Short-term changes in feeding and digestion by the copepodCalanus pacificus

The planktonic marine copepodCalanus pacificus exhibits an enhanced feeding rate, or hunger response, when exposed to food following short periods of starvation. In a scries of laboratory experiments with copepods collected from the main basin of Puget Sound, Washington, during 1982 and 1984, we measured maximum ingestion rate, assimilation efficiency, and digestive enzyme activity to determine the time scales over which the feeding behavior ofC. pacificus responds to increases in food. These laboratory results were then compared to field studies of diel fluctuations in digestive enzymes and gut fluorescence ofC. pacificus in Dabod Bay, a fjord of Puget Sound, during September, 1980, and the closely relatedC. marshallae off the Washington coast, in August, 1981. Laboratory experiments demonstrated that the hunger response ofC. pacificus lasts approximately 6 h before ingestion rate returns to a steady state level of about one-half maximum. On the order of 12h of starvation were required to induce the maximum ingestion rate of the hunger response. Digestive enzyme activities did not change over these time scales. Assimilation efficiency peaked within a few hours of the onset of feeding, with low initial rates possibly related to the period of starvation prior to feeding. These results were consistent with diel patterns observed in the field. The hunger response ofC. pacificus appears to be controlled by processes within the gut, and our results are discussed in relation to recent studies of the digestive processes of calanoid copepods.Contribution No. 1772 from the School of Oceanography, University of Washington, Seattle, Washington 98195, USA     

Effects of algal diet on digestive enzyme activity in Calanus helgolandicus

Adult female Calanus helgolandicus were transferred, immediately after collection in the English Channel in June 1984, to two unialgal diets one of which, the flagellate Cryptomonas maculata, was rich in starch, while the other, the diatom Thalassiosira weissflogii, contained no starch. The activity of the digestive enzymes amylase, trypsin and laminarinase was measured in these two populations under foodsaturating conditions over an acclimation period of eight days. Ingestion rates were measured on a daily basis and the results confirmed, together with a constant level of body protein, that the experimental conditions were above the incipient limiting concentration. In the long-term (4 to 8 d), the activities of all three enzymes were significantly elevated in the C. maculata-fed copepods, whereas ingestion rates were lower than those on T. weissflogii. These results observed under food-saturating conditions indicate a compensatory mechanism between digestive enzymes and the substrate ingested. They are consistent with previous work on Artemia sp. suggesting that the rate of assimilation is a function of the digestive enzyme activity and ingestion rate. Enzyme activity exhibited differing shortterm responses (<48 h) on transfer to the two algal diets, which are interpreted in relation to the in situ activity of the field population.Communicated by J. Mauchline, Oban     

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.     

Particle-Size-dependent maximum grazing rates for Temora longicornis fed natural particle assemblages

The hypothesis that lower retention efficiencies of filter-feeding copepods for small particles should result in different ingestion rate versus food concentration curves for different-sized foods was tested using Temora longicornis (Müller) fed natural phytoplankton. The copepods were fed different natural phytoplankton assemblages, which varied in their species and size distribution. Volume ingestion rates were an asymptotic function of food concentration, with maximum ingestion rates measured at food concentrations exceeding 5 to 10x 10⁶ m³ ml^-1, which were less than those occurring in the natural waters in which the copepods and phytoplankton were collected. Maximum volume ingestion rates increased linearly by a factor of 3.5, as the diameter of the particle forming the peak in the food size distribution increased fron 5 m (primarily microflagellates) to 30 m (mostly large diatoms). These results suggest that natural and pollutant-induced size reductions in natural phytoplankton could markedly decrease the volume of food consumed by filter-feeding copepods.Contribution No. 243 of the Marine Sciences Research Center     

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.     

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.     

Diel variability of feeding activity in haddock (Melanogrammus aeglifinus) larvae in the East Shetland area, North Sea

Investigations of factors affecting feeding success in fish larvae require knowledge of the scales of variability of the feeding process itself and the indices used to assess this variability. In this study, we measured short-term (diel) variability in feeding rates of wild haddock (Melanogrammus aeglifinus) larvae four times per day during a 10-d cruise in the northern North Sea. Feeding activity was evaluated using indices of gut fullness, prey digestive state and biochemical measurements (tryptic enzyme activity). The gut fullness and the enzyme activity indices indicated moderate to high rates of food consumption throughout the cruise. Time series analysis of the three indices showed significant diel variability in all indices and enabled identification of significant lags between food uptake and peak digestive enzyme activity. The typical pattern of food consumption and digestion was characterized by maximal ingestion of prey early in the evening (19:00 hrs) and peak digestive enzyme activity at 01:00  hrs. The time scale over which enzyme activities reacted to prey ingestion was ca. 6 h, and is consistent with expectations from controlled laboratory experiments with other larval fish species. Significant diel variability in tryptic enzyme activity suggests that attempts to relate this measure of feeding success to other variables (e.g. food concentrations) should take care to accommodate natural cycles in feeding activity before making statistical comparisons. Received: 29 October 1998 / Accepted: 18 June 1999     

Seasonal differences in citrate synthase and digestive enzyme activity in larval and postlarval antarctic krill, Euphausia superba

Antarctic krill maintain large population sizes despite dramatic seasonal fluctuations in food availability, but the mechanisms for this are still debated. The aim of this study was to compare seasonal differences in enzyme activity and respiration rates of larval and postlarval krill to provide insights into their overwintering strategies. Respiration rates, activity of the metabolic enzyme citrate synthase (CS), and those of the digestive enzymes laminarinase and total proteinase were measured in austral summer west of the Antarctic Peninsula, and in autumn in the southwestern Lazarev Sea. The 100-fold difference in chlorophyll a concentrations between the two studies is representative of the classic transition from a summer bloom to sparse winter conditions. Correspondingly, adult krill showed reduced respiration rates and CS activity in autumn. However, their digestive enzyme activity was significantly higher, suggesting more efficient assimilation of food at low food levels. Similar-sized larvae showed no summer-autumn differences in respiration rates and enzyme activity, supporting suggestions that they need to feed and grow year-round. However, trends in enzymatic activity varied between the larval stages measured, implying ontogenetic changes in body structure and function. Published online: 1 August 2002     

Concentration of ascorbic acid and antioxidant response in early life stages of Engraulis ringens and zooplankton during the spawning seasons of 2006–2009 off central Chile

This study reports changes in ascorbic acid (AA) in anchoveta eggs, copepods and zooplankton during the 2006, 2007 and 2009 main spawning seasons in the coastal area of the central Humboldt Current System, Chile. Anchoveta eggs, copepods and total zooplankton community shared a seasonal variation and an increasing trend in AA concentration from winter through spring which was associated with the spring diatom bloom. The lineal relationship observed between AA concentration in anchoveta eggs, chlorophyll a and Sea Surface Temperature (SST) suggests that the increase in phytoplankton abundance could also increase the amount of AA in the spawning female anchoveta incorporated through tissue, thus increasing the concentration in their eggs. Ascorbic acid concentrations in copepods presented size (weight) dependence. Small copepods (e.g. Acartia, Oithona) had AA concentrations two orders of magnitude higher than the heavier weight class copepods (e.g. Calanus, Rhincalanus). Results of the determination of glutathione and the antioxidant potential showed a similar trend in interannual variations, suggesting that cold SST conditions observed in the 2007 spawning season could increase the consumption of antioxidants in early stages. Potential connections between AA concentration in the food web on anchoveta reproduction and egg hatching and embryo malformations are discussed.     

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%.     

Gut pigment accumulation and destruction by arctic copepods in vitro and in situ

The results presented here were obtained at six locations during three cruises in 1985 (off the coast of Labrador), 1986 (at the eastern end of Viscount Melbourne Sound) and 1988 (off the coast of Labrador). In situ chlorophyll maximum concentrations were >7 gl^-1 at depths of between 0 and 30 m in all sampling areas. In feeding experiments copepods attained higher gut pigment concentrations the longer they had been previously starved and higher concentrations when fed in the dark than when fed in the light. Community ingestion rates calculated from changes in particulate chlorophyll were higher than estimates derived from gut pigment data except when copepods had been starved for 24 h. Differences between estimates by the two methods suggested pigment destruction. In feeding experiments pigment: biogenic silica ratios in food and faecal pellets suggested that the length of starvation period affected the degree of pigment destruction differently at different stations and that feeding in the light greatly increased pigment destruction. A comparison of pigment: silica ratios in the water column, and in faecal pellets collected from copepods which had fed there, suggested that pigment destruction may occur in situ sometimes and that the degree to which it occurs may be affected by feeding history, light, diel feeding behaviour and species composition.     

Copepod grazing during a declining spring phytoplankton bloom in the Northern North Sea

Grazing rates of larger (Calanus finmarchicus) and smaller (Acartia clausii Pseudocalanus elongatus etc.) copepods on naturally occurring phytoplankton populations were measured during a declining spring phytoplankton bloom. During the initial period, dominated by Chaetoceros spp. diatoms, constant ingestion rates were observed in Calanus finmarchicus at suspended particulate concentrations above 300 g carbon l^-1. Average daily intake during this time amounted to 35 to 40% of body carbon and reached a maximum of 50%. The feeding response of the smaller copepods was not so well defined, although a maximum daily intake of 56% body carbon was recorded. In both groups, feeding thresholds were at particulate concentrations around 50 g C l^-1. The feeding response of C. finmarchicus was correlated with both a change in their own population and in the food cell type. Linear regressions describing the concentration-dependent feeding response were: ingestion rate (IR)=1.16 total particulate volume (TPV)-36.15 during the initial part of the period compared with IR=0.41 TPV-12.18 for the latter period. C. finmarchicus filtered out slightly larger (x 1.2 diameter) particles than the small copepods and, in both groups, some filtering adjustment was made to accomodate to modal changes in the phytoplankton population from 20–30 m to 10 m diameter cells. Particle production during feeding was frequently evident in the smallest size ranges of particles and the ratio of particle production to ingestion rate was greater at low feeding rates.     

Effects of exposure to sublethal concentrations of crude oil on the copepod Centropages hamatus

Female copepods of the species Centropages hamatus show decreased ingestion rates and decreased egg viability when exposed to crude oil/seawater dispersions having crude oil concentrations of 10–80 ppb. However, rates of egg production were not significantly affected by these exposure levels. In addition, we found no evidence for accumulation of petroleum hydrocarbons by copepods exposed to 200 ppb of South Louisiana crude oil. The results imply that biosynthetic pathways involved in oogenesis may be influenced by sublethal concentrations of crude oil or that petroleum hydrocarbons directly affect the viability of eggs. Recruitment into field populations of copepods could be severely reduced as a consequence of exposure to low levels of physically dispersed crude oil.     

Quantification of copepod gut content by differential length amplification quantitative PCR (dla-qPCR)

Quantification of feeding rates and selectivity of zooplankton is vital for understanding the mechanisms structuring marine ecosystems. However, methodological limitations have made many of these studies difficult. Recently, molecular based methods have demonstrated that DNA from prey species can be used to identify zooplankton gut contents, and further, quantitative gut content estimates by quantitative PCR (qPCR) assays targeted to the 18S rRNA gene have been used to estimate feeding rates in appendicularians and copepods. However, while standard single primer based qPCR assays were quantitative for the filter feeding appendicularian Oikopleura dioica, feeding rates were consistently underestimated in the copepod Calanus finmarchicus. In this study, we test the hypothesis that prey DNA is rapidly digested after ingestion by copepods and describe a qPCR-based assay, differential length amplification qPCR (dla-qPCR), to account for DNA digestion. The assay utilizes multiple primer sets that amplify different sized fragments of the prey 18S rRNA gene and, based on the differential amplification of these fragments, the degree of digestion is estimated and corrected for. Application of this approach to C. finmarchicus fed Rhodomonas marina significantly improved quantitative feeding estimates compared to standard qPCR. The development of dla-qPCR represents a significant advancement towards a quantitative method for assessing in situ copepod feeding rates without involving cultivation-based manipulation.     

Ecological success of the cladoceran <Emphasis Type="Italic">Penilia avirostris</Emphasis> in the marine environment: feeding performance,gross growth efficiencies and life history

Marine cladocerans are important contributors to the zooplankton community of tropical and temperate coastal ecosystems during the warmer months, when they show explosive population growth. Despite this fact, little information is available on their ecology compared with the extensive studies on their freshwater relatives. The main objective of this study was to determine the in situ feeding and growth rates, and life history parameters of Penilia avirostris in São Sebastião Bay (Brazil) during austral summer 2004, as a premise to understand the advantages of this cladoceran in oligotrophic waters. Culture development experiments, monitored for a period of 12 days, showed that maximum juvenile release occurred after 2 days, and that the development duration of a complete cohort was around 9 days. From bottle incubation grazing experiments, significant ingestion rates upon flagellates, ciliates, dinoflagellates and diatoms were detected. Flagellates were the most important contributors to P. avirostris diet (ca. 80%). P. avirostris ingested between 28 and 97% of its own carbon biomass per day (daily ration) and individual growth rates of this marine cladoceran (0.10–0.24 d^?1) increased with prey availability. The combination of ingestion rates of natural prey and growth rates provided gross growth efficiencies (GGE) of 15–53%, on a carbon basis. Our results suggest that P. avirostris has similar GGE to copepods, although at low food conditions the values for the marine cladocerans seems slightly higher. However, this characteristic alone does not explain the explosive growth and community dominance shown by P. avirostris. Therefore, other traits related to the reproductive biology of the species, such as short generation time, parthenogenetic reproduction, and continuous somatic growth, seems to be mostly responsible for the success of P. avirostris in many marine ecosystems during their seasonal occurrence.     

Feeding four species of pelagic copepods under experimental conditions

Different qualitative and quantitative aspects of feeding with varied compositions of diets have been studied in 4 species of pelagic copepods: Calanus helgolandicus, Centropages typicus, Temora stylifera and Acartia clausi. By feeding copepods different algal concentrations, it was shown that when food concentration increases grazing rate decreases; the ingestion rate remains fairly constant in the lower range of concentrations, but then increases, reaching a plateau at higher algal concentrations. There is a significant correlation between daily food intake and fecal pellet production. On a pluri-algal diet, selective grazing is observed: larger phytoplankton cells are more efficiently removed than smaller ones. Using Artemia nauplii, it is shown that the copepods studied are also able to eatch and ingest animal prey. Increased daily food intake affects respiration and oviposition. Metabolic requirements, gross growth-efficiency, and food assimilation have been ealculated in Calanus helgolandicus, Centropages typicus and T. stylifera for a large range of algal concentrations.     

In situ feeding rhythms of herbivorous copepods,and the effect of starvation

In situ diel variations in gut pigment contents of neritic (Acartia omorii andPseudocalanus minutus) and oceanic copepods (Calanus plumchrus andC. cristatus) were analyzed.A. omorii andP. minutus were sampled in Onagawa Bay on the east coast of Japan in May and August 1987, andC. plumchrus andC. cristatus were sampled in the Bering Sea in June 1986. Gut pigments were generally high at night, and bimodal feeding rhythms were observed in all species. The first peak of gut pigments occurred between sunset and midnight and was followed by a midnight decrease in gut pigment levels, resulting in eventual evacuation of the gut. The second peak was observed a few hours after sunrise. Incubation experiments indicated that ingestion rates of starved copepods were higher than those of acclimated copepods. This phenomenon was most notable at high food concentrations. Gut pigments of starved copepods rapidly increased after exposure to high concentrations of culturedThalassiosira decipiens. These findings suggest that in situ feeding behavior of herbivorous copepods includes periods of cessation or reduction in feeding during the night, and consequently, feeding activity is periodically enhanced with starvation. Starvation enhanced feeding behavior is most obvious in the large oceanic species,C. plumchrus andC. cristatus and is not distinct in small coastal species such asA. omorii.     

Zooplankton abundance and grazing at Davies Reef,Great Barrier Reef,Australia

Zooplankton abundance and grazing on autotrophic and heterotrophic particulate matter were measured along a transect across Davis Reef (18°5S; 147°39E) and in the back-reef lagoon over tidal and diel cycles during austral winter (August 1984). Zooplankton entering the reef from the surrounding shelf waters decreased in abundance over the reef flat, presumably because of predation. Within the reef lagoon, maximum daytime densities of pelagic copepods occurred during high water, suggesting an external input. At night, water-column zooplankton biomass increased by a factor of 2 to 3 due to the emergence of demersal reef zooplankton. Zooplankton grazing rates on heterotrophic particulate matter (bacteria + detritus and Protozoa) compared to phytoplankton were higher on the reef flat than on the fore-reef or lagoon. Within the lagoon, zooplankton grazing rates on heterotrophic material were maximum during high water, coincident with maximum tidal concentrations of particulate organic carbon. The combined demersal and pelagic zooplankton community were often able to crop 30% of the daily primary production by >2µm phytoplankton. However, >50% of phytoplankton biomass was in cells <2µm, presumably unavailable to these zooplankton. Our particulate production and ingestion measurements, together with zooplankton carbon demand extrapolated from respiration estimates, suggest that the zooplankton community of Davies Reef derives much of its nutrition from detritus.Joint contribution from the University of Maryland, Center for Environmental and Estuarine Studies (No. 2015), and the Microbial Ecology on a Coral Reef Workshop (MECOR No. 19)     

Effects of prey concentration,prey size,predator life stage,predator starvation,and season on predation rates of the carnivorous copepod Euchaeta elongata

Adult females of the large carnivorous copepod Euchaeta elongata Esterly were collected from 1977 to 1980 in Port Susan, Washington, USA. Predation rates of the adult females increased with increasing prey abundance when fed the following 4 sizes of copepods: adult females of Calanus pacificus (average prosome length [PL] of 2 650 μm), adults of Aetideus divergens (PL of 1 560 μm), adult females of Pseudocalanus spp. (PL of 1 060 μm), and nauplii of C. pacificus (PL of 410 μm). Saturation feeding levels were reached when adult females of the predator were fed the small adult copepod, Pseudocalanus spp. Maximum biomass ingested of this small copepod was more than the maximum amount ingested of the larger copepods. Predation rates of the predatory copepodids at Stages IV and V also increased with increasing concentration of the 1 060 μm (PL) prey. High feeding rates exhibited by both adults and copepodids at Stage V of the predator indicate their importance as sources of mortality on populations of small copepods. Ingestion efficiency E _i (prey wholly consumed [prey attacked]^-1) varied as follows: adults of E. elongata were more efficient than copepodids of E. elongata; adults were more efficient than copepodids when ingesting smaller prey; starved adults were more efficient than fed ones; and both adults and copepodids were more efficient at low food concentrations. For adults of E. elongata, there were no marked seasonal variations in predation or respiratory rates that would represent acclimatory responses; however, small adults obtained during winter were more efficient at ingesting prey than were the larger adults gathered in summer. This seasonal variation in the efficiency of ingestion may be a useful indicator of physiological state: high E _i values could indicate that predators are starving in winter, and low E _i values could indicate that predators are satiated in summer.     

Effects of a transitory,low food supply in the early life of larval herring (Clupea harengus) on mortality,growth and digestive capacity

In order to assess possible effects of a transitory, low food supply on later development, three groups of Clyde herring larvae (Clupea harengus L.) were exposed in 1989 to different feeding regimes immediately after yolk resorption. Group 1 received a high daily ration of 80 copepods larvae^–1 for 31 d, Group 2 a low daily ration of 15 copepods larva^–1 for 10 d followed by a high ration (80 copepods larva^–1) for 21 d and Group 3 a low ration of 15 to 20 copepods larva^–1 for 31 d. After 31 d of feeding, digestive capacity, expressed as the sum of trypsin and trypsinogen, was markedly reduced in Group 2 compared to Group 1, while Group 3 had an even lower digestive capacity. After the switch from low to high ration Group 2 exhibited compensatory growth and caught up with Group 1 both in standard length and content of soluble protein. Group 3 had the lowest growth rates. Mortality was equal in Groups 1 and 2, while Group 3 showed an excess mortality of 40% of the start population. Although Group 2 larvae had caught up with Group 1 in growth at the end of the study, content of trypsin and trypsinogen in Group 2 was only half of that found in Group 1. Thus, comparing effects of a short period of food limitation on future growth, mortality and content of digestive enzymes, the study indicates content of trypsin and trypsinogen to be the most sensitive variable for detection of food limitation in the early stages of exogenous feeding.