Variation of the locomotor activity rhythms in three species of talitrid amphipods, Talitrus saltator, Orchestia montagui, and O-gammarellus, from various habitats

The expression of biological rhythms was investigated in five populations of three different species of talitrid amphipods from various habitats in the Maremma Regional Park, Grosseto, Italy: Talitrus saltator (from a sandy beach and a canal), Orchestia montagui (form a Posidonia banquette), and O. gammarellus (from a cave entrance and a river bank). Locomotor activity rhythms were recorded in individual animals over 21 days in constant dark at a temperature of 18 degrees +/- 1 degrees C. A high variability in rhythm expression was evident, not only among species and populations but also within populations and the activity pattern of individuals. Activity rhythms of T. saltator and O. montagui were similar, with a good definition and precise circadian periodicity, whereas O. gammarellus showed a high variability and low definition of the circadian rhythm. Significant differences were also observed between two populations of O. gammarellus and T. saltator from different habitats. Within the O. gammarellus species, a significantly higher percentage of active animals (p <0.001) was observed in the cave than the river-bank population; within T. saltator, a significantly lower percentage of active animals (p <0.01) and higher percentage of periodic animals (p <0.05) was found in the canal than the sandy beach population. With reference to environmental stability and variability, the differences observed are explained as a need for plasticity to adapt to environmental changes.