Aspects of the biology of the vermetid mollusc Serpulorbis arenaria

Abstract

The gross morphology of S.arenaria is described. This animal can either live as a solitary animal and in this case the shell is of the typical coiled type, or it can occur in groups of two to twenty individuals in which case the shell deviates from its typical coiling pattern. Characteristic structures are the pedal tentacles that are employed in mucous net production. The pallial complex extends for fifty per cent the length of the animal. The ctenidium is well developed and is involved in the ciliary feeding mechanism. Females can be distinguished from males by a median slit in the mantle that is absent in males. Two varieties, either with dark brown or dark red colouration with light yellow specks, are found in a population of S.arenaria. The dark brown variety is the rarer of the two. The histology of the major organs and tissues is described in detail and illustrated with drawings and photomicrographs. Diagrams of serial section through the animal reveal in detail the internal structure of S.arenaria. The histology of the pedal gland shows that it consists of two longitudinal lobes composed of small tubules involved in mucous secretion. These mucous secretions are transferred to a number of ciliated ridges that convey the mucous strings to the pedal tentacles. The radula is of the Taenioglossan type and can be represented by the formula 2 1 R 1 2. The transformation of the gonad from testes to ovary is illustrated by a series of three photomicrographs. S.arenaria is protandreus. Finally a shell secreting epithelium at the posterior end of the visceral mass consists of large goblet cells that unlike mucoytes, are preferentially stained by Ponceau Red rather than light Green. The ecology of S.arenaria is also dealt with. A review of the ecology of three Mediterranean vermetids namely Vermetus cristatus, Vermetus triqueter and Dendropoma petraeum is followed by a description of the fauna and flora at Il-Ponta l-Kbira, the site where the ecology of S.arenaria was studied. The application of the Tine-intercept sampling technique to determine the zonation pattern, showed that S.arenaria is a member of the Infralittoral zone only, unlike Dendropoma glomeratus that occupies a definite Upper Mediolittoral zone on the same shore line. A list of associated fauna and flora is included Finally an experimental investigation shows that the presence of turbulance and detritus together stimulate the release of the mucous net feeding mechanism as opposed to ciliary feeding. The two feeding mechanisms used by S.arenaria are described. A list of the feeding methods used by a variety of vermetid molluscs is included. S.arenaria is a mixed ciliary and mucous net feeder but the prominence of the ctenidium and observations made on living animals indicate that it is predominantly a ciliary feeder. The ciliary of the ciliary feeding mechanism consists of a filtering mechanism powered by the ctenidial filaments which have been shown histologically to possess powerful lateral cilia and numerous mucocytes on the frontal and abfrontal surfaces. Finally the digestive system of gastropod microherbivores like S.arenaria is described in detail and compared to that of S.arenaria. The structure of the digestive system resembles that of a typical mesogastropod. The histology of the oesophagus, stomach intestine and digestive glands is illustrated by photomicrographs and camera lucida drawings. The digestive tubules contain several excretory spherules and this emphasises their role in excretion. In fact two types of cell can be identified: the excretory cells and the digestive cells. The digestive cells are more numerous. The absorption phase cells described by Morton (1955) were not observed in sections. Experiments for detection of enzyme activity revealed the absence of a protease in the stomach and digestive glands.