Field data, stable isotope analyses of the Baltic Sea food web structure and experimental studies of the functional response indicate that the omnivorous crustacean Mysis mixta (Crustacea, Mysidacea) has no distinct trophic position, but can potentially affect plankton community structure and compete with herring and sprat for the zooplankton. To evaluate field data on mysid body constituents, abundance, and population structure and that of their potential prey, basic knowledge of feeding behaviour, growth mechanisms, and reproductive capabilities are required. This study on mysid growth, energetics, and stable isotope dynamics is based on a set of laboratory observations and experiments. Several aspects of mysid physiology were considered: (1) estimation of feeding rates, (2) energy allocation within the organism, (3) analysis of moulting as a function of growth and as a potential tool for determining the in situ growth rates, and (4) trophic isotopic fractionation.

First, to develop experimental methods, I examined artificial factors affecting feeding rates of M. mixta. The estimate of consumption rates under laboratory conditions was found to be largely a function of the following factors: light intensity, duration of the experiment, predator density, and starvation status prior to the experiment. Ingestion rates decreased when the mysid density increased (i.e., predator-density-dependent functional response was observed).

Further, reproductive life-history traits, chronology of ontogenetic development, sexual differentiation, and lifetime variations of biomass and body composition in terms of ash, carbon, and nitrogen were studied in mysids reared under laboratory conditions. Ontogenetic variations in body composition were related to embryo development, gonadogenesis, and reproduction. The weight-specific female investment in reproduction increases with body size. Gravid females are capable of intersegmental growth during brooding period, while males appear to store energy only for copulation and die after mating.

A growth model of M. mixta was developed by testing and revising an existing model based on literature data and presented by Rudstam (1989). The experimental estimate of daily energy intake, together with obtained data on fecundity and energy allocation between somatic and reproductive tissues, allowed for reliable predictions of both the growth dynamics of mysids and energy costs of embryogenesis. The bioenergetics model, combined with a functional response model and data on the population size and structure, can be used to estimate the food consumption and production of M. mixta in the Baltic.

Determining the in situ growth rates and stable isotope incorporation of mysids is critical to understanding energy transfer, application of the bioenergetics model, and interpreting the isotope signatures. A moult staging system and a precise timing of the moult cycle was established for M. mixta and Neomysis integer. Effects of temperature and feeding regimes on the chronology of the moult cycle were investigated. These results can be used to analyze moulting activity in the wild populations, allowing predictions of moult cycle duration and seasonal growth. As growth and turnover rates of different tissues affect their isotopic composition, the fractionation of mysid muscle tissue, feces and exoskeleton in response to a change in the isotopic composition of the diet was examined. The isotopic composition of these metabolic products may form a basis for diet reconstruction of M. mixta and N. integer in the field studies. The feces delta13C and delta15N values mirror diet over last few hours, exuviae delta13C represent nutrients metabolized 2-3 weeks ago, and muscle tissues integrates isotopic signal over a relatively long period (6-8 weeks for delta15N and >3 months for delta13C).

This study demonstrates that a combined approach, including (1) bioenergetics, (2) morphological observations, (3) behavioural responses, and (4) elements and isotope dynamics, can be useful for answering specific questions about individual organisms and for making predictions about how these organisms will interact with other trophic levels.