Ingestion and conversion of phytoplankton by the larval stages of a spider crab (Hyas araneus) were studied in laboratory experiments, testing three different diatom species as food: Biddulphia sinensis, Thalassiosira rotula and Skeletonema costatum. Ingestion (expressed in mu-g carbon, C) was measured in daily intervals and compared with that in a carnivorously fed control group (given Artemia sp. nauplii). Digestive enzyme (trypsin, amylase) activities, growth, and biochemical composition (dry weight, CHN, protein, lipid) were also measured in short intervals (every 2-4 days) in herbivorous larvae, as well as in two control groups that were fed with Artemia sp. nauplii and deprived completely of food, respectively. Only with Artemia or B. sinensis as food were daily feeding rates high enough to detect patterns of variation during individual larval moult cycles: in both of these experimental groups, early postmoult and late premoult ingestion rates were consistently lowest, while maximum values occurred in the middle (zoea I), somewhat earlier (zoea II), or near the beginning (megalopa) of the moult cycle. Similarity between carnivorous and herbivorous larvae suggests that these patterns of variation in daily feeding rates are controlled by internal factors, whereas the average quantities ingested depend on food quality. Total amounts of ingested C decreased in all larval instars consistently in the same sequence of experimental conditions: Artemia sp., B. sinensis, T. rotula, S. costatum, reaching in the herbivorous groups 21-31%, 7-13% and 4-7%, respectively, of C ingested by the carnivorous control larvae. Digestive enzyme activities showed significant variations during the course of larval moult cycles, with some conspicuous differences in relation to nutritional conditions. Average activities per individual decreased in both enzymes in the same order of experiments as total amounts of C ingested (see above), however, these differences were less conspicuous in the final larval instar, the megalopa. Individual activities of trypsin and amylase showed in most cases (i.e. larval instars, experimental conditions) significant positive correlations. Protein-specific activities (units per gram protein) showed complicated patterns of variation, reflecting differential changes in individual activities and biomass. In spite of positive correlations between individual activities of these two enzymes, shifts were observed in the amylase/trypsin activity ratio. In general, there was a relative increase in amylase under conditions of malnutrition (again, this was more conspicuous in the zoeal stages than in the megalopa). These shifts are interpreted as a response of the digestive system to changes in food quality (substrate availability) and instar requirements; in the case of amylase, co-regulation with another carbohydrase is presumed, as diatoms do not contain starch, which is the substrate for amylase. Growth (expressed as dry weight or any chemical constituent measured) showed a decrease in the same sequence of experimental condition as average ingestion and individual digestive enzyme activities. Strongest nutritional effects were observed in the lipid and protein fractions, where in starved control larvae losses of as much as 88 and 73% of the initial (early postmoult) values, respectively, occurred. Larvae feeding on S. costatum or T. rotula revealed similar losses, whereas those eating B. sinensis lost much less biomass. In the zoea I instar, B. sinensis allowed some (22%) gain in protein, however, this was accompanied by a significant (36%) loss in the lipid fraction. In the carnivorous control group, lipid and protein increased by up to 150 and 178%, respectively, with a decreasing percentage gain in subsequent larval instars. Gross growth efficiency (K1, C-based) in zoea-I larvae eating B. sinensis or T. rotula was only little lower than that in the carnivorous control (0.24-0.27 vs. 0.31); the same applied to zoea II eating B. sinensis (0.21 vs. 0.31). Consumption of S. costatum, in contrast, caused in all larval stages loss of carbon and consequently, negative K1 values. Our results indicate that all larval stages of H. araneus are in principle able to ingest and convert diatoms, with a clearly decreasing conversion efficiency in subsequent instars. Phytoplankton as a sole food is not sufficient for successful development and growth of spider crab larvae, but in natural pelagic communities it should play a major role as an additional energy source.