Separated from the cradle: A new species of Falcidens (Mollusca, Aplacophora, Caudofoveata) reveals weird patterns of distribution in the deep-sea

The shell-less, worm-like caudofoveate aplacophorans are common inhabitants of the deep-sea. A recent series of studies has demonstrated their phylogenetic, ecological and biogeographical importance for the understanding of marine environments. Falcidens Salvini-Plawen, 1968 is among the most speciose genera of Caudofoveata, including species with a wide range of morphological diversity. Here, F. amiae sp. nov. is described based on its body shape, sclerites, and radula; it is compared with other similar species, and distributions are also modelled to envisage other suitable areas of occurrence. Collected over a large area of the southwestern Atlantic Ocean, along the Brazilian coast, most specimens were found in bottom samples from the upper continental slope. Falcidens amiae sp. nov. has a stoutish body without externally perceptible divisions, a "U"-shaped oral shield, a radula with two pairs of teeth, and sclerites with a wide base and acute blade ornamented with a strong central longitudinal keel and weak grooves. While almost all species of Falcidens have elongated bodies with externally perceptible divisions, only two known species have a more-stoutish and homogeneous body shape, F. limifossorides Salvini-Plawen, 1992 and F. halanychi Schander, Scheltema and Ivanov, 2004, comparable to that of F. amiae sp. nov.; they are also similar in the radula structure and F. amiae sp. nov. has an oral shield and sclerites that are markedly similar to those of F. limifossorides. These taxa appear to form a clade, but if so, they apparently became separated long ago, as they now have very distinct distributions. Maps of potential distributions showed that F. amiae sp. nov. and F. halanychi do not overlap in the Atlantic. This study fills a small gap in knowledge of the morphology of this intriguing molluscan group, revealing part of its puzzling distribution in the deep sea.