One for each ocean: revision of the Bursa granularis (Roding, 1798) species complex (Gastropoda: Tonnoidea: Bursidae)

Bursa granularis (Roding, 1798) is a tonnoidean gastropod that is regarded as broadly distributed throughout the Indo-Pacific and tropical western Atlantic. Because of its variable shell it has received no less than thirteen names, now all synonymized under the name B. granularis. We sequenced a fragment of the cox1 gene for 82 specimens covering a large part of its distribution and most type localities. Two delimitation methods were applied, one based on genetic distance (ABGD) and one based on phylogenetic trees (GMYC). All analyses suggest that specimens identified as B. granularis comprise four distinct species: one limited to the tropical western Atlantic, another to southwestern Western Australia and two in the Indo-Pacific (from the Red Sea to the open Pacific) that are partly sympatric-but not syntopic-in Japan, the Philippines, Vanuatu and New Caledonia. Based on comparison of shell characters, we applied the following available names to the four species, respectively: B. cubaniana (d'Orbigny, 1841), B. elisabettae Nappo, Pellegrini & Bonomolo, 2014, B. granularis s. s. and B. affinis Broderip, 1833. We provide new standardized conchological descriptions for each of them. Our results demonstrate that a long planktotrophic larval stage, common among Tonnoidea, does not necessarily ensure a circumtropical species distribution.