Cardiomya knudseni ( Allen & Morgan, 1981 )

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 : 8-9

Cardiomya knudseni ( Allen & Morgan, 1981)

( Fig. 4 A-C)

Cuspidaria (Cardiomya) knudseni Allen and Morgan 1981: 466 , figs 30, 31.

Cardiomya knudseni — Scarabino 2003: 238; Olabarria 2005: table 1; Allen 2008: appendix 3; Allen 2011: 440; Scarabino et al. 2016: supplementary material 6; Oliveira et al. 2017: 289.

Type material: According to Allen and Morgan (1981), the material was deposited in the malacological collection of the MCZ; however, we failed to find it asser intensive searching.

Type locality: Atlantis II expedition, station 262 (38°46 ʹ N, 52°17 ʹ 54″W), 2440 m.

Distribution

Literature records: Northwestern Atlantic, off Biscay Bay and Portugal, to Northeastern Atlantic, off Washington, DC and off NJ, USA ( Allen and Morgan 1981, Olabarria 2005). Southwestern Atlantic: Off Río de la Plata, Uruguay ( Allen and Morgan 1981).

Bathymetry: 1661–3806 m.

Material examined: 36°49 ʹ S, 53°15 ʹ 24″W, 1661 m ( MCZ 352388 ) two fragments of a valve ; 38°46 ʹ N, 70°06 ʹ W, 2886 m / 38°00 ʹ N, 71°47 ʹ W, 2864 m ( MCZ 352389 ; the label has two stations written) one individual ; 57°58 ʹ 60″N, 10°48 ʹ W, 2081 m (MNHN-IM-2016-5660) one individual.

Description

Shell small (≤ 8.1 mm L), white, fragile. Numerous (≤30) almost equally prominent radial riblets covering the entire shell, except on the dorsal side of the rostrum. Riblets slightly more defined on the posterior side of the disc and inconspicuous on the anterior side. Umbo small, subcentral, prosogryrus. Posterior dorsal margin straight, anterior dorsal margin straight (forming a shoulder), anterior margin acute, posteroventral margin with a gently marked rostral sinuation. Rostrum moderately long (approximately half of the shell length), tube shaped, covered by ≤10 radial lines including the rostral depression. Right posterior lateral tooth low and long. Resilifer triangular. Interior nacrous.

Remarks

Oliveira et al. (2017) emphasized the strong resemblance, both in shell morphology and in size, between Cardiomya knudseni and Cardiomya perrostrata (Dall, 1881) , in addition to other peculiarities: the presence of the former in the Eastern Atlantic, where the latter had not been recorded at the time; the inclusion of the same illustration of Cardiomya abyssicola Verrill & Bush, 1898 , in both Cardiomya knudseni and C. perrostrata synonym lists by Allen and Morgan (1981); and the absence of Cardiomya knudseni records since its description, except in the paper by Olabarria (2005). Oliveira et al. (2017) did not find the Cardiomya knudseni specimens studied by Allen and Morgan (1981) and were unable to perform a close comparison of the species. As a result of all of these problems, they assigned Cardiomya knudseni as ‘ nomen dubium ’. However, we, located two lots from the Atlantis II in the MCZ: MCZ 352388 and MCZ 352389. The studied material allowed for a better comparison of Cardiomya knudseni with other Cardiomya species, although some features from the original description (30 radial ribs, interior nacreous, and a triangular resilifer) could not be verified because they were missing from the inspected material. As a result, it was observed that C. perrostrata has a stronger right posterior tooth, more prominent posterior ribs, and a longer rostrum (approximately three-quarters of the shell length) than Cardiomya knudseni .

Only three lots were found of the material originally studied by Allen and Morgan (1981); none of them is the holotype. MCZ 352389 contains a specimen with the shell almost destroyed except for the rostrum, which does not seem to be as long or tube shaped as in the original illustration. In addition, the label indicates some ambiguity in its coordinates and identity, i.e.: ‘Sta. 72 (HH’) myonerid B or sp. 3 of Sta. 64’. Owing to this uncertainty, this lot was not considered for the analysis of the species. In contrast, MCZ 352388 ( Fig. 4A ) has two fragments of a right valve from station 239 (Continental Slope of Uruguay, 1660–1680 m deep), which correspond perfectly to the original description and illustration of Cardiomya knudseni ( Allen and Morgan 1981: fig. 31). This correspondence is also fulfilled by MNHN-IM-2016-5660. This last lot contains a well-preserved specimen that could be a candidate for a neotype; however, it is a juvenile, noticeable by its shorter rostrum and fewer riblets.

As mentioned by Oliveira et al. (2017), the bathymetric distribution of Cardiomya knudseni and Cardiomya perrostrata overlaps in one record, station 239 (1661 m), represented by MCZ 352388, a specimen verified here as Cardiomya knudseni . Allen and Morgan (1981) reported a bathymetric range between 1661 and 2886 m; however, station 126, referenced in the same work, reached a depth of 3806 m.

According to Oliver et al. (2016), specimens identified as Cardiomya knudseni from the entire Atlantic margin fall within the range of morphological variation of Cardiomya costellata (Deshayes, 1835) , Cardiomya cadiziana Huber, 2010 , and Cardiomya striata (Jeffreys, 1876) . It is important to emphasize that many records of Cardiomya costellata were then identified as Cardiomya striolata ( Carvalho de Lima et al., 2020), whose rostrum is noticeably shorter than Cardiomya knudseni and lacks radial lines in the rostral depression. In relation to the other species, Cardiomya knudseni exhibits a shorter anterior region and weaker riblets in comparison to Cardiomya striata , and it also has a long rostrum and radial lines in the rostral depression, as opposed to Cardiomya costellata and Cardiomya striolata . Additionally, Cardiomya cadiziana has a much shorter rostrum, and the anterior radial ornamentation is weaker.

Figures

Figure 4. Cardiomya spp. A, Cardiomya knudseni MNHN-IM-2016-5660:less and right view of the individual. B, Cardiomya knudseni MCZ 352388:fragments of right valve. C, Cardiomya knudseni illustration of the material studied by Allen and Morgan (1981). D, Neaera ornatissima perrostrata syntype USNM 63119: less and right view of the shell.Scale bars: 1 mm in A; 5 mm in B–D.