Weak 'top-down' control of dinoflagellate growth in the coastal Skagerrak

Effects of planktivorous fish (Sprattus sprartus), copepods(mainly Oithona sp.) and benthic filter feeders (Mytilus edulis)on the biomass and species composition of a natural marine planktoncommunity, dominated by dinoflagellates, was studied experimentallyin September 1988. Plastic 300 I cylinders were filled withSkagerrak surface water, filtered through a 90 µm nylonnet in order to remove mesozooplankton. Copepods were addedin three different concentrations (0, 1x and 6x the ambientdensity) with and without nutrient additions (nitrate, silicateand phosphate). In two nutrient-enriched cylinders with addedcopepods (1x and 6x), 5 cm sprats were also added (10 per cylinder).In two nutrient-enriched cylinders without added copepods. bluemussels (20 per cylinder. 3 cm length) were introduced. Chlorophylla. inorganic nutrients and densities of phytoplankton, ciliatesand copepods were measured throughout the experimental periodof 10 days. Gyrodinium aureolum increased in cell numbers inall but the mussel cylinders, while the other dominating dinoflagellate,Ceratium furca, decreased. This decrease was not due to grazing,but probably a container artefact. Diatoms, Rhizosolenia fragilissinia,Leptocylindrus danicus and Chaetoceros spp., increased slowlyin treatments without fish. In the fish treatments, diatomshad appreciably higher growth rates and cell numbers, probablydue to predation by fish on the copepods. Gyrodiniun aureolunalso had a somewhat higher growth rate in the presence of fish.although this was probably not due to reduction of copepod grazing.The results show that planktivorous fish can affect the biomassand species composition of natural phytoplankton communitiesin marine waters, but that the effect on the late summer-autumndinoflagellate dominated community is weak. Blue mussels areefficient grazers of the whole phytoplankton community and arenot particularly selective with respect to either size or species.     

Trophic interactions of the freshwater jellyfish Craspedacusta sowerbii

The predatory impact and the trophic role of the freshwaterjellyfish, Craspedacusta sowerbii, was studied using microcosmand enclosure experiments as well as a 3-year pond survey. Theresults showed a significant decrease of small herbivorous crustaceans,i.e. Bosmina longirostris and juvenile cyclopoid copepods, inthe medusa treatments of the microcosms and the enclosure experiments.Chlorophyll concentrations in the enclosure experiment weresignificantly increased in the medusa treatment, suggestingthat C. sowerbii may cause cascading effects in the food chain.A comparison of daily zooplankton losses during the pond surveycaused by medusae and fish (roach, Rutilus rutilus), and theirfood selectivities suggest food separation of these two predatorsand reveal a strong negative impact of medusae on the copepodpond community. In the case of a jellyfish bloom, our resultsshow that both food chains can co-occur in lakes because ofa weak interaction between these top predators, fish and jellyfish,with simultaneous impacts on the zooplankton structure.     

Feeding patterns of post-larval and juvenile notothenioids in the southern Weddell Sea (Antarctica)

Summary The food of 163 juvenile specimens of 13 species of notothenioid fishes collected in the southern Weddell Sea (Antarctica) was analyzed. Investigated fish size range was 3–13 cm SL. Principal food items were calanoid copepods Metridia gerlachei, Calanoides acutus, and Calanus propinquus; all developmental stages of Euphausia crystallorophias, and post-larval nototheniid fish Pleuragramma antarcticum. Diet of juvenile channichthyids is limited to few species of euphausiids and fish in the size > 10 mm, but does not include significant numbers of copepods. Pelagic stages of nototheniids feed on copepods and/or larval euphausiids smaller than 10 mm. At similar size, nototheniids and bathydraconids take smaller prey items than channichthyids.     

Copepod reaction to odor stimuli influenced by cestode infection

The cestode Schistocephalus solidus uses copepods as first andsticklebacks as second intermediate hosts. For transmission,an infected copepod has to be preyed upon by a stickleback.We used copepods of the species Macrocy albidus to test whetherinfected and uninfected copepods differ in their reaction totwo kind of simultaneously presented odors: odors of sticklebacksand odors of sticklebacks and conspecificz. By giving this choice,we attempted to force the copepods to make a trade-off betweenthe benefit of risk dilution and possible predator confusionand the costs of food competition and other disadvantages inducedby conspecifics. Within 1–8 h after last feeding, uninfectedcopepods clearly preferred the odors of conspeciflcs under thechemically simulated threat of predation. This was in contrastto the infected copepods, who tended to avoid the odor of conspecifics.When the time between experiment and last feeding varied, infectedcopepods showed an increas preference for fish water only (oravoided conspecthcs) with increasing hunger level This suggeststhat S. solidus benefits from hunger-induced behavioral changesof its copepod host by influencing its microhabitat selection.The same effect could be found in both sexes; however, it wassignificantly more pronounced in male than in female copepods.We propose several hypotheses that could explain the differencebetween the sexes in their infection-dependent microhabitatselection.     

Is mating of copepods associated with increased risk of predation?

In laboratory experiments, we tested whether the cyclopoid copepodCyclops vicinus is more vulnerable to predation by juvenile(young-of-the-year) roach (Rutilus rutilus) and/or larvae ofthe phantom midge Chaoborus flavicans during mating than individualadult copepods of the same species. We also tested whether C.vicinusavoided mating, displayed shorter mating times and/or loweractivity when a predator (Chaoborus) was present, or when exposedto water in which predators (Chaoborus) were previously held.Fish ate the same quantity of pairs in copulation and individualegg-bearing females but significantly fewer females withouteggs. Chaoborus ate similar quantities of pairs and femalesbut fewer males. Pairs in copulation and egg-bearing femaleswere detected earlier by fish than individual non-ovigerousfemales or males, and pairs in copulation were captured mosteasily by fish. Cyclops vicinus mated less often when Chaoboruslarvae were present than in water in which Chaoborus was previouslyheld, or in pure tap water. However, predator presence or exposureto predator-treated water had no effect on (i.e. did not reduce)mating duration. Activity of C.vicinus pairs during copulationwas significantly lower when a predator was present, or in predator-treatedwater, than in pure tap water, and activity of pairs decreasedwith increasing activity of the predator. Although our experimentswere carried out under artificial conditions which cannot betransferred to the complex conditions in the field, they suggestthat mating in copepods is dangerous and that copepods haveevolved particular strategies to reduce the risk of predationduring mating.     

Feeding and survival rates of the copepods Euterpina acutifrons Dana and Acartia grani Sars on the dinoflagellates Alexandrium minutum Balech and Gyrodinium corsicum Paulmier and the Chryptophyta Rhodomonas baltica Karsten

Here we studied the feeding performances and survival rates of the copepods Euterpina acutifrons and Acartia grani exposed to cultures of the toxic dinoflagellates Alexandrium minutum and Gyrodinium corsicum and the nontoxic Chryptophyta Rhodomonas baltica. The presence of hemolysins in G. corsicum was also analysed. For both copepod species, high ingestion rates on A. minutum and lower but similar rates on G. corsicum and R. baltica were found. After 120 h, only the E. acutifrons that grazed on G. corsicum or R. baltica showed 100% survival, whereas survival of the copepods that grazed on A. minutum was significantly lower. A. grani was more sensitive to the dinoflagellates, and at 120 h showed survival rates of 70% on G. corsicum and 60% on A. minutum. These results support the hypothesis that the feeding interactions between copepods and toxic dinoflagellates are highly species-specific. Long-term observations indicated that E. acutifrons can graze on G. corsicum without impairing their feeding or growth performances. Both copepod species consumed A. minutum in the short-term, but their long-term impact on this and other toxic marine dinoflagellates would be limited by the toxic effects of these flagellates. We did not find any evidence of haemolytic activity in extracts of G. corsicum. Toxins other than haemolysins may be responsible for the deleterious effects observed on A. grani.     

The importance of plankton to Cassin's auklets during breeding

The breeding chronology, food habits, growth and reproductivesuccess of Cassin's Auklets, Ptychoramphus aleuticus, were investigatedon Triangle Island, British Columbia. The breeding of Cassin'sAuklets coincides with the plankton bloom in the eastern NorthPacific. Cassin's Auklets in British Columbia feed their youngmostly large copepods, Calanus cristatus, and euphausiids, chieflyThysanoessa spinifera, but fish was an important food item inone summer. Chicks which hatched early, grew and survived betterthan late-hatched chicks and this may relate to food availability.Adults and chicks on Triangle Island were heavier than thoseon South Farallon Island, California. The heavier weight andgreater numbers of Cassin's Auklets in British Columbia as comparedwith regions to the south may relate to Calanus cristatus availabilityand abundance in northern areas.     

Response of two zooptankton grazers to an ichthyotoxic estuarine dinoflagellate

The dinoflagellate Pfiesteria piscicida (gen. et sp. nov.).a toxic ‘ambush predator’, has been implicated asa causative agent of major fish kills in estuanne ecosystemsof the southeastern USA. Here we report the first experimentaltests of interactions between P.piscicida and estuarine zooplanktonpredators. specifically the rotifer Brachionus plicatilis andthe calanoid copepod Acartia tonsa. Short-term (10 day) exposureof adult B.plicatilis to P.piscicida as a food resource, aloneor in combination with the non-toxic green algae Nannochlorisand Tetraselmis. did not increase rotifer mortality relativeto animals that were given only non-toxic greens Similarly,short-term (3 day) feeding trials using adult A.tonsa indicatedthat the copepods survived equally well on either P.piscicidaor the non-toxic diatom Thalassiosira pseudonana. Copepods giventoxic dinoflagellates exhibited erratic behavior, however, relativeto animals given diatom prey. The fecundity of B.plicatiliswhen fed the toxic dinoflagellate was comparable to or higherthan that of rotifers fed only non-toxic greens We concludethat, on a short-term basis, toxic stages of P.piscicida canbe readily utilized as a nutritional resource by these commonestuarine zooplankters. More long-term effects of P.piscicidaon zooplankton, the potential for toxin bioaccumulation acrosstrophic levels, and the utility of zooplankton as biologicalcontrol agents for this toxic dinoflagellate. remain importantunanswered questions.     

Microscale patchiness of plankton within a sharp pycnocline

Microscale distributions of plankton around the pycnocline werestudied over a 24 h period in the southern Kattegat, using agradient sampler which collects 20 samples over a 3 m depthinterval. Moderately elevated concentrations of phyto- and zooplanktonwere observed at the pycnocline. Microscale variance was highestfor adult copepods. Nauplii and copepodites were equally wellrepresented by sampling with ordinary vertical 5 1 water bottlesas with the horizontal 1.5 1 bottles of the gradient sampler.Adult copepods were underestimated by the vertical bottles.No vertical migration of dinoflagellates was observed over the3 m interval covered by the gradient sampler. Microscale correlationsbetween copepods and phytoplankton within the gradient samplerwere weak. Copepodites (mainly Oithona sp.) and the dinoflagellateProrocentrum micans showed the best correlation.     

Zooplankton diversity and the predatory impact by larval and small juvenile fish at the Fisher Banks in the North Sea

The biomass and diversity of the mesozooplankton and fish larvaecommunity were investigated across a frontal zone in the centralNorth Sea in the early summer, to investigate whether larvalfish predation is a regulator of mesozooplankton production.Pronounced changes in the mesozooplankton community were observedacross the front off the Jutlandic coast. The biomass and thediversity of copepods changed across the front as the populationof Calanus finmarchicus became abundant in the deeper water.The crustaceans (Acartia spp. and Evadne spp.) and polychaetelarvae dominated the mesozooplankton in the coastal water. Thebiomass of fish larvae was dominated by gadoid larvae. As inthe copepods, a shift in fish diversity was observed in thefrontal zone. On the coastal side of the front, whiting (Merlangiusmerlangus) dominated the biomass, while offshore from the frontwhiting were absent and cod (Gadus morhua) was the dominantlarval fish species on the deeper stations. The present investigationdemonstrates two different trophic pathways related to hydrographyin the central North Sea. First, in the shallow coastal water,the abundant small neritic copepods are predominately predatedby whiting, while in the offshore region the larger oceaniccopepods are predated by cod larvae. However, the predationpressure by the fish larvae was in general low (<10%) relativeto copepod production per day. Consequently, in the early summer,the copepod production potentially results in a build-up ofcopepod biomass on both sides of the front.     

Food and feeding ecology of postlarval and juvenile Pleuragramma antarcticum (Pisces; Notothenioidei) in the seasonal pack ice zone off the Antarctic peninsula

Summary Stomach contents of 275 postlarval and 269 one year old juvenile Pleuragramma antarcticum caught in February 1982, March 1981 and November 1977 in the Bransfield Strait and adjacent waters were investigated. Juveniles in November 1977 fed mainly on calanoid copepods and Oithona spp. The principal food of postlarvae in February 1982 were Oncaea spp., eggs of calanoid copepods and tintinnids whereas the staple food of juveniles in February 1982 consisted of calanoid copepods of which copepodites of Calanus propinquus were most abundant. Eggs of Euphausia superba were frequently ingested by postlarvae and were the main component of the juvenile diet in the Antarctic Sound. Postlarvae in March 1981 preyed on Oncaea spp. and calanoid eggs. There was a shift from feeding on cyclopoid copepods by postlarvae to feeding on calanoids by juveniles. The food particle size increased as a function of fish length. Maximum prey size was controlled by mouth width up to at least 45 mm standard length. Prey selection was apparently food density dependent with an inverse relationship between food abundance and selective feeding. Density dependent size selection may have resulted in resource partitioning among postlarval and juvenile fishes in February. This specific feeding behaviour is likely to be an adaptation to the low zooplankton stocks of the high Antarctic shelf waters to minimize food competition among the youngest age classes. Predation on krill eggs indicated that the frequent association of one year old juvenile Pleuragramma antarcticum with the Antarctic krill is related to local spawning events.     

Feeding preferences and grazing rates of Pfiesteria piscicida and a cryptoperidiniopsoid preying on fish blood cells and algal prey

The grazing rates and feeding preferences of the dinoflagellates Pfiesteria piscicida and a cryptoperidiniopsoid on the alga Rhodomonas sp. and fish blood cells were calculated at different ratios of the two food types and at different total food densities. Data from 6 h grazing periods within microcosms were used to calculate grazing rates. Grazing rates of both dinoflagellates increased linearly with an increased ratio of blood cells to Rhodomonas, and P. piscicida had a higher maximum grazing rate than the cryptoperidiniopsoid. The grazing rate of P. piscicida on Rhodomonas also increased with increased Rhodomonas densities relative to the blood cells, but increased densities of Rhodomonas did not increase the grazing rate of the cryptoperidiniopsoid, suggesting a lower feeding threshold for this species. Both dinoflagellates demonstrated a preference for fish blood cells over Rhodomonas cells, with no significant difference in the index of preference between the two species. Total food abundance affected the degree of preference differently for each dinoflagellate species. A higher index of feeding preference was attained by P. piscicida when resource levels were high, while the cryptoperidiniopsoid did not show this response. A preference for fish blood cells occurred at all food ratios for both dinoflagellates, including when blood cells were scarce relative to the alternate food type (15% of total available food). These results suggest that these strains of P. piscicida and the cryptoperidiniopsoid share similar feeding preferences for the prey types tested, although cryptoperidiniopsoids have not been associated with fish kills.     

Differential use of zooplankton prey by Ancient murrelets and Cassin's auklets in the Queen Charlotte Islands

The distribution at sea and the food of two similar sized plankton-feedingalcids were examined during the 1981 breeding seasons in thenorthwestern Queen Charlotte Islands, British Columbia. Thetwo alcids, the Ancient murrelet (Synthliboramphus antiquus)and the Cassin's auklet (Prychoramphus aleuticus) have differentchick-rearing strategies. Both species fed predominantly atthe shelf break, although the Cassin's auklet also foraged overseamounts. The feeding distributions of the species appear tobe related to those of their prey. Zooplankton sampling indicatedthat each alcid selects a small and different portion of thezooplankton available in surface waters. The Ancient murrelet'smain foods were euphausiids (Thysanoessa spinifera and Euphausiapacifica) and larval and juvenile fishes. The Cassin's aukletchicks fed chiefly on calanoid copepods (Neocalanus cristatus).euphausiids (mostly Thysanoessa longipes in 1981, but in otheryears also Thysanoessa spinifera), and larval and juvenile fishes.The Cassin's auklets took smaller prey than the Ancient murrelet.Differences in the diets of the two alcid species were associatedwith differences in morphology and chick-rearing strategies.     

Growth, development and condition of Dendraster excentricus (Eschscholtz) larvae reared on natural and laboratory diets

Feeding invertebrate larvae may be food limited while developingin the ocean. If they are, then their time in the plankton isprolonged, which likely increases mortality. Food limitationcould be due to the quantity and/or quality of the food available.In an effort to answer how food type influences larval nutrition,we compared growth, development and lipid deposition for Dendrasterexcentricus larvae reared in natural seawater from two depths(1 and 20 m) and in filtered seawater on a monoculture laboratorydiet of 6 cells µL^–1 of the green alga Dunaliellatertiolecta (Butcher). Five days post-fertilization, larvaereared on the laboratory diet had developed to the latest stage,were the largest and had lipid deposits. Larvae reared on naturalsurface water were intermediate in size and developmental stage,and larvae reared in the water from 20 m depth were the smallestand developed the slowest. This trend continued at 8 days post-fertilizationwhen surface water diet larvae were similar in size to laboratorydiet larvae, but their juvenile rudiments were significantlysmaller. To assess food availability in each food treatment,we compared the concentration of chlorophyll (Chl) a, b andc in natural seawater from each depth and in D. tertiolectaculture in filtered seawater. Natural seawater collected fromthe surface had the highest concentration of Chl a and c, whereasChl b was not significantly different between treatments. IncreasedChl concentrations in the surface water are likely due to higherconcentrations of diatoms and dinoflagellates, which are typicallynot high-quality food items for echinoid larvae. Our resultssupport a hypothesis that echinoid larvae in the water columnmay be limited by food quality.     

Marine microalgae attack and feed on metazoans

Free-living microalgae from the dinoflagellate genus Karlodinium are known to form massive blooms in eutrophic coastal waters worldwide and are often associated with fish kills. Natural bloom populations, recently shown to consist of the two mixotrophic and toxic species Karlodinium armiger and Karlodinium veneficum have caused fast paralysis and mortality of finfish and copepods in the laboratory, and have been associated with reduced metazooplankton biomass in-situ. Here we show that a strain of K. armiger (K-0688) immobilises the common marine copepod Acartia tonsa in a density-dependent manner and collectively ingests the grazer to promote its own growth rate. In contrast, four strains of K. veneficum did not attack or affect the motility and survival of the copepods. Copepod immobilisation by the K. armiger strain was fast (within 15 min) and caused by attacks of swarming cells, likely through the transfer and action of a highly potent but uncharacterised neurotoxin. The copepods grazed and reproduced on a diet of K. armiger at densities below 1000, cells ml⁻¹, but above 3500 cells ml⁻¹ the mixotrophic dinoflagellates immobilised, fed on and killed the copepods. Switching the trophic role of the microalgae from prey to predator of copepods couples population growth to reduced grazing pressure, promoting the persistence of blooms at high densities. K. armiger also fed on three other metazoan organisms offered, suggesting that active predation by mixotrophic dinoflagellates may be directly involved in causing mortalities at several trophic levels in the marine food web.     

Mesozooplankton grazing under conditions of extreme eutrophication in Guanabara Bay, Brazil

The objective of the present study was to quantify mesozooplanktongrazing in the eutrophic waters of Guanabara Bay. Mesozooplankton(>200 µm) was dominated by the copepods Acartia lilljeborgi,Acartia tonsa, Parvocalanus crassirostris and Paracalanus furcatus.Dinoflagellates, specifically the species Prorocentrum triestinum,were an important group for mesozooplankton nutrition, beingingested in significant amounts during all experiments. On average,12.3 ± 2.9 P. triestinum cells were ingested copepod^–1min^–1 (other dinoflagellates: 11 ± 8 cells copepod^–1min^–1). Filamentous cyanophyceae and nanoplankton wereingested in one experiment each, but the mesozooplankton communitygenerally preferred dinoflagellates to these groups, which werealways abundant in the water column. Euglenophyceae were notingested, although they dominated in one experiment. Mesozooplanktoningested, on average, only 0.2% of the nano- and microplanktonbiomass per day. The results suggest that grazing was not acontrolling process for the nano- and microplankton communityin the study area. Addition of zoeae larvae of Chasmagnatusgranulata (Decapoda: Brachyura: Grapsidae) in one experimenthad a significant effect on the mortality of adult copepods,probably due to a predator–prey relationship.     

Photophobic responses of calanoid copepods: possible adaptive value

A genual pattern of photophobic responses has been observedwhich differs for calanoid copepods from freshwater, estuarineand oceanic environments. Using a video-computer system formotion analysis, the photophobic responses of light and darkadapted calanoid copepods were compared. Dark-adapted copepodswere exposed to 600 ms flashes of dim blue light at 5 s intervalswhich simulated the flashes of biolumines-cent marine zooplankton.Light-adapted copepods were exposed to 600 ms intervals of darknessat 5 s intervals to simulate the shadows of organisms passingoverhead. Four species of coastal marine copepods (Acartia hudsonica,Centropages hamatus, Pseudocalanus minutus and Temora longicornis)all showed photophobic responses to both flashes and shadows.These responses may have adaptive value to the copepods sincethey live in an environment with predators that are bioluminescentat night and cast shadows on their prey during the day (e.g.ctenophores and cnidarian medusae). Two species of oceanic copepods(Euchaeta marina, Pleuromamma abdominalis) showed strong photophobicresponses to flashes but no response to shadows. This may correspondto the abundance of bioluminescent predators on copepods inthe oceanic environment (fish, ctenophores, siphonophores, etc.)and their lack of exposure to the shadows of predators, sinceboth these species are rarely found in the euphoric zone duringthe day. Two species of freshwater copepods (Diaptomus sanguineus,Epishwa massachusettsensis) showed no similar photophobic responseto flashes of light. This lack of startle response may relateto the lack of bioluminescence in the freshwater environment.Freshwater copepods showed a weak photophobic response to shadows.The adaptive value of this behavior is unclear, however, sincethe responses seem to be too weak to function for escape, andthe dominant predators large enough to cast shadows (fish) tendto approach their prey laterally. ¹Present address: Marine Science Institute, University of Texasat Austin, Port Aransas, TX 78373–1267, USA     

Selective predation by herring and mysids, and zooplankton community structure in a Baltic Sea coastal area

In lakes, fish and invertebrate predation are recognized asstrong structuring forces on zooplankton communities. The objectof this investigation was to study whether predation has a similarpotential in a coastal area of the brackish Baltic Sea and ifit could explain observed differences in zooplankton communitystructure between a reference area and an eutrophied area. Speciescomposition and daily vertical migration of zooplankton andzooplanktivores, and the diets of the latter, were studied inJuly and August at two 30 m deep stations differing in primaryproductivity. The biomass of zooplankton >35 µm wasdominated by copepods, but cladocerans and rotifers also occurredin significant numbers. The dominating zooplanktivores wereherring (Clupea harengus) and the mysid shrimp Mysis mixta.They fed almost exclusively on zooplankton, mainly copepods,and their estimated food consumption equalled or exceeded thesummer copepodite production. The structure of the zooplanktoncommunity cannot be explained by effects of predation or byfeeding conditions alone. Increases in rotifer and cladoceranabundances with increased primary production suggests effectsof food supply. However, a generally rapid decline in the annualsummer peak of cladocerans may be caused by predation. The totalabundance of copepods did not increase with improved feedingconditions, but there was a shift in species dominance. Thecopepod Ewytemora affinis hirundoides, which was intensivelypreyed upon, increased with increased phytoplankton production,while Acartia bifilosa and/or A.longiremis, which was less eaten,decreased. Predation may explain a pronounced daily verticalmigration of the most predated copepods. They occurred in deeperwater during the day, when the visually feeding herring wereactive, and moved closer to the surface at night when M.mixtaleft the bottom, to forage in the water column.     

Naupliar growth versus egg production in the calanoid copepod Centropages typicus

Growth rates of the copepod Centropages typicus were assessedduring May 1998 in the Alboran Sea. The goal of the work wasto compare field growth rates of juvenile (artificial cohortmethod) and adult copepods (female egg production rates) underan extensive range of natural conditions. The results showedthat adult and juvenile growth rates were similar in some stations,while in others no such relationship was apparent. In contrastto the common belief that juvenile growth may be saturated undernatural conditions whereas adults may be food-limited, in mostof our study, adult growth rates were greater than juveniles.We discuss these results in the light of food availability atthe surveyed stations. In summary, the assumption that copepodgrowth rates estimated through egg production rates of adultsare equivalent to juvenile growth is not always valid.     

Egg production, growth and development of the cyclopoid copepod Oithona similis

Egg production, growth and development rates of Oithona similiswere measured in the laboratory as a function of food concentrationand composition. On an optimum diet, development is isochronaland growth is near exponential. The maximum juvenile growthrate at 15C (0.2 day^–1) is similar to juvenile growthin calanoid copepods. The maximum weight-specific egg productionrate (0.1 day^–1), on the other hand, is substantiallyless than in free-spawning calanoids, but similar to that inegg-carrying calanoids. In the Kattegat, Oithona spp. egg productionis strongly limited by food during summer and controlled bytemperature during winter. The seasonal signal in fecundityand population biomass is much weaker than in the co-occurringfree-spawning calanoid genera, where fecundity and populationbiomass undergo dramatic seasonal viaration. ¹Present address: Instituto Nacional de Investigacin y DesarrolloPesquero, INIDEP, CC 175, Playa Grande, Mar del Plata, Argentina