Bivalve Veligers Research Articles

An experimental evaluation of the efficacy of imaging flow cytometry (FlowCam) for detecting invasive Dreissened and Corbiculid bivalve veligers

Hassett W, Zimmerman J, Rollwagen-Bollens G, Bollens SM, Counihan TD. 2021. An experimental evaluation of the efficacy of imaging flow cytometry (FlowCam) for detecting invasive Dreissened and Corbiculid bivalve veligers. Lake Reserv Manage. 37:406–417. Zebra (Dreissena polymorpha) and quagga (D. bugensis) mussels, first introduced from central Asia into the Great Lakes of North America in the late 1980s, have crossed the continental divide and more recently spread across western North America. At the same time, several new technologies have been developed for the early detection of dreissenids, including the FlowCam, a digital imaging-in-flow instrument, intended to detect dreissenid planktonic larvae (veligers). However, the efficacy of this technology has rarely been tested. We experimentally evaluated the FlowCam’s ability to capture identifiable images of quagga mussel veligers under 2 different types of conditions: (i) deionized water, and (ii) Columbia River Basin water (CRBW), including natural sediment and native plankton. We further evaluated the FlowCam’s ability to distinguish between dreissenid veligers and corbiculid veligers (Asian clam, Corbicula fluminea). We interpret our results to indicate that the FlowCam can consistently detect dreissenid veligers across a range of veliger densities. Moreover, the presence of other plankton and detritus only slightly affected dreissenid detection by the FlowCam. However, the orientation of individual bivalve veligers as they were imaged by the FlowCam precluded specific identification of a substantial proportion (24.8%) of veligers as either dreissenid or corbiculid. We suggest that the FlowCam is an important detection tool best utilized as part of a multifaceted approach, including traditional microscopy and possibly environmental DNA.

Spatiotemporal patterns and impact of a cyclone on the zooplankton community structure in a brackish coastal lagoon

Zooplankton communities in response to hydrological (salinity, turbidity, water temperature, nitrate, phosphate, silicate) and biological (phytoplankton density) variables were studied in the Asia’s largest brackish water lagoon, Chilika (India). Water samples were collected from 13 stations on a monthly frequency between July 2014–June 2015. A total of 141 species of holoplankton including Ciliophora, Foraminifera, Rotifera, Rhizopoda, Gastropoda, Cladocera, Copepoda, Ostracoda, Malacostraca, Chaetognatha, Chordata, Nematoda, and 18 types of planktonic larvae were identified. Copepods were the most dominant and diverse zooplankton in terms of their species richness, numerical abundance, and widespread distribution in the lagoon. The major holoplanktonic taxa were Acartia sp., Acrocalanus gibber, Acrocalanus gracilis, Acrocalanus longicornis, Euterpina acutifrons, Lecane sp., Oithona sp., Pseudodiaptomus serricaudatus, and Tintinnopsis tubulosa. The dominant planktonic larvae were bivalve veliger, copepod nauplii, cyphonautes larvae, gastropod veliger, and protozoea of lucifer. Canonical correspondence analysis showed that distribution of zooplankton assemblages was influenced by salinity, turbidity, and phytoplankton density. Indicator analysis demarcated 26 indicator zooplankton taxa. Of these, the most prominent indicator zooplankton such as Belzebub hanseni, Mesopodopsis orientalis, Chydorus sphaericus, and Corycaeus catus were characteristic to salinity gradient in southern sector, sediment resuspension in central sector, eutrophic conditions in northern sector, and tide-induced intrusion of marine forms in outer channel, respectively. Hudhud, a category 4 cyclone resulted decline in the abundances of zooplankton population. Copepod nauplii and copepodites were dominant during post-Hudhud and their succession was linked to the increased phytoplankton density due to an upsurge in the nutrient level in the northern sector of the lagoon. The present study provided a comprehensive detail on the zooplankton composition and their environmental drivers including the impact of a tropical cyclone from a tropical coastal lagoon system.

Marine zooplankton of Coronation Gulf and Bathurst Inlet, Nunavut, Canada

Here we report a survey of marine zooplankton in Coronation Gulf and Bathurst inlet located in the Kitikmeot Region of Nunavut, Canada. Zooplankton were collected at nineteen stations with a 236 μm mesh net in September 2007. The zooplankton samples were examined for species composition, abundance and spatial heterogeneity. Taxa were characteristic of the Arctic zooplankton fauna but not distributed homogeneously. Overall, from west to east, the abundance of cyclopoid and harpacticoid copepods decreased as calanoid copepods increased. Multivariate cluster analysis revealed 5 sub-groups based on species composition and abundance which are located in West, Central, and Eastern Coronation Gulf, and mid- and southern Bathurst Inlet. The West group had three stations and a high number of the cyclopoid copepods Oithona similis and Triconia borealis. The Central group contained six stations, which were distinguished as having a mixture of Oithona and juvenile Pseudocalanus as well as high percentage of Calanus juveniles. One station of the Central group was found in northeastern Dolphin and Union Strait likely tracing the outflow of water from Coronation Gulf. The Eastern group had a high number of Pseudocalanus and Acartia juveniles. Acartia hudsonica, Eurytemora, and Tortanus discaudatus were found in Bathurst Inlet in association with conditions created by riverine freshwater input. Also, a high number of bivalve veligers were observed in the southern reaches of Bathurst Inlet. This report describes a heterogeneous distribution of typical Arctic zooplankton taxa in the region and serves as a baseline from which to measure change.

Diet, prey selection, and individual feeding rates of the jellyfish Lychnorhiza lucerna (Scyphozoa, Rhizostomeae)

Large scyphomedusae can deplete zooplankton communities when occurring in high densities and the assessment of their trophic impacts relies on basic information of the species’ feeding habits. We quantified in situ gut contents of the South American jellyfish Lychnorhiza lucerna Haeckel, 1880 and described the procedures to determine the diet, prey-selectivity patterns, and feeding rates of this species. Specimens were collected between 2008 and 2011 from surface waters along the southeastern coast of Brazil (23°–25°S, 45°–48°W), where they were immediately preserved simultaneously with plankton samples near aggregations of medusae. Most prey items (~ 70%) were extracted from the central cruciform stomach by rinsing, although ~ 16% remained in the gastric cavity even after several rinses. Non-digestive body regions (oral arms and umbrellar canals) accounted for a small proportion of the prey found (< 10%). Calanoid copepods were the most abundant (53%) prey, followed by cyclopoid (15.1%) and poecilostomatoid (11.4%) copepods and bivalve veligers (~ 7%). The dietary composition was mostly similar to the proportional abundances in the surrounding mesozooplankton. As medusa size increased, the proportion of calanoids increased, but dietary diversity decreased. The ingestion rates quantified did not supply the species minimum carbon requirements as estimated from oxygen consumption rates; therefore, nutritional resources (e.g., dissolved and particulate organic matter) in addition to mesozooplankton must be considered in further studies. We estimated that from 110 to 102,871 copepods were ingested daily by medusae (5–30 cm diameter), which indicates the species have one of the highest feeding rates among scyphomedusae. Therefore, the aggregations of L. lucerna along the southwestern Atlantic coast must be better studied to understand what are the predatory impacts and the role of this species in local production process.

Towards a ground pattern reconstruction of bivalve nervous systems: neurogenesis in the zebra mussel Dreissena polymorpha

Bivalvia is a taxon of aquatic mollusks that includes clams, oysters, mussels, and scallops. Within heterodont bivalves, Dreissena polymorpha is a small, mytiliform, freshwater mussel that develops indirectly via a planktotrophic veliger larva. Currently, only a few studies on bivalve neurogenesis are available, impeding the reconstruction of a ground pattern in Bivalvia. In order to inject novel data into this discussion, we describe herein the development of the serotonin-like and α-tubulin-like immunoreactive (lir) neuronal components of D. polymorpha from the early trochophore to the late veliger stage. Neurogenesis starts in the early trochophore stage at the apical pole with the appearance of one flask-shaped serotonin-lir cell. When larvae reach the veliger stage, four flask-shaped serotonin-lir cells are present in the apical organ. At the same time, the anlagen of the cerebral ganglia start to form at the base of the apical organ. From the apical organ, one pair of cerebro-visceral connectives projects posteriorly and connects to a posterior larval sensory organ that contains serotonin- and α-tubulin-like flask-shaped cells. Additional, paired serotonin-lir neurites originate from the apical organ and project into the velum. One unpaired stomatogastric serotonin-lir cell develops ventrally to the stomach at the veliger stage. The low number of serotonin-lir cells in the apical organ of bivalve veligers is shared with larvae of basally branching gastropods and scaphopods and is thus considered a feature of the last common ancestor of Conchifera, while the overall simplicity of the larval neural architecture appears to be a specific trait of Bivalvia.

Spatial patterns of mesozooplankton communities in the Northwestern Iberian shelf during autumn shaped by key environmental factors

The autumn mesozooplankton community structure and spatial distribution was investigated in relation to the prevalent hydrographic conditions at the Northwestern Iberian shelf (38.5°–41.6°N), a major spawning ground for small pelagic fish species. Copepods clearly dominated the mesozooplankton community, followed by Cladocera, Appendicularia, Cirripedia, Bivalvia larvae and Chaetognata. Three different groups were identified according to their distribution: a northern coastal group (A), a southern coastal group with two stations in the northern area (B), and a shelf group (C), each delimited to areas with distinct oceanographic conditions. The boundary between the coastal groups A and B was coincident with areas of prominent geographic features, particularly a submarine canyon (Nazaré canyon) and a cape (Cabo Carvoeiro), both inducing abrupt changes in the local/regional patterns of water mass circulation. Group B includes a few northern stations that can be related to river discharges while the southern stations can be defined by upwelling conditions at that time. The distribution of some relevant species can be related to biophysical variables (temperature, salinity, Chlorophyll a). Some copepod taxa were generally coincident with high concentrations of chlorophyll a abundance (e.g. Temora spp., Oncaea spp.), while the distribution of other groups such as Cladocera or bivalve veliger larvae was correlated to temperature and/or salinity gradients caused by river discharges. Other important taxa as Paracalanus and Clausocalanus spp. or Oithona spp. showed no particular influence of any of the measured hydrological variables. This work is the first description of mesozooplankton assemblages and their distribution pattern in a broad area off Western Iberia during autumn. It contributes to understanding the dynamics of zooplankton in the area, increasing the knowledge of the drivers of plankton community variability and providing information about their potential as food for small pelagic fish species spawning in the area throughout the season.

Veligers of the invasive bivalveLimnoperna fortuneiin the diet of indigenous fish larvae in a eutrophic subtropical reservoir

AbstractLarval fish development depends largely on their ability to capture and ingest food items, and on food availability. In this context, invasive species, eutrophication and river impoundments have complex impacts on fish larvae. Using samples collected in 2005–2009 in the Salto Grande reservoir (Argentina–Uruguay), periodically affected by cyanobacterial blooms, we studied the impact of the larvae of the exotic bivalve Limnoperna fortunei (Dunker, 1857) (Bivalvia) on larval fish diets. Compared with other nearby waterbodies, the abundance of fish larvae was scarcer in the reservoir, especially during algal bloom periods. Only 20% of the larval fish with gut contents fed on L. fortunei veligers. Seven fish taxa (of a total of 12) consumed veligers of L. fortunei, but only two showed a preference for this prey. Taxonomic changes in the larval fish assemblages due to the river's impoundment, and temporal uncoupling between veliger densities (affected by the toxigenic effects of Microcystisspp.) and ichthyoplankton could account for the comparatively low trophic importance of the invasive bivalve's veligers. These results reflect the complexity of interactions brought about when the same invasive species invades different environments, underscoring that the impacts involved depend as much on the invader, as on the regional and ecological settings of the area invaded.

Seasonal dynamics of meroplankton in a high-latitude fjord

Knowledge on the seasonal timing and composition of pelagic larvae of many benthic invertebrates, referred to as meroplankton, is limited for high-latitude fjords and coastal areas. We investigated the seasonal dynamics of meroplankton in the sub-Arctic Porsangerfjord (70°N), Norway, by examining their seasonal changes in relation to temperature, chlorophyll a and salinity. Samples were collected at two stations between February 2013 and August 2014. We identified 41 meroplanktonic taxa belonging to eight phyla. Multivariate analysis indicated different meroplankton compositions in winter, spring, early summer and late summer. More larvae appeared during spring and summer, forming two peaks in meroplankton abundance. The spring peak was dominated by cirripede nauplii, and late summer peak was dominated by bivalve veligers. Moreover, spring meroplankton were the dominant component in the zooplankton community this season. In winter, low abundances and few meroplanktonic taxa were observed. Timing for a majority of meroplankton correlated with primary production and temperature. The presence of meroplankton in the water column through the whole year and at times dominant in the zooplankton community, suggests that they, in addition to being important for benthic recruitment, may play a role in the pelagic ecosystem as grazers on phytoplankton and as prey for other organisms.

The effect of the ctenophore Mnemiopsis leidyi A. Agassiz, 1865 (Ctenophora: Lobata) on the population density and species composition of mesoplankton in inshore waters of the Crimea

This paper studies the qualitative and quantitative food composition of the ctenophore Mnemiopsis leidyi A. Agassiz, 1865, as well as its feeding intensity and predatory impact on selected species of mesoplankton in the inshore waters of the northwestern Black Sea in 2008–2010. Interannual and interspecies differences in the intensity of the predatory impact on zooplankton were found. Copepod nauplii, the cladocerans Penilia avirostris and Pleopis polyphemoides, and bivalve veligers experienced the highest predation pressure. The daily consumption rate for adult copepods did not exceed several percent of their total abundance. The mass development of M. leidyi defines the qualitative and quantitative composition of the zooplankton community through the control of the abundance of copepod nauplii and cladocerans, which, in turn, can result in changes in the trophic structure of the entire food chain.

Ontogenetic changes in predator–prey interactions between two species of larval fishes and oyster veligers

The naked goby (Gobiosoma bosc), striped blenny (Chasmodes bosquianus), and eastern oyster (Crassostrea virginica) are common resident species in temperate Atlantic tidal waters. All three species produce planktonic larvae, and interactions between their early life stages likely influence recruitment success. Fish predation on oyster veligers requires co-occurrence as well as a match between prey behavior, size, and morphology and the ability of the predator to capture and engulf the veliger (prey). Wild plankton, oyster veligers, and wild plankton enriched with oyster veligers (mixed) were fed to cultured larval fishes during laboratory experiments. Three hypotheses were tested: (1) no fish species-specific differences in oyster veliger consumption exist within similar predator–prey development stage combinations, (2) postflexion blennies and gobies consume more veligers than preflexion fishes, (3) veligers reach a size refuge from fish predation by the umbo developmental stage. The resulting data provide a quantitative description of larval fish feeding behavior with respect to fish (preflexion, postflexion) and veliger (straight hinge, umbo) developmental stage. Selectivity for bivalve veligers as well as ontogenetic effects on the predator–prey interaction were examined. Fish nests were cultured to provide larval fish predators of known age. Overall, 70% of striped blennies and 59% of naked gobies consumed at least one prey item during the 3h experiments. There were fish species-specific differences in oyster veliger consumption between striped blennies and naked gobies. Striped blennies consumed more prey items including a wider range of veliger ages than larval naked gobies of the same developmental stages, regardless of the prey treatment. Fish selectivity was high for copepod nauplii and bivalve veligers and usually low for polychaete larvae, barnacle nauplii, and diatoms. There was a significant positive relationship between standardized consumption and predator development stage in the wild treatment for both fishes and in mixed and veliger treatments for blennies. All fishes consumed straight hinge veligers more often than umbo stage veligers. The largest veligers appear to reach a predation refuge from both fishes. Larval gobies and blennies are generalist predators that use seasonal pulses of zooplankton prey within temperate estuaries. Potential mismatches between predator–prey spatial and temporal overlap that might reduce fish recruitment are likely offset by adult spawning strategies that result in multiple nests within a long spawning season.

Population dynamics of the ctenophore Mnemiopsis leidyi and its impact on the zooplankton in the coastal regions of the Black Sea of the Crimean coast in 2004–2008

The abundance, biomasses, and population structure of the ctenophore Mnemiopsis leidyi were monitored in the coastal waters of the northern part of the Black Sea (Sevastopol Bay and the adjacent continental shelf regions) in 2004–2008. The abundance and species composition of the comb jelly’s food in the sea were obtained along with experimental data on the digestion time. Based upon these data, the feeding intensity of the ctenophore in situ was estimated. This information was used to calculate the predatory impact of the comb jelly population on certain groups of forage organisms and the forage zooplankton community as a whole. The predatory impact of the M. leidyi population on the bivalve veligers was the highest (up to 90% of the abundance a day) compared to the Copepoda and Cladocera (30% and 40%, respectively). In the summers of 2004–2008, the daily consumption rates of the zooplankton by the ctenophore population in the shelf zone and in the bay were similar to each other: up to 15% and 12% of the abundance a day, respectively. The highest pressure of the ctenophore upon the zooplankton was observed in 2004 and 2008, when M. leidyi was especially abundant in the plankton for a long time.

Characterization of the central nervous system and various peripheral innervations during larval development of the oyster Crassostrea virginica

. Although a few investigations have considered neuronal aspects of bivalve larval ontogeny, the central and peripheral nervous systems of bivalve veligers have yet to be described in detail. This study provides a detailed description of central and peripheral components of the larval nervous system in the Eastern oyster, Crassostrea virginica. Larvae at the D-hinge, newly eyed and pediveliger stages were examined at the light level by means of serial histological section sets. D-hinge larvae were found to have limited neurogenesis with only an anterior gangliar rudiment being present. As development progressed to the newly eyed larval stage, a single apical ganglion and paired cerebro-pleural, pedal, and visceral ganglia were identified, forming a typical bivalve central nervous system loop. The nervous system of pediveligers was similar to that of newly eyed larvae, but added an additional accessory ganglion on the left and right sides, posterior to each gill rudiment. A posterior visceral commissure was also present. Various peripheral innervations were also documented, including nerves extending from the pedal and visceral ganglia in both newly eyed and pediveliger larvae. Furthermore, several neuronal processes, such as the mantle, ventral osphradial, anterior adductor and dorsal esophageal nerves were seen connecting to ganglia. This investigation provides an essential histological characterization of the complex nervous system present in bivalve oyster larvae.

Shifting abundance of the ctenophore Mnemiopsis leidyi and the implications for larval bivalve mortality

Along the mid-Atlantic coast of the US, the ctenophore Mnemiopsis leidyi (Agassiz) appears to be increasing in abundance and undergoing shifts in its historical seasonal distribution. We provide new data on shifting ctenophore abundance in Long Island estuaries and its implications for top-down control of the plankton community. Peak mean biovolume estimates of M. leidyi in Long Island estuaries in 2006 revealed ctenophore abundance values that were a factor of two to five times greater than previous studies conducted two decades ago. Furthermore, peak M. leidyi densities in 2006 occurred 2–3 months earlier than previously documented, suggesting a shift in the seasonal maxima of M. leidyi. Application of daily ingestion rates to zooplankton abundance indicates that, at its highest densities M. leidyi can remove an overall average of 20–89% per day of bivalve veligers and other zooplankton taxa, including adult copepods, nauplii, and tintinnids. Increasing ctenophore abundance, especially during a period when they were not historically abundant (i.e., June) may have significant consequences for species which spawn at this time. For example, current populations of M. leidyi represent a major source of larval mortality for bivalves which may inhibit recovery of shellfish populations and reinforce their low abundance state in Long Island estuaries.

Particle retention efficiency of asari clam Ruditapes philippinarum larvae

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 6:281-287 (2009) - DOI: https://doi.org/10.3354/ab00132 Theme Section: Directions in bivalve feeding Particle retention efficiency of asari clam Ruditapes philippinarum larvae Naoaki Tezuka1,*, Erika Ichisaki2,3, Masaei Kanematsu1, Hironori Usuki1, Masami Hamaguchi1, Kazuo Iseki2 1National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Maruishi 2-17-5, Hatsukaichi, Hiroshima 739-0452, Japan 2Hiroshima University, Kagamiyama 1-4-4, Higashi-hiroshima, Hiroshima 739-8528, Japan 3Present address: Nagasaki Prefectural Institute of Fisheries, Taira 1551-4, Nagasaki 851-2213, Japan *Email: tezukan9@fra.affrc.go.jp ABSTRACT: Larval food abundance is one potential cause of recruitment variation in benthic invertebrates, including bivalves. Bivalve veliger larvae can clear particles <~10 µm. However, the particle retention efficiency (PRE) of bivalve larvae differs among species, so is necessary to determine larval food abundance in nature. We investigated the PRE of larvae of the asari clam Ruditapes philippinarum, which recently has greatly decreased in numbers in Japan. Artificially hatched larvae from the umbo to the full grown stage were exposed to natural food assemblages. Larval clearance rates were determined for particles in 32 size ranges from 0.8 to 18 µm. We observed significant clearance of particles in the ~1 to 8 µm size range, with the highest clearance rates being on 1.4 to 2.0 µm particles. Larval ingestion of the natural food assemblages ranged from 50 × 102 to 120 × 102 µm3 ind.–1 h–1, which is below the hatchery food ration, suggesting larval food limitation in nature. KEY WORDS: Ruditapes philippinarum · Clam larvae · Clearance rate · Particle retention efficiency Full text in pdf format PreviousNextCite this article as: Tezuka N, Ichisaki E, Kanematsu M, Usuki H, Hamaguchi M, Iseki K (2009) Particle retention efficiency of asari clam Ruditapes philippinarum larvae. Aquat Biol 6:281-287. https://doi.org/10.3354/ab00132 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 6. Online publication date: August 11, 2009 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2009 Inter-Research.

Seasonal change of community structure and size spectra of zooplankton in the Kaw River estuary (French Guiana)

The zooplankton community of the Kaw River estuary in French Guiana and its relationship to its physical and chemical environment are described. Sampling was carried out in November 1998, June 1999, and November 2001, corresponding to the end of the dry season in 1998 and 2001 and to the end of the rainy season in 1999. The Kaw is a small coastal river that drains a vast swamp and flows into the Atlantic Ocean, close to the equator and to the mouth of the Amazon. The hydrodynamic and hydrochemical functioning of the estuary, and the development of littoral mangroves, is strongly constrained by the transport of sediments from the Amazon. During the rainy season the zooplankton, originating from the highly oligotrophic upstream sectors of the Kaw River, was of a strictly freshwater type. It was highly dispersed within the flooding water and dominated (in terms of density) by highly diversified rotifer taxa and (in terms of biomass) by cladocerans. During the dry season the estuary, turbid but enriched by exports from adjacent mudflats and mangroves, was colonized by a large and abundant zooplankton community that was dominated by tintinnids, with copepods as their main associated taxa. Exploiting the rich autochthonous phytoplankton and allochthonous phytobenthos, the microzooplankton components were only constrained when strong hydrodynamic exchanges allowed mixing between the opportunistic estuarine community and coastal mesozooplankton (copepods, chaetognaths, bivalve veligers). Between 1998 and 2001, these two communities were partially isolated from each other due to the gradual arrival of a mud bank, causing the blockage of the estuary. This isolation resulted in the under-exploitation of the microzooplankton. The intensification of tidal currents (spring tide) that occurred during the subsequent stabilization phase of the mudflat induced a more balanced zooplankton community. While the “estuarine” zooplankton of the Kaw River estuary therefore relies on the relative isolation of its water mass, its contribution to the coastal ecosystem also implies the existence of strong tidal currents that temporarily break this isolation.

SPATIAL DISTRIBUTION OF VELICHONCHA LARVAE (BIVALVIA) IDENTIFIED BY SSNM-PCR

Bivalve veligers are difficult to identify morphologically, and when it comes to strait hinge (~90 μm) or small velichoncha veligers (>90–300 μm) it is practically impossible. However, the use of new molecular based methods allows the identification of individual larvae, in a size independent manner (Larsen et al. 2005). In this contribution we show small-scale spatial variations among species of velichoncha veligers, in the size range of 120–135-μm shell length. Larvae were sampled quasi-simultaneously in a squared matrix at Løgstør Broad, Limfjorden, Denmark. The matrix consisted of 46 sampling sites, situated within 500–750 m apart. Velichoncha larvae were enumerated and sized. Larval concentrations ranged from <50 to >350 Ind. per liter, and found not to be coupled to any environmental parameter recorded. At 25 of the sampling sites larvae were identified to species/genera level by Singe Step Nested Multiplex PCR (SSNM-PCR). The bulk of the velichoncha was Mytilus edulis/Musculus marmoratus (52% to 91%), which was found to be present in all samples tested. In contrast Ensis spp. was observed in 24 of the tested samples, making up between 4% to 31%, whereas species of the order Myoida and species belonging to the Cardiidae family only was observed occasionally in low relative abundance. At a few sampling sites a difference in the vertical distribution among the composition of velichoncha assemblages was suggested. Hence, SSNM-PCR was useful for identifying evidence for spatial scale differences in species composition within bivalve larval assemblages. Because of the ecologic and economic importance of some species, there are significant perspectives in identifying bivalve veligers in large scale studies.

Stage-specific spatial and temporal variability in the diets of larval winter flounder (Pseudopleuronectes americanus) in a northeastern U.S. estuarine nursery

Collections of winter flounder (Pseudopleuronectes americanus) larvae were made biweekly from March to May in 1999 and 2000 in the Navesink River-Sandy Hook Bay estuarine system, New Jersey, to determine stage-specific spatial and temporal variability in diets. Relatively high percentages of larvae with empty guts were found at low water temperatures. Percentages of empty guts did not differ by larval stage (preflexion and postflexion) or region of collection (river and bay). There was high intraregional variability in percentages of larvae with empty guts. Nauplii, invertebrate eggs, and polychaete larvae were dominant prey items followed by tintinnids, bivalve and gastropod veligers, and diatoms. Ontogenetic dietary shifts were evident in both years. Preflexion winter flounder largely consumed nauplii, invertebrate eggs, and tintinnids; postflexion winter flounder consumed the largest prey (polychaete larvae) but also retained small prey items in their diets. Water temperature significantly affected the percentages of larvae feeding on nauplii (p<0.05) and tintinnids (p<0.08) in 2000. Region of collection was not significantly related to diets because of high intraregional variability. Fine scale spatial (within regions, stations were approximately 3 km apart) and temporal (weeks) dietary variation of larval winter flounder could result in accompanying variation in development, growth stage duration, and survival.

Observations on the distribution of meroplankton during an upwelling event

The distribution of the larvae of benthic invertebrates was investigated relative to hydrographic structures as a test of the hypothesis that larvae behave as if they are passive particles. Observations of larval and oceanographic distributions were made off Duck, North. Carolina, USA in August 1994. Conditions were characterized by wind-driven coastal upwelling; flow was generally offshore near the surface and onshore below the pycnocline. Within 5 km of the shore the pycnocline was bent upward by the upwelling and it intersected the surface along most of the transects. In zooplankton samples, 20 taxa of larvae were counted (10 bivalve veligers nine gastropod veligers and one polychaete larvae). Using cluster analysis, larvae were separated into groups with similar patterns of distribution and similar affinities to water properties. The larvae in Cluster 3 did not display a consistent distribution pattern beyond that they tended to be found in warmer surface waters. An earlier paper described the distribution of larvae in the same location during a downwelling event [A. Shanks et al. (2002) J. Plankton Res., 24, 391-416]. Two of the clusters identified during this previous study were quite similar in composition to Clusters I and 2 in this study. In both studies, Cluster I larvae were found below the pycnocline, but during the upwelling event they were transported shoreward with the advection of the subpycnocline waters by the upwelling circulation. Within 5 km of the shore Cluster 1 larvae were found at depths shallower than the base of the pycnocline and were often found in patches of high larval concentration. The patches were located where the waters were upwelling. Cluster 2 larvae were found within 5 km of the shore in both studies and tended to be highly concentrated in convergences or divergences, Larvae, in Cluster I generally appeared to be dispersing as passive particles, except within the zone of upwelling where they may have been swimming against the upwelling flow leading to higher larval concentrations, Cluster 2 larvae appeared to be consistently concentrated in areas of vertical currents, suggesting that they may be attempting to maintain a preferred depth in the face of the vertical flow which would lead to high larval concentration and nearshore larval distributions despite extensive cross-shelf movement of water. Despite their slow swimming speeds, the larvae in Clusters 1 and 2 were not swept offshore by the upwelling event.

Low rates of predation on planktonic marine invertebrate larvae

We conducted in situ observations and experiments to evaluate predation on inverte- brate larvae in near-natural plankton assemblages captured in large-volume in situ corrals. In these captured assemblages, we placed known numbers of marked larvae and determined their fate after 24 h. Recovery of marked larvae averaged 99% (±0.25%, SE), enabling a thorough and direct deter- mination of predators and predation rates. The highest predation rate observed was on bivalve veligers by the heterotrophic dinoflagellate Noctiluca scintillans (7% lost in 24 h). Gastropod veligers experienced no predation and echinoplutei experienced only rare predation. While N. scintillans preyed on bivalve veligers in some runs, most runs yielded little or no predation. These observations suggest that larvae can encounter relatively safe assemblages. To investigate low predation, corral experiments were conducted which presented prey at near-natural and unnaturally high densities and in the presence or absence of natural background plankton. Predation observed at high prey densities decreased or disappeared at near-natural prey densities. This suggests that low encounter rates may explain some of the low predation. Predation rates also decreased in the presence of nat- ural background plankton. Background plankton may occupy the predator's time and decrease opportunities for encounters with larvae, obscure larvae from detection or capture, or serve as sub- stitute food. Since predation was usually low or absent in diverse corral assemblages, we recommend confirming natural relationships and predation rates for suspected predator-prey combinations before making assumptions about predator effects. It may frequently be the case that planktonic larvae suffer little or no predation by planktonic predators.

Observations on the distribution of meroplankton during a downwelling event and associated intrusion of the Chesapeake Bay estuarine plume

We investigated the dispersal of larvae of benthic invertebrates and tested the hypothesis that larvae behaved as if they were passive particles. Observations were made off Duck, North Carolina, USA during a period of wind driven downwelling at the coast and an intrusion of estuarine water from the Chesapeake Bay. The plume of estuarine water (salinity < 30 psu) was strongest at the shoreward stations in the more northern transects. Wind driven shoreward surface flow converged at the seaward edge of the plume and downwelled. Offshore flow was present below the thermocline and caused the thermocline to bend downward and contact the bottom at between 5 and 10 km offshore. In the zooplankton samples, we enumerated 33 taxa of larvae (17 taxa of bivalve veligers, 10 taxa of gastropod veligers, and 6 taxa of polychaete larvae). Using cluster analysis, larvae were separated into groups with similar patterns of distribution. If larvae were acting as passive particles then we hypothesized that: 1) Their distribution should remain tied to a water mass and 2) around a convergence or divergence, there should be no change in larval concentration. The distributions of larvae in Clusters 1, 4, 5, and 6 were consistent with the hypothesis that they were acting as passive particles. Larvae in Clusters 2 and 3, however, did not appear to be acting as passive particles. Larvae in Cluster 2 did not remain tied to a water mass. They entered the study area in the estuarine plume waters, but within 20 km they were nearly absent from the plume water and were found seaward of the plume and at greater depth. Larvae in Cluster 3 were most abundant in areas of converging currents where the shoreward flowing surface waters downwelled at the plume front or against the shore. We hypothesized that larvae of organisms which as adults live in the intertidal or shallow subtidal zones would have more nearshore distributions than the larvae of adults that are broadly distributed across the shelf. We compared the depth of the habitat of the adult bivalves from which the bivalve larvae in the different clusters were derived. The results were consistent with the hypothesis; larvae with distributions closer to shore tended to come from adults found at shallower depths or in the intertidal zone.