How many species of Hexabranchus (Opisthobranchia : Dorididae) are there?

Abstract

Anatomical examination of several specimens of Hexabranchus, collected from the extreme boundaries of its geographic range and other localities, revealed that there are two distinct species within this genus. One of these species is widely distributed throughout the tropical Indo-Pacific. The oldest available name for the Indo-Pacific species is Hexabranchus lacer (Cuvier, 1804). The study of the original type material of H. lacer confirmed that it is a senior synonym of other names introduced subsequently, including Hexabranchus sanguineus (Rüppell & Leuckart, 1830), which is the name most commonly used for this species. However, according to the provisions of the International Code of Zoological Nomenclature (Article 23.9.1), the name H. sanguineus has precedence over H. lacer, which has never been used as valid, except for the original description. Hexabranchus praetextus Ehrenberg, 1828 is also a senior synonym of H. sanguineus that has not been used as valid since its original description and it is also invalidated under the provisions of Article 23.9.1.

The other species, Hexabranchus morsomus Ev. Marcus & Er. Marcus, 1962, appears to be endemic to the Caribbean Sea. Hexabranchus sanguineus and H. morsomus are clearly distinguishable by the morphology of the reproductive system and the radula. Hypotheses on the speciation process that took place in Hexabranchus, the possible origin of the two species and the geographic range of the genus are discussed.