Advancing molecular crustacean chronobiology through the characterisation of the circadian clock in two malacostracan species, Euphausia superba and Parhyale hawaiensis

The ability to entrain to environmental cycles and therefore anticipate and prepare for the changes they predictably bring is the preserve of the endogenous biological clock, most widely studied at the circadian level. Despite a rich history of research into the behavioural and physiological rhythms shown by many crustacean species, the underlying molecular system driving such traits is not well understood. The aim of this research was to develop our understanding of crustacean clocks through the study of two species, one of major ecological importance and the other a powerful model organism. The Antarctic krill Euphausia superba is a keystone species in the Southern Ocean ecosystem, and evidence suggests that the clock may influence both daily and seasonal rhythms. Using a variety of approaches, including the creation of a de novo assembled head transcriptome, a full suite of clock-related genes have now been cloned and characterised. Unlike many species Euphausia superba possesses orthologs of every canonical core clock gene, and cell culture assays indicate that the central feedback loop has the capacity for complete transcriptional inhibition via two separate pathways, raising the possibility that the krill clock may be an ancestral type or employ multiple oscillators to control rhythms of differing periods. In contrast to the relatively intractable krill, the amphipod Parhyale hawaiensis has simple maintenance requirements and an extensive genetic toolkit with the potential to enable sophisticated dissection of the molecular clock. With the aim of laying the groundwork for future research the clock genes of this species have also been identified, along with the development of a locomotor activity assay. Parhyale hawaiensis shows evidence of bimodal patterns of activity under the control of a molecular clock that combines mammalian-like characteristics with some unique features worthy of further investigation.