The genus Dallocardia (Mollusca: Bivalvia: Cardiidae) in the Southwestern Atlantic Ocean

The taxonomy of the species assigned to the genus Dallocardia (Cardiidae: Trachycardiinae) recorded from the Argentinean and Magellanic biogeographical provinces was reviewed, revealing that two species are present: Dallocardia muricata (Linnaeus, 1758), well known as widely distributed along the western Atlantic, and here considered as having its living southernmost limit of distribution in the southern Brazilian coast; and Dallocardia delicatula (EA Smith, 1915), a species resurrected as valid, to which D. manueli (Prado, 1993) is regarded as a junior synonym, occurring along southeastern and southern Brazil, Uruguay and in temperate waters of Argentina. The shells of both species are characterized in detail, differing in outline and rib morphology. Apart from this, a study of the functional anatomy of D. muricata was performed. This species is a moderately rapid shallow burrower from intertidal and subtidal regions in coarse, grassy and muddy sand bottoms, presenting adaptations to live in low-energy environments of sheltered bays and inlets. It has short siphons, with the incurrent crowned with simple, finger-like tentacles, which prevent, but do not block completely excess material and/or large particles from entering the pallial cavity. In addition to presenting a wide pedal gape and the two siphonal openings, D. muricata is the first known cardiid having a temporary fourth mantle gape, this acting as a supplementary incurrent opening, and also serving for expulsion of pseudofaeces. Compared to the labial palps, the ctenidia in this species are large and involved in processing the bulk of material entering the mantle cavity. The densely folded labial palps are indicative that D. muricata deals with moderate to considerable amounts of material. D. muricata is a hermaphroditic bivalve species.