New archaeogastropod limpets from hydrothermal vents; Superfamily Lepetodrilacea. II. Anatomy

Abstract

The anatomy of nine species (two genera) of lepetodrilacean limpets is described and the mode of life deduced therefrom. The unique action of the odontophore sets them apart from all prosobranchs yet known: its initial ventral movement is all but suppressed and the dorsal movement, before withdrawal, exaggerated. This action is facilitated by the freedom of the odontophore from the restraining action of the snout which is controlled by special cephalic levator and retractor muscles. Evidence indicates that the jaws are used as scrubbers, loosening particles from the substratum over the animal’s head which are then collected by the specialized rhipidoglossate radula. This is the only method of feeding in the more primitive of the two genera, Gorgoleptis , in which the possession of a metapodium with an operculum indicates that the transformation to the limpet form is incomplete. All members of the superfamily have a single left gill typically bilamellate and with a pectiniform skeleton. The dorsal (left) lamellae are reduced in number and size, particularly in Lepetodrilus (lost in Lepetodrilus ovalis ), and the ventral (right) ones exaggerated and their numbers increased with the enlargement of the mantle cavity. Although the ciliary bands on the lamellae of Lepetodrilus are as in other prosobranchs, at the tips of the lamellae the frontals and abfrontals expand to form pads and the laterals encircle the tips between them. This unusual modification relates to suspension feeding: the two methods of feeding may proceed concurrently. The nervous system has a high degree of fusion of the postcephalic ganglia. The pedal ganglia are large, their short commissure continuous with a pleural commissure, and the supra- and suboesophageal ganglia are contiguous with the right and left pleurals respectively. Nerves to the mantle and foot are compound. The epipodium is well developed, confined to the region of the foot in Gorgoleptis and with elongated tentacles, but spreading on to the head and forming a fold surrounding the base of each cephalic tentacle in Lepetodrilus , which has shorter tentacles round the foot; cilia on the right cephalic fold pass particulate matter from the mantle cavity to the mouth. The penis of Lepetodrilus is also of epipodial origin as opposed to its development from the left side of the snout in Gorgoleptis. In both genera a right pallial tentacle is associated with the exhalant passage from the mantle cavity, though this is greatly reduced in Lepetodrilus . The shell of Gorgoleptis species indicates that these limpets have followed a different evolutionary course from that of Lepetodrilus in that the columellar lip remains as an integral part of the peristome. In species of both genera the mantle edge has two folds, the inner probably represents the fusion of the inner and middle folds of zeugobranch limpets: the remoteness of the periostracal and shell secreting areas results in an inturned band of periostracum particularly broad in Lepetodrilus species. In each, the shell muscle is approximately bilaterally symmetrical, but details of its constituent parts, pedal and pallial, differ. In addition to the bilamellate gill and epipodium, the alimentary, circulatory, nervous and excretory systems indicate that relationships are at the archaeogastropod level. Although similar to the trochaceans, these limpets differ from them in a number of respects. The reproductive system approaches that of monotocardians in the length of the gonadial duct, which in the male is a vesicula seminalis; in the separation of that part of the right kidney through which gametes pass to the urinogenital opening; in the presence of a prostate and penis in the male, and in the female Lepetodrilus of a receptaculum seminis. The female has no hypertrophied pallial oviduct, and eggs with no secondary investments are apparently fertilized in the mantle cavity, and then shed. Although the anatomy of Neomphalus indicates that it had a different origin and evolution from the more conservative lepetodrilaceans, all these prosobranchs share a common characteristic in the attainment of the limpet form by having a narrow section between the enlarged head and the visceral mass marked by the end of the oesophageal pouches and tight chiastoneury.