Microanatomy Of Hedylopsis Ballantinei, A New Interstitial Acochlidian Gastropod From The Red Sea, And Its Significance For Phylogeny

Abstract

This is the first description of an acochlidian gastropod species from the Red Sea. More than 20 specimens of Hedylopsis ballantinei n. sp. have been extracted from subtidal coral sand in the Gulf of Aqaba. As a model organism for this poorly known, highly aberrant and enigmatic opisthobranch group, the anatomy of this new species is described in detail. Major organ systems have been reconstructed from histological semithin serial sections. The central nervous system has also been studied by means of immunocytochemical staining and confocal laser scanning techniques; dorsal bodies and rhinophoral ganglia are described for the first time for Acochlidia. Hardparts were examined using SEM, the midpiece of an acochlidian sperm is described with aid of TEM for the first time. Our results show Hedylopsis ballantinei n. sp. to be similar to the European H. spiculifera (junior synonym H. suecica). Differences include tropical occurrence, larger rhinophoral ganglia, different arrangement of ampulla and genital glands, and the probable absence of anterior male genitalia. Hedylopsis ballantinei n. sp. plays an important role in understanding acochlidian phylogeny, since it combines an array of morphological features that appear plesiomorphic for Acochlidia, e.g. a (small) mantle cavity, a unidirectional stomach lacking any cuticular elements, a tube-like, holohepatic digestive gland, a prepharyngeal, epiathroid central nervous system with separate cerebral, pleural and pedal ganglia, dorsal bodies associated with the cerebral ganglia, a (short) visceral loop with three ganglia, a monaulic and hermaphroditic reproductive system, and elongate spiral sperm. On the other hand, the absence of anterior male genitalia appears to be a derived condition that is shared with microhedylacean species. The aberrant possession of three glycogen helices in the sperm midpiece appears unique for opisthobranchs. With present, limited knowledge, the Acochlidia is regarded as a monophyletic group that is supported by several autapomorphies, with a hypothetical origin from acteonoidean opisthobranchs, i.e. diaphanid-like ancestors.