Calanus in the North Atlantic: species distribution and genetic population structure in space and time

"It is hard to imagine that such minute animals could be of such importance in the economy of a country", said Norwegian naturalist A. Boeck already in 1871 about Calanus finmarchicus, a tiny crustacean (ca. 3 mm) drifting in the ocean. Indeed Calanus species are likely the most numerous animals in the world and are central in marine food webs. Calanus eat phytoplankton and are at the same time an important food source for many marine organisms, including herring, sardine, cod and salmon. Since the 1950s, due to increase of sea temperature in the North Atlantic, Calanus species have moved northwards, affecting the abundance of several species of fish, and, potentially leading to ecosystem changes. So-called Next Generation Sequencing technologies allow us to sequence large amount of DNA and RNA quickly and cheaply. Partial sequencing of the genome of Calanus was used to design new tools to reliably identify species. Calanus species look very similar but differ in their amount of energy-rich lipids and, thus, support different food webs. Species identification is crucial and needed to predict how species responds to climate change. This development of genomics resources has led to the recent finding of local, permanent resident populations of the arctic Calanus glacialis in fjords all along the Norwegian coast. Such findings deeply challenge our current understanding of Calanus ecology, life cycle, responses to climate change, but also our basic understanding of fjords ecosystem and dynamics. Furthermore, looking at the genes responding to temperature stress revealed striking differences between the boreal C. finmarchicus and the Arctic C. glacialis that seems to lack the molecular mechanisms to deal with elevated temperature. This, combined with the genetic differentiation between arctic and costal populations of , C. glacialis suggested that this wide ranging species might be locally adapted.

Understanding how climate change will affect the planet is a key issue worldwide and addressing the consequences of global warming for the biosphere has become a research priority. The rise of temperature has affected all Earth´s subsystems including the oceans. Copepods of the genus Calanus are predominant in the zooplankton biomass of the North Atlantic and the Arctic. They play a key role in marine food webs both as main primary consumers and as prey species for many commercially important species. As a response to ocean warming, a northward movement of warm-water Calanus species has been observed and C. finmarchicus appears to have declined throughout much of the North Atlantic. Calanus sp are often separated based on their size and on the geographic location of collection. As changes in temperature are expected to affect both the distribution and the size of Calanus, is clear that molecular tools for the identification of Calanus species are essential. It is also crucial to access the ecological (r ange shift) and evolutionary (population genetic / selection / genetic diversity & effective population sizes) response to climate change of Calanus. The main goal of this project is to use new molecular mnethods in combination with historical samples to address how climate change has affected the distribution and the evolutionary response of Calanus species in the North Atlantic. By developing Calanus spp as beacons of climate change, the research will contribute toward a better understanding of the inte raction between climate change and plankton communities; and a better forecasting of the changes in food web structure of the North Atlantic and Arctic Oceans. Such knowledge will be crucial for an appropriate societal response.