Feeding in Solenogastres (Mollusca)

Research project P 14330 Feeding in Solenogastres (Mollusca) L. SALVINI-PLAWEN 06.03.2000 Investigations on the primary shell-less Solenogastres (Mollusca) are rare and by now mostly restricted to anatomical descriptions and species identifications. The present study is designed to extend the knowledge on the feeding biology of this small molluscan class by means of modern techniques. Transmission electron microscopy and diverse in situ histochemical methods will be used for the functional analysis of glands and epithelia within the digestive tract. The mode of food uptake will be studied by observations of feeding behavior in living specimens and, indirectly, via morphological features. Solenogastres mostly appear to feed on cnidarians (Hydrozoa and Anthozoa). An analysis of gut contents including the identification of nematocysts and a comparison to other cnidariavorious mollusks (e.g. aeolid gastropods) in respect to digestive enzymes is planned. The digestive system in Solenogastres appears to be exceptional within the mollusks because there is no separate midgut gland and no true radular ribbon developed. Both characters can be judged to be plesiomorphic in mollusks. The primitive status of the Solenogastres within the phylum thus gives the class a crucial position in respect to molluscan phylogeny and the concept of a molluscan "archetype". Moreover, in Solenogastres the elaboration of foregut glands and especially the type of lateroventral tubular organs is of importance for systematics on the family level. It will help to answer some open questions within the phylogeny of the class if these histologically different gland organs produce secretions composed of different chemical compounds. Therefore specimens of the primitive order Pholidoskepia and of the more evolved order Cavibelonia will be studied.

This project considerably enlarged the knowledge on prey spectra in Solenogastres and on the function of their digestive tract, including feeding mechanics and digestion (intracellular digestion and cell cycles of midgut cells). Of special interest are results on the fine structure of multicellular foregut glands because these organs are important for considerations on relationships and evolution within the Solenogastres. Solenogastres are exclusively marine, vermiform molluscs (Mollusca). Most of their body surface is covered by a chitinous cuticle with calcareous sclerites (scales, needles, hooks) but ventrally they bear a narrowed gliding sole (foot). Most species of Solenogastres feed on cnidarians (Cnidaria). This is known from occasional direct observations but mostly from analyses of gut contents. Several species live on the colonies of their host species, for example soft corals or hydrozoans. In the free-living species, however, which are found on the surface of muddy sediments or between the grains of subtidal sands, nettle capsules detected within the digestive tract are the only indication of their actual prey. The discharge of nettle capsules is prevented during ingestion and their functional ability continues even within the midgut. The mechanisms of discharge inhibition is unknown, although it can be assumed that the secretions of foregut glands are of importance. The present project focuses on eight species of Solenogastres from diverse habitats in the coastal areas of Norway, Bermuda, and northern Spain, as well as from hydrothermal vent sites of the eastern Pacific Ocean. The animals measured between 0.75 and 5 mm in total body length. Although the general structure of their digestive tracts is comparable, there are important differences concerning the foregut glands and radula (feeding apparatus). Histologic cross-sections (2-8 m thick) of paraffin- or resin-embedded specimens were produced to analyse the anatomy and histology of the digestive tract. Foregut gland secretions (acid or neutral mucus, protein secretions) were characterised by means of selective staining methods applied on histologic sections. The fine structure (ultrastructure) of digestive tract cells was investigated by means of ultrathin (70 nm) sections which were observed with a transmission electron microscope. This included several different receptor cells and numerous types of glandular cells of the foregut, the digestive cells of the midgut, and the ciliary cells of the hindgut.