Differences in the Endogenous Rhythm of Locomotion of Two Hermit Crab Species Helps Facilitate Their Coexistance

Abstract

Circadian rhythms are evolved mechanisms that allow organisms to anticipate environmental changes through internally controlled processes, increasing survival and fitness. In addition to abiotic conditions, rhythms can be shaped by biotic pressures like interspecific competition. The temporal partitioning of activity between ecologically similar species can reduce agonistic encounters, which is especially beneficial to 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. Within each species, circadian locomotor rhythms were similar between the two light regimes, confirming self-sustaining activity patterns. However, the species differed in their peak activity times: Cal. californiensis was active mainly at sunset (crepuscular), while Cli. albidigitus had activity peaks at both sunrise and sunset (cathemeral) resulting in higher total locomotor activity. This temporal separation is expected to reduce the likelihood of agonistic encounters, particularly around 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, suggesting that it invests more heavily in encountering resources to compensate its lower resource holding potential Our findings suggest that many generations of interspecific competition have driven the evolution of distinct activity rhythms, promoting species coexistence through temporal niche differentiation.

Keywords: Crustaceans, endogenous rhythm, exploration, dominant, subordinate, sunset, sunrise, temporal partitioning, aquatic, fitness