Food selection capabilities of the estuarine copepod Acartia clausi

Existing viewpoints and theories of selective grazing by copepods are briefly reviewed in order to formulate explicit hypotheses to be tested experimentally. Based on these hypotheses, a series of grazing experiments was run to determine (1) the extent of the selective ingestion capabilities of Acartia clausi and (2) how these capabilities were affected by previous feeding histories. Groups of copepods were separately preconditioned on a small diatom (Thalassiosira pseudonana), a large diatom (T. fluviatilis), or a plastic sphere. The ingestive behavior was then examined on various combinations of spheres and food particles. Spheres offered alone were not ingested. In mixtures of diatoms and spheres, the copepods avoided ingesting spheres intermediate in size between the sizes of the diatoms. The copepods either ingested particles on either side of the spheres, or ignored all particles less than the size of the largest spheres. The pattern observed depended upon the size of the preconditioning food. However, if the spheres were larger than the largest food particles, the copepods still selectively ingested the food particles. The above results demonstrate that A. clausi has a complex grazing behavior consisting of (1) more efficient grazing on larger particles within its particle-size ingestion range; (2) the ability to alter effective setal spacing to optimize feeding behavior (i.e., the ability to increase efficiency of capture of food particles, and to avoid non-food particles); and (3) the ability for post-capture rejection of non-food particles when they interfere with the ingestion of food particles on which the copepod has been preconditioned. The behavioral patterns observed depend heavily on the food preconditioning and the presence or absence of non-food particles. These results clearly indicate that a simple mechanistic explanation of selective grazing is insufficient.     

Particle-size selection by Pseudopolydora paucibranchiata (Polychaeta: Spionidae) in suspension feeding and in deposit feeding: influences of ontogeny and flow speed

Pseudopolydora paucibranchiata Okuda suspension feeds and deposit feeds at the sediment-water interface, where it is exposed to a variety of particles differing in physical characteristics and nutritional value. In flume experiments (conducted in August 1994 and May 1995) with two sizes of either suspended or deposited beads, I measured particle-size selection separately in each feeding mode. The same influences of palp width and of ambient flow speed were observed in each mode. At velocities 0.74 cm s^-1 there were no relationships between palp width and the proportion of gut contents composed of large beads. At velocities 1.8 cm s^-1 worms with narrower palps ingested relatively fewer large beads (and more small beads) than did worms with wider palps. Palp width and body length were linearly related, and results were similar when analyzed with body length as the independent variable. As flow speed increased, selectivity changed in a worm-size-specific manner: worms with a palp width -1. Assuming that in the field (1) particle size is the principle criterion for selection, and (2) the amount of digestible food component in deposited and suspended particles, respectively, is related to particle surface area and volume, I hypothesize that changes in selectivity as velocity rises can cause juveniles to experience a decreasing profitability of suspension feeding and a simultaneously increasing profitability of deposit feeding. Juveniles could maintain a diet of high food value despite flow variations by adjusting the proportion of time they spend suspension feeding relative to deposit feeding.     

Particle size,flow velocity,and suspension-feeding by the erect bryozoansBugula neritina andB. stolonifera

Patterns of feeding by the erect bryozoansBugula neritina andB. stolonifera were studied by assessing ingestion rates of mixtures of polystyrene particles of three sizes present in equal densities at two ambient water-flow velocities. Particle size was found to influence feeding byB. neritina, while feeding byB. stolonifera was influenced by an interaction between particle size and flow velocity. Large and mediumsized particles were ingested at rates disproportionate to their numerical abundance, and small particles were always ingested in low numbers. Disproportionate feeding did not appear to be due to the greater likelihood of directly intercepting or of detecting particles of larger sizes, but may be explained by other size-dependent particle behaviors, active selection or rejection by the bryozoans, and/or the utilization of different feeding techniques. Comparison with a parallel study indicated that patterns of feeding on single-sized suspensions cannot be used to predict patterns of feeding on mixed suspensions. This is one of the few studies to test the combined influence of variation in both suspended particulate matter and in properties of the fluid medium. Such investigations will provide more realistic views of suspensionfeeding performance.     

Seasonal grazing of Pseudocalanus minutus on particles

Grazing by Pseudocalanus minutus on naturally occurring partieles has been investigated over a 2 year period at 5 m depth, in a small coastal embayment in Nova Scotia, Canada. Large variations in the standing stock of particulate matter in the water from 5 m depth were associated with seasonal changes in phytoplankton, and at times with runoff from rivers. Each size group displayed seasonal changes, with high amplitudes occurring in the large-particle size ranges. P. minutus consumption was associated with seasonal changes in total particle concentration as well as with the concentration in each size group. The food uptake was correlated (P>0.01) with the particle concentration in all particle categories, except in the size range below 3.57 . Frequency of positive electivity indices increased with particle sizes up to 57 and then decreased. Observations on particle spectra revealed considerable seasonal variability in both particle sizes and copepod feeding patterns. P. minutus adapted to seasonal variations within the particle spectrum by shifting its grazing pressure from one size range to another. By taking advantage of every particle peak concentration, P. minutus revealed a strong opportunistic feeding behaviour, and a very efficient utilization of the standing stock. Maximum consumption was recorded in early spring, when P. minutus fed on large-size particles. Feeding took place in the medium and small-size ranges during the summer and part of fall. Food uptake was rarely less than 2.26% of body weight during the winter, and reached up to 55% and sometimes more in spring. The unselective feeding patterus demonstrated by P. minutus suggest certain ecological implications of feeding pressure on standing stock.     

Effects of feeding experience in the copepod Eucalanus pileatus: a cinematographic study

High-speed microcinematography was used to examine the effects of prior experience with particular cell types on the feeding efficiency of a calanoid copepod. Female Eucalanus pileatus were fed monocultures of either the 5-m diatom Thalassiosira pseudonana or the 11-m diatom T. weissflogii during a 2-to 3-d preconditioning period. The smaller diatoms are accumulated passively by the second maxillae while the larger diatoms are detected and actively captured as individual cells. Four females from each preconditioning culture were transferred to a monoculture of the large cells and their behavior filmed at five intervals over a 24-h period to determine whether a loss of efficiency occurs when the copepods must shift capture modes. Ingestion rates for females experienced with the larger cells were approximately 2.5 times higher than those of inexperienced females. Six sequential behavioral steps in the feeding process could alter ingestion rates: (1) amount of time spent flapping the feeding appendages. (2) rate of flapping of the feeding appendages, (3) ability to detect individual cells, (4) success rate of capture attempts, (5) capture and handling time per cell and (6) rejection rate of captured cells. An increased ability to detect cells and a decreased rejection rate contributed significantly to the higher ingestion rate of experienced feeders, indicating that copepods have the ability to learn during the feeding process. Grazing rates may be seriously underestimated in experiments which do not include a preconditioning period, especially those which calculate ingestion over short time intervals. Such effects may also influence the feeding of copepods in the field when encountering changes in particle spectra through vertical migration or horizontal displacement.     

Prey selection by larval weakfish (Cynoscion regalis): the effects of prey size,speed, and abundance

We examined feeding by larval weakfish, Cynoscion regalis (Bloch and Schneider), in laboratory experiments conducted during the 1991 spawning season. under natural conditions weakfish larval development is ca. 3 wk, and we ran separate experiments with larvae of five different ages (5, 8, 11, 14, and 17 d post-hatching). We used two different size classes of rotifers (Brachionus plicatilis) and brine shrimp nauplii (Artemia sp.) as prey organisms. Contrary to results of previous research, weakfish larvae did not select prey based on size alone. When prey abundance was above 100 itemsl^-1 weakfish, larvae always chose large rotifers (length = 216 m) over small rotifers (length = 160 m). At 11 d post-hatching, larvae switched their diet from large rotifers to small brine shrimp nauplii (length = 449 m); however, when fed small rotifers and small brine shrimp nauplii the change in diet occurred at 14 d post-hatching. This pattern of selectivity was maintained in each larval age class. Early-stage larvae (5 and 8 d post-hatching) did not feed selectively when prey abundance was less than 100 itemsl^-1. Late-stage larvae (17 d post-hatching) fed selectively at abundances ranging from 10 to 10000 items^-1. Lwimming speeds of prey items, which ranged from 1 to 6 mms^-1, had no consistent effect on prey selection. These results suggest that weakfish larvae are able to feed selectively, that selectivity changes as larvae age, and that selectivity is also influenced by prey abundance.     

Grazing by adult estuarine calanoid copepods of the Chesapeake Bay

Grazing by adult female Eurytemora affinis, Acartia tonsa and A. clausi on natural distributions of particles from the Chesapeake Bay has been investigated. During the course of a year's sampling, a wide variety of particle size-biomass distributions were observed as seasonal shifts in detritus, and over 150 algal species occurred. These distributions were grouped into 5 basic types in the analyses of feeding. All three species demonstrated similar capabilities for feeding over a broad range of particle size with selection (higher filtering rates) on larger particles and on biomass peaks. Feeding on multiple-peak distributions resulted in strong selection or tracking of each biomass peak with reduced filtering rates between peaks. Evidence is presented which suggests that the copepods first feed on large particles and then successively switch to biomass peaks of the smaller size categories. Comparisons of the feeding behavior of Eurytemora affinis and the Acartia species showing that the Acartia species have greater capabilities for taking large particles may be associated with modifications of their mouth parts for raptorial feeding. The results suggest considerable flexibility in copepod feeding behavior which cannot be explained solely by the mechanism of a fixed sieve.     

Particle size-selection of two deposit feeders: the amphipod Corophium volutator and the prosobranch Hydrobia ulvae

The feeding biology of the deposit-feeding amphipod Corophium volutator is compared to that of the coexisting, deposit-feeding prosobranch Hydrobia ulvae. Regarding ingestion of particles, both forms show size selection which alone can explain their coexistence. Particle size-selection also explains some qualitative differences in the composition of the food of the two forms; thus, diatoms play a relatively larger role in the diet of H. ulvae than in the diet of c. volutator, whereas bacteria are probably relatively more important for the latter. Results of experiments with feeding of C-14 labelled microorganisms are in accordance with the findings on particle size-distribution of the gut contents, and show that (1) C. volutator can only utilize bacteria adsorbed to particles within the size range 4 to 63 (this is why the presence of clay and silt particles in the sediment are necessary for efficient feeding of this amphipod); (2) C. volutator can utilize bacteria suspended in the water pumped through its burrow for respiration if silt and clay particles are present in the sediment. (3) H. ulvae can utilize large particles, and also browses on surfaces, and some evidence is brought forward that it also utilizes mucus for trapping microorganisms. The coexistence of deposit-feeding animals is discussed. It is concluded that the number of coexisting, closely related species is usually small, and that their resource partitioning is probably mainly based on particle-size selectivity. In the case of unrelated forms (e.g. H. ulvae and C. volutator, a number of behavioural, physiological and morphological differences, and also the widespread ability of deposit feeders to utilize alternative feeding mechanisms may also lead to resource partitioning. Thus, there are often several niche dimensions related to feeding allowing a certain diversity of coexisting deposit feeders.     

Particle retention and selection by larvae and spat of Ostrea edulis in algal suspensions

Grazing rates and electivity indices of larvae and spat of Ostrea edulis L. were, measured and examined in relation to certain physical parameters using a flow-through system. Retention and size-selection were determined for the major particle sizes present in cultures of Isochrysis galbana Parke, an alga used frequently as food for bivalves. Cultures of the algae Dunaliella tertiolecta Butcher and Phaeodactylum tricornutum Bohlin were used as sources of particle suspensions of various sizes and shapes, respectively. While increases in flow rate caused increased grazing, the mode of selection of I. galbana particles remained constant. Filtration rate, F _f was related to body size, W, by the general allometric equation R _f =aW ^b,while particle-size preference in suspensions of I. galbana by both larvae and spat of O. edulis was independent of W. Grazing rates increased with temperature to an optimum temperature, which was related to the acclimation temperature. Increases above this optimum caused a reduction in feeding activity. No significant change in particle size-preference in the I. galbana suspension with temperature was observed. Grazing rates and selection were dependent, however, on particle number and volume. Both larvae and spat displayed maximum retention at optimum particle concentrations which tended to decrease with increasing particle size. Variations in cell shape of P. tricornutum had no measurable effect on selectivity by O. edulis.     

Feeding behaviour and prey size selection of gilthead seabream,Sparus aurata,larvae fed on inert and live food

Prey selection shortly after the onset of feeding by laboratory-reared gilthead seabream, Sparus aurata L., larvae was studied using larvae fed on two types of microcapsule (hard- and soft-walled) having diameters ranging from 25 to 300 m. Preferences between inert food and live prey (rotifers and Artemia sp. nauplii) were also studied. Seabream larvae were able to ingest inert food from first feeding. Larvae of all size classes ingested hard microcapsules with diameters in the range 25 to 250 m. However, larvae with a total length (TL) below 4 mm preferentially selected particles 25 to 50 m in diameter, larvae of TL 4 and 5 mm preferred particles 51 to 100 m in diameter, while larvae above 5 mm TL preferred particles 101 to 150 m in diameter. With soft microcapsules, larvae always preferred particles larger than in the previous case, and above 4.5 mm TL they preferentially selected particles 201 to 250 m in diameter. In addition, the gradual increase of preferred diameters with increasing TL was more pronounced when larvae were increasing TL was more pronounced when larvae were fed on soft particles. Mean values for prey width/mouth width ratios were approximately 0.24 and 0.30 when larvae were fed on hard-walled and soft-walled microcapsules, respectively, irrespective of the absolute value of larval length. When a mixed diet of live and inert food items was offered, live prey were always preferentially selected, even if the prey width/mouth width ratio was apparently not favourable. Only a physical constraint such as excessive prey width could counter this preference for living prey vs inert microcapsules. These results contribute to our knowledge in larval feeding behaviour, especially in the presence of inert food, and represent a fundamental step in developing prepared food for marine fish larvae.     

Selective particle ingestion by oyster larvae (Crassostrea virginica) feeding on natural seston and cultured algae

I investigated selective particle ingestion by oyster larvae (Crassostrea virginica) feeding on natural seston from Chesapeake Bay and laboratory-cultured algae of different sizes or chemical content. In 15 of 16 experiments with complex natural suspensions as food, small(<150 m) and large (>150 m) larvae selected most strongly for small (2 to 4 m) food particles, but in the presence of a large (>10 m)-cell dinoflagellate bloom, large larvae strongly selected much larger (22 to 30 m) food material (presumably dinoflagellates). When fed simplified mixtures of four cultured algal species (Synechococcus bacillaris, Isochrysis sp., Dunaliella tertiolecta, and Prorocentrum minimum) ranging in size from 1 to 11 m, small larvae preferred 1 m algae while large larvae preferred 11 m algae. In experiments with algal mixtures, and with suspensions of natural particles and added algae, large larvae preferred algal species harvested from exponential-phase cultures over other species from stationary-phase cultures. Larval ingestion rates of the cultured alga Thalassiosira pseudonana were about three times higher for cells with a low carbon:nitrogen ratio (7.2:1) than for high C:N ratio (16.2:1) cells when these cells were offered separately in suspensions of equal concentration. As a result, more algal cells, algal C, and algal N was ingested by larvae fed low C:N cells. However, larvae did not show a significant preference for either type of cell when they were offered in a 1:1 cell mixture. Feeding patterns of C. virginica larvae in natural food suspensions can vary with the composition of these complex suspensions, and ingestion seems dependent not only on the size, but on the growth rate and chemical quality of food particles.     

Feeding ecology ofLeitoscoloplos fragilis

The present study examines the effects of density ofLeitoscoloplos fragilis and of fine sediment on benthic microalgal abundance and production in laboratory microcosms, and the effects of fine sediment on diffusive transport of ammonia. Microcosms having different densities ofLeitoscoloplos fragilis (Verrill, 1873) were determined in sediment collected from one of two field stations (each containing a different amount of fine particles <125 m) from Cape Henlopen, Delaware, USA, in August 1986. The worms were acclimated in a recirculating seawater system for two months prior to experiments. Chlorophylla concentrations were highest in sediments with less fine particles (<125 m). Benthic diatom production, total microbenthic metabolic activity, and concentrations of pore-water ammonia were higher in sediment microcosms containing high densities of worms.L. fragilis grew more in microcosms containing less fine particles and higher worm densities. The upward flux of ammonia across the sediment-water interface was higher in sediments with less fine particles. A greater abundance of fine particles in these sediments impedes the upward flux of ammonia to surface and nearsurface diatoms. The coupling between population density and diatom production, which can be altered by fine-particle abundance could control the distribution and stability of populations ofL. fragilis.     

Measurement of in situ clearance rates of Oikopleura vanhoeffeni (Appendicularia: Tunicata) from tail beat frequency, time spent feeding and individual body size

In situ observations (1994 to 1996) of the behavior of Oikopleura vanhoeffeni Lohmann in combination with a previously published model can be used to give a good approximation of clearance rates. The model is based on tail beat frequency, time spent feeding and subsequent measurements of individual size (trunk length). These estimated clearance rates are in close agreement with clearance rates determined in the laboratory under static and flow-through conditions. A comparison of various techniques (such as measurement of gut pigment, and various particle removal and particle uptake methods) with the behavior-model approach, revealed a convergence of all rates within a threefold range. Most of this discrepancy can be explained by the number of non-feeding individuals and the deterioration of the feeding filters in the field. The main reasons for the similarity of the various clearance rate estimates are the low variability of the behavior of appendicularians in response to environmental variables, such as temperature and particle concentration, and their non-selective retention of particles. The suggested mechanistic approach has great value for estimating the flux of material and energy through populations of appendicularians for which only size and abundance data exist, or for appendicularian species that cannot be assessed empirically. Received: 26 August 1997 / Accepted: 28 September 1998     

Grazing in juvenile stages of some estuarine calanoid copepods

The grazing of juvenile Eurytemora affinis, Acartia tonsa and A. clausi from the Chesapeake Bay (USA) was investigated using natural particle distributions and freshly caught copepods, live-sorted into stages. Data were analyzed in 110 size channels using an electronic particle counter, and filtering rates (FR) were estimated based on total particle removal (mean FR), and that for each size channel (giving maximum FR). Mean and maximum filtering rates increased from NVI (Nauplius Stage VI) through CVI (Copepodid Stage VI). Both rates plotted against weight satisfied a log fit best for A. tonsa, and a linear fit best for E. affinis. Results for A. tonsa were quite variable, apparently due to differences in temperature between experiments. Particle selection was investigated from the shape of the filtering rate curve over particle size. We define selective feeding by a FR curve which is higher in some size categories, and non-selective feeding by a flat FR curve. The general pattern was one of selective feeding in all copepodid stages of the three calanoid copepods investigated. E. affinis tended to track biomass peaks while Acartia spp.'s feeding was more variable, including feeding in size ranges of greatest particle concentration, on larger particles, and in other size categories as well. Experiments with nauplii tended to yield flat FR curves, and it may be that selective grazing appears with, or is greatly accentuated by, metamorphosis from NVI to CI (Copepodid Stage I).University of Maryland, Center for Environmental and Estuarine Studies Contribution No. 762.     

Feeding,tube-building and particle-size selection in the terebellid polychaete Eupolymnia nebulosa

The present study was aimed at obtaining new information on the feeding biology of the terebellid polychaete Eupolymnia nebulosa (Montagu). Feeding, tube-building, particle-size selection, and tentacular morphology are described. Experiments were carried out during the fall of 1984 on specimens collected in the vicinity of Banyuls-surmer, France. Feeding activity takes place almost entirely in darkness, and E. nebulosa shows a marked preference for smaller particles when feeding and selects larger ones for tube-building. The control of its activity pattern by light intensity may result in seasonal modification of its energy balance. The mechanisms controlling particle-size selection in this species are discussed.     

Modification of the feeding behavior of marine copepods by sub-lethal concentrations of water-accommodated fuel oil

The feeding behaviors of Acartia clausi and A. tonsa were measured in samples of water containing low levels of a water-accommodated fraction of No. 2 fuel oil. The copepods fed normally at a hydrocarbon concentration of 70 g l^-1, but their feeding behavior was altered both quantitatively and qualitatively at a concentration of 250 g l^-1. Three types of response to the higher oil level were found. The first was total suppression of feeding. Both other types involved suppression of feeding on particles between 7 and 15 m diameter, but one showed no change in the ingestion of larger particles, whereas the other displayed increased feeding on particles larger than 15 m diameter. These results suggest that the species of Acartia studied use three different modes of feeding, each on a different size range of particulate material. Low-level hydrocarbon pollution affects each feeding mode differently.Contribution No. 973, Center for Environmental and Estuarine Studies of the University of Maryland     

Digestive mechanics and gluttonous feeding in the feather starOligometra serripinna (Echinodermata: Crinoidea)

The movement and digestion of food in the gut ofOligometra serripinna (Carpenter) were studied at Lizard Island (14°3842S; 145°2710E) in the austral winter of 1986. Feather stars in the laboratory were fed a brief, small meal of brine shrimp nauplii and killed at increasing time intervals thereafter. Histological reconstructions showed that the ingested nauplii progressed along the digestive tract surprisingly quickly. Some nauplii were found in the mid and hind intestine in only 30 min, and all of the nauplii had reached the hind intestine and rectum in 1 h. Digestion of the nauplii had started at 1 h, and only a few fragments of naupliar exoskeleton remained in the hind intestine and rectum 5 h after the start of feeding. Videotape analysis showed that no fecal pellets were released during this experiment. In the natural environment ofO. serripinna, ingested particles may similarly be transported quickly to the hind part of the gut and digested there — when feather stars were fixed in the field, most of the gut contents were found in the hind intestine and rectum.O. serripinna, which efficiently rejects inert particles before they are ingested, usually defecates infrequently (probably not more than once over a span of many hours) and differs from some other feather stars that ingest numerous inert particles and defecate much more frequently. When specimens ofO. serripinna were fed continuously on brine shrimp nauplii,Artemia sp. (San Francisco strain), in the laboratory, the feather stars fed gluttonously, packing their guts with several hundred nauplii in 1 to 2 h. Thereafter, superfluous feeding began (i.e., further ingestions appeared to force undigested nauplii, some of them still living, out of the anus). These observations suggest thatO. serripinna usually feeds at relatively modest rates in its natural habitat, but can feed gluttonously to take advantage of infrequent patches of highly concentrated, nutritious particles (e.g. copepod swarms, migrating demersal zooplankton, and invertebrate gametes from mass spawnings). It is likely that such patches of nutritious particles are usually small enough to drift out of reach of the feather stars before gluttonous feeding proceeds to superfluous feeding. Opportunities for superfluous feeding in nature are probably very infrequent (e.g. ingestion of coral gametes and embryos after a mass spawning), and the feather stars evidently have no behavior that stops further ingestions after the gut becomes filled to capacity.     

Factors influencing particle selection and feeding rate in the polychaete Cistenides (Pectinaria) gouldii

Feeding behavior of the deposit feeding polychaete Cistenides (Pectinaria) gouldii was examined to determine factors affecting particle selection and feeding rate. Worms were found to select large particles preferentially and particle size selection increased with worm size. Particle selection behavior was unaffected by changes in sediment bacterial abundance. Feeding rates were affected by sediment size, bacterial density and worm size. Generally feeding rates increased in sediment containing more food, although the response was worm size specific. When viewed in a theoretical construct these results were inconsistent with predictions of deposit feeder optimal foraging models. Alternative explanations, such as morphological constraints placed upon the polychaete, may explain C. gouldii feeding behavior.     

Experimental analysis of byssus thread production by Mytilus edulis and Modiolus modiolus in sediments

Laboratory experiments have been conducted on byssus thread production by two species of mussel, Mytilus edulis L. and Modiolus modiolus (L.), representing the epibyssate and endobyssate species of the Mytilidae, respectively. Mussels were placed in seven particle sizes of sediment ranging from 50 m to 16 mm for 12 d. The number of byssus threads per mussel, length of threads, number of threads per particle and size of pads were then recorded. Modiolus modiolus (endobyssate) produced more threads than Mytilus edulis (epibyssate). M. edulis produced most threads in the size range 2 to 16 mm and Modiolus modiolus in the size range 500 to 1000 m. M. modiolus produced longer threads than Mytilus edulis. Both species produced longer threads in particle sizes finer than 2 mm. M. edulis produced smaller pads than Modiolus modiolus. Both species produced larger pads in sediments coarser than 1 000 m. Results were also analysed in terms of attachment units (a thread attached to one or more particles, or a particle to which more than one thread is attached). The thread:particle ratio of the attachment units ranged from ca. 1:10 in the finest sediments to ca. 10:1 in the coarsest sediments. Mytilus edulis attached more threads to single particles (72% of attachment units) than Modiolus modiolus (37% of attachment units). M. modiolus had a wider spread of ratios in the finer particle sizes than Mytilus edulis. The significance of our results are discussed in relation to the ecology and palaeoecology of epibyssate and endobysste species.     

Herbivory and detritivory among gammaridean amphipods from a Florida seagrass community

The gammaridean amphipods Cymadusa compta (Smith), Gammarus mucronatus Say, Melita nitida Smith and Grandidierella bonnieroides Stephensen from a seagrass community in the Indian River estuary of Florida (USA) fed variously upon large drift algae, small algae epiphytic on seagrasses and seagrass leaf debris and detritus. Consumption was measured in the laboratory using an index (CI) equivalent to mg ingested mg^-1 amphipod day^-1. Observations revealed that the amphipods fed by macrophagy, an attack upon large algae and seagrass debris, and by microphagy, small particle detritus feeding and scraping of plant surfaces for diatoms and other epiphytic algae. C. compta was a macrophagous feeder with a generalized diet of algae and seagrass debris, preferring epiphytic algae and drift algae at mean rates of 1.10 and 0.87 CI, respectively. Gammarus mucronatus fed upon epiphytes and seagrass debris equally at mean rates of 0.90 and 0.97 CI, respectively. The diet of M. nitida condisted primarily of epiphytes, consumed at an average rate of 1.05 CI. Grandidierella bonnieroides fed in a specialized microphagous manner, grooming plant surfaces for small particle detritus and diatoms at an approximate CI rate of 1.45. Assimilation of plants ingested, as reflected by carbon-14 uptake, varied similarly among the 4 amphipods. Epiphytic algae appeared to be most useful as food, providing means of 41 to 75% carbon-14 uptake as ingesta. Drift algae and seagrass debris were of less value, with means varying between 11 and 24 % of carbon-14 uptake by the amphipods. The data show a pattern of feeding which resembles resource partitioning of food both by size and kind. Other evidence, however, including population limitation by predators and an apparent overabundance of food, indicate that resource partitioning as seen may be an artifact, and one which has no co-evolutionary basis among the present species.Contribution No. 102 of Harbor Branch Foundation, Inc.