Mysis Mixta Research Articles

In situ evidence of the role of Crangon crangon in infection of cod Gadus morhua with nematode parasite Hysterothylacium aduncum in the Baltic Sea.

Cod was one of the most important fish species in the Baltic Sea, but its condition is deteriorating for several reasons, including an increasing parasite burden. The aim of this study was to determine the source of infection of Baltic cod with parasites by examination of invertebrates found in situ in the cod stomach. A total of 1681 cod were sampled during four research cruises in the southern Baltic Sea in 2012, 2013 and 2014 and the composition of their diet was analysed. Each prey item from cod stomach was identified to the lowest possible taxonomic level and a parasitological analysis of all invertebrates collected was performed. Crangon crangon, Saduria entomon and Mysis mixta were the most commonly represented invertebrates among food items. Hysterothylacium aduncum was found only in C. crangon. This host–parasite system is reported here for the first time in situ in the stomach of cod from the Baltic Sea, confirming the role of C. crangon in cod infection with H. aduncum.

Diet of the twaite shad Alosa fallax (Lacépède, 1803) (Clupeidae) in the Gulf of Gdansk, the Baltic Sea

Abstract Twaite shad Alosa fallax (12–48 cm total length) from the Gulf of Gdansk feed on Mysids: Mysis mixta, Neomysis integer, and fish: Sprattus sprattus, Osmerus eperlanus, and Pomatoschistus sp.. In the stomachs of A. fallax there also occur Hydrobia sp., Diptera and small stones, particles of wood and plastic. The food of inshore twaite shad measured from 12 to 21 cm centered on M. mixta. Fish are the main prey for A. fallax from the class length 22–31 cm from the Vistula River mouth and for all shads longer than 32 cm.

Effect of sinking spring phytoplankton blooms on lipid content and composition in suprabenthic and benthic invertebrates in a cold ocean coastal environment

Lipids were measured in 19 suprabenthic and benthic invertebrate taxa from 5 phyla to determine their responses to a sinking spring diatom bloom. In Conception Bay, as in many areas of coastal Newfoundland, Canada, benthic organisms experience sub-zero temperatures year round. Among specimens collected at depths between 210 and 220 m, the small mysid Erythrops erythrophtalma had, on average, the highest lipid concentration (6% of wet weight), while a holothurian (Order Apodida) had the lowest (0.3%). In E. erythrophtalma, 73% of the lipids were found in the neutral fraction, while in the sea cucumber 68% were polar lipids. The euphausiid Thysanoessa raschii had the highest proportion of essential ω3 fatty acids (41% total fatty acids) while the holothurian had the lowest (21%). Pseudocalanus spp. showed the highest mean value for the C16 polyunsaturated fatty acid (PUFA) ratio indicative of lipids from nutrient-replete diatoms. The amphipods Acanthostepheia malmgreni and Anonyx spp. responded immediately to each spring bloom with increased total and neutral lipid contents. The mysid Pseudomma truncatum also showed a transient increase in total lipid content, but this occurred much later in the summer. However, P. truncatum, together with the decapod Pandalus borealis, the mysid Mysis mixta, and the polar bivalve Yoldia hyperborea, did respond immediately based on increased proportions of ω3 fatty acids, while the asteroid Ctenodiscus crispatus showed increased ω3 fatty acid proportions in its gonads much later. The chaetognath Parasagitta elegans differed from most other species in that it maintained very similar total lipid and C16 PUFA proportions throughout the sampling period, suggesting little direct response to the bloom. Thus, we found a variable response to the bloom with respect to phenology, even within orders, which depended upon feeding behaviour and gonadogenesis.

Ecology of mysid shrimps in the Bornholm Basin (central Baltic Sea)

Mysid shrimps are an important trophic link in the food web of the Baltic Sea. In 2002 and 2003 we investigated species composition, distribution, life cycle and prey in the Bornholm Basin (central Baltic). Three species and one genus were identified: Mysis mixta, Neomysis integer, Mesopodopsis slabberi and Praunus sp. The dominant M. mixta and N. integer occurred in the whole study area but were more abundant in the peripheral, more shallow regions. Size frequency distribution indicated one generation for M. mixta and probably two for N. integer. Main prey of all size classes in both species was the cladoceran Bosmina coregoni maritima during summer and autumn. In spring and winter also copepods of the species Temora longicornis and Pseudocalanus acuspes were important prey items. Detritus and phytoplankton cells were of minor importance and many individuals had empty guts. Mysid shrimps in the deep basin might be advected from more shallow areas, as the abundance of all species was much lower than in coastal regions. The deep Bornholm Basin does not seem to be a favourable habitat, probably due to the inaccessibility of the oxygen deficient bottom layer. Under current hydrographic conditions mysids are probably not important predators of zooplankton and thus not competitors of planktivorous fish in the Bornholm Basin.

Winter food utilisation by sympatric mysids in the Baltic Sea, studied by combined gut content and stable isotope analyses

We studied the winter dietary characteristics of two sympatric mysid species, Mysis mixta and M. relicta, which exploit both benthic and pelagic habitats during diel vertical migrations. Samples collected before and after the ice-covered period in the northern Baltic Sea were investigated using both stomach content analyses and stable isotope analyses of carbon and nitrogen. Both of the mysid species were omnivorous during winter and utilised both benthic and pelagic food sources. The main food source before the ice period was calanoid copepods (40 and 36% for M. mixta and M. relicta, respectively), and after ice-out calanoid copepods (23%) and zooplankton resting eggs (23%) for M. mixta and diatoms (44%) and calanoids (25%) for M. relicta. Their patterns of food utilisation broadly followed seasonal fluctuations in the abundance of the main prey groups. Although pelagic food availability is low in winter both mysid species utilised pelagic prey widely. We also show that when combining these different diet analysis methods it is important to take into account the time lag in isotopic signatures, otherwise the obtained results do not correspond but instead show the feeding history at different times.

Habitat modification mediated by motile surface stirrers versus semi-motile burrowers: potential for a positive feedback mechanism in a eutrophied ecosystem

We performed a 4 wk laboratory experiment with the semi-motile burrowers Macoma balthica and Marenzelleria spp. and the motile surface sediment stirrers Monoporeia affinis and Mysis mixta to study their effects on the transport of tracer particles (mean diameter () 1 µm) in the sediment, sediment parameters (depth of the oxidized layer, water content, organic matter content), water turbidity, and nutrient fluxes across the sediment-water interface (PO4 3- , NO2 - + NO3 - , NH4 + ; measured weekly). Two densities were included for each taxon, representing a low and a high field density. M. balthica significantly increased particle mixing, the rates being 59.6 and 61.9 × 10 -3 cm 2 d -1 . While all taxa increased turbidity in the overlying water, the strongest effects were caused by M. affinis and M. mixta, resulting in 112- and 45-fold increases, respectively. In addition, these 2 motile species increased oxidation of the sediment surface layers. A distinctive difference in the nutrient fluxes was observed between the semi-motile (M. balthica, Marenzelleria spp.) and the motile taxa (M. affinis, M. mixta). The former increased the efflux of both PO4 3- and NH4 + , while the latter sup- pressed the efflux of NH4 + and decreased the sediment uptake of NOx, indicating enhanced N removal. Higher nutrient exchange rates were observed at the higher animal densities. We conclude that a shift in the benthic communities from the 2 motile to the 2 semi-motile taxa, observed through- out the northern Baltic Sea, may notably alter the regime of benthic nutrient cycling and thereby the performance of the entire ecosystem.

Toxic cyanobacteria Nodularia spumigena in the diet of Baltic mysids: Evidence from molecular diet analysis

A PCR-based method was used to detect toxic cyanobacteria Nodularia spumigena in the diet of Baltic mysids, Mysis mixta and Mysis relicta. The decay in detectability of Nodularia DNA in mysid stomachs and feces following the cyanobacterium consumption was examined in laboratory with special references to (1) marker size (780 bp vs. 200 bp), (2) mysid developmental stage (juveniles vs. subadults), and (3) feeding regime after consuming the cyanobacteria (continuous vs. interrupted feeding). The Nodularia DNA could be reliably detected in mysid stomachs and feces by PCR technique. In the mysid with interrupted feeding, the calculated half-lives of N. spumigena DNA in the mysid stomachs were 1.2 and 6.1 h for 780 and 200 bp fragments, respectively. Continuous feeding, however, facilitated decay in the detectability, most likely due to increased gut evacuation rate. In stomachs of the field-collected mysids, the Nodularia DNA was detected with high frequencies, 60% in M. mixta and 51% in M. relicta. Moreover, it was higher in immature mysids than in adults and correlated with stomach fullness in age-specific manner: in juveniles and subadults, stomachs containing Nodularia were significantly fuller, while in adults, the presence of the cyanobacteria was associated with empty stomachs. This suggests greater habitat overlap for juvenile mysids and N. spumigena and thus higher encounter and consumption rates. These findings contribute to a growing body of evidence that cyanobacteria in the Baltic Sea are important food for grazers.

Effects of bioturbation by three macrozoobenthic species and predation by necto-benthic mysids on cladoceran benthic eggs

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 336:131-140 (2007) - doi:10.3354/meps336131 Effects of bioturbation by three macrozoobenthic species and predation by necto-benthic mysids on cladoceran benthic eggs Satu Viitasalo* Tvärminne Zoological Station, University of Helsinki, 10900 Hanko, Finland, and Finnish Institute of Marine Research, PO Box 2, 00561 Helsinki, Finland *Email: satu.viitasalo@fimr.fi ABSTRACT: The effects of bioturbation and predation by the amphipod Monoporeia affinis, the bivalve Macoma balthica and the polychaetes Marenzelleria spp. were studied on the number, vertical distribution and hatching of benthic eggs (ephippia) of the cladoceran Bosmina longispina maritima. It was hypothesised that the 3 functionally different macrobenthic species affect Bosmina ephippia to different degrees. In addition, it was hypothesised that macrofaunal bioturbation either inhibits or enhances predation by the necto-benthic mysid Mysis mixta on cladoceran benthic eggs. M. affinis and M. balthica caused a 48 and 23% decrease in the total number of Bosmina ephippia, respectively, indicating egg predation. In addition, M. balthica extended the distribution of eggs into deeper sediment layers. Both M. affinis and M. balthica suppressed the daily hatching rate of Bosmina, and the effect of the bivalves (reduction 92%) was stronger than that of the amphipods (34 to 78%). None of the benthic species promoted the predation of mysids on Bosmina eggs. Instead, both M. affinis and M. balthica reduced the number of ephippia in the sediment surface (by 79 and 43%, respectively), implying that these species decrease the ability of mysids to feed on cladoceran benthic eggs. M. balthica was the most harmful species for the recruitment of Bosmina, whereas the nonindigenous Marenzelleria spp. did not affect ephippia. KEY WORDS: Bosmina ephippia . Bioturbation . Macrofauna . Mysids . Recruitment Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 336. Online publication date: April 27, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

A combined approach to understand trophic interactions between Cercopagis pengoi (Cladocera: Onychopoda) and mysids in the Gulf of Finland

We use molecular markers, feeding experiments, and stable isotopes to understand trophic interactions between native mysids, Mysis mixta and Mysis relicta, and a recent invader to the Baltic Sea, the cladoceran Cercopagis pengoi. In feeding experiments, both mysid species ingested C. pengoi at a maximum of 16 prey mysid‐1 d‐1 and a specific consumption rate of 0.15 d‐1, similar to rates reported for predation on other prey species. The frequency of predation on C. pengoi assayed by molecular diet analysis of field‐collected mysids varied between 5% and 43% depending on mysid size and species. Surprisingly, it was consistently higher in juveniles than in adults and in M. mixta than in M. relicta. The results of stable isotope analysis corroborate those of feeding experiments and molecular analysis and indicate a higher contribution of C. pengoi to the nutrition of juveniles and M. mixta. These ontogenetic and interspecific differences in mysid predation on C. pengoi are likely to reflect differences related to their different migratory behavior. Thus, despite its low relative abundance, C. pengoi is readily consumed by mysids; this may impact lower food web interactions and the nutrition of mysids in the invaded ecosystems.

Cercopagis pengoi and Mysis spp. alter their feeding rate and prey selection under predation risk of herring (Clupea harengus membras)

The anti-predator behaviour of Baltic crustacean planktivores was studied in feeding experiments under predation pressure of herring. The experiments were conducted with pelagic mysids: Mysis mixta and Mysis relicta, and with Cercopagis pengoi, a non-indigenous cladoceran, which invaded the Baltic Sea in 1992. Zooplankton was offered as prey. Two kinds of experiments were performed in the absence and presence of chemical predator cues: (1) two-prey experiments with prey, which have poor or good escape responses and all three planktivores and (2) natural prey experiments with mysids in natural zooplankton assemblages. The results showed that all three species reacted to the chemical cue of herring by decreasing their feeding rate and altering prey selection. C. pengoi selected easily captured prey (rotifers) in two-prey experiments under predation risk while selection for any prey was evident in mysids in natural prey experiments only in the absence of predator cues. This indicates that planktivores have different anti-predator strategies, which are modified by their own prey capture abilities. C. pengoi was a very efficient predator on small prey with size-specific prey consumption rate 5 to 18 times the rate of mysids. Results show that the studied planktivores are capable of adjusting their feeding behaviour to decrease their conspicuousness in order to increase survival under predation risk. Further, results support the view that C. pengoi has adapted well to the Baltic ecosystem, sharing food niche with pelagic mysids and most probably having a strong influence on the whole pelagic food web.

Seasonal and developmental variation in the fatty acid composition of Mysis mixta (Mysidacea) and Acanthostepheia malmgreni (Amphipoda) from the hyperbenthos of a cold-ocean environment (Conception Bay, Newfoundland)

Fatty acid composition in different life-history stages of Mysis mixta (Crustacea, Mysidacea) and Acanthostepheia malmgreni (Crustacea, Amphipoda) from Conception Bay, Newfoundland, was examined throughout 1999 and 2000. The primary aim was to relate the seasonal fatty acid dynamics to each species’ life cycle and to published information on the occurrence and quality of the annual phytoplankton bloom. Divergent patterns in fatty acid composition and specific fatty acid marker ratios reflected different life styles, diets and critical periods of energy accumulation and utilization in M. mixta and A. malmgreni. Changes in fatty acids reflected the sequence of plankton taxa during and following the spring bloom, starting with diatoms and dinoflagellates and ending with copepods. Immature mysids exhibited a particularly rapid accumulation of diatom-associated fatty acids at the start of the spring bloom, probably owing to the high degree of motility of M. mixta and the broad range of prey types available to this species. In contrast, immature amphipods did not begin to accumulate significant amounts of phytoplankton fatty acids until after the spring bloom material had settled to the hyperbenthos in May. Differences in fatty acid composition indicated that A. malmgreni was restricted to a lower quality diet than was M. mixta and that the trophic connection between A. malmgreni and production in the euphotic zone was weaker.

Seasonal changes in the lipids of Mysis mixta (Mysidacea) from the hyperbenthos of a cold-ocean environment (Conception Bay, Newfoundland)

Total lipid (TL) and lipid class composition of Mysis mixta Lilljeborg from 240-m depth in a cold-water marine bay were studied throughout 1999 and 2000. The objective was to relate the seasonal lipid dynamics in an abundant and lipid-rich hyperbenthic species to its life cycle and to the annual phytoplankton bloom. Mysis mixta accumulated high levels of lipid reserves similar to those reached by other cold-water zooplankton. Greatest accumulation occurred in maturing females and males upon initiation of the spring bloom and continued for 6 months. The lowest TL level was ~5% dry mass (DM) in small juveniles, post-spawned males, and spent females, and the highest level was 32% DM in females approaching full maturity (overall content range 0.3–16 mg·mysid–1). Most females accumulated lipid for 2 years before spawning, although a few females spawned 7 months early with a very low TL content. Seasonal variation in TL resulted primarily from net changes in triacylglycerol, the predominant neutral lipid. Seasonal and interannual variation in TL quantities in individuals and in areal concentrations of lipid in the hyperbenthic population indicates a strong trophic link between M. mixta and the pelagic production cycle.

Effects of grazing and excretion by pelagic mysids (Mysis spp.) on the size structure and biomass of the phytoplankton community

The aim of this study was to determine the effects of pelagic mysids (Mysis mixta and M. relicta) on the biomass and size-structure of the phytoplankton community during the period following the spring bloom. Mysids excreted phosphate (4.5 ± 0.7 nmol ind−1 h−1) and ammonium (123.6 ± 31.6 and 45.0 ± 3.2 nmol ind−1 h−1) and increased the total chlorophyll-a concentration of phytoplankton slightly. However, the presence of mysids affected different size-classes of phytoplankton differently. Mysids mainly grazed on large-sized (>10 μm) phytoplankton cells. Small-sized (<10 μm) algal cells avoided grazing, gained a competitive advantage and were able to utilize the nutrients excreted by mysids. According to this study, both top-down and bottom-up mechanisms simultaneously mould the structure of the phytoplankton community. A large zooplankton biomass might promote the increase of small flagellates by a combination of repleting nutrient stores, selective grazing on large algal cells and heavy predation on protozoa which, consequently, might have a cascading effect on the most favoured protozoan food source, small flagellates.

Predation by the mysid shrimps Mysis mixta and M. relicta on benthic eggs of Bosmina longispina maritima (Cladocera) in the northern Baltic Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 281:155-163 (2004) - doi:10.3354/meps281155 Predation by the mysid shrimps Mysis mixta and M. relicta on benthic eggs of Bosmina longispina maritima (Cladocera) in the northern Baltic Sea Satu Viitasalo1,2,*, Markku Viitasalo3 1Department of Biological and Environmental Sciences, University of Helsinki, PO Box 65, 00014 Helsinki, Finland 2Tvärminne Zoological Station, 10900 Hanko, Finland 3Finnish Institute of Marine Research, PO Box 33, 00931 Helsinki, Finland *Email: stmviita@mappi.helsinki.fi ABSTRACT: The abundance of Bosmina longispina maritima (Cladocera), the most common water flea in the Baltic Sea, shows considerable interannual fluctuations.The number of Bosmina resting eggs (ephippia) in the sediment also fluctuates from year to year. Biotic as well as abiotic factors have been suggested to contribute to these fluctuations, but the impact of benthic predation by pelagic mysids has not yet been considered. The objective of this study was to investigate the effect of benthic predation and bioturbation by Mysis mixta and M. relicta on the ephippia of B. longispina maritima on the SW coast of Finland. The results show that mysids can significantly affect the number of Bosmina eggs in the sediment. All size groups of both mysid species preyed upon ephippia of B. longispina maritima both in the water column and directly from the top layer of sediment, whereas they rejected the eggs of another cladoceran species, Pleopsis polyphemoides. The predation rate on B. longispina maritima ephippia was significantly lower when the eggs were buried under a sediment layer of ca. 1 cm. This suggests that factors affecting the vertical distribution of ephippia in the sediment determine the magnitude of the impact that mysids have on B. longispina maritima benthic eggs. KEY WORDS: Bosmina longispina maritima · Resting eggs · Mysis mixta · M. relicta · Benthic predation · Baltic Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 281. Online publication date: November 01, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Feeding preferences of herring (Clupea harengus) and sprat (Sprattus sprattus) in the southern Baltic Sea

Abstract No field studies have been performed on the selectivity of herring and sprat in the southern Baltic Sea in relation to their entire range of prey. Accordingly, we tested in the field the following hypotheses: (i) sprat and herring are selective feeders and (ii) sprat and herring selectivity is size- and season-dependent. The results show that (i) smaller herring and all size classes of sprat are strictly zooplanktivorous, selecting principally Temora longicornis and Bosmina maritima during the autumn and Pseudocalanus elongatus in winter; (ii) larger herring are essentially nektobenthos feeders, predating on Mysis mixta during the autumn and amphipods and polychaetes during the winter; and (iii) herring and sprat seem to avoid Acartia spp. in both autumn and winter. During the autumn, herring are zooplanktivorous up to 18–20 cm, whereas in winter herring feed on nektobenthos starting from 14–15 cm. Selectivity was not an absolute process but it was related to prey relative abundance in the sea and, possibly, to prey profitability (e.g. size, conspicuousness, and reaction time).

Population biology of hyperbenthic crustaceans in a cold water environment (Conception Bay, Newfoundland). I. Mysis mixta (Mysidacea)

Seasonal population dynamics of Mysis mixta Lilljeborg were studied from December 1998 to November 2000 at a 240 m deep site in Conception Bay, Newfoundland. At this depth, temperature was <0°C and salinity between 32.0 and 34.0 psu year-round. The spring phytoplankton bloom began in early or late March and reached a maximum in late April to mid-May. M. mixta exhibited a highly synchronised life cycle, with spawning and mating occurring in October to November, embryos brooded for ~5 months, and juveniles released during spring bloom sedimentation in April and May. Females were semelparous and died at age 2.5 years, following release of juveniles in spring, whereas the majority of mature males died at age 2 years, following mating in November. The biennial life cycle of this population resulted in the presence of two cohorts in the hyperbenthos at any given time. Variation in density and biomass was low among cohorts but high within cohorts, the latter probably due to the high motility of mysids. Densities in 1999 and 2000 were 242±379 and 544±987 ind. per 100 m3 (mean±SD), respectively. Although growth rates were similar between years, rates measured from changes in dry mass differed both seasonally and among life-history stages (range from −4 to 7 mg month−1). Annual secondary production was estimated at 29–73 mg C m−2 in 1999 and 53–205 mg C m−2 in 2000. The annual P/B ratios were 1.62 and 1.19 in 1999 and 2000, respectively.

Moult cycle and its chronology in Mysis mixta and Neomysis integer (Crustacea, Mysidacea): implications for growth assessment

The moult-staging technique was used to determine the main moult stages (MS) and their duration in laboratory-reared juveniles and subadults of Mysis mixta and Neomysis integer. The relative duration of each stage was similar for both species with premoult occupying the major part of the moult cycle (51% and 44% for M. mixta and N. integer, respectively), followed by postmoult (26% and 34%) and intermoult (23% and 22%). Effects of temperature and feeding regimes on the chronology of the moult cycle were investigated. When the duration of the moult cycle (MCD) was extended by manipulating the feeding regime, animals prolonged their late postmoult and early premoult stages. At 12 and 5 °C, no specific moult stage varied in relative duration as long as food supply was high. Field application of moult staging for growth assessment was tested using wild-caught M. mixta. The MCD estimated via moulting experiments was compared to that obtained by analyzing moult stage distribution combined with the experimentally obtained data on stage duration. Close correspondence between two methods of MCD assessment was observed with moult-staging technique being particularly useful in situations when conducting of experiments immediately after collection is not feasible.

Effect of light on the feeding rates of pelagic and littoral mysid shrimps: a trade-off between feeding success and predation avoidance

It has been shown that freshwater pelagic mysid shrimps capture zooplankton at a higher rate in light than in darkness. This has been suggested to be due to facilitation of visual predation on evasive zooplankton prey. To test this hypothesis with Baltic mysid shrimps, and to see whether pelagic (migrating) and littoral (non-migrating) mysids differ in this respect, we compared the feeding rates of Mysis mixta and Praunus flexuosus on the copepod Acartia spp. As light conditions change radically from the beginning of summer to late autumn at the Baltic latitudes, we conducted the experiments during three different times of the year to determine if there was a seasonal response to light in mysids. The feeding rates of pelagic mysids were significantly higher in total darkness than in light. In contrast, the feeding rates of littoral mysids did not differ in the dark and the light in the three different seasons. The decreased feeding of pelagic mysids under well-lit conditions may be an adaptation to avoid visual predation by pelagic fish. In contrast, littoral mysids, which live in the well-lit layer throughout the day, do not show a similar response. The fact that light did not increase feeding in either of the mysid species indicates that these mysid species do not use vision in capturing prey, but rely mainly on mechano-reception.

Temporal variability in functional responses and prey selectivity of the pelagic mysid, Mysis mixta , in natural prey assemblages

Predation rates and prey selection of the pelagic mysid shrimp, Mysis mixta, were studied experimentally in the northern Baltic Sea in 1998 during their most intensive growth period, from June to October. Functional responses during 5 months were determined by providing the mysids with a natural zooplankton assemblage, diluted to several different concentrations. The results show that ingestion rate increased, along with mysid growth, from early summer to autumn and that saturation level was reached between 400 and 500 μg C l−1. Ingestion rates increased with increasing prey concentration, and sigmoidal curves explained mostly the variation in ingestion rates (explanatory levels of 86–97%). Prey selection was evident in June, July and August, though weaker during the latter 2 months. Selection differed between the studied months but, generally, copepods were more positively selected than cladocerans. Rotifers were the main prey during June and July, when mysids were small, while larger mysids fed on copepods and cladocerans. Of the copepods, Eurytemora affinis was a truly selected species. This study shows that mysids feed on many zooplankton taxa and that they undergo ontogenetic diet shifts.

Effects of toxic and non-toxic cyanobacteria on grazing, zooplanktivory and survival of the mysid shrimp Mysis mixta.

We measured the effect of toxic and non-toxic cyanobacteria strains on grazing, predation rates and survival of the mysid shrimp Mysis mixta by means of laboratory experiments. Juvenile and adult M. mixta fed most actively on the non-toxic strains Aphanizomenon flos-aquae and Nodularia sphaerocarpa as on high quality food, the green flagellate Brachiomonas submarina, whereas grazing on the toxic N. spumigena was significantly lower than on the other strains. The mysids showed normal functional responses; decreasing clearance rates at increasing concentrations of cyanobacteria. In a separate predation experiment, the feeding rates on copepods declined in the presence of aggregated non-toxic cyanobacteria. However, we could not demonstrate increased mortality in a 7-week experiment where mysids were exposed to high concentrations of toxic N. spumigena