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In recent decades, the rate of introduction of non-indigenous macroalgae has increased. While invasive seaweeds often outcompete native species for substrata, their direct effects on marine life are rarely described. Here, we describe 'red water' events caused by the decay of blooms of the invasive red seaweed, Dasysiphonia japonica, in Great South Bay, NY, USA, and the ability of water from such events to induce rapid and significant mortality in larval and juvenile fish (Menidia beryllina, Menidia menidia, and Cyprinodon variegatus) and larval bivalves (Mercenaria mercenaria and Crassostrea virginica). All species studied experienced significant (p<0.05) reductions in survival when exposed to macroalgae in a state of decay, seawater in which the alga was previously decayed, or both. Both bivalve species experienced 50-60% increases in mortality when exposed to decaying D. japonica for ∼ one week, despite normoxic conditions. Among fish, significant increases (40-80%) in mortality were observed after 24 h exposure to decayed D. japonica and one-week exposures caused, on average, 90% mortality in larval M. beryllina, 50% mortality in juvenile (∼3 cm) M. menidia, and 50% mortality in larval C. variegatus. All fish and bivalve mortality occurred under normoxic conditions (dissolved oxygen (DO) >7 mg L -1 ) and low ammonium levels (< 20 µM), with the exception of C. variegatus, which expired under conditions of decayed D. japonica coupled with reduced DO caused by the alga. Screening of water with decayed D. japonica using liquid chromatography-mass spectrometry revealed compounds with mass-to-charge ratios matching caulerpin, a known algal toxin that causes fish and shellfish mortality, and several other putative toxicants at elevated levels. Collectively, the high levels of mortality (50-90%) of larval and juvenile fish and bivalves exposed to decaying D. japonica under normoxic conditions coupled with the observation of 'red water' events in estuaries collectively indicate the red seaweed, D. japonica, can create harmful algal blooms (HABs)., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022. Published by Elsevier B.V.)

Abstract: The continuing increase of carbon dioxide (CO2) levels in the atmosphere leads to increases in global temperatures and partial pressure of CO2 (PCO2) in surface waters, causing ocean acidification. These changes are especially pronounced in shallow coastal and estuarine waters and are expected to significantly affect marine calcifiers including bivalves that are ecosystem engineers in estuarine and coastal communities. To elucidate potential effects of higher temperatures and PCO2 on physiology and biomineralization of marine bivalves, we exposed two bivalve species, the eastern oysters Crassostrea virginica and the hard clams Mercenaria mercenaria to different combinations of PCO2 (~400 and 800μatm) and temperatures (22 and 27°C) for 15weeks. Survival, bioenergetic traits (tissue levels of lipids, glycogen, glucose and high energy phosphates) and biomineralization parameters (mechanical properties of the shells and activity of carbonic anhydrase, CA) were determined in clams and oysters under different temperature and PCO2 regimes. Our analysis showed major inter-species differences in shell mechanical traits and bioenergetics parameters. Elevated temperature led to the depletion of tissue energy reserves indicating energy deficiency in both species and resulted in higher mortality in oysters. Interestingly, while elevated PCO2 had a small effect on the physiology and metabolism of both species, it improved survival in oysters. At the same time, a combination of high temperature and elevated PCO2 lead to a significant decrease in shell hardness in both species, suggesting major changes in their biomineralization processes. Overall, these studies show that global climate change and ocean acidification might have complex interactive effects on physiology, metabolism and biomineralization in coastal and estuarine marine bivalves. [Copyright &y& Elsevier]

Abstract: Brevetoxin metabolites were identified and characterized in the hard clam (Mercenaria sp.) after natural exposure to Karenia brevis blooms by using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Principal brevetoxins BTX-1 and BTX-2 produced by K. brevis were not detectable in clams. Metabolites of these brevetoxins found in clams included products of oxidation, reduction, hydrolysis and amino acid/fatty acid conjugation. Of highest abundance were cysteine and taurine conjugates. We also found glutathione, glycine–cysteine, and γ-glutamyl-cysteine conjugates. A series of fatty acid derivatives of cysteine-brevetoxin conjugates were also identified. [Copyright &y& Elsevier]

Abstract: The immune response of the hard clam (quahog) Mercenaria mercenaria following challenge with live bacteria (Vibrio alginolyticus) and the protist QPX (Quahog Parasite Unknown) was investigated. The study also compared immune responses following QPX challenge in two different hard clam broodstocks exhibiting different degrees of susceptibility toward this parasite. Different immune and stress-related cellular and humoral factors were assessed including general hemocyte parameters (total and differential hemocyte counts, percentage of dead cells, reactive oxygen production, phagocytosis), parameters geared toward QPX (anti-QPX activity in plasma and hemocyte resistance to the cytotoxicity of QPX extracellular products). Two genes (ferritin and metallothionein) previously shown to be modulated following QPX exposure were molecularly characterized by rapid amplification of cDNA ends (RACE) and their transcription levels were determined in resistant and susceptible clams in response to QPX and bacterial challenge. Results indicated that both V. alginolyticus and QPX challenge triggered significant immune responses in clams with similar trends for most measured parameters. However, specific responses were observed for anti-QPX activity in plasma and hemocyte resistance to QPX products as well as ferritin and metallothionein expression according to each inoculum. Similarly, different response patterns were detected following QPX challenge in susceptible and resistant clam stocks. Resistant clams were able to elicit effective response against the parasite leading to the elimination of QPX and the restoration of constitutive immune status whereas QPX-susceptible clams triggered a strong immune modulation characterized by an acute phase response and associated acute phase protein but appeared to be less active in eliminating the parasite. These results suggest that different signaling pathways are triggered during V. alginolyticus and QPX challenge. Moreover, differences in the immune response toward QPX might be linked to the susceptibility or resistance of different clam stocks to the infection by this parasite. [Copyright &y& Elsevier]

Abstract: The predator functional response is an important mechanism determining the persistence of prey species; however, little is known about the effects of spatial scale on the functional response. We used a manipulative field experiment to quantify the effects of plot size on the guild functional response on the clam Mercenaria mercenaria, replicating the experiment in the summer in Chesapeake Bay, Virginia, and in the spring and fall in Indian River Lagoon, Florida, to examine the effects of predator and alternative prey abundance. In Virginia, the predation rate increased with both patch size and predator density, and was described by a modified sigmoid Type III functional response model that incorporated the effects of patch size. In Florida in the spring, the predator functional response was a Type III and did not vary with plot size, but in the fall it was a linear Type I at small plot sizes, and a Type III at a larger plot size. We hypothesize that the difference is primarily driven by changes in predator abundance and species between sites. In showing that the functional response can vary with plot size and season, our results indicate that small-scale experiments do not always scale up spatially or temporally. We suggest that the predictive power of such experiments may be limited by the complexity of the food web. [Copyright &y& Elsevier]

Abstract: External shell surface (ESS) of bivalve mollusks is known to adsorb various metals dissolved in ambient water in high concentration. It is hypothesized here that the surface microtopography of the thin organic coating layer, periostracum, or calcareous shell (if periostracum was destroyed) plays a major role in the adsorption of actinides on ESS. Thorium (natural alpha-emitter) was used in short-term biosorption experiment with shell fragments of five bivalve mollusks. After a 72h exposure to Th (∼6kBqL−1), thorium concentration was measured on ESS using laser ablation inductively coupled plasma mass spectrometry; the distribution and density of alpha tracks were subsequently visualized by α-track autoradiography. A trend in reduced Th concentrations on the ESS was observed depending upon the species tested: (group 1 ∼4000μgg−1) Chlamys islandica (M.), Mercenaria mercenaria (L.), Dreissena polymorpha (P.)>(group 2 ∼1200μgg−1) Crassostrea virginica (G.)≫(group 3 ∼150μgg−1) Mytilus edulis L. The microtopography of ESS was characterized by scanning electron microscopy revealing the high porosity of the calcareous surface of C. islandica and M. mercenaria, lamellate surface of periostracum in D. polymorpha, uneven but a weakly porous surface of periostracum of C. virginica, and a nearly smooth surface of the periostracum of M. edulis. This work has demonstrated, for the first time, the presence of a strong correlation between concentration of adsorbed Th and ESS microtopography, and the role of the periostracum in this process is discussed. [Copyright &y& Elsevier]

Rising CO2 concentrations and water temperatures this century are likely to have transformative effects on many coastal marine organisms. Here, we compared the responses of two life history stages (larval, juvenile) of three species of calcifying bivalves (Mercenaria mercenaria, Crassostrea virginica, and Argopecten irradians) to temperatures (24 and 28°C) and CO2 concentrations (∼250, 390, and 750 ppm) representative of past, present, and future summer conditions in temperate estuaries. Results demonstrated that increases in temperature and CO2 each significantly depressed survival, development, growth, and lipid synthesis of M. mercenaria and A. irradians larvae and that the effects were additive. Juvenile M. mercenaria and A. irradians were negatively impacted by higher temperatures while C. virginica juveniles were not. C. virginica and A. irradians juveniles were negatively affected by higher CO2 concentrations, while M. mercenaria was not. Larvae were substantially more vulnerable to elevated CO2 than juvenile stages. These findings suggest that current and future increases in temperature and CO2 are likely to have negative consequences for coastal bivalve populations. [ABSTRACT FROM AUTHOR]

Abstract: Within estuaries of New York, USA, recurrent harmful brown tides (Aureococcus anophagefferens) have contributed toward the collapse of two major bivalve fisheries (bay scallop, Argopecten irradians and northern quahog or hard clam, Mercenaria mercenaria), while populations of the Atlantic slipper limpet Crepidula fornicata have persisted in the presence of these blooms. Rates of suspension feeding by C. fornicata and M. mercenaria in the presence of different strains of A. anophagefferens and wild brown tide blooms were quantified to assess the potential impact of brown tides on slipper limpet and hard clam populations. Slipper limpets were capable of clearing all strains of A. anophagefferens at biomass-specific rates similar to an ideal food source (Isochrysis galbana), whereas clearance rates of M. mercenaria fed toxic clones of A. anophagefferens were significantly lower than when fed I. galbana or non-toxic clones of A. anophagefferens (p <0.01). During brown tide blooms (104–106 cells mL −1) on the south shore of Long Island during 2008 and 2009, clearance rates of C. fornicata when fed bloom water were an order of magnitude greater than those of M. mercenaria (p <0.001). Finally, during mesocosm experiments, C. fornicata reduced bloom densities of A. anophagefferens (106 cellsmL−1) by an order of magnitude in 3–4 days, whereas densities of A. anophagefferens in mesocosms stocked with equal or greater biomasses of M. mercenaria were unchanged. These results demonstrate that C. fornicata can actively feed in the presence of A. anophagefferens and suggest that this species could serve as a top-down control of blooms in shallow, poorly flushed estuaries. [Copyright &y& Elsevier]

The article discusses a study which determined the size-dependent pH effect on calcification in post-larval hard clam Mercenaria spp. using the alkalinity anomaly method. In the study, experimental water acidity was controlled by bubbling with air-carbon dioxide blends to obtain pH values of 8.02, 7.64 and 7.41, which are typically found in many near-shore terrigenous marine sediments. The study found that calcification rate decreased with lower pH in all 5 classes measured, and also a significant effect of size on calcification rate.

The article discusses a study which investigated the δ13 values of filter-feeding hard clams Mercenaria mercenaria from 13 locations in Greenwich Bay, Rhode Island. The values of δ13 of the clams showed a linear gradient of two percent over the four kilometer (km) length of Greenwich Bay. Study authors conclude that their findings suggest that δ13 values may be sensitive to the changes in inorganic carbon in estuarine systems. They add that the δ13 values in Mercenaria mercenaria may depict ecosystem metabolism variation.

Abstract: Glutathione S-transferases (GSTs) are universally xenobiotic detoxifying enzymes which have been shown to play a unique detoxifying role in northern quahogs. GST consists of two distinct domains: N-terminal domain which contributes residues to form G-site (GSH-binding site) and C-terminal domain which provides residues to form a hydrophobic H-site (second substrate-binding site). In this study, glutathione S-transferases (GSTs) gene and cDNAs of two isotypes from the northern quahog (Mercenaria mercenaria) were cloned and characterized for the first time. Two full-length GST cDNAs were obtained and named as GST Pi-1 and Pi-2, respectively. Both cDNAs have an open reading frame of 624 bp, which encodes a 207-amino acid protein. Multiple sequences alignment analysis shows that the deduced amino acid sequences of GST Pi-1 and Pi-2 have high homology with other Pi class GSTs. The evolutionary relationship assessment indicates that the two deduced amino acid sequences are closely related to Pi class GSTs. The GST Pi-1 gene is an intronless gene so that it has the same sequence as the transcript. The digested peptide fragments of the purified northern quahog GSTs were analyzed by tandem mass spectrometry and the results confirm the existence of the translation products of the GST Pi-1 and Pi-2 transcripts. The predicted three-dimensional structures of GST Pi-1 and Pi-2 showed that both proteins have two domains, N-terminal domain and C-terminal domain. The conserved phenylalanine-48 serving as the ball in ball and socket style interface in all reported Pi class GSTs was found in both proteins. These results suggest that GST Pi-1 and Pi-2 from the northern quahog belong to the Pi class GSTs which may be involved in polychlorinated biphenyl (PCB) dechlorination. [Copyright &y& Elsevier]

Abstract: Induced triploidy has been used in oyster culture to improve growth, but has not been fully explored for the hard clam Mercenaria mercenaria notata. Therefore, growth was examined in approximately 14 week-old (Exp I) and 15–18 week-old (Exp II) triploid juvenile hard clams in two 3-week experiments. Triploidy was induced chemically (cytochalasin B, 1.0 mg/l) by inhibiting polar body I (PBI) or polar body II (PBII). Growth, as a percentage change in live weight (LW), of triploids was significantly (P <0.001) less compared to diploids in both experiments. In Experiment I, LW increased 250% and 269% for PBI and PBII triploids (initial average LW 93.6±19.0 and 59.5±11.7 mg/clam), respectively, and 341% for diploids (initial average LW 72.0±16.7). Additionally, diploids within triploid groups of Experiment I had lower LW increase (218–296%) as compared to untreated control diploids (341%). In Experiment II, LW increased 422% for PBII triploids (initial avg. LW 11.8±1.6 mg/clam) and 549% for diploids (initial average LW 11.7±1.9 mg/clam). Juvenile triploid clams did not exhibit better growth than diploids in these laboratory trials, but triploid clams may have a growth advantage during stressful conditions or as adults during reproduction as triploids are virtually sterile, which would allow for somatic growth during a time when diploids are spawning and losing mass. Additionally, the use of untreated control diploids is recommended for ploidy experiments rather than diploids found within triploid groups. [Copyright &y& Elsevier]

Abstract: Quahog parasite unknown (QPX) is a fatal protistan parasite affecting cultured and wild hard clams Mercenaria mercenaria along the northeastern coasts of the USA and maritime Canada. Field investigations and laboratory transmission studies revealed some variations in the susceptibility of different hard clam stocks to QPX infection. In this study, we used in vitro QPX cultures to investigate the effect of plasma and tissue extracts from two different clam stocks on parasite survival and growth. Results demonstrated the presence of factors in clams that significantly modulate QPX growth. Extracts from gills and mantle tissues as well as plasma inhibited in vitro QPX growth, whereas foot and adductor muscle extracts enhanced parasite growth. Investigations of anti-QPX activities in plasma from two clam stocks displaying different susceptibility toward QPX disease in vivo demonstrated higher inhibition of QPX growth by plasma from New York (resistant) clams compared to Florida (susceptible) clams. Some clams appeared to be deficient in inhibitory factors, suggesting that such animals may become more easily infected by the parasite. [Copyright &y& Elsevier]

Natural stress gradients can reduce predation intensity and increase prey abundances. Whether the harsh conditions of anthropogenic habitat degradation can similarly reduce predation intensity and structure community dynamics remains largely unexplored. Oxygen depletion in coastal, waters (hypoxia) is a form of degradation that has recently emerged as one of the greatest threats to coastal ecosystems worldwide due to increased rates of eutrophication and climate change. I conducted field experiments and surveys to test whether relaxed predation could explain the paradoxically high abundance of clams that have' sustained a fishery in a degraded estuary with chronic hypoxic conditions. Hypoxia reduced predation on all experimental species but enhanced the long-term survivorship of only sufficiently hypoxia-tolerant prey due to periodic extreme conditions. As a consequence, only the harvested quahog clam (Mercenaria inercenaria) thrived in hypoxic areas that were otherwise rendered dead zones with depauperate diversity and low abundances of other species. This suggests that enhanced populations of some key species may be part of a predictable nonlinear community response that sustains ecosystem services and masks overall downward trends of habitat degradation. [ABSTRACT FROM AUTHOR]

During the last several decades, the waters of mid Narragansett Bay, Rhode Island have increased in temperature and decreased in chlorophyll concentration, and it is possible that these changes affected the growth and success of a common benthic filter feeder, the hard clam, Mercenaria mercenaria. We determined recent hard clam growth rates through a sclerochronological analysis and compared them to the rich historical record of Narragansett Bay growth rates in order to understand how these opposing changes influenced hard clam growth. We found no significant differences in short-term growth between 1985 and 2000. Long-term juvenile growth showed a significant decrease between the 1960s and 1990s, while long-term adult (mature) growth showed a significant increase over this same time period. While it is not clear why the changes in juvenile and adult growth rates differ, it appears as though the decrease in chlorophyll concentration, together with a change in phytoplankton community composition, increasing water temperature, and an increase in predator abundance, may all have influenced hard clam growth between the 1960s and the 1990s. [ABSTRACT FROM AUTHOR]

Abstract: Quahog Parasite Unknown (QPX) disease is attributed with hard clam mortalities from Atlantic Canada down to Virginia. Prior field investigations discovered that resiliency to QPX infection varied considerably among different clam stocks. QPX descriptions and pathologic severity have also varied among field surveys and diagnostic reports, raising the question of whether there is more than one strain of the parasite throughout its known geographic range. In this study we used a recently developed experimental transmission methodology to test the hypothesis that genotypic variability in the host and/or in the pathogen was responsible for differences in the severity of QPX infections. Inoculation methods were applied in a trial that utilized three QPX isolates, geographically or morphologically distinct, and naïve juvenile clams obtained from culture facilities in Massachusetts (MA), New York (NY), Virginia (VA), and Florida (FL). Trends in prevalence and disease severity were significantly associated with seed origin and QPX isolate. Results show clams from FL and VA to be noticeably more susceptible toward QPX infection than clams from MA or NY. QPX isolated from infected NY clams appeared more virulent than the QPX isolated from infected MA clams. Hard clam stock susceptibility differences are important considerations for aquaculture applications in the field, it is also important to be aware that different QPX strains represent different potential threats of disease. [Copyright &y& Elsevier]

The article focuses on the intentionally released larvae of Mercenaria spp. in the northernmost basin of the Indian River Lagoon, Florida in July 1998. The purpose of the short-term dispersal of larval Mercenaria spp. is to characterize the initial dispersion from a point source at time scales of hours to days. Larval densities, which showed close concentrations were enhanced by 10 larvae L-1 near surface drifters discharged with the larvae. Surface distributions from samples garnered near the drifters showed that larval patches developed during the first day. It is suggested that high-resolution methods for mapping larvae are crucial to better answer in spatial distribution evolution at time scales of hours to days, and spatial scales of tens to hundreds of meters.

The hard clam is an economically important bivalve and is abundant along the East Coast of the US. The goal of this research was to evaluate the sensitivity of this test species as compared to that of other benthic and epibenthic organisms. Toxic effects of cadmium (inorganic metal), DDT (organochlorine pesticide), and fluoranthene (polycyclic aromatic hydrocarbon) exposure in sediments (10-day) and seawater (24-h) on juvenile (212–350-μm) hard clams Mercenaria mercenaria were determined. The aqueous 24-h LC50 values were 0.42mg/L cadmium (95% CL=0.35–0.45mg/L), 0.61mg/L DDT (95% CL=0.40–0.95mg/L), and 0.65mg/L fluoranthene (95% CL=0.44–1.23mg/L). Results of sediment toxicity tests indicated that the 10-day LC50 values were 1.66mg/kg cadmium (95% CL=1.21–2.28mg/kg), 5.8mg/kg DDT (95% CL=4.8–8.3mg/kg), and 1.75mg/kg fluoranthene (95% CL=1.38–2.09mg/kg). Based on comparisons to toxicity data for other marine species, these findings suggest that the juvenile clam is one of the more sensitive species to a variety of contaminants and may be a valuable indicator for potential sediment toxicity. [Copyright &y& Elsevier]

Fipronil is a phenylpyrazole insecticide used in agricultural and domestic settings for controlling various insect pests in crops, lawns, and residential structures. Fipronil is chiral; however, it is released into the environment as a racemic mixture of two enantiomers. In this study, the acute toxicity of the (S,+) and (R,-) enantiomers and the racemic mixture of fipronil were assessed using Simulium vittatum IS-7 (black fly), Xenopus laevis (African clawed frog), Procambarus clarkii (crayfish), Palaemonetes pugio (grass shrimp), Mercenaria mercenaria (hardshell clam), and Dunaliella tertiolecta (phytoplankton). Results showed that S. vittatum IS-7 was the most sensitive freshwater species to the racemic mixture of fipronil (LC50 = 0.65 μ g/L) while P. pugio was the most sensitive marine species (LC50 = 0.32 μ g/L). Procambarus clarkii were significantly more sensitive to the (S,+) enantiomer while larval P. pugio were significantly more sensitive to the (R,-) enantiomer. Enantioselective toxicity was not observed in the other organisms tested. Increased mortality and minimal recovery was observed in all species tested for recovery from fipronil exposure. These results indicate that the most toxic isomer of fipronil is organism-specific and that enantioselective toxicity may be more common in crustaceans than in other aquatic organisms. [ABSTRACT FROM AUTHOR]

Karyotype and chromosomal location of the major ribosomal RNA genes were studied in the hard clam ( Mercenaria mercenaria Linnaeus) using fluorescence in situ hybridization (FISH). Metaphase chromosomes were obtained from early embryos. Internal transcribed spacers (ITS) between major RNA genes were amplified and used as FISH probes. The probes were labeled with digoxigenin-11-dUTP by polymerase chain reaction and detected with fluorescein-labeled anti-digoxigenin antibodies. FISH with the ITS probes produced two to four signals per nucleus or metaphase. M. mercenaria had a haploid number of 19 chromosomes with a karyotype of seven metacentric, four metacentric or submetacentric, seven submetacentric, and one submetacentric or subtelocentric chromosomes (7M + 4M/SM + 7SM + 1SM/ST). Two ITS loci were observed: one located near the centromere on the long arm of Chromosome 10 and the other at the telomere of the short arm of Chromosome 12. FISH signals on Chromosome 10 are strong and consistent, while signals on Chromosome 12 are variable. This study provides the first karyotype and chromosomal assignment of the major RNA genes in M. mercenaria. Similar studies in a wide range of species are needed to understand the role of chromosomal changes in bivalve evolution. [ABSTRACT FROM AUTHOR]

Several species of bivalves coexist at the lower intertidal of large tidal flats located in the enclosed or inland coast of the northern area of the Nord-Patagonic archipelagos on the Chilean coast (ca. 40–42°S): Tagelus dombeii (Lamarck), Mulinia edulis (King & Broderip), Venus antiqua King & Broderip, Semele solida (Gray), Gari solida (Gray) and Diplodonta insconspicua Philippi. To explore possible spatial variation in the community structure of the macroinfauna inhabiting sediments with different assemblages of these bivalves, seasonal sampling was carried out during 2003–2004 at two tidal flats of that area. Higher species richness and specimen densities of the macroinfauna occurred in sediments with the higher densities of bivalves, especially in sediments where the deep burrower T. Dombeii reaches its greatest abundances. Our results suggest that, apart from presence of bivalves, the burrowing depth of these organisms is also important in promoting the abundance of macroinfauna. Our results are in contrast with earlier conceptualizations for community organization of the soft bottom macroinfauna inhabiting intertidal flats, related to biological interactions occurring among different phyletic groups, such as that arguing that suspension feeding bivalves (such as T. Dombeii and V. Antiqua) will negatively affect the recruitment of species with planktonic larvae, by filtering them before they become established in the substrate. Thus, it is concluded that beneficial effects of bivalve bioturbation overcome that negative effects on the macroinfauna, although detrimental effects may well occur at bivalve densities higher than those studied here. [ABSTRACT FROM AUTHOR]

Abstract: Since the collapse of populations of northern quahogs (hard clam), Mercenaria mercenaria, in Long Island bays, brown tide blooms have been proposed to pose a barrier to recovery. We tested whether the brown tide alga, Aureococcus anophagefferens, affects survivorship, development or growth in the larvae of M. mercenaria. There was no effect of A. anophagefferens (clone CCMP1708) on survivorship of hard clam larvae, even at bloom concentrations. Under most experimental conditions, larvae fed a mixed diet of Isochrysis galbana (T-Iso) and A. anophagefferens or a single species diet of A. anophagefferens, developed faster than those fed a single species diet of Isochrysis. A mixed diet of I. galbana and A. anophagefferens either had no effect on larval growth, or produced enhanced growth at moderate cell densities (8×104 cellsml−1 of A. anophagefferens). Similarly, moderate cell densities of a single food diet of A. anophagefferens (1.6×105 cellsml−1) generally had no effect on the growth of larvae. When fed bloom concentrations (106 cellsml−1) of A. anophagefferens, larvae developed faster, but growth was reduced, compared to those fed an equal biovolume of Isochrysis. Larvae fed slow growing or near stationary phase cultures of A. anophagefferens experienced reduced growth and slowed development. These data suggest a qualitative difference between slow or stationary phase and fast growing cultures of the brown tide alga. They also suggest that impacts of A. anophagefferens, when present, are likely to be due to the nutritional quality of this alga as a food source for hard clam larvae, which could have a lasting legacy through ontogeny. Additional studies are needed to test whether our findings apply to more recently isolated strains of A. anophagefferens. [Copyright &y& Elsevier]