fig. 28— Soot-Ryen 1951: 21; Powell 1958: 178; 1960: 184; Dell 1964: 227; Egorova 1982: 67, fig. 294; 1985: 21, fig. 24.

Poromya cf. adelaides Hedley 1916 (in error pro adelaidis )— Hain 1990: 102, pl. 16, fig. 1.

Poromya antarctica — Voss1988: 44; Hain1990: 102,pl.16, fig.2a–c.

Poromya adelaidis — Dell 1990: 61, fig. 107; Pouthiers and Bernard 1995: 156; Zelaya 2005: 117, table 1; Aldea and Troncoso 2008: 105, fig. 112; Aldea and Troncoso 2010: 206, fig. 245.

Poromya adelaidis auctt . (non Hedley 1916)— Zelaya 2016: 254, table 2.

Cetoconcha spinulosa — Machado et al. 2019: 62.

Type material: [ Poromya spinulosa ] lectotype: ZMB Moll 63.117a, one less valve; paralectotypes: ZMB Moll 63.117b, six valves. [ Pholadomya adelaidis ] syntypes: AM Malacology 46525, two fragmented valves; AM Malacology 105659, a fragment of one valve. [ Pholadomya antarctica ] syntype: AM Malacology 46526, fragments of a right valve. In order to clarify the taxonomic status of Cetoconcha spinulosa , we designate the specimen AM Malacology 46525, illustrated here ( Fig. 6B) as a lectotype of Pholadomya adelaidis , in agreement with article 74.7 of the ICZN, with type locality off the Eastern Barrier of Adelie Land. Type locality: [ Poromya spinulosa ] Gauss Station, Davis Sea, 385 m. [ Pholadomya adelaidis ] off the Eastern Barrier of Adelie Land (66°52 ʹ S, 145°30 ʹ E), 288 fathoms(527 m). [ Pholadomya antarctica ] off Shackleton Ice-Shelf (65° 20°S, 95°27 ʹ E), 240 fathoms (439 m).

Distribution

New records: Mar del Plata Submarine Canyon (146–2934 m), Argentina.

Literature records: Mar del Plata Submarine Canyon, Argentina to Antarctica ( Hedley 1916, Powell 1958, Dell 1964, Egorova 1982, Dell 1990, Hain 1990, Aldea and Troncoso 2008).

Bathymetry: 110–2934 m.

Material examined: 37°57 ʹ 51″S, 54°57 ʹ 24″W, 647 m (MACN-In 44329) five individuals and two valves; 37°59 ʹ 42″S, 54°41 ʹ 51″W, 851 m (MACN-In 44330) one individual and eight valves; 37°57 ʹ 54″S, 54°31 ʹ 55″W, 1144 m (MACN-In 44331) one individual; 38°00 ʹ 59″S, 54°30 ʹ 19″W, 1006 m (MACN-In 44332) one individual and five valves; 37°56 ʹ 40″S, 54°10 ʹ 59″W, 1508 m (MACN-In 44333) seven individuals; 38°08 ʹ 04″S, 53°50 ʹ 42″W, 2081 m (MACN-In 44334) one individual; 37°51 ʹ 40″S, 54°10 ʹ 33″W, 1950 m (MACN-In 44335) one individual; 38°02 ʹ 37″S, 53°54 ʹ 45″W, 1770 m (MACN-In 44336) one individual and two valves; 37°58 ʹ 03″S, 54°31 ʹ 42″W, 1144 m (MACN-In 44337) one individual; 37°53 ʹ 33″S, 54°42 ʹ 56″W, 780 m (MACN-In 44338) two valves; 38°01 ʹ 55″S, 53°39 ʹ 16″W, 2934 m (MACN-In 44339) four individuals and 12 valves; 37°58 ʹ 48″S, 53°43 ʹ 27″W, 2711 m (MACN-In 44340) one individual; 38°01 ʹ 26″S, 53°51 ʹ 01″W, 2212 m (MACN-In 44341) one individual; 37°52 ʹ 37″S, 53°54 ʹ 14″W, 1763 m (MACN-In 44342) two valves; 37°49 ʹ 40″S, 54°07 ʹ 56″W, 1395 m (MACN-In 44343) two individuals; 37°49 ʹ 40″S, 54°07 ʹ 56″W, 1395 m ( ZUEC 7002 ) one individual ; 54°36 ʹ 59″S, 62°50 ʹ 55″W, 647 m (MACN-In 44344) one valve; 54°30 ʹ 25″S, 58°39 ʹ 59″W, 146 m (MACN-In 44345); 36°32 ʹ 24″S, 53°22 ʹ 58″W, 993–1011 m ( MCZ 353553 ) one individual ; 51°59 ʹ 31″S, 56°37 ʹ 58″W, 646–845 m ( USNM 870352 ) two valves ; 61°19 ʹ 01″S, 56°09 ʹ 28″W, 220–240 m ( USNM 886285 ) one individual ; 51°59 ʹ 31″S, 56°37 ʹ 58″W, 646–845 m ( USNM 886374 ) one individual ; 74°52 ʹ 30″S, 174°55 ʹ 58″W, 2143–2154 m ( USNM 886578 ) one individual ; 64°47 ʹ 17″S, 64°06 ʹ 51″W, 110 m ( USNM 904332 ) one individual ; 53°22 ʹ 58″S, 37°16 ʹ 01″, 1299–1400 m ( USNM 904505 ) one individual .

Description

Shell medium to large size (L, 18.2 ± 3.3 mm; H, 13.2 ± 2.3 mm; W, 4.6 ± 0.9 mm; N = 58), thin, ovate, elongated, inequilateral. Surface with ~50 weak, closely and regularly spaced radial lines interspaced by groups of four to eight granules, usually covered by debris, and commarginal growth lines. Periostracum brownish. Umbo central and prosogyrus. Antero- and posterodorsal margin gently descendent. Anterior margin somewhat rounded, posterior truncate. Ventral margin widely curved. Right cardinal tooth prominent, conical. Less valve with an elongated longitudinal ridge near the end of the posterodorsal margin. Resilifer opisthodetic. Interior iridescent, with numerous close-spaced radial lines not corresponding to the external lines. Adductor muscle scars marked, anterior larger than posterior. Small scars dorsal to adductor scars associated with anterior pedal, septal, and probably visceral muscles. Palial line noticeable, siphonal sinus shallow. Ring of 13–15 siphonal tentacles. Ŋe anteriormost groups of septal pores consist of three to four pores; the middle has five to six and the posteriormost six to seven.

Remarks

Hedley (1916) differentiated between P. adelaidis and P. antarctica based on ornamentation, with the former having radial lines, whereas the laưer has radially ordered granules. Although the original illustrations of these specie s clearly depict this difference, the type material of both species exhibits both radial lines and granules ( Fig. 6B, C). Ŋe valve of P. antarctica has less conspicuous radial lines, but, as explained by Dell (1964), the specimen is probably ‘a somewhat worn valve of adelaidis ’. In addition, both type specimens have a similarly developed right cardinal tooth, suggesting the same specific assignment.

Dell (1990) differentiated between P. adelaidis and P. spinulosa based on differences in contour. However, the author did not reference the type specimens of P. spinulosa as material examined, suggesting that he might have based his assertion solely on the original description and illustration by Ŋiele (1912). According to Engl (2012), the name P. spinulosa has been neglected in many studies, which typically focused on P. adelaidis and/or P. antarctica ( Dell 1964, 1990, Mühlenhard-Siegel 1989, Hain 1990, Aldea and Troncoso 2008). Engl (2012) compared the type material of the three species and synonymized them, a decision with which we agree, because these specimens have the same ornamentation, hinge, and contour. Engl (2012) also designated a lectotype from the best-preserved material of the syntypes of P. spinulosa ( Fig. 6A), which he indicated were all juveniles, with the largest one measuring 14 mm in length. Machado et al. (2019) assigned this species to Cetoconcha owing to the presence of three groups of septal pores, in contrast to the two observed in other Poromyoidea genera, a feature also noted in our specimens ( Fig. 7G).

Ŋere are two syntypes of Pholadomya adelaidis , i.e. AM Malacology 46525, which is here designated as a lectotype, while AM Malacology 105659 then becomes a paralectotype. Ŋis assignation is aưributable to the necessity of depriving AM Malacology 105659 of the syntype status. Ŋis specimen has radial riblets instead of lines and lacks granules, and is here identified as a Cardiomya , probably Cardiomya fragilissima , because this is a Southern Ocean species, and the valve shows the same subtle cancellated ornamentation as Cardiomya fragilissima .

Cetoconcha sarsii (E. A. Smith, 1885) was originally collected near the Mar del Plata submarine Canyon, at 4846 m, in addition to (1000 miles) off Southwest Australia at 3560 m. Ŋe original description and illustrations resemble Cetoconcha spinulosa ; however, the type material ( Fig. 8A, B) is in very poor condition and consists of soss parts with a fragment of shell aưached, lacking an umbo or hinge. Ŋis fragment exhibits external ornamentation similar to Cetoconcha spinulosa but with radial lines almost inconspicuous, noticeable only near the anteroventral margin. Ŋe original description mentioned a toothless hinge, and the original illustration ( Fig. 8C) shows a small and less prominent umbo, but owing to the state of the specimen, it was not verified. Currently, Cetoconcha spinulosa and Cetoconcha sarsii appear to be distinct species.