3D- microanatomy of the semiterrestrial slug Gascoignella aprica Jensen, 1985—a basal plakobranchacean sacoglossan (Gastropoda, Panpulmonata)

Abstract

The monophyly of the panpulmonate, usually marine benthic Sacoglossa—and its basal division into shelled Oxynoacea and shell-less Plakobranchacea—is undisputed, but family relationships are in doubt. Of particular interest is the potentially basal plakobranchacean family Platyhedylidae, comprising morphologically aberrant members lacking head tentacles or body appendages. Herein we re-describe the type species of the genus Gascoignella, G. aprica Jensen, 1985, from Hong Kong. Morphological data was generated by three-dimensional reconstruction from serial semi-thin sections using Amira software. Our microanatomical results largely confirm the original description. The anterior digestive system is sacoglossan-like but modified, e.g. the ascus is not demarcated externally and pharyngeal pouches are lacking. The digestive gland is bipartite, with two rami separated by a longitudinal, muscular, median septum, but fused in the rear end. The postpharyngeally situated nerve ring contains fused cerebropleural ganglia; the short visceral loop has three ganglia. Two major cerebral nerves were identified as rhinophoral and labiotentacular nerves, innervating sensory areas on the head velum. Gascoignella aprica is a hermaphrodite with a truly androdiaulic genital system of which some originally ambiguous characters are clarified. Bursa and prostate insert into a fertilization chamber proximal to a sac-like albumen gland and a tubular mucus gland. The cephalic copulatory apparatus contains a penis armed with a short and straight stylet and an accessory gland of unclear function; the presumed mode of sperm transfer is discussed. A well-developed heart and a large H-shaped kidney are present; the nephroduct opens into the intestine. Epidermal glands and body tissues are described for the first time. The presence of a unique longitudinal, median septum is considered diagnostic for Platyhedylidae, multiple further apomorphies are given. Morphological evidence supports the molecular phylogenetic hypothesis that the Platyhedylidae could be a basal non-shelled sacoglossan lineage.