New observations of the gills of Placopecten magellanicus (Mollusca: Bivalvia), and implications for nutrition

The organization, general anatomy, and surface microanatomy of all regions of the gills of a representive bivalve mollusc, Placopecten magellanicus Gmelin, were studied using stereo-microscopic, histological, and scanning electron microscopic techniques. Individuals were collected in May and November 1985 from Chamcook Bay, New Brunswick, Canada. In addition to correcting earlier accounts of this structure, a number of new observations are reported. The orientation of the ciliated spurs appears to be responsible for the sinusoidal arrangement of the gill filaments. Micrographs showing the structure of the dorsal respiratory expansion are presented. The entire abfrontal surface of the principal filament, including the dorsal respiratory expansion, is densely ciliated and mucosecretory. These characteristics may aid in the establishment of a respiratory current and in the prevention of gill damage during escape responses. All nonciliated regions of the gill filaments are covered with microvilli, thus greatly increasing the surface area of the gill. The feeding mechanism is discussed in relation to the dorsal and ventral ciliated tracts. Symbiotic ciliate protozoans are constantly dislodged from the gill filaments and transported via the ventral mucus string to the buccal region. The nutritional implications of these observations are discussed.     

Fluid motion and particle retention in the gill of Mytilus edulis: Video recordings and numerical modelling

Particle trajectories of 6.4m Latex spheres were recorded by video, both near an isolated blue mussel, Mytilus edulis, gill filament and, in place of an intact interfilamentary canal, in a model canal of width 200, 100 or 70m, formed by a transparent plate positioned next to a gill filament. Each arrangement was placed in a 2 x 10 x 10 cm test vessel filled with seawater. Serotonin (nerve-transmitter) stimulation was used to activate lateral cilia and to either lock latero-frontal cirri at the end of an active stroke (10^-5 M), or to activate them (10^-6 M), yielding lateral cilia beat frequencies of 19 and 16 Hz, respectively. With latero-frontal cirri locked, image analysis of particle tracks gave maximum velocities of ca. 2.9±0.2mm s^-1 close to the tips of lateral cilia, for both isolated filament and model canal cases. Experimental velocity profiles along the 200-m wide model canal were recorded and used as good approximations to the fluid velocity because of the low Reynolds number. A two-dimensional steady model was proposed for the gill pump, assumed to only comprise lateral cilia. This model was solved numerically for the experimental model, canal in the vessel and the results showed satisfactory agreement with experimental volocity profiles from particle tracks. The numerical approach was also applied to a model of a single interfilamentary canal in the vessel. The resulting mean velocity in the canal was 1.70 mm s^-1, but the resistance to flow in the model was less than that in an intact mussel gill. Video graphs of particle tracks indicated that active latero-frontal cirri play a role in the transfer of particles from through current to frontal current, probably by means of a strong interaction through the motion of intervening fluid rather than through a direct physical contact. M. edulis specimens used in the present study were collected in 1990 at Helsingør and in 1991 at Kerteminde, Denmark.     

Ultrastructure of the gill of the hydrothermal-vent mytilid Bathymodiolus sp.

Specimens of Bathymodiolus sp. were collected at 2 620 m depth during the Biocyarise Cruise (12°5880N; 13°5660W) in March 1984, and samples of the gill were fixed for ultrastructural observations. The gill of this hydrothermal-vent mytilid constitutes the main organ in the nutritional processes. The lamellae display abundant ciliation, normally comprised of frontal cilia, compound latero-frontal cirri and lateral cilia. At the ventral margin of each demi-branch, a longitudinal, ciliated, feeding groove is present. the lamellae are composed of numerous homorhabdic filaments connected by tufts of cilia. Each filament is made of a thin wall overlying a central lumen containing amoebocytes. Ultrastructural observations revealed the filament wall to be composed of four types of cells: (1) The ciliated cells of the frontal, latero-frontal and lateral ciliation, characterized by an abundance of mitochondria. (2) Mucous cells present to some degree among the ciliated cells, but more abundant on the distal edge and containing dense droplets of mucus. (3) Cells colonized at their apical pole by numerous bacteria enclosed in membrane-delimited clear spaces and composing the major part of the filament wall. (4) Thin ciliated cells separating the bacterial cells and characterised by a dense fringe of microvilli at their apical pole. The lumen of the filament contains amoebocytes of different morphological aspects which seem to accumulate electron-dense granules, possibly related to detoxification processes.     

Particle capture in the musselMytilus edulis: The role of latero-frontal cirri

Microscope video graphs of particle paths near one-filament-thick mussel gill preparations, stimulated with a nerve transmitter (10^–6 M serotonin which restores normal ciliary activity), were used to disclose the capture of 6 m algal cells. Suspended algal cells carried with the water were stopped for a while at the entrance to the interfilament gap by the action of the latero-frontal cirri (Ifc), and transferred to the frontal side of the filament to be transported towards the marginal food groove. The event of transfer took place during approximately a time interval of 1150 to 1/25 s. To gain a better understanding of the capture mechanism and retention efficiency versus particle size, the flow through and around the Ifc was theoretically estimated. Normally beating Ifc create periodic, unsteady, three-dimensional flows at the entrance to the interfilament canal. During the active beat most of the water is deflected to flow around the branching cilia of the Ifc while some of the water is strained by these. Large particles (> 4 m) are stopped and transferred to the frontal current, whereas smaller particles either follow the flow around the Ifc and escape or they are stopped by the branching cilia.     

Particle capture in the crown of the ciliary suspension feeding polychaete Sabella penicillus: videotape recordings and interpretations

An experimental setup was designed for in situ videotape recording of the particle capture process in the crown of the polychaete Sabella penicillus. Intact individuals of S. penicillus (collected in the Gullmornfjord, Sweden in 1992) were exposed to either 6 m Latex spheres or Rhodomonas sp. flagellate cells (6 m). The capture of the added particles was recorded on video. From frameby-frame analyses particle velocities were estimated and the shape of the three-dimensional particle paths was inferred. The mean velocity of particles approaching the crown was estimated to be ca. 1 mm s^-1, increasing to ca. 1.7 mm s^-1 in the interpinnule channel. At the moment of capture the particles were seen to follow a curved, near circular path close to the tips of the latero-frontal cilia. The transport velocities on the frontal side of the pinnules and filaments were estimated to be up to 0.15 and 0.5 mm s^-1, respectively. Counting captured particles relative to particles arriving within the area of the pinnules gave a rough, direct estimate of nearly 100% retention rate when the polychaete was feeding undisturbed. Together with results from clearance measurements in the literature this implies that the worm is able to capture particles down to 3 m entering the interpinnule channel almost 100% effectively. In view of the 80-m wide interpinnule channel and 40-m spacing between the tips of the latero-frontal cilia on both sides of the channel, this result cannot be explained by mechanisms based solely on direct mechanical contact between cilia and particles but must involve fluid mechanical mechanisms. The present work is the experimental basis for ongoing numerical simulations of the particle motion in the interpinnule channel.     

Acquisition of particle processing capability in juvenile oyster <Emphasis Type="Italic">Crassostrea gigas</Emphasis>: ontogeny of gill mucocytes

Acquisition of particle processing capability in postlarval oysters depends upon the structural development of the pallial organs, as well as the development of cilia and mucocytes used (either directly or indirectly) in particle capture and transport. Mucocyte mapping was therefore used to identify mucocyte types and distributions throughout gill development in juvenile oyster Crassostrea gigas (Thunberg 1793) specimens from 2.9 mm to 2.4 cm in shell length. Three categories of gill filaments were identified: apical, lateral and principal filaments, corresponding to filament location or future location in gill plicae. Mucocyte densities were recorded per linear μm (l μm) of frontal surface, and converted to potential total volumes, using the mean volumes of each of the two major mucocyte types: acid mucopolysaccharide (AMPS)-mucocytes and mixed mucopolysaccharide (MMPS)-mucocytes. While AMPS secretions were dominant up to 1.0 cm (flat homorhabdic gill, to semi-heterorhabdic differentiation and plication), MMPS secretions increased progressively, dominating in 2.4 cm and adult specimens (fully heterorhabdic and plicated). Mucus composition, and hence mucus viscosity, thus appears to evolve in relation to the degree of enclosure of the gill frontal surfaces. Total (AMPS + MMPS) potential mucus secretion increased allometrically with juvenile growth, characterized by a sharp increase between 10 and 24 mm shell length, suggesting a marked improvement in particle processing capability. Mucocyte distributions on the gill were heterogeneous from the onset of heterorhabdic differentiation (7.5 mm): the apical filaments of the plicae contained much greater mucocyte total volumes, compared to the lateral and principal filaments. In addition to mucus composition, total potential mucus volume thus also evolved in relation to the degree of enclosure of the gill frontal surfaces. These results show that functional specialization in mucocyte distribution precedes the complete anatomical heterorhabdic differentiation. The completely functional adult gill system is thus attained in 2.4 cm juveniles. This information should be of use in understanding the dynamics of juvenile feeding, growth, and mortality, both in natural systems and in rearing operations.     

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.     

Functional morphology and feeding behavior of Paraprionospio pinnata (Polychaeta: Spionidae)

The functional morphology and feeding behavior of Paraprionospio pinnata (Ehlers) were studied. The tentacular feeding palps of P. pinnata possess four groups of cilia: laterals, latero-frontal cirri, frontals and basal transverse rows. The lateral cilia beat in metachronal waves creating in current that flows toward the frontal surface of the palp. The latero-frontal cirri deflect suspended particles onto the frontal surface and potentially resuspend deposited particles. The frontal cilia line the groove of the palp and transport particles to the ciliated pharynx. The basal transverse cilia also beat in metachronal waves and together with the frontal cilia of the branchiae produce a U-shaped current that removes particles rejected by the pharynx from the burrow. The significance of particle selection at the site of the pharynx in spionid polychaetes is hypothesized to be constrained by the interaction of specific morphologies and behaviors that reduce the effects of epifaunal predation and browsing. The relationship between sediment permeability and type of respiratory or ventilation current is hypothesized to be a potentially important factor in understanding the distribution of spionid polychaetes.     

Effects of exposure to petroleum hydrocarbons on the gill functions and ciliary activities of a marine bivalve

The effects of long-term exposure to low levels of water-accommodated fractions of Kuwait Crude oil, or to shortterm exposure to chemically dispersed oil, on the gill performance of the clam Venus verrucosa were investigated. Reduced pumping activities of the lateral cilia as well as interference with the normal beating activities of the eulaterofrontal cirri led to reduced clearance rates and retention efficiencies of food particles less than 6 m in diameter. On the other hand, frontal ciliary activities were significantly accelerated, while any retained oil droplets were conducted to the mouth region as food particles. The activities of terminal and sensory cilia were also enhanced and mucus production increased. The significance of these responses to the clam's energy budget is discussed.     

Form and feeding mechanism of a living Planctosphaera pelagica (phylum Hemichordata)

We describe aspects of the anatomy and suspension-feeding mechanism of a single Planctosphaera pelagica captured from the plankton in June 1992 off Bermuda in the western Atlantic. We also describe several unusual features of the larva, including its occurrence in surface waters, unusually large size, and limited swimming ability. Our account of the form and feeding behavior of P. pelagica is the first based on observations of a specimen captured and observed alive. Our limited observations suggest that the planctosphaera may use a suspension-feeding mechanism much like that of the other feeding deuterostome larvae (the pluteus and bipinnaria larvae of echinoderms and the tornaria larva of enteropneust hemichordates) known to capture food particles using a single ciliated band. Although we could not observe cilia directly, the movement of dye streams and food particles and the structure of the ciliated band suggest that some particles may be captured at the ciliated band by the reversal of ciliary beat. The planctosphaera possesses many prominent mucous glands near the food grooves. This suggests an important role of mucus in the biology of the larva, but we were not able to observe directly any role of mucus in particle capture.     

Particle interception,transport and rejection by the feather star Oligometra serripinna (Echinodermata: Crinoidea), studied by frame analysis of videotapes

Suspension feeding by a stalkless crinoid (Oligometra serripinna) was studied at Lizard Island, Australia, in 1985. The crinoids were placed in a laboratory flume with a slow, unidirectional current of seawater. Nutritive and non-nutritive particles (15 to 180 m) were introduced upstream from the crinoid, and feeding behavior was recorded at high magnifications on videotape for frame analysis. These direct observations showed that each intercepted particle (whether a dejellied clam egg, Sephadex bead or latex sphere) contacts a single, evidently adhesive tube foot and is rapidly transferred to the pinnular food groove by a bend of the tube foot. The tube foot bends in about 0.1 s and returns to its extended position in 1 to 2 s. Spheres less than 20 m in diameter cause only the intercepting tube foot to bend. In contrast, larger spheres cause the coordinated bending of the intercepting tube foot plus many of the neighboring tube feet: the stimulus spreads through the reacting group of tube feet at about 1 cm s^-1. After transfer to the pinnular food groove, the nutritive particles (dejellied clam eggs) travel at about 1 cm min^-1 to the arm axis and thence down the arm food groove at about 4 cm min^-1 to the mouth; in contrast, non-nutritive particles (Sephadex beads and latex spheres) are discarded from the pinnular food groove between 1 and 30 s after capture. Tube-foot bending is presumably triggered when arriving particles (whether nutritive or non-nutritive) are detected by sensory cells in the tubefoot epithelium: mechanoreception by itself appears sufficient to initiate bending, although chemoreception may modify the reaction. Then, soon after captured particles have been transferred to the pinnular food groove, the crinoid discards those judged unsuitable (probably by contact chemoreceptors in the food-groove epithelium). Clam eggs with intact jelly layers temporarily hang up on tube feet they contact and then float away in the curent: the jelly evidently interferes with mechanoreception and/or chemoreception by the tube-foot epithelium. Some previous studies of crinoid feeding have suggested that particles are trapped in extensive nets or strands of mucus: we found no evidence for this in O. serripinna, which captures particles predominantly be the direct interception method of the aerosol filtration model.     

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     

Ciliary feeding of tornaria larvae of Ptychodera flava (Hemichordata: Enteropneusta)

Ciliary feeding by tornariae of Ptychodera flava (Eschholtz) and other tornariae from the plankton is compared with Garstang's (1939) account of feeding by these larvae, which account contains errors, and with ciliary feeding by echinoderm larvae. Some details of ciliation are also described. As in echinoderm larvae, band cilia beat away from the food grooves and retain particles on the upstream side of the ciliated band, but tornariae use muscles less in ingestion and rejection. In early-stage and most late-stage echinoderm larvae, the ciliated band functions in both swimming and feeding, but in tornariae the ciliated band is arranged meridionally so that few portions of the ciliated band produce a posteriorly-directed current and a locomotory teletroch is needed for swimming. Faster-swimming tornariae observed in bowls achieved higher ingestion and clearance rates. These observations raise questions about form and function in the giant Planctosphaera pelagica. Cilia of the locomotory telotroch increase in length as tornariae increase in size, but there is little increase in length of cilia which capture food. Instead, the length of the ciliated band increases relative to other larval tissues by means of increasing convolution of the band. Hence, the volume of water processed for food probably increases relative to energy expended by larvae during development to the tentaculate stage. However, the length of the ciliated band may decrease relative to other larval surfaces with continued increase in size beyond this stage. These interpretations of growth and feeding efficiency are consistent with the reported geographic distribution of tornariae with and without tentacles.     

Histopathology of cerebro neuronal cells in freshwater snail Bellamya bengalensis: impact on respiratory physiology,by acute poisoning of mercuric and zinc chloride

Mercuric (Hg) and zinc (Zn) chloride toxicity was investigated in cerebroneuronal cells and gills of Bellamya bengalensis using sublethal concentrations under lab conditions. Freshwater snail B. bengalensis was exposed to mean LC₅₀ concentration (1.56 ppm and 12.7 ppm) of Hg and Zn chloride, respectively. Bioaccumulation of Hg and Zn was observed in nervous and gill tissue in proportion to the time of exposure. Respiratory mechanisms and rate of oxygen consumption was depleted by both metals. Histopathological alterations in cerebro neuronal cells (giant, large, medium, and small) and gill filamental epithelia were apparent in Hg and Zn-exposed snails. Histopathology demonstrated increased cytoplasmic basophilia, extreme indentation of plasma membrane, karyolitic and eccentric nuclei, nuclear envelope with irregular size, and shrunken appearance of cerebroneuronal cells. Histologically, gill filamental epithelia showed hypertrophy, enlarged ciliated margins reduced length of cilia, nuclear dilations, thickening of basal lamina, and hemocytic accumulations in induced cells and severe loss of goblet mucus cells at the tip. Histopathology was accompanied by dysfunctioning cilia with decreased rate of respiration. Overall, neuronal impairment with damaged gill filament produced improper gaseous exchange leading to sluggish movement.     

Relationships between porewater nutrients and seagrasses in a subtropical carbonate environment

The feeding, diet and egg production of the copepod Acartia tonsa were dermined during ten experiments in Los Angeles Harbor, California, between November 1986 and October 1987. Copepods were incubated in situ, in quasi-natural food environments. Water temperatures ranged from 14.6 to 21.5°C. Particulate organic carbon and nitrogen (POC and PON) were high (534 to 3710 g Cl^-1, 51 to 459 Nl^-1) but dominated by small (<8 m diam) particles. Plankton (phytoplankton and microzooplankton) C-biomass composed about 10% of the total POC and was usually dominated by particles >8 m. Plankton biomass was always low. Daily ingestion rates ranged from 3 to 96% of body C; egg production ranged from 4 to 35% of body carbon. Mean ingestion and egg production rates during spring-summer were 1.9 and 1.5 times higher than average for the entire study, respectively. The average gross efficiency of egg production for the study was 80%; the spring-summer mean was 41%. Bivariate and multiple-regression analyses revealed that the ingestion rate was dependent upon both temperature and food availability, but that, below 21°C, egg production depended more upon temperature than upon food concentration. To detect dietary preferences, the composition of diet was compared with that of the food supply. Selective feeding was infrequent, but the diet was often dominated by dinoflagellates and ciliates. It would appear that within metabolic limits governed by temperature, the feeding response of A. tonsa is dependent upon food concentration, while egg production depends more on qualitative attributes of the food supply.     

Microheterotrophic utilization of mucus released by the Mediterranean coral Cladocora cespitosa

The amount of mucus released by the Mediterranean coral Cladocora cespitosa (L.) was determined in laboratory experiments and the incorporation of mucus into bacterial biomass was investigated by means of incubation experiments in 1984. Mean mucus release was 8.5 g (mucus dry wt) pclyp^-1 h^-1 and amounted to 44% of the respiratory carbon losses of the coral since mean organic carbon content of freshly collected mucus was 102.2g C mg (mucus dry wt)^-1. Due to the abundance of C. cespitosa in the shallow littoral of the Bight of Piran, the energy content of mucus released is estimated to correspond to about 20% of the phytoplankton primary production in this area. Furthermore, the carbon conversion efficiency of 20% obtained from the bacterial population during decomposition of mucus indicates the high nutritional value of C. cespitosa mucus, although bacterial carbon onto mucus particles contributes less than 0.1% to the total organic carbon pool of the mucus.     

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     

Fluid mechanics of the mussel gill: The lateral cilia

Quantitative data on the water currents produced by the ciliary tracts of the gill filaments are needed to understand the fluid mechanics of suspension feeding in bivalves, as well as in other ciliary suspension feeders. This paper investigates the water currents produced by the bands of lateral cilia, as studied on isolated gill filaments, gill fragments, and intact gills of the mussel Mytilus edulis L with severed adductor muscles. The metachronally beating cilia produce oscillatory currents near the oscillating enveloping surface of the ciliary bands and rectilinear currents, the interfilamentary through-currents, farther from the surface. It is suggested that the oscillatory currents play an important role in the fluid mechanical capture of suspended particles. In the intact gill the interfilamentary currents pass the bands of lateral cilia at velocities that are two or more times higher than those generated by the bands of isolated filaments. The mussel gill is compared with an optimized peristaltic pump.     

Delivery of dietary components to larval shrimp (Penaeus vannamei) by means of complex microcapsules

Efficient delivery of nutrients is necessary for the successful study of aquatic larval nutrition. Conventional artificial food particles for larval shrimp (Penaeus vannamei) have poor water stability and poor nutrient retention or both. We developed a novel food particle type that retained low-molecular weight, water-soluble nutrients (vitamins and glucose) within lipid-wall microcapsules embedded with dietary ingredients in particles of gelled alginate-gelatin. The combination of lipid-wall microcapsules (LWMs) embedded in gelled food particles was termed complex microcapsules (CXMs). Eighty-five percent of ¹⁴C-activity associated with encapsulated ¹⁴C-glucose was retained by CXMs (after 18 h of suspension in seawater). Bioavailability of CXM-encapsulated molecules was demonstrated by release of encapsulated dye marker into the gut lumen of larval shrimp, and by uptake of ¹⁴C from encapsulated ¹⁴C-glucose. Minimum ingestion rates, calculated from ¹⁴C-uptake for larval shrimp (Mysis-1 through Postlarva-2) fed CXMs, ranged from 48 to 99 g dry wt larva^-1 d^-1, and were similar to literature values reported for ingestion of live rotifers by penaeid larvae. Complex microcapsules described in this study will be a valuable new tool for studying nutrition of suspension-feeders in that both micro-and macronutrients can be delivered to these animals by one particle type.     

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.