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

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

Further, reproductive life-history traits, chronology of ontogenetic development, sexual differentiation, andlifetime variations of biomass and body composition in terms of ash,carbon, and nitrogen were studied in mysids reared under laboratoryconditions. Ontogenetic variations in bodycomposition were related to embryo development, gonadogenesis, andreproduction. The weight-specific female investment in reproductionincreases with body size. Gravid females are capable of intersegmentalgrowth 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 andrevising an existing model based on literature data and presented byRudstam (1989). The experimental estimate of daily energy intake, together with obtained data on fecundity and energy allocation between somaticand reproductive tissues, allowed for reliable predictions of both thegrowth dynamics of mysids and energy costs of embryogenesis. Thebioenergetics model, combined with a functional response model and data on the population size and structure, can be usedto estimate the food consumption and production of M. mixta in theBaltic.

Determining the in situ growth rates and stable isotopeincorporation of mysids is critical to understanding energy transfer, application of the bioenergetics model, andinterpreting the isotope signatures. A moult staging system and a precisetiming 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 resultscan be used to analyze moulting activity in the wild populations, allowingpredictions of moult cycle duration and seasonal growth. As growth andturnover rates of different tissuesaffect their isotopic composition, the fractionation of mysid muscle tissue,feces and exoskeleton in response to a change in the isotopic compositionof the diet was examined. The isotopic composition of these metabolicproducts may form a basis for dietreconstruction of M. mixta and N. integer in the fieldstudies. The feces (13C and (15N values mirror diet over last few hours, exuviae (13C represent nutrients metabolized 2-3 weeks ago, and muscle tissuesintegrates isotopic signal over a relatively long period (6-8 weeks for(15N and >3 months for (13C).

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 predictionsabout how these organisms will interact with other trophic levels.