The Complex Hydrodynamics of Swimming in the Spanish Dancer

The lack of a vertebra seems to have freed marine gastropods to explore and exploit a stupendous~variety of swimming kinematics. In fact, examination of just a few animals in this group reveal~locomotory~modes ranging from insect-like flapping, to fish-like~undulatory~swimming, jet propulsion, and~rajiform~(manta-like) swimming. There are also a number of marine gastropods that have bizarre swimming gaits~with~no equivalent among fish or marine mammals. In this latter category is the Spanish Dancer (\textit{Hexabranchus~sanguineus})~a sea slug~that swims with~a complex combination of body undulations and flapping~parapodia.~~ While the neurobiology of these animals has been~relatively~well-studied, less is known about their propulsive mechanism~and swimming energetics. In this study,~we focus on the hydrodynamics of two distinct swimmers: the Spanish Dancer, and the sea hare~Aplysia;~the latter~adopts a~rajiform-like mode of swimming~by passing travelling waves along~its~parapodia.~In the present study an immersed boundary method is employed to examine~the~vortex~structures,~hydrodynamic~forces and energy costs of~the swimming in~these animals.~~