Studies on the ontogeny of Tritonia hombergi cuvier (gastropoda opisthobranchia)

Abstract

Tritonia hombergi Cuvier is the largest British nudibranchiate mollusc; it is always found in association with the colonial cnidarian Alcyonium digitatum , in depths from 6 to 38 fm. Cleavage and development up to gastrulation resemble the pattern described for other nudibranchs. The heavily yolked stereoblastulae gastrulate by epiboly. As the veliger form is assumed, a slow movement of the anal cells (which predict the site of the proctodaeal invagination) represents the sole vestige of the morphogenetic processes of gastropod torsion and visceral flexure. This movement is completed before the differentiation of any recognizable muscle elements or of the organs of the visceral mass. Hatching occurs 36 to 38 days (at 10 °C) after oviposition; the lecithotrophic larvae are at first negatively geotactic but after 1 or 2 days this is reversed and searching behaviour begins. Searching larvae could be induced to metamorphose only if a live healthy colony of A. digitatum was provided. The shell and operculum are cast, the velar lobes resorbed, and reflexion and differential growth of the mantle fold bring about a movement of the anal complex which reverses the embryonic torsion and visceral flexure. The adult dorsal integument is derived from the layer of the mantle fold which lined the embryonic mantle cavity. It is concluded that the tritoniid nudibranchs probably arose via stages in which the ancestral shell was enclosed by folds of the mantle. Both right and left embryonic midgut diverticula contribute to the formation of the adult digestive gland. Larval metamorphosis involves also concentration of the main ganglia and the enlargement of the rudiments of the definitive kidney and pericardium, while the larval retractor muscle and the metapodial gland begin to atrophy. The young benthic stages browse on the tissues of the cnidarian. Thousands were reared in the laboratory for several months until, when some had reached a length of 3 cm, lack of sufficient food prevented their being cultured further. Shortly after metamorphosis, the histogenesis of the adult skin begins, the pleural ganglia make their first appearance, and the original embryonic otoliths are augmented. The larval kidney disappears and its functions are taken over by the definitive kidney. The ventricle and the auricles develop in the pericardium and, as the pallial branchiae are formed, the adult circulation of the blood is established. Changes in the radular dentition illustrate the taxonomic identity of T. hombergi Cuvier with T. alba Alder and Hancock. The primordia of the reproductive system are first detectable in post-larvae I 3 4 mm in length. The benthic stages feed exclusively on Alcyonium . Previous accounts of the feeding mechanism were incorrect in attributing to the jaws the function merely of prehension and in stating that the food is torn up by the radula. Digestion begins in the stomach after only a few minutes; the cnidarian spicules fall to the stomach floor from whence they are carried by cilia into the hindgut. In the juvenile T. hombergi partially digested matter is sucked into the lobules of the digestive gland by passive dilation following pulsating contractions of their muscle-sheaths; probably a similar mechanism exists in the adult tritoniid. In young animals absorption of the products of digestion, both particulate and liquid, takes place through the walls of the stomach, hindgut, and digestive gland. In older stages, particulate matter is taken up solely by the digestive gland, but fluids may be absorbed also through the stomach wall. Experiments with young individuals (in length 8 to 12 mm) at 15 °C showed that, while individual digestive cells pass through a sequence of changes, the complete digestive gland acts arhythmically under conditions of constant access to food. The conclusion is reached that Pelseneer was in error when he stated that all opisthobranchs undergo during their development a torsion identical with that shown by prosobranchs. Only vestiges of gastropod torsion remain both in veliger and adult stages of many (perhaps most) living Opisthobranchia.