Notes: Abstract Efficient grazing by marine bivalve larvae has been thought to be limited to particles larger than 4 μm in diameter, thereby eliminating photosynthetic and non-photosynthetic picoplankton as contributers to larval diets. Documentation of ingestion, carbon retention and growth of laboratory-reared larvae of the bivalve Mercenaria mercenaria L. on Synechococcus sp. (WH7803), a small unicellular cyanobacterium 1 μm in diameter, was facilitated using 14C-labelled cells in pulse/chase experiments and growth of larvae on diets of cell mixtures of both Synechococcus sp. and the haptophyte Isochrysis aff. galbana (TISO). Clearance rates on Synechococcus sp. ranged between 2 and 23 μl larva-1 h-1 depending on ambient cell concentration and larval age. Retention efficiency of cell carbon after gut evacuation was about 55% for both prey species. Growth rates of larvae fed on monocultures of Synechococcus sp. at typical summer concentrations in coastal waters (1×105 cells ml-1, ∼29 μg C l-1) was two-fold lower than on monocultures of Isochrysis galbana at 1×104 cells ml-1 (∼120 μg C l-1). Larval growth was inhibited and atrophy of the digestive gland was observed when Synechococcus sp. was offered at concentrations at or exceeding 8.6×105 cells ml-1. Larval growth was enhanced, however, in the presence of Synechococcus sp. (5×104 cells ml-1) when Isochrysis galbana was limiting. During the diurnal study of Synechococcus sp. population dynamics conducted by Waterbury et al. (1986) in Vinyard Sound, Massachusetts, the abundance of bivalve larvae was sufficient to account for 12 to 24% of the calculated grazing activity on Synechococcus sp. When nanoplankton are scarce, invertebrate larvae may exert considerable grazing pressure on Synechococcus sp. and derive benefit from ingestion of these cyanobacteria.

Notes: Abstract In January and February 1992 an experiment was conducted in a 10.5-m deep tank (diameter: 3.7 m, volume: 117 m3) to examine the effects of food distribution with respect to a stable thermocline, depth, and substratum type on the settlement and metamorphosis of larvae of the giant scallop, Placopecten magellanicus (Gmelin). Polyethylene tube bags (diameter: 0.60 m) were used to enclose 9-m deep columns of seawater which were then used as treatment replicates. A sharp thermocline (i.e. 7 to 11°C gradient) was created between a depth of 4.0 and 5.0 m. At the beginning of the experiment, one million 6-d old larvae were added to the surface of each tube. Two or three replicate tubes of each of four feeding treatments were established: (1) food (Isochrysis galbana) added to the top 1 m of the water column (“top-fed”, n=3); (2) food added to the bottom 1 m of the water column (“bottom-fed”, n=3); (3) food added throughout the water column (“mixed”, n=3); and (4) no food added (“unfed”, n=2). Settlement collectors were placed in two replicate tubes of each treatment at depths of 0.1, 4, 5, and 9 m and contained two different substrata, Polysiphonia lanosa (a red filamentous alga) and aquarium filter-wool as an algal mimic. Spat settlement in the different feeding treatments was a function of larval growth rate. Most spat were collected in the mixed tubes. Fewer individuals were collected in the top-fed treatment and fewer still in the bottom-fed treatment; minimal numbers of spat were found in the unfed tubes. Filter-wool collected more spat than P. lanosa, but this was evident only in the 4-m deep collectors in the mixed tubes. Most spat were found in the 0.1-m or 4-m deep collectors; generally few were located below the thermocline in collectors at 5 or 9 m. We suggest that, in areas of intense stable stratification, spat collection of the giant scallop may be enhanced by the placement of collectors with appropriate substratum material at or above the zone of stratification, rather than near the bottom. Furthermore, we propose that natural settlement may be increased in areas where a stratification layer intersects with the sea floor or where the layer is disrupted by turbulent mixing.

Notes: Abstract Food limitation is likely to be a source of mortality for fish larvae in the first few weeks after hatching. In the laboratory, we analyzed all aspects of foraging in cod larvae (Gadus morhua Linnaeus) from 5 to 20 d post-hatching using protozoa (Balanion sp.) and copepod nauplii (Pseudodiaptomus sp.) as prey. A camera acquisition system with two orthogonal cameras and a digital image analysis program was used to observe patterns of foraging. Digitization provided three-dimensional speeds, distances, and angles for each foraging event, and determined prey and fish larval head and tail positions. Larval cod swimming speeds, perception distances, angles, and volumes increased with larval fish size. Larval cod swam in a series of short intense bursts interspersed with slower gliding sequences. In 94% of all foraging events prey items were perceived during glides. Larval cod foraging has three possible outcomes: unsuccessful attacks, aborted attacks, and successful attacks. The percentage of successful attacks increased with fish size. In all larval fish size classes, successful attacks had smaller attack distances and faster attack speeds than unsuccessful attacks. Among prey items slowly swimming protozoans were the preferred food of first-feeding cod larvae; larger larvae had higher swimming speeds and captured larger, faster copepod nauplii. Protozoans may be an important prey item for first-feeding larvae providing essential resources for growth to a size at which copepod nauplii are captured.

Notes: Abstract Suspension feeding is an important mechanism by which many marine organisms obtain food. A nonparametric method for analyzing experimental results on capture efficiency in suspension feeding is described. This approach can be used to estimate capture efficiency along the length of a cilium and to compare the capture efficiencies of two different cilia. The method is applied to experimental data on feeding by larvae of the clamMerenaria merenaria (Linné), to examine changes in capture efficiency through time, and to assess the effect of particle electrostatic charge on capture efficiency.

Notes: Abstract To understand how thermal stratification and food abundance affects the vertical distribution of giant scallop larvae Placopecten magellanicus (Gmelin), a mesocosm study was conducted in January and February 1992. The position of larvae was followed over 55 d in replicated 9-m deep tanks in relation to a sharp thermocline and the presence or absence of phytoplankton. Growth and vertical position of larvae were monitored in separate treatments which included phytoplankton added above the thermocline, below the thermocline, throughout the mesocosm, or absent from the mesocosm. Changes in the vertical position of larvae over time were quantified with a new, profiling, video-optical instrument capable of semi-automatically identifying, counting and sizing larvae. The strong diurnal migration of scallop larvae resulted in aggregations at two interfaces: the air/water interface during the night, and at the thermocline during the day. At times, the concentration of larvae within cm of the surface was 〉 100 times that in the remaining water column. The formation of bioconvective cells of swimming larvae at the air/water interface allowed larval aggregations to persist throughout the period of darkness. Regardless of the distribution of food, larvae remained above the thermocline during most of the experiment. Therefore, only in those treatments where food was also present above the thermocline was larval growth relatively high. Larger larvae penetrated the thermocline only after reaching a shell length of about 200 μm; thus larval size, rather than chronological age, was more important in describing their vertical distribution. The rapid increase in kinematic viscosity with decreasing water temperature at the thermocline may retard the movement of larvae and contribute to aggregation at this interface. The influence of larval size on their vertical distribution, and the resulting potential for horizontal transport to settlement sites, points to the importance of persistent hydrographic features as critical factors contributing to settlement variance in scallops.