Description: Zooplankton grazing on bacterio- and phytoplankton was studied in the Gulf of Aqaba and the Northern Red Sea during Meteor Cruise Me 44-2 in February-March 1999. Protozoan grazing on bacterioplankton and autotrophic ultraplankton was studied by the Landry dilution method. Microzooplankton grazing on phytoplankton 〉6 µm was studied by incubation experiments in the presence and absence of microzooplankton. Mesozooplankton grazing was studied by measuring per capita clearance rates of individual zooplankton with radioactively labelled food organisms and estimating in situ rates from abundance values. Protozoan grazing rates on heterotrophic bacteria and on algae 〈6 µm were high (bacteria: 0.7 to 1.1 d-1, ultraphytoplankton: 0.7 to 1.3 d-1), while grazing rates on Synechococcus spp. were surprisingly low and undetectable in some experiments. Mesozooplankton grazing was weak, cumulative grazing rates being ca. 2 orders of magnitude smaller than the grazing rates by protozoans. Among mesozooplankton, appendicularians specialised on smaller food items and calanoid copepods on larger ones.

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.