Insemination by a kiss? Interactive 3D-microanatomy, biology and systematics of the mesopsammic cephalaspidean sea slug Pluscula cuica Marcus, 1953 from Brazil (Gastropoda: Euopisthobranchia: Philinoglossidae)

Increasing molecular evidence suggests that the phylogeny of euthyneuran gastropods differs greatly from centenary textbook concepts. The presence, homology and evolution of characters in major subgroups thus need to be reinvestigated. Traditionally basal opisthobranch Cephalaspidea ("head-shield snails and slugs") were pruned to a new taxon concept, with benthic euopisthobranch and tentacle-bearing cephalaspidean lineages basal to burrowing, head-shield bearing philinoidean species. Among the latter, mesopsammic "microslug" lineages evolved at least twice. Herein we explore in 3D microanatomical detail the putatively basal philinoglossan Pluscula cuica (Marcus, Boletim da Faculdade de Filosofia, Cincias e Letras. Universidade de So Paulo 164:165-203, 1953a) from its type locality in Brazil. The species possesses several "accessory" ganglia and a reduced posterior mantle cavity that retains some putative shell-building tissue and an osphradium. The hermaphroditic, monaulic genital system opens in a posterior position; it retains a bursa copulatrix but lacks a distinct receptaculum seminis. Autosperm is transferred to the cephalic copulatory organ via an external sperm groove, not through the hemocoel, as suggested in the original description. The penis opens through the oral tube, sperm is transferred by a "kiss". A conspicuous yellow gland is discussed as a modified Blochmann's gland. Retaining several putative symplesiomorphies with philinoids, Pluscula is discussed as the most basal offshoot in meiofaunal Philinoglossidae. However, the supposed "primitiveness" of the fused rather than separate cerebropleural ganglia and the triganglionate rather than pentaganglionate visceral nerve cord was based on misobservations. Higher categories such as Philinoglossacea for Philinoglossidae, and a separate family Plusculidae for P. cuica are no longer warranted. Inner cephalaspidean relationships and a scenario of more or less successive philinoglossid adaptation to meiofaunal environments should be investigated by molecular studies with more comprehensive taxon sampling.