The cerata of Melibe leonina (Gould, 1859) (Mollusca; Nudibranchia): morphogenesis, neurogenesis, autotomy, and repugnatorial glands
Date
1988
Authors
Page, Louise Roberta
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Abstract
Larval and metamorphic development of Melibe leonina. as determined from histological sections, is similar to that of other planktotrophic nudibranch larvae except that the rudiments of the postmetamorphic cerata appear during the larval stage. Using semi-serial thin sections, I examined ceratal development from the larval mantle fold and neurogenesis of the pleural/ceratal (PC) nerve. The proximal segment of the PC nerve is established initially by outgrowing axons of central motoneurons. Morphological evidence suggests outgrowing axons are guided by a supporting cell and by the basal lamina of the mantle fold epithelium. The distal portion of the PC nerve is pioneered by ingrowing axons from peripheral sensory cells.
Ceratal autotomy, which allows escape from crab predators, requires breakage of the epidermis, longitudinal muscle bands. ceratal nerve and digestive gland. Ultrastructural observations show that two circular nerves, originating from the ceratal nerve, lie within the autotomy plane; one runs beneath the epidermis, the other surrounds the ceratal branch of the digestive gland. Innervated granule-filled cells (GC) lie within the perineurium of these nerves and extend processes to the basal laminae and associated connective tissue of the four structures that cross the autotomy plane. Degranulation of GC and disruption of basal laminae accompany autotomy. I suggest that autotomizing stimuli (pinching the ceras) elicit neurally-mediated release of GC granules, which disrupts basal laminae and connective tissue structures within the autotomy plane. Tissue separation is assisted by strong muscular contractions.
Each ceras of M. leonina contains a peripheral nervous system consisting of several small ganglia that give rise to distal nerves. Efferent neurons within the ganglia are recruited by strong afferent stimuli and fire semisynchronously to produce a train of large amplitude spike bursts in extracellular recordings from all nerves emanating from the ganglia. The spike bursts precede ceratal autotomy. I argue that these neurons are presynaptic to ceratal muscles that help accomplish ceratal autotomy.
The repugnatorial glands of the cerata are composed of two types of secretory cells invested with muscle cells. Glandular secretion requires direct touch to ciliated sensory cells adjacent to the secretory pore and sea star tube feet reduce the stimulus threshold for secretion.