Different circadian locomotor rhythms of two competing hermit crab species.

Circadian rhythms enable organisms to anticipate and adapt to daily environmental fluctuations. These rhythms are thought to have been shaped by natural selection in response to both abiotic factors and biotic interactions, including interspecific competition. The coexistence of ecologically similar species can promote the divergence of activity rhythms, allowing temporal partitioning that reduces agonistic encounters and minimizes competitive pressure, particularly benefiting the weaker competitor. We tested these ideas by considering the locomotor activity of two hermit crab species that compete for gastropod shells -Calcinus californiensis (dominant) and Clibanarius albidigitus (subordinate). We first determined whether activity rhythms are endogenously controlled by comparing individuals' locomotor activity patterns under a 12:12 h light:dark photoperiod (LD) to their activity under constant darkness (DD). We then compared the locomotor rhythms between species under the LD treatment to assess whether their activity patterns are differentiated. Circadian rhythms observed under the LD photoperiod were maintained under DD conditions, confirming the presence of self-sustained oscillations for both species; although, the rhythms amplitudes were reduced under DD. Both species increased their activity around sunset (Zeitgeber Time 10). Calcinus californiensis remained active until approximately ZT 13, exhibiting a crepuscular activity pattern, whereas Clibanarius albidigitus remained active throughout the night until sunrise (ZT 23), displaying a nocturnal activity pattern. This temporal separation is expected to reduce the likelihood of agonistic encounters, particularly around late night and sunrise when Cal. californiensis is inactive. Meanwhile, the increased movement of Cli. albidigitus could compensate for its weaker fighting ability by allowing it to be a better exploitation competitor. Our findings suggest that many generations of interspecific competition have driven the evolution of distinct activity rhythms, promoting species coexistence through temporal niche differentiation.