Neuroethology of Melibe leonina swimming behavior

American Zoologist, Sep 2001 by Watson, Winsor H III, Lawrence, Kaddee A, Newcomb, James M

Nitric oxide in Melibe

Staining of the Melibe brain with the NADPH-diaphorase histochemical technique reveals a pair of symmetrical neurons on either side of the cerebropleural ganglion (Fig. 6; Newcomb and Watson, 2001). The putative nitrergic neurons project past the eye to the pedal ganglia. Control experiments, in which NADPH is replaced with NAD , yield no staining in the brain. Immunocytochemical staining with antibodies directed against a conserved region of NOS also reveals the same pair of neurons, confirming the results obtained with the diaphorase method (Fig. 6).

In addition to the pair of NOS-positive neurons in the brain, the diaphorase method stains processes in the tentacles, neurons in small ganglia at the base of the tentacles, and the neuropil in the cerebral ganglia. It appears as if all of the staining in the cerebral ganglia is due to processes emanating from cells in the ganglia at the base of the tentacles. But the function of these neurons is unknown.

Application of the NO-donors sodium nitroprusside (SNP, 1.0 mM) and S-nitrosoN-acetyl-penicillamine (SNAP, 1.0 mM) to isolated brains that are expressing the swim motor program, causes a dramatic slowing of the swim rhythm (Fig. 7). In isolated brains, the rate of bursting of swim interneurons and motoneurons goes from 1 cycle every 2-5 sec to 1 cycle every 20-30 sec. Preliminary experiments with semi-intact Melibe indicate that NO-donors slow down the rate of swimming, rather than activating a different type of behavior, such as feeding or crawling. The role of NO in Melibe and the function of the nitrergic neurons in the brain and tentacles are currently under investigation.

DISCUSSION

Melibe swimming behavior provides an excellent model system for investigating the production, control and modulation of a rhythmic behavior. Melibe swim in response to a distinct natural stimulus (seastar tube foot); it is possible to record from both swim motoneurons and interneurons in semi-intact, swimming animals; the isolated brain spontaneously expresses the swim motor program; the CPG circuit is relatively simple and accessible for manipulation; and the behavior is modulated by a variety of different inputs. In the coming years, along with other comparative studies of opisthobranch swimming, this system is likely to contribute a great deal to our understanding of the general principles of motor control and modulation.

Possible reasons for swimming in opisthobranchs

It is generally accepted that a number of opisthobranchs swim to avoid predators, such as the seastar Pycnopodia helianthoides, although this is rarely observed in their natural habitat. In fact, Ajeska and Nybakken (1976) and Bickell-Page (1991) have reported that Pycnopodia and 3 other seastar species all exhibited aversive responses to contact with Melibe, presumably in response to secretions from the repugnatorial glands (Ayer and Andersen, 1983; BickellPage, 1991). The only seastar that has been observed eating a Melibe is Crossaster paposus (Mauzey et aL, 1968; Bickell-Page, 1991). Therefore, it seems odd that Melibe so readily swim in response to seastar tube feet, given the low probability of actually being attacked by seastars. In contrast, Ajeska and Nybakken (1976) and Mauzey et al. (1968) have observed Pugettia producta attacking Melibe in the field and Bickell-Page (1991) has reported that Pugettia and two other crab species capture and eat Melibe in captivity. Melibe will swim in response to pinches and when their cerata are pinched they autotomize them (BickellPage, 1989). This combination of responses may help them escape from predatory crabs that are not deter-red by chemical defenses.