Two new species of Adontorhina Berry, 1947 (Bivalvia: Thyasiridae) from the Porcupine Bank, off the west coast of Ireland

Abstract

A minute bivalve, Adontorhina keegani, new species (Thyasiridae) is described from the Porcupine Bank, west of Ireland. The new species occurs between 300 and 789 m on the continental slope. The shell is elongate and compressed, with a flattened posterior and complete lack of radial sulcus. The shell has a hinge margin bearing irregular granules and noticeably low umbones, located in the posterior. Hydroids were found growing on the margins of the shell, indicating a shallow burrowing habit. Adontorhina similis, new species, was previously recognized as a European variety of Mendicula pygmaea Verrill & Bush, 1898, but is here shown to be a distinct species. The hinge bears irregular granules, which precludes classification as a Mendicula species. The shell is elongate, moderately inflated with prominent umbones and a pointed posterior margin. The posterior flank of the shell is flattened and does not bear hydroids. In comparison with previously described species of Adontorhina, the new species are more elongate and less inflated.

Treatise Online no. 61: Part N, Revised, Volume 1, Chapter 4: The early shell: Ontogeny, features, and evolution

“…All examined Thyasiridae have early shells of the 2D type, most of which are less than 200 µm in length (maximum length 270-300 µm) (oliver & killeeN, 2002;Barry & MccorMack, 2007). Brooding has not been observed in this group even though PayNe and alleN (1991, p. 486) mention lateral brood pouches for the genus Thyasira laMarck, 1818 in 1818-1822.…”

On the Generic Allocation of“Aligena” PisumDall, 1908

“…These granules are stronger beneath the beak -corresponding to what Harry (1969) interpreted as "rounded bosses" -and anteriorly become slender, running parallel to the dorsal margin (i.e., the "triangular extension" and "incised lines" of Harry (1969)). This peculiar hinge plate morphology reveals that the species actually belongs to Adontorhina, the single thyasirid Barry & McCormack, 2007).…”

“…The species was subsequently reported as also living by Howard (1952). Other species currently regarded under Adontorhina are: A. sphaericosa Scott, 1986, A. lynnae Valentich Scott (in Coan, Valentich Scott & Bernard), 2000, A. keegani Barry &McCormack, 2007, andA. similis Barry &McCormack, 2007. In this paper, Aligena pisum is redescribed, properly illustrated, and reallocated to Adontorhina.…”

“…Adontorhina lynnae, A. keegani, and A. similis differ from A. pisum in their ovate shell outline, originated by their widely projected anterior end. According to Barry & McCormack (2007) the hinge plate of Adontorhina similis has an additional "small cardinal tubercle in the right valve", which is absent in A. pisum A. similis and A. keegani have a minor number A. pisum (seven to eight in A. keegani 11 in A. similis 15 17 in A. pisum).…”