Locomotion

All Aplysia are characterized by wing-like projections called parapodia that are actually thin, lateral extensions of the animal’s mantle cavity (Beesley, Ross and Wells 1998). The size ratio of the foot or parapodia to the rest of the body can often be used to determine the primary locomotary action of the species (Beesley, Ross and Wells 1998). Those with a large muscular foot will often use horizontal undulatory movements as their principal form of locomotion, while those with large parapodia will more often be seen swimming (refer to picture below) (Moore 2006). In addition to swimming however, it is thought that the majority of Aplysia species may use their parapodia as a means of propelling themselves by a sharp downthrust in order to quickly avoid or distract predators. Parapodia are also thought to serve a third purpose, and that is to more efficiently filter water over the gills (which occur to the right of the embedded shell and mantle cavity) (Beesley, Ross and Wells 1998). This is believed to be a particularly important function especially for those species that occur in particulary turbid sandy reef flats (Beesley, Ross and Wells 1998; Moore 2006).


Sample of one 'swimming species', Aplysia oculifera, reproduced courtesy of Neville Coleman.

Aplysia dactylomela are one of the ‘heavier-set’ species that have a large foot, and are believed to use their parapodia primarily for increasing the water flow over their gills as opposed to swimming (Carefoot 1987).