Swimming in the sea hare Aplysia brasiliana: Cost of transport, parapodial morphometry, and swimming behavior

The energetics and behavior of the parapodial-swimming Aplysia brasiliana were investigated in order to compare net cost of transport (COT_net) between swimming and crawling, and to compare transport costs with other swimmers. Oxygen consumption (V_O2) increased with increasing animal mass for resting, crawling, and swimming animals. Slopes of the regressions of log V_O2 on log mass were 0.90, 0.91, and 0.89 for resting, crawling, and swimming, respectively. The regression for resting V_O2 on mass was significantly lower than regressions of crawling and swimming on mass, which fell into a statistically homogenous subgroup. During 4-h swimming bouts, parapodial beat frequency dropped by less than 10% of starting values after 2 h and then stabilized for the remainder of the trial, whereas velocity steadily decreased to about 70% of starting values over the 4-h period. Initial beat frequency (at the start of a swimming bout) was negatively related to body mass, varying from 1.1 beat s^− 1 for a 34 g individual to 0.7 beats s^− 1 for a 500 g individual. Final beat frequency (at the end of a swimming bout) was also negatively related to body mass, but had a significantly lower intercept than initial beat frequency. Neither initial swimming velocity nor final swimming velocity was related to mass, but final velocity was significantly lower than initial velocity. A 250 g A. brasiliana swam at 345 m h^− 1 and crawled at 7 m h^− 1. Swimming COT_net (0.1 ml O₂ kg^− 1 m^− 1) for a 250 g A. brasiliana was 50 times less than crawling COT_net (5.3 ml O₂ kg^− 1 m^− 1). While the crawling COT_net for A. brasiliana fell within the range of other marine gastropods, swimming COT_net was less than that of swimming crustaceans, and much less than another gastropod, Melibe leonina, that uses lateral bending to swim.