Molt cycle alterations in behavior, feeding and diel rhythms of a decapod crustacean, the spiny lobster Panulirus argus

Abstract

The crustacean molt cycle manifests extensive behavioral changes in addition to physiological and integumentary modifications. The paucity of quantitative studies led us to characterize molt stage dependent alterations in rhythmic locomotor, feeding, and agonistic behavior of subadult spiny lobster Panulirus argus held grouped and solitarily in simulated natural conditions. Non-disruptively determined molt stages were defined as proportions of intermolt duration. Significant nocturnal rhythmicity persisted through the full molt cycle, though daily form varied as a function of activity level and molt stage. A distinct early evening peak signaled initiation of foraging and walking behavior. Thereafter, rhythm amplitude either declined progressively (low activity: unimodal pattern), remained high (high activity: nocturnal plateau), or exhibited a secondary early morning peak (intermediate activity: bimodal and polymodal patterns). Activity ceased at or immediately prior to sunrise. Ecdysis was predominantly nocturnal, probably reflecting species specific spatiotemporal movement patterns and social behavior. A selective advantage of nocturnal ecdysis was postulated concerning avoidance of cannibalism and diurnal predators. Locomotor activity and feeding rates were not equivalent through the molt cycle, though both peaked in stages B2-C1. Locomotor activity remained high in metecdysis, decreased sharply at proecdysis initiation (D0), reaching lowest levels in D1-D3, then increased in D3 through B1. Activity dropped steeply at ecdysis, though lobsters were capable of intense and coordinated activity. Feeding decreased slowly through metecdysis after the B2-C1 maximum, then increased temporarily in C4 and D0, indicating heightened feeding motivation. This contrasts with the locomotor activity decrease at proecdysis. Food consumption declined rapidly in D1 and D2 and ceased at the D2-D3 transition. Feeding resumed in B1 or B2, intensifying to maximum in late B2. Feeding remained relatively constant within stages, whereas locomotor activity varied greatly, though both correlated with metabolic needs. Grouped and solitary lobsters displayed similar patterns of foraging and walking, equivalent to those of locomotor activity and feeding of solitary individuals. Frequency of agonistic interactions (not aggression per se) remained relatively constant through the cycle, peaking in metecdysis, though the highest relative proportion occurred near ecdysis. Lobsters then were submissive and avoided physical contact with conspecifics. Clearly, locomotor activity, feeding, and social behavior of P. argus are not simply determined. Indeed, behavior is distinctly phase coordinated with varying metabolic requirements dependent on the proximity to ecdysis and ecological pressures.