Ontogenetic differences in the circadian locomotor activity rhythm of the talitrid amphipod crustacean Orchestoidea tuberculata

 Adult talitrid amphipods often display a nocturnal activity pattern, which has been shown in a number of cases to be partly controlled by an endogenous rhythm of circadian period. Juvenile talitrids have been little studied in the past, but evidence from field studies indicates that they may display a different diel pattern of locomotor activity from that of adults. This study presents evidence from the field in south–central Chile that juvenile Orchestoidea tuberculata Nicolet show crepuscular activity peaks, contrasting with the nocturnal peaks shown by adult conspecifics. Under constant conditions in the laboratory, both juveniles and adults exhibit endogenous rhythms of circadian periodicity. However, the phasing of activity differs in juveniles and adults, corresponding to the differences observed in the field. It is suggested that differing phasing of peak locomotor activity may be a mechanism to reduce contact between juvenile and adult talitrids. Experimental evidence indicates a significant negative influence on survival of juveniles in the presence of adult conspecifics by intraspecific predation. Other factors influencing the timing of surface activity are also discussed. Received: 3 August 1999 / Accepted: 26 May 2000