Microsetella norvegica: a rare report of a potentially abundant copepod on the Scotian Shelf

Fine mesh (0.080 mm) zooplankton samples were collected along an onshore-offshore transect during three cruises (February to May, 1983) off southwest Nova Scotia, Canada. The small harpacticoid copepod Microsetella norvegica was numerically the dominant copepod during the winter (February) cruise, where it composed up to 50% of the total zooplankton, and was consistently among the three most abundant copepods during the spring cruises. Due to its small size (<0.45 mm in length), M. norvegica has rarely been reported from plankton surveys of the northwest Atlantic. However, existing reports indicated it varies interannually from being a rare to an abundant member of the zooplankton community in the northwest Atlantic.     

Ingestion of natural particulate organic matter and subsequent assimilation,respiration and growth by tropical lagoon zooplankton

Rates of ingestion of natural particulate organic matter and subsequent assimilation and respiration by zooplankton at Enewetak Atoll lagoon (Marshall Islands) were measured using a flow-through system. Maximum daily ingestion rates of carbon and nitrogen, expressed as a percentage of the body content, were 79 and 37%, respectively, for the large copepod Undinula vulgaris; 112 and 65%, respectively, for a group of mixed small copepods; and 61 and 34%, respectively, for the pteropod Creseis acicula. Daily metabolic carbon losses, expressed as above, were 63% for U. vulgaris, 88% for the small copepods, and 50% for C. acicula. Assimilation efficiences of carbon and nitrogen ranged from about 86 to 91%. The above rates are generally higher than in previous reports for similar sized zooplankton in temperate waters, while the daily growth increments, expressed as a percentage of the body carbon content (4.8% for U. vulgaris, 8.6% for the small copepods, and 2.6% for C. acicula), are comparable. It appears that the high rates of ingestion and assimilation of organic matter are compensated by high metabolic losses. These results indicate that at least for carbon, tropical zooplankton may have low growth efficiencies ranging from 4 to 9%.     

Osmoacclimation in Enteromorpha intestinalis: long-term effects of osmotic stress on organic solute accumulation

The fate of the protease trypsin in intestines of individual herring larvae Clupea harengus L. was studied following digestion of the copepod Acartia tonsa. Trypsin was retained in the intestine during two consecutive pulses of feeding and defaecation of copepods. Quantification of herring trypsin in digested, defaecated copepods showed that ca. 1% of larval intestinal enzyme was defaecated along with 1 to 3 copepods. Following ingestion of a single meal, the level of intestinal trypsin post-ingestion declined to pre-ingestion levels within 1 to 2 d of starvation. All enzyme data thus indicated that trypsin, released in response to ingestion of a meal, was retained. In addition, analysis of fed subgroups of starved larvae clearly indicated that release of trypsin from the pancreas stopped after 6 to 8 d of starvation. As the fish still contained substantial amounts of trypsinogen, the underlying cause might be defective release mechanisms. Daily secretion of trypsin and processes responsible for enzyme retention in the gut are discussed. Assimilation efficiency in herring larvae was estimated for copepodite prey. Average carbon assimilation was 90%.     

Composition biochimique des copépodes de l'hyponeuston de Méditerranée Nord occidentale. Poids sec et analyse élémentaire du carbone,de l'hydrogène et de l'azote

Dry weight, total carbon, hydrogen and nitrogen contents were studied in Anomalocera patersoni, Pontella mediterranea, P. lo biancoï and Labidocera wollastoni as a function of sex and developmental stage. 629 individuals were analysed over the year. Protein content was calculated from the nitrogen values, and results are presented as percent dry weight. C:H and C:N ratios were also determined. The lowest contents of carbon (32.4%) and nitrogen (9.3%) were determined for female L. wollastoni, the highest carbon content (43.3%) for female P. lo biancoï, and the highest nitrogen content (11.5%) for female P. lo biancoï and male A. patersoni. This range agrees with the data in the literature for marine copepods. According to available data on biochemical composition of zooplankton in relation to depth, the Pontellidae contain a high amount of proteins. The carbon:nitrogen ratio displays great stability within a species, indicating a constant elementary composition during development from copepodite to adult. Nevertheless, there is a statistically significant discrepancy between the C:N ratios for A. patersoni and P. mediterranea which is due to a higher rate of increase in carbon content in A. patersoni. As a whole, interspecific variations were small (C:N ranged between 3.4 and 3.8) compared to those recorded in true planktonic species. This appears to be an important characteristic of the Pontellidae, in contrast with other, more widely distributed copepods, and probably is related to the peculiarities of their biotope, the ultrasuperficial layer.     

Spatial and temporal feeding patterns of copepods from the North Pacific centra gyre

Spatial and temporal feeding patterns (determined from an index of gut fullness) are described for 10 typical species of calanoid copepods collected from the North Pacific central gyre (September 1968 to June 1977), an area where the zooplankton is food limited and there were a-priori reasons to suspect that feeding and competition for food were important in regulating zooplankton community structure. Over 100 samples from 11 cruises to the eastern part of the gyre were examined, and patterns of gut fullness were related to environmental variables and the copepod species structure. The copepods studied all tended to be omnivores and food generalists. Males had lower indices of gut fullness than females but both males and females of a species had similar spatial and temporal feeding patterns. Guts were usually fuller at night than during the day, even in nonmigrating species; however, within nighttime depth distributions, no depths were preferred for feeding. There were also differences between species in mean gut fullness, but different species tended to have similar spatial and temporal feeding patterns. There was considerable spatial variability, and locales could be identified in which most species had higher indices of gut fullness. The copepods were not necessarily more abundant in these locales, nor did these tend to be areas of above average chlorophyll concentration. These patterns were consistent with relatively nonselective feeding, and there was no evidence that these species separate their niches by feeding at differing places or times.     

Mosaic horizontal distributions of three species of copepods in the subarctic Pacific during spring

 Continuous abundance estimates (510 m resolution) of the copepods Neocalanus cristatus, N. flemingeri and Metridia pacifica were obtained with an electronic particle counter along cruise tracks in the subarctic western North Pacific in spring. For all three species, the number of patches decreased exponentially with increasing patch size. Most patches (63 to 83%) were dominated by one species, and patches of the same species more closely spaced than patches of different species. The patches of M. pacifica tended to coexist with those of N. cristatus. In contrast, patches of N. flemingeri rarely co-occurred with those of other copepods. These patterns were more clearly observed in fine-scale observations with sampling intervals of <31 m. Coherence analysis of copepod species pairs showed no characteristic scale at 2 to 50 km wave lengths. At shorter wave lengths (<2 km), frequent positive correlations were observed between N. cristatus and M. pacifica. Thus, the distribution of copepods appears to be a mosaic assemblage of patches of each copepod species. These results suggest that copepods have a mechanism to form species-specific aggregations, and the aggregation and segregation processes are maintained at a scale of <2 km. Received: 24 February 1999 / Accepted: 25 April 2000     

Trophic implications of cross-shelf copepod distributions in the Southeastern Bering Sea

Spring distributions of some numerically dominant copepods reflect associations with two distinct water masses separated along the 80- to 100-m isobaths. Seaward of this middle shelf front, the oceanic Bering Sea hosts populations of Calanus cristatus, C. plumchrus, and Eucalanus bungii bungii; Metridia pacifica, Oithona similis, and Pseudocalanus spp. are also present. The large oceanic species are much less abundant in waters shallower than 80 m where the community is seasonally dominated by smaller copepods, O. similis, Acartia longiremis, and Pseudocalanus spp. Experimental and field-derived estimates of carbon ingestion indicate that the oceanic/outer shelf copepods can occasionally graze the equivalent of the daily plant production and probably routinely remove 20–30% of the primary productivity. Conversely, stocks of middle shelf copepods rarely ingest more than 5% of the plant carbon productivity. During 45 d between mid April to late May, 1979, approximately three times more organic matter was ingested m^-2 by the outer shelf/oceanic copepod community than by middle shelf species. This imbalance in cross-shelf grazing permits middle shelf phytoplankton stocks to grow rapidly to bloom proportions, and to sink ungrazed to the seabed. Over the outer shelf and particularly along the shelf break, a much closer coupling to phytoplankton supports a large biomass of oceanic grazers. Here, copepod stocks approaching 45 g dry wt m^-2 occur in late spring as a narrow band at the shelf break.Supported by National Science Foundation Grant DPP 76-23340Contribution no. 485, Institute of Marine Science, University of Alaska, Fairbanks     

Metabolism and elemental composition of a giant chaetognath Sagitta gazellae from the Southern Ocean

Oxygen consumption, ammonia excretion and phosphate excretion rates were measured on Sagitta gazellae Ritter-Zahony, in conjunction with body composition analyses (water, ash, carbon, hydrogen, nitrogen and phosphorus). Both water content (94.7% of wet weight) and ash content (53.0% of dry weight) recorded on S. gazellae were the highest and the lowest, respectively, among the chaetognath data being reported. Contents of carbon, hydrogen and nitrogen of S. gazellae were the lowest among published values of chaetognaths. Metabolic comparison with other chaetognaths living in similar subzero water temperature revealed reduced rates in S. gazellae, while no appreciable differences were seen in the metabolic quotients (O:N, N:P and O:P ratios). The O:N atomic ratios were 10.5 to 15.9 indicating protein oriented metabolism. Reduced metabolic activity of S. gazellae is not due to their body composition as calculated daily metabolic losses of body carbon (0.50%), body nitrogen (0.38) and body phosphorus (1.6%) were also found to be lower than respective values reported on other congeners and even those of other zooplankton living in the Antarctic waters.     

Omnivorous feeding behavior of the Antarctic krill Euphausia superba

Feeding experiments were conducted at Palmer Station from December 1985 to February 1986 to examine the potential role of copepod prey as an alternative food source for Euphausia superba. Copepod concentration, copepod size, phytoplankton concentration, the duration of krill starvation and the volume of experimental vessels were altered to determine effects on ingestion and clearance rates. Krill allowed to feed on phytoplankton and copepods in 50-litre tubs showed greatly increased feeding rates relative to animals feeding in the much smaller volumes of water traditionally used for krill-feeding studies. Clearance rates on copepods remained constant over the range of concentrations offered, but clearance rates on phytoplankton increased linearly with phytoplankton concentration. Feeding rates increased when larger copepods were offered and when krill were starved for two weeks prior to experiments. Clearance rates of krill feeding on copepods were higher than, but not correlated with, their clearance rates on phytoplankton in the same vessel. E. superba may have a distinct mechanism for capturing copepods, perhaps through mechanoreception. Although our observed clearance rate of 1055 ml krill^-1 h^-1 indicates that krill can feed very efficiently on copepod prey, such feeding would meet less than 10% of their minimum metabolic requirements at the typical copepod concentrations reported for Antarctic waters. However, substantial energy could be gained if krill fed on the patches of high copepod concentrations occasionally reported during the austral summer, or if krill and copepods were concentrated beneath the sea ice during the winter or spring months. Our results, indicating efficient feeding on zooplankton and higher clearance rates on phytoplankton than previously believed, represent a step towards balancing the energy budget of E. superba in Antarctic waters.     

Metabolism and body composition of a copepod (<Emphasis Type="Italic">Neocalanus cristatus</Emphasis>: Crustacea) from the bathypelagic zone of the Oyashio region,western subarctic Pacific

Metabolism [respiratory oxygen consumption, electron-transfer-system (ETS) activity] and body composition [water, ash, carbon (C), nitrogen (N), carbon/nitrogen (C/N) ratio] of stage C5/C6 Neocalanus cristatus from 1000 to 2000 m depth of the Oyashio region, western subarctic Pacific, were determined during the period of July 2000 through June 2003. Compared with the C5 specimens from shallow depths (<250 m), those from 1000 to 2000 m were characterized by quiescent behavior, reduced respiration rates (30% of the rates at active feeding), very low water content (61–70% of wet weight), but high C content (56–64% of dry weight) and C/N ratios (7.2–10.6, by weight). Artifacts due to the recovery of live specimens from the bathypelagic zone appeared to be unlikely in this study, as judged by the consistent results between re-compression (100 atm) and non-compression (1 atm) respiration experiments, and between ETS activities and respiration rates directly measured. In addition, the respiration rates of C6 males and females of N. cristatus from the same 1000–2000 m depth were two to three times higher than the rates of C5 individuals, but were similar to the rates of a bathypelagic copepod, Paraeuchaeta rubra. Combining these results with literature data, C budgets of: (1) diapausing C5 specimens, weighing 6–10 mg dry weight; (2) molt to C6 females; and (3) the complete the life span were established, taking into account assorted losses in respiration during diapause at stages C5 and C6, molt production and egg production. Respiratory C losses by C5 and C6 specimens were estimated on the basis of body N as adjusted metabolic rates [AMR; µl O₂ (mg body N)^–0.843 h^–1], then N budgets were also computed subtracting N lost in the form of cast molts and eggs from the initial stock. Calculations revealed that allocation of the C stock was greatest to egg production (34–57%), followed by respiration (27%) and cast molts (3%), leaving residual C of 13–36% in spent C6 females. The present results for N. cristatus from the North Pacific are compared with those of Calanus spp. in the North Atlantic.Communicated by O. Kinne, Oldendorf/Luhe     

Survival of copepods passing through a nuclear power station on northeastern Long Island Sound,USA

About 70% of the copepods entering the cooling water system of a nuclear power plant on northeastern Long Island Sound (USA) are not returned to the Sound in the effluent. Copepod mortalities are caused by the mechanical or hydraulic stresses of passage, although our experimental design could not determine whether heat or chlorination could cause mortality in the absence of mortality induced by hydraulic stress. After passing through the power plant, copepods sink rapidly (ca. 2.5 times faster than controls). This leads to an increase in concentrations of copepods suspended in the deep water (25 to 30 m) of the effluent pond. About half of the live copepods collected at the discharge and held in situ died within 3.5 days, and 70% died within 5 days, whereas only 10% of those from the intake died in 5 days. About 60% of the copepods observed suspended in deep water in the pond were dead. The copepod mortality caused by the power plant reflects the loss in secondary production occurring below about 270×10³ m² of sea surface in Long Island Sound annually. This loss represents a reduction of about 0.1% in the annual secondary production over a 333 km² area of Long Island Sound adjacent to the power plant. Highest losses occurred during the spring (April, 1.4×10⁶ g dry weight), the lowest in autumn (November, 45.8×10³ g). If the same copepod loss rate exists for all power plants in Long Island Sound, then secondary production in 1.69×10⁶ m², or 0.05% of the total copepod production may be lost annually. A comparison of the surface outflow from Long Island to Block Island Sound with the water entrained through Millstone Unit One, and the 70% copepod loss rate in the latter area, indicates that Unit One eliminates about 0.1 to 0.3% of the copepod production in eastern Long Island Sound. This calculation compares favorably with losses computed from production data.     

Influence du mode de conservation sur la composition chimique élémentaire de Pontella mediterranea (Copepoda: pontellidae)

A comparative study of the elementary chemical composition (C, H, N) of Pontella mediterranea (males, females and copepodites) was carried out in two series: in Series 1, live copepods were briefly rinsed with distilled water, dried for one night (12 h) at 60°C, and then stored in a desiccator; in Series 2, the copepods were preserved in a 5% formalin-freshwater solution neutralized with borax; they were then rinsed with distilled water, and subsequently dried and stored in a desiccator as in Series 1. The results reveal that the dry weight of preserved copepods (Series 2) is 20 to 25% less than that of fresh, dried individuals. Carbon loss was estimated as about 10% in males and 17% in females, hydrogen loss as 14% in males and 23% in females, and nitrogen loss as 20% in males and 21% in females. The organic compounds lost seem to be mainly proteinaceous. Nevertheless, relative carbon content expressed as percent dry weight, and C:N and C:H ratios were all significantly higher in the preserved specimens. These results are compared with literature data on two other crustancean species.     

Metabolism and elemental composition of zooplankton from the Barents Sea during early Arctic summer

Rates of oxygen consumption, ammonia excretion and phosphate excretion were measured on a hydromedusae (Aglantha digitale), pteropods (Limacia helicina, Clione limacina), copepods (Calanus finmarchicus, C. glacialis, C. hyperboreus, Metridia longa), an amphipod (Parathemisto libellula), a euphausiid (Thysanoessa inermis) and a chaetognath (Sagitta elegans), all of which were dominant species in the Barents Sea during early summer 1987. Water and ash contents and elemental composition (C and N) were also analysed on the specimens used in these metabolic experiments. Between species variations were 67.8% to 94.7% of wet weight in water content, 6.4% to 56.5% of dry weight in ash content, 16.7% to 61.0% of dry weight in carbon content, and 4.3% to 11.2% of dry weight in nitrogen content. Oxygen consumption rates ranged from 0.33 to 13.8 l O₂ individual^-1 h^-1, ammonia excretion rates, from 0.0072 to 0.885 gN individual^-1 h^-1 and phosphate excretion rates, from 0.0036 to 0.33 g P individual^-1 h^-1. In general, higher rates were associated with larger species, but considerable differences were also seen between species. The ratios between the rates (O : N, N : P, O : P) exhibited a wide species-specific variation, indicating differences in dominant metabolic substrates. Typical protein oriented metabolism was identified only in S. elegans. From the results of metabolic rate measurements and elemental analyses, daily losses of body carbon and nitrogen were estimated to be 0.50 to 4.15% and 0.084 to 1.87%, respectively, showing faster turnover rates of carbon than that of nitrogen. Comparison of daily loss of body carbon of the Barents Sea zooplankton with that of the Antarctic zooplankton indicated reduced rates of the former (63% on average).     

Impact of copepod grazing on the red-tide flagellate Chattonella antiqua

Although planktonic copepods are major suspension feeders in the sea, the impact of their grazing pressure upon red-tide flagellates has not been fully investigated. In the present study, the grazing of adult females of several copepod species is examined using three food types: viz. natural suspended particles, natural suspended particles mixed with cultured Chattonella antiqua, and cultured C. antiqua. The functional response on C. antiqua was investigated for five species of copepods (Acartia erythraea, Calanus sinicus, Centropages yamadai, Paracalanus parvus and Pseudodiaptomus marinus). Ingestion rates increased linearly with increasing cell concentrations until a maximum level was reached, beyond which the rates were constant. This cell concentration was higher for larger copepods. The weight-specific maximum ingestion rates were higher in the small species. In general, copepods tended to feed selectively on larger particles when feeding on natural particles. This tendency was strongest in a simulated red-tide environment. Thus, it can be surmised that copepods may selectively graze on C. antiqua during the outbreak of a red tide. Grazing pressure by the natural copepod community in Harima Nada, the Inland Sea of Japan, was calculated by integration of the laboratory determined feeding rates and field measurements of zooplankton biomass. The daily removal rate was 3.4 to 30.8% (mean: 12.3%) of C. antiqua biomass at 20 cells ml^-1 and decreased to 0.6–4.3% (mean: 1.8%) at 500 cells ml^-1. Therefore, the grazing pressure by the copepod community is important at the initial stage of the red tide.     

Population structure of the planktonic copepod <Emphasis Type="Italic">Calanus pacificus</Emphasis> in the North Pacific Ocean

The pelagic copepod Calanus pacificus ranges nearly continuously across temperate-boreal regions of the North Pacific Ocean and is currently divided into three subspecies—C. pacificus oceanicus, C. pacificus californicus, C. pacificus pacificus—based on subtle morphological differences and geographic location. The relation between geography and genetic differentiation was examined for 398 C. pacificus individuals sampled from six widely distributed locations across the North Pacific, including an open ocean site and coastal sites on both sides of the North Pacific basin. For each individual copepod, the DNA sequence was determined for a 421-bp region of the mitochondrial coxI gene (mtCOI). A total of sixty-three different mtCOI sequences, or haplotypes, were detected, with a sequence divergence between haplotypes of 0.2–3.1%. The number and distribution of haplotypes varied with sampling location; 12 haplotypes were distributed across multiple sampling locations, and 51 occurred at only one location. Five genetically distinct populations were detected based on F _ST values. Haplotype minimum spanning networks, nucleotide divergence and F _ST values indicated that individuals from coastal sites in the North Pacific Ocean were more closely related to each other than to individuals from the open ocean site at Station P. These results provide genetic support for the designation of two subspecies—a coastal subspecies that consists of what is currently referred to as C. p. pacificus and C. p. californicus and an open ocean subspecies C. p. oceanicus. This work also indicates that planktonic copepods with potentially high dispersal capacity can develop genetically structured populations in the absence of obvious geographic barriers between proximate locales within an ocean basin.     

Population structure of the planktonic copepod Calanus pacificus in the North Pacific Ocean

The pelagic copepod Calanus pacificus ranges nearly continuously across temperate-boreal regions of the North Pacific Ocean and is currently divided into three subspecies—C. pacificus oceanicus, C. pacificus californicus, C. pacificus pacificus—based on subtle morphological differences and geographic location. The relation between geography and genetic differentiation was examined for 398 C. pacificus individuals sampled from six widely distributed locations across the North Pacific, including an open ocean site and coastal sites on both sides of the North Pacific basin. For each individual copepod, the DNA sequence was determined for a 421-bp region of the mitochondrial coxI gene (mtCOI). A total of sixty-three different mtCOI sequences, or haplotypes, were detected, with a sequence divergence between haplotypes of 0.2–3.1%. The number and distribution of haplotypes varied with sampling location; 12 haplotypes were distributed across multiple sampling locations, and 51 occurred at only one location. Five genetically distinct populations were detected based on F _ST values. Haplotype minimum spanning networks, nucleotide divergence and F _ST values indicated that individuals from coastal sites in the North Pacific Ocean were more closely related to each other than to individuals from the open ocean site at Station P. These results provide genetic support for the designation of two subspecies—a coastal subspecies that consists of what is currently referred to as C. p. pacificus and C. p. californicus and an open ocean subspecies C. p. oceanicus. This work also indicates that planktonic copepods with potentially high dispersal capacity can develop genetically structured populations in the absence of obvious geographic barriers between proximate locales within an ocean basin.     

Characterizing dietary variability and trophic positions of coastal calanoid copepods: insight from stable isotopes and fatty acids

The spring zooplankton community in the Strait of Georgia (British Columbia, Canada) is characterized by the presence of several calanoid copepod species which collectively make up ~90% of the mezozooplankton biomass. Here, we investigate interspecific, interannual, and geographic variability in the diets and trophic positions of these copepods using a combination of fatty acids and stable isotopes. To characterize geographic variability in diet, we compare our findings from the Strait of Georgia with similar data from Ocean Station P in the subarctic northeast Pacific. Both fatty acid and stable isotope signatures indicate the existence of three trophic levels, even within the limited size range of these copepods: Neocalanus plumchrus and Calanus marshallae are primarily omnivorous, while Euchaeta elongata is carnivorous and Eucalanus bungii is herbivorous. Fatty acid markers of trophic position (e.g., DHA/EPA, 18:1n-9/18:1n-7) correlate significantly with δ¹⁵N, while markers indicating the proportion of diatoms to flagellates in the diet (e.g., 16PUFA/18PUFA and DHA/EPA) correlate significantly with δ¹³C, after the effect of lipid concentration on δ¹³C is accounted for. Despite the general correlation between stable isotopes and fatty acids, the former are not sensitive enough to capture the range of interannual variability observed in the latter, and can only capture substantial shifts in the diet over geographic scales. However, regardless of variability in food quality, the relative trophic positions of these copepods do not change significantly either spatially or temporally.     

In situ grazing rate of the copepod population in the western subarctic North Pacific during spring

Time series sampling with a multi-layer plankton sampler was carried out in the western subarctic North Pacific during spring 1991. Neocalanus cristatus, N. flemingeri and Eucalanus bungii dominated and accounted for 88.5% of the copepod population in volume. Neocalanus spp. were distributed in the upper mixed layer, while E. bungii was mainly distributed between 120 and 300 m throughout the day and night. In contrast, Metridia pacifica, Pleuromamma scutullata and Gaetanus simplex showed clear diel vertical migration. Grazing activities were estimated simultaneously by gut fluorescence. Nocturnal grazing was observed for diel migrating species. Neocalanus spp. did not have a diel feeding rhythm and had relatively low gut fluorescence. E. bungii was considered to be dormant during the observation period. The estimated grazing rate of the copepod population on phytoplankton was 1.4 to 2.0% of the primary production while the metabolic requirement was 8.3 to 12.4% of the primary production. These facts suggest that the copepod population was unimportant as primary consumers and that microzooplankton plays a much more important role in sustaining low standing stock of phytoplankton and a high nutrient concentration in the western subarctic Pacific Ocean.     

Zooplankton feeding ecology: contents of fecal pellets of the copepods Temora turbinata and T. stylifera from continental shelf and slope waters near the mouth of the Mississippi River

In-situ feeding habits of the copepods Temora turbinata and T. stylifera were investigated by scanning electron microscope examination of fecal pellets, the contents of which reflected copepod gut contents upon capture. Pellet contents were compared with assemblages of available phytoplankton in the water column at the times of zooplankton sampling. Samples were collected in continental shelf and slope waters of the Gulf of Mexico near the mouth of the Mississippi River. Both species ingested a wide size range and taxonomic array of phytoplankters, and to a lesser extent, other crustaceans. Fecal pellets contained primarily the remains of the phytoplankters that were most abundant in the water at times of collection. There was considerable overlap in the food items ingested by adult females of both copepod species, or two stages of T. turbinata copepodites. Thus, T. turbinata and T. stylifera are omnivores, but primarily opportunistic herbivores.     

Food selection by copepods: discrimination on the basis of food quality

The copepod Acartia tonsa displayed nearly two-fold higher ingestion rates on faster-growing cells of the diatom Thalassiosira weissflogii compared to ingestion rates on slower-growing cells of that species at the same cell concentration. Ingestion rates on slow-growing cells were also enhanced by the addition of cell-free aliquots of algal exudate to the experimental feeding chambers. In addition, the faster-growing algal cells were selectively ingested by the copepod when the two cell types were mixed together in different proportions, indicating that physiological differences between growing cells are a critical factor in the food detection/selection process of zooplankton. Consideration of cell carbon, nitrogen, and protein composition suggests that the copepods are maximizing nitrogenous ingestion (total protein and/or nitrogen). Selectivity for cells with higher protein content results in a higher daily protein ration, even if the selection process results in a decreased rate of ingestion in mixtures of cell types.