Abra americana (Verril & Bush, 1898)

Simone, Luiz Ricardo L., 2024, New species, misidentifications and problematic taxonomy of some Atlantic South American marine mollusks: a review, Papéis Avulsos de Zoologia 64, pp. 1-104 : 88-89

Abra americana (Verril & Bush, 1898) revalidated, new stratum ( Fig. 59 B-E)

Abra longicallis americana Verrill & Bush, 1898: 778 (pl. 83, fig. 6-7); Warmke & Abbott, 1961: 201; Abbott, 19745: 515 (fig. 5805).

Abra longicallis : Morretes, 1949: 44; Rios,1975:249 (pl. 79, fig. 1194), 1985: 257 (pl. 91, fig. 1278); Díaz & Puyana, 1994: 14: 97 (fig. 271) (non Scacchi, 1835).

Abra longicallus : Rios, 2009:556 (fig. 1551); MolluscaBase, 2023; Passos et al., 2024: 58 (non Scacchi, 1835).

Type locality: 6 stations between 39°49′N 68°28′30″W and 36°16′30″N 68°21′W, 924-2,620 fms.

Remarks: Until now, Abra americana has been identified as the Pliocene fossil Abra longicallus (Scacchi, 1835) , originally classified within the genus Tellina Linné, 1758 ( Fig. 59A ), and described from Puglia, in the southern region of Italy. Initially proposed as a subspecies (technically a variety) of Abra longicallis (an unjustified emendation of longicallus ) by Verrill & Bush in 1898, some authors adopted this classification, while most considered it a synonym (refer to synonymy above). In the original description, Verrill & Bush (1898: 778) emphasized that the variety then designated as americana differs from the European [fossil] form ( longicallis ) "in having the lateral tooth less remote, and the cartilage-pit or chondrophore longer, the antero-dorsal margin more convex, and the whole shell relatively broader."

By examining the original image of the fossil ( Fig. 59A ) and comparing it to a typical shell of the living A. americana ( Fig. 59 B-E), it becomes evident that, in addition to the features highlighted in the original description, the shells of the living population have a more pointed and centrally positioned umbo (in contrast to the fossil, which has a rounded umbo located more posteriorly). The sub-umbonal cardinal teeth are more developed, while the posterior cardinal teeth are shorter. Furthermore, the posterior edge is more rounded in the living species, whereas the fossil species exhibits a more angular shape. Additionally, the out- er surface appears smoother in the living population, while the fossil species appears more concentrically sculptured.

Taking into account the temporal and geographical separation, along with the conchological differences explored above,the most appropriate taxonomic approach appears to be retaining A. longicallus solely for the Italian fossil species and designating the living Atlantic deep-water species as A. americana .

Figures

Figure 59. Studied Abra spp.: (A) replication of figure by Scacchi (1834: fig. 6) of his Italian fossil Tellina longicallus; (B-E) shells details of A. americana; (B) MZSP 117015,off Rio de Janeiro,Brazil,left valve,outer view (L 15.4 mm); (C) same,inner view;(D) same,detail of hinge;(E) MZSP 116911, right valve, detail of hinge. Scale:= 2 mm.