Biogenic amines, behaviour, and the multifunctional depressor muscle in the squat lobster Munida quadrispina (Anomura, Galatheidae)




Antonsen, Brian L.

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The biogenic amines serotonin (5-HT) and octopamine (OA) have many roles in neurological systems in decapod crustaceans influencing processes as diverse as sensitivity of individual sensory neurons and agonistic behaviours. I examined aspects of the aminergic system in the squat lobster Munida quadrispina (Anomura, Galatheidae) and compared my results with data on aminergic systems and behaviours in more extensively studied species. M. quadrispina has a complex set of agonistic behaviours, and in comparisons with crayfish and lobster behaviour one major difference stands out: squat lobsters do not normally fight. Injecting carefully controlled doses of 5-HT induces M. quadrispina to perform stereotypical aggressive behaviours in the absence of any additional stimulation, and animals under the influence of injected 5-HT will fight. Animals under the influence of injected OA are much more likely to initiate escape responses to a standardized stimulus than are untreated animals, and assume, under certain circumstances, a submissive stance in the absence of additional external stimulation. The distributions of serotonergic and octopaminergic neurons in M. quadrispina are overall fairly similar to those of crayfish, lobsters, and crabs. However, several important differences, such as a lack of unpaired medial serontonergic neurons and far fewer octopaminergic “crotch” cells in M. quadrispina than in lobsters may relate to functional differences in the aminergic systems and other systems influenced by the amines. The pereiopod depressor muscles lift the body of the animal above the substrate and, therefore, are important in aggressive, and other behaviours. In M. quadrispina, as in all decapods, the depressor muscle and its antagonist, the levator muscle, are composed of multiple anatomically distinct heads. Most published studies have treated the depressor muscle as a single functional unit, despite documented differences in the population of depressor excitatory motor neurons. In M. quadrispina, each head has individualistic patterns of excitatory innervation, and the heads are activated differentially during walking and maintained stance. These differences reveal a functional subdivision among the heads of the depressor muscle, with different combinations of heads responsible for movement of the leg, stance maintenance, and joint tension. Injecting 5-HT into freely moving animals increases the excitatory input to all of the heads of the depressor muscle, whereas injecting OA decreases excitatory input.



Lobsters, Munida quadrispina