Uniting what belongs together—Reevaluation of the isopod species Macrostylis grandis and M. ovata using ontogenetic, morphological and genetic evidence

Abstract

Solid taxonomy is the primary basis for many further-reaching studies in biology, such as biogeography. Here we reevaluate the taxonomic status of two species of the isopod crustacean family Macrostylidae Hansen, 1916, Macrostylis grandis Birstein, 1970 and M. ovata Birstein, 1970. Although they differ significantly in size and other morphological characteristics they share, amongst other character states, an oval habitus which makes them stand out from congeners. We test the hypothesis that M. grandis and M. ovata may represent different life stages of a single species. While some of the newly available specimens collected in the northwest Pacific Ocean during the KuramBio I and SokhoBio expeditions could be morphologically identified as either of the two described species, others represented intermediate forms. Based on this new dataset we provide (1) a complemented morphological taxonomic description of M. grandis and (2) reconstruct parts of the postmarsupial development and (3) analyse their genetic distances. Results of all three analyses were used to reevaluate the taxonomic status of both species and provided justification for a transfer of M. ovata into synonymy with M. grandis. The updated biogeography resulting from the new taxonomic status and samples reveals a distribution at abyssal and upper hadal depths across the Kuril Kamchatka Trench with no indication that the trench may represent a barrier for this species.