Finding Nemo's clock reveals switch from nocturnal to diurnal activity.

Timing mechanisms play a key role in the biology of coral reef fish. Typically, fish larvae leave their reef after hatching, stay for a period in the open ocean before returning to the reef for settlement. During this dispersal, larvae use a time-compensated sun compass for orientation. However, the timing of settlement and how coral reef fish keep track of time via endogenous timing mechanisms is poorly understood. Here, we have studied the behavioural and genetic basis of diel rhythms in the clown anemonefish Amphiprion ocellaris. We document a behavioural shift from nocturnal larvae to diurnal adults, while juveniles show an intermediate pattern of activity which potentially indicates flexibility in the timing of settlement on a host anemone. qRTPCR analysis of six core circadian clock genes (bmal1, clocka, cry1b, per1b, per2, per3) reveals rhythmic gene expression patterns that are comparable in larvae and juveniles, and so do not reflect the corresponding activity changes. By establishing an embryonic cell line, we demonstrate that clown anemonefish possess an endogenous clock with similar properties to that of the zebrafish circadian clock. Furthermore, our study provides a first basis to study the multi-layered interaction of clocks from fish, anemones and their zooxanthellae endosymbionts.

Behavioural and neuronal basis of olfactory imprinting and kin recognition in larval fish.

Imprinting is a specific form of long-term memory of a cue acquired during a sensitive phase of development. To ensure that organisms memorize the right cue, the learning process must happen during a specific short time period, mostly soon after hatching, which should end before irrelevant or misleading signals are encountered. A well-known case of olfactory imprinting in the aquatic environment is that of the anadromous Atlantic and Pacific salmon, which prefer the olfactory cues of natal rivers to which they return after migrating several years in the open ocean. Recent research has shown that olfactory imprinting and olfactory guided navigation in the marine realm are far more common than previously assumed. Here, we present evidence for the involvement of olfactory imprinting in the navigation behaviour of coral reef fish, which prefer their home reef odour over that of other reefs. Two main olfactory imprinting processes can be differentiated: (1) imprinting on environmental cues and (2) imprinting on chemical compounds released by kin, which is based on genetic relatedness among conspecifics. While the first process allows for plasticity, so that organisms can imprint on a variety of chemical signals, the latter seems to be restricted to specific genetically determined kin signals. We focus on the second, elucidating the behavioural and neuronal basis of the imprinting process on kin cues using larval zebrafish (Danio rerio) as a model. Our data suggest that the process of imprinting is not confined to the central nervous system but also triggers some changes in the olfactory epithelium.