LARVAL AND METAMORPHIC MORPHOGENESIS IN THE NUDIBRANCHMELIBE LEONINA(MOLLUSCA: OPISTHOBRANCHIA)

Abstract

Larval development and metamorphosis in the nudibranch Melibe leonina (Gould) are described from observations of living animals and from one micrometer histological sections. Larval morphogenesis is similar to that previously described for other species of planktotrophic opisthobranch larvae except the rudiments of the primary cerata and the oral hood of the post-metamorphic stage appear in the late stage larva. Unlike many other opisthobranch larvae, M. leonina does not appear to require a specific exogenous cue to induce metamorphosis. Metamorphosis involves loss of the shell, operculum, velar ciliated cells, and certain components of the larval stomach but the left and right digestive diverticula are retained. A rapid expansion of the primary cerata and the oral hood occurs and is accompanied by a large volume increase of the internal hemocoel of these structures and a flattening and vesiculation of their epithelial cells. Several neuronal somata within the pleural ganglia become notably larger than their neighbors during metamorphosis. At approximately 2.5 days after shell loss, M. leonina begins to employ the oral hood to capture ciliates and small benthic nauplii. Morphogenesis in M. leonina is compared to that of other opisthobranchs and the premetamorphic appearance of the cerata and the lack of an exogenous metamorphic trigger are discussed.