In a comfort zone and beyond – ecological plasticity of key marine mediators

Copepods of the genus Calanus are the key components of zooplankton.
Understanding their response to a changing climate is crucial to predict
the functioning of future warmer high-latitude ecosystems. Although
specific Calanus species are morphologically very similar, they have
different life strategies and roles in ecosystems. In this study, C.
finmarchicus and C. glacialis were thoroughly studied with regard to their
plasticity in morphology and ecology both in their preferred original
water mass (Atlantic vs. Arctic side of the Polar Front) and in suboptimal
conditions (due to, e.g., temperature, turbidity, and competition in
Hornsund fjord). Our observations show that ‘at the same place and time’,
both species can reach different sizes, take on different pigmentation, be
in different states of population development, utilize different
reproductive vs. lipid accumulation strategies, and thrive on different
foods. Size was proven to be a very mutable morphological trait,
especially with regard to reduced length of C. glacialis. Both species
exhibited pronounced red pigmentation when inhabiting their preferred
water mass. In other domains, C. finmarchicus individuals tended to be
paler than C. glacialis individuals. Gonad maturation and population
development indicated mixed reproductive strategies, although a
surprisingly similar population age structure of the two co-occurring
species in the fjord was observed. Lipid accumulation was high and not
species-specific, and its variability was due to diet differences of the
populations. According to the stable isotope composition, both species had
a more herbivorous diatom-based diet in their original water masses. While
the diet of C. glacialis was rather consistent among the domains studied,
C. finmarchicus exhibited much higher variability in its feeding history
(based on lipid composition). Our results show that the plasticity of both
Calanus species is indeed impressive and may be regulated differently,
depending on whether they live in their ‘comfort zone’ or beyond it.