Description: Tolerance of organisms towards heterogeneous and variable environments is highly related to physiological flexibility. An effective strategy to enhance physiological flexibility is the expression of polymorphic enzymes. This seems to be the case in the brown shrimp Crangon crangon. It shows high reproduction rates, feeds opportunistically on endo- and epibenthic organisms, and is apparently well adapted to variable environmental conditions. Previous electrophoretic studies revealed a high level of polymorphism and no consistent phenotype of digestive enzymes between individuals. In order to understand the underlying biochemical processes, we carried out a transcriptome-based study of digestive enzymes of C. crangon. Detailed sequence analyses of triacylglycerol lipase, phospholipase A2, alpha amylase, chitinase, trypsin and cathepsin L were performed to identify putative isoforms. The number of isoforms, and thus the degree of polymorphism varied among enzymes: lipases and carbohydrases showed higher numbers of isoforms in enzymes that besides their extracellular function also have diverse intracellular functions. Furthermore, cysteine proteinases showed a lower polymorphism than serine proteinases. We suggest that the expression of enzyme isoforms improves the efficiency of C. crangon in gaining energy from different food sources.

Description: Calanoid copepods in boreal and polar habitats are confronted with a strong variability in food supply. The abundant species in the Southern North Sea, Temora longicornis, cannot accumulate extensive energy stores to buffer longer periods of food paucity. These copepods react immediately to dietary changes and therefore, recent trophic conditions must strongly influence their metabolic and functional responses. To elucidate the implications of feeding history, we collected two cohorts of T. longicornis females, which were characterized by different feeding histories and consequently different physiological backgrounds. In the North Sea in April 2005, females fed at a higher trophic level than in May, were poor in lipids, showed low proteinase activity and produced 41 eggs female-1 d-1. In May, females were significantly smaller than in April, contained more lipids, had a higher proteinase activity and produced 26 eggs female-1 d-1. In the laboratory, females from each group were fed with either autotrophic diatoms or heterotrophic dinoflagellates for three days. Irrespective of different initial conditions, all T. longicornis females incorporated diet-specific fatty acids within 24 h. Also in both experiments and each dietary treatment, egg production increased after 24 h indicating that females were food-limited in situ in April and in May. Responses differed, however, with regard to lipid accumulation and enzyme activities . Total lipid contents increased significantly in females during experiment I (April) but not during experiment II (May), despite higher algal lipid levels during experiment II. Proteolytic activity increased during experiment I, but decreased during experiment II. These deviating responses of T. longicornis females to food conditions suggest that detailed knowledge about the initial physiological state of specimens is required, when investigating adaptive mechanisms and metabolic performances of copepods by means of experiments.

Description: The Southern Ocean, in particular the southwest Atlantic sector, is experiencing rapid environmental changes. A long-term trend of density changes of key pelagic species has been noted over the last decades: Antarctic krill populations are declining whilst salps are on the rise and shifting their distribution poleward. A similar poleward expansion is anticipated for a third key player, the hyperiid amphipod crustacean Themisto gaudichaudii, leading to an increasing overlap of the distributions of these three species. Due to major knowledge gaps in the ecology, and genetic connectivity of T. gaudichaudii, the likelihood of this shift and its consequences for the pelagic food web structure remain largely unexplored. In this context, Themisto’s genetic and trophic connectivity as well as thermal response were investigated with state-of-the-art molecular methods. Phylogeographic analyses showed genetic homogeneity between localities in the Southern Ocean and Atlantic waters combined with high degree of phenotypic plasticity enabling different lineages to thrive in regions further south. Diet analyses using DNA metabarcoding were applied to characterize regional variation in diet. These analyses showed a diet predominantly composed of krill, in particular in the Antarctic Peninsula region, showing that Themisto’s poleward range expansion can further impact the already declining krill stocks. It also unexpectedly revealed ctenophores to be an important prey, despite their reputation as “trophic dead-end”. Transcriptome analyses were used to study the thermal response of Themisto individuals from different geographic populations that were experimentally exposed to heat and cold treatments. The analysis of differentially expressed genes showed that genetic lineages differ in thermal tolerances. It also revealed a wide range of molecular mechanisms in Themisto amphipods to cope with thermal stress. These findings contribute to better predict the impact of climate-driven range shifts on the pelagic ecosystems in the Southern Ocean.