Life on a leaf: 3D-reconstruction and description of a new limapontiid sacoglossan (Gastropoda: Heterobranchia: 'Opisthobranchia') living on the seagrass Halophila ovalis

Sap-sucking sea slugs, Sacoglossa, are almost all suctorialherbivores feeding on siphonalean green algae. Ercolaniais probably the most species-rich genus in the family Limapontiidae, containing at least 25 species, most of which feed on filamentous green algaeof the order Cladophorales. Here we describe an ewErcolania species from the Andaman Sea coast of Thailand. The species is very small, generally less than 1 mm long in sexually mature animals. It is found exclusively on the leaves of the seagrass Halophila ovalisand accordingly we name it Ercolania halophilaen. sp. Anatomy and histology have been studied by serial sectioning and 3D-reconstruction. This is the first limapontioid species studied in full 3D-microanatomical detail. Ercolania halophilaeshows several special features. A heart and pericardiumare absent, as is also seen in a few other species of Sacoglossa. Salivary glands are very prominent and unequal on left and right sides. There is a system of highly vacuolated cells situated along the main digestive gland tubules. Those cells are apparently associated with the kidney and we assume that they are involved in excretion. Radular teeth are sabot-shaped and more similar in shape to those of the enigmatic Japanese Alderiopsis nigra than to any known species of Ercolania. The reproductive system is typical limapontiid, with a closed seminal receptacle, a penetrant penial styletand a bilobed prostate gland. Branches of the albumen gland enter the cerata, as seen in some but not all species presently referred to Ercolania. We therefore place the species in Ercolaniarather than the poorly defined Alderiopsis. We briefly compare limapontiid genera and show that revision is needed.