A study of Melibe leonina during culture¹ to adulthood provides insight into the ontogenetic development of behaviours such as crawling, swimming, and feeding.  The author divides Melibe’s development into 5 arbitrary stages², shown on the Right, and notes that each definitive adult behaviour develops through precursor behaviours arising in the early stages. 

For example, normal crawling starts with locomotion by cilia but, by the advent of Stage IV, is replaced by muscular crawling (see lower part of figure on Right). Note, however, that an escaping individual resorts to precocious employment of muscular crawling.  True escape “galloping”, effected entirely by muscles, commences late in Stage III.

Swimming involves, in order, hood crumpling, cerata inflation and erection, lateral body flexions, and foot folding³, and develops by increments during ontogeny (see figure on Left).  As expected, swim cycles are also faster during the early stages than in later ones. 

Feeding also develops ontogentically, from holding the hood in a stationary or fixed position in early stages, to a behaviour involving lowering the hood to scrape the substratum surface in middle stages ("incipient" fishing), to true fishing where the hood is raised into the water column then closed to an interlocked tentacle position in cyclical rhythm in later stages (see figure lower Right). Early-stage Melibe will hold the “fixed” feeding position until the hood is externally stimulated by the touch of particles, then the hood swivels.  In later stages, the feeding rhythms are intrinsic. 

Finally, defensive secretions from ceratal repugnatorial glands appear at Stage III.  Page 1993 Mar Behav Physiol 22: 141.

NOTE¹  eggs from adults in Patricia Bay, British Columbia are cultured in the laboratory at 12°C.  After hatching, the juveniles are fed on ciliate protists and naupliar copepods; later, they are given brine shrimp Artemia nauplii, copopodites, and adult copepods as food

NOTE²  Stage I includes post-metamorphic individuals possessing one pair of cerata (the primary pair) with unbranched digestive diverticula (this stage lasts 0-2wk).  Stage II (2-4wk post-metamorphosis) begins with the appearance of secondary cerata and branching of the digestive diverticula.  Stage III (4-7wk post-metamorphosis) starts with the appearance of the tertiary cerata, and features increased branching of the diverticula and appearance of rhinophore sheaths.  Stage IV (6-10wk) features the appearance of inner tentacles (note that the primary and secondary cerata are resorbed).  Stage V (minimum 8wk) features the appearance of quaternary cerata, and the juvenile now resembles a small adult

NOTE³  the foot folds in on itself longitudinally, presumably for better streamlining