The molecular basis of daily and seasonal migration behaviour in the copepod Calanus finmarchicus in the face of climate change

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Climate-change-induced increases in oceanic temperatures are causing poleward distribution range shifts in many temperate marine organisms. These shifts have far-reaching effects on marine food webs and oceanic carbon fluxes. Zooplankton are a central component of oceanic ecosystems, where they capture primary carbon fixed by phytoplankton and provide the major carbon source to higher trophic levels, including commercially important fish stocks. The cosmopolitan and lipid-rich copepod Calanus finmarchicus dominates zooplankton biomass in the North Atlantic and has emerged as a powerful model organism for understanding the biogeochemical processes in oceanic carbon cycling.Calanus displays two behaviours that are crucial for its role in the oceanic food web and carbon pump. First, diel vertical migration (DVM), which is a circadian behaviour whereby the animals dive to depth during the day and migrate to the surface to feed at night. This behaviour is a predator avoidance response and is driven primarily by light. Second, long resting stage (diapause), which is a crucial part of the life cycle of Calanus whereby juveniles in their last pre-adult copepodid stage in late spring migrate from their surface feeding grounds down to deeper epipelagic waters (~400-1000m) where they remain inactive for 7-8 months until January. The factors governing timing of diapause entry and exit remain unclear, but are suspected to include daylength, temperature and lipid reserves.Both DVM and diapause structure populations in space and time and both behaviours are sensitive to changes in environmental cues. Climate change is facilitating range expansions into higher latitudes where the cues (light, temperature) governing these behaviours are likely to differ from those at lower latitudes. Therefore, the capacity of individuals and populations to respond to these phenological shifts via phenotypic plasticity and/or adaptation will be a crucial factor in affecting realised range shifts, biomass flux and broader biogeochemical processes in the North Atlantic.A major hindrance in predicting population responses to phenological shifts is a lack of insight into the physiological and molecular processes affecting behavioural variability at the individual level. DVM timing, speed and depth vary measurably among individuals and show some correlation with among-individual variability in photo-responsiveness (unpublished ongoing work in KSL's group) and metabolic factors such as respiration rates and lipid reserves. Similarly, individuals vary in their timing of diapause entry, though diapause exit is often remarkably synchronised. Very little is known about the molecular genetic basis of this among-individual variation, of the physiological basis of behavioural synchronicity, and how adaptive genetic diversity among Calanus populations is structured in space and time.This project will examine the molecular genetic variation associated with variation in DVM and diapause behaviour and environmental factors likely to drive behavioural synchronicity. The principal aims are twofold: First, to examine the links between gene expression, behavioural phenotypic plasticity and environmental factors hypothesised to trigger and synchronise DVM and diapause. Second, to examine genetic diversity and genetic structure among populations from different latitudes from nearshore coastal to open ocean environments. We hypothesise that, first, environmental triggers of DVM/diapause cause changes in expression at key genes involved in biological rhythms consistent with phenotypic plasticity, and, second, that these genes show allele-frequency differences among populations from different latitudes, consistent with an adaptive evolutionary response to range shifts.