Soft-Shell Clam, Mya arenaria, a Convenient Laboratory Animal for Screening Pathogens of Bivalve Mollusks

Attempts to introduce infectious or foreign material into oysters and other bivalve mollusks usually involve force or trauma because of immediate, prolonged adduction of the tightly closing valves. The soft-shell clam, Mya arenaria, is unable to seal its valves completely and relaxes readily, exposing soft tissue and a large siphon. This species is free from fouling organisms and is readily available at all seasons in the New England and mid-Atlantic areas. Suspensions of five strains of Vibrio sp. that cause bacillary necrosis in larval and juvenile bivalve mollusks were injected into the heart, siphon tissue, and the incurrent and excurrent siphon lumina of soft-shell clams. All vibrio strains caused significant mortality, usually within 2 days. Heaviest losses resulted from heart and excurrent siphon injections. No mortality occurred in control clams injected with seawater, broth, Serratia sp., and Escherichia coli. The soft-shell clam appears to be a useful animal for testing the pathogenicity of marine microorganisms for bivalve mollusks.     

Location Is Everything: Evaluating the Effects of Terrestrial and Marine Resource Subsidies on an Estuarine Bivalve

Estuaries are amongst the world’s most productive ecosystems, lying at the intersection between terrestrial and marine environments. They receive substantial inputs from adjacent landscapes but the importance of resource subsidies is not well understood. Here, we test hypotheses for the effects of both terrestrial- and salmon-derived resource subsidies on the diet (inferred from stable isotopes of muscle tissue), size and percent nitrogen of the soft-shell clam (Mya arenaria), a sedentary estuarine consumer. We examine how these relationships shift across natural gradients among 14 estuaries that vary in upstream watershed size and salmon density on the central coast of British Columbia, Canada. We also test how assimilation and response to subsidies vary at smaller spatial scales within estuaries. The depletion and enrichment of stable isotope ratios in soft-shell clam muscle tissue correlated with increasing upstream watershed size and salmon density, respectively. The effects of terrestrial- and salmon-derived subsidies were also strongest at locations near stream outlets. When we controlled for age of individual clams, there were larger individuals with higher percent nitrogen content in estuaries below larger watersheds, though this effect was limited to the depositional zones below river mouths. Pink salmon exhibited a stronger effect on isotope ratios of clams than chum salmon, which could reflect increased habitat overlap as spawning pink salmon concentrate in lower stream reaches, closer to intertidal clam beds. However, there were smaller clams in estuaries that had higher upstream pink salmon densities, possibly due to differences in habitat requirements. Our study highlights the importance of upstream resource subsidies to this bivalve species, but that individual responses to subsidies can vary at smaller scales within estuaries.     

Epibiosis and parasitism of coquina clam Donax spp.: Location,location, location!

Cryptic species of coquina clams Donax fossor and D. variabilis carry the hydroid epibiont Lovenella gracilis and are infected with metacercariae of the monorchiid parasites Lasiotocus trachinoti and L. choanura. The associations among this host–epibiont–parasite system were investigated. Fifty clams were collected at low tide over 3 days in June 2020 in South Carolina from each of three groups: clams with no hydroid from the upper intertidal zone and clams with and without hydroids from the swash zone. Clams were measured, identified using a newly developed PCR-RFLP, and examined for infection by metacercariae. Parasites were identified based on cercarial morphology and on metacercariae habitat in the clams. D. fossor was most often found in the swash zone and D. variabilis in the upper intertidal zone. The hydroid was most often associated with D. fossor, which was more infected by both digeneans than D. variabilis. Mean abundance of metacercariae of L. choanura was higher than that of L. trachinoti in both clams and increased over time for both parasites, because higher infection was correlated with larger clams. Greater time spent in the water by individuals of D. fossor appears to best explain these results, with the presence of the hydroids also being associated with higher infection by metacercariae in this coquina. Integration of D. variabilis in both digenean life cycles appears to lead to a positive outcome for the parasites as prevalence and abundance of infection were high; however, because D. variabilis is most frequent in the upper intertidal, more emersed, zone, it is likely deleterious to the epibiont to establish on this clam.     

Population status of giant clams (Mollusca: Tridacnidae) in the northern Red Sea,Egypt

Throughout their range, giant clams (family Tridacnidae) are increasingly threatened by anthropogenic impacts and natural disasters, but little is known about their population status. In this first assessment of the tridacnid population at Abu Sauatir in the northern Red Sea, a total of 491 m² were surveyed and >200 clams recorded. Tridacna maxima was the only species found. The population's live:dead ratio was 3:1. Overall clam density was 0.08?±?0.008 live and 0.02?±?0.007 dead individuals per 0.25 m². Greatest densities occurred on the reef flat in 1 m depth (live), and on the northern reef slope in deeper waters (dead). On the slope, live clam density decreased significantly, whereas dead clam density increased significantly with depth. Sizes of live and dead individuals differed significantly. Live clams ranged from 1 to 30 cm (median 5 cm). Juveniles ≤2 cm (8.2% of the population) and individuals >11 cm occurred on the reef slope but not on the reef flat. Live clam sizes did not differ significantly between reef sites. Dead clam size ranged from 2 to 15 cm (median 6.5 cm). 2.1% of the empty shells were ≤2 cm long. Dead clam sizes differed significantly between 5 and 10 m depth on the northern reef slope. The low clam abundance (live and dead) in the shallowest and most easily accessible areas of the reef flat, combined with small sizes, strongly suggest artisanal reef-top gathering for meat and shells.     

Virulence of parasites in hosts under environmental stress: experiments with anoxia and starvation

Most environments periodically impose severe stress that may cause high mortality and alter population structure, for example, by removing sick and old individuals. We examined how anoxic conditions and starvation of the host affect virulence of two closely related trematode parasites, Rhipidocotyle campanula and R. fennica . These parasites differ by prevalence of infection and by exploitation rate of individual hosts (freshwater clam, Anodonta piscinalis ). Infection by R. campanula is rare (<5% prevalence of infection) and destroys on average 90% of the gonad tissue of the individual host. Infection by R. fennica is more common (20–60% prevalence of infection) and leads to on average 30% gonad destruction. In the end, both infections lead to host infertility. We predicted that R. campanula induces higher host mortality than R. fennica under host stress. In two laboratory experiments, we exposed naturally-infected and uninfected clams to anoxia and to starvation. Anoxia occasionally takes place during winter in eutrophic lakes, while some degree of starvation should occur seasonally. We found that mortality rate of clams was much higher under anoxia than under starvation, and that infection increased mortality rate under both types of host stress. As predicted, R. campanula induced higher host mortality than R. fennica . Host survival was population-specific, suggesting that clams of different origins carried different amount of energy reserves. Severe environmental perturbation may remove R. campanula infected individuals from the host population, but recolonization from the fish host is likely to prevent extinction of the parasite suprapopulation. The observed high host mortality induced by R. campanula may be one ecological explanation for the consistently lower prevalence of infection of R. campanula when compared to R. fennica .     

Environmental distribution and persistence of Quahog Parasite Unknown (QPX)

Quahog Parasite Unknown (QPX) is the cause of mass mortality events of hard clams Mercenaria mercenaria from Virginia, USA, to New Brunswick, Canada. Aquaculture areas in Massachusetts, USA, have been particularly hard hit. The parasite has been shown to be a directly infective organism, but it is unclear whether it could exist or persist outside of its clam host. We used molecular methods to examine water, sediment, seaweeds, seagrass and various invertebrates for the presence of QPX. Sites in Virginia and Massachusetts were selected based upon the incidence of QPX-induced clam die-offs, and they were monitored seasonally. QPX was detectable in almost all of our different sample types from Massachusetts, indicating that the parasite was widely distributed in the environment. Significantly more samples from Massachusetts were positive than from Virginia, and there was a seasonal pattern to the types of samples positive from Massachusetts. The data suggest that, although it may be difficult to completely eradicate QPX from the environment, it may be possible to keep the incidence of disease under control through good plot husbandry and the removal of infected and dying clams.     

Hearing loss in stranded odontocete dolphins and whales

The causes of dolphin and whale stranding can often be difficult to determine. Because toothed whales rely on echolocation for orientation and feeding, hearing deficits could lead to stranding. We report on the results of auditory evoked potential measurements from eight species of odontocete cetaceans that were found stranded or severely entangled in fishing gear during the period 2004 through 2009. Approximately 57% of the bottlenose dolphins and 36% of the rough-toothed dolphins had significant hearing deficits with a reduction in sensitivity equivalent to severe (70-90 dB) or profound (>90 dB) hearing loss in humans. The only stranded short-finned pilot whale examined had profound hearing loss. No impairments were detected in seven Risso's dolphins from three different stranding events, two pygmy killer whales, one Atlantic spotted dolphin, one spinner dolphin, or a juvenile Gervais' beaked whale. Hearing impairment could play a significant role in some cetacean stranding events, and the hearing of all cetaceans in rehabilitation should be tested.     

The course and mortality of a hematopoietic neoplasm in the soft-shell clam,Mya arenaria

Results from two experiments containing approximately 280 Mya arenaria indicated that significantly higher (P < 0.05) mortality occurred within the neoplastic clam population than in the nonneoplastic clam population. Using an in vivo blood cytological technique, five levels of severity were established. The levels were based on the number of neoplastic cells in the circulation with level 1 as the lowest severity and level 5 as the highest severity. Neoplastic clams that were diagnosed as the lowest level had higher survival rates (60%) than those clams diagnosed as the highest level (0%). The hematopoietic neoplasm in M. arenaria followed one of three courses: (1) in approximately 50% of the neoplastic clams the disease progressed to a higher severity and resulted in death; (2) in approximately 40% of the neoplastic clams the disease was chronic, i.e., remained at a stable level; and (3) in approximately 10% of the neoplastic clams the disease diminished in severity or disappeared entirely. In addition, 10% of the clams diagnosed as nonneoplastic at the beginning of the experiments were neoplastic by the termination of the experiment. The contraction of the disease may have been de novo or by stimulation of latent infections.The prevalence of the neoplasm in clams collected from an epizootic area followed a biphasic seasonal pattern. The highest prevalences occurred in October, November, and in May. The hematopoietic neoplasm in M. arenaria was also age and species specific.     

Reproduction of an alien Ruditapes clam (Bivalvia: Veneridae) on recreational clam-gathering grounds in Tokyo Bay,Japan

The Manila clam Ruditapes philippinarum is an important fisheries resource and popular recreational native species in Japan. To counter the domestic shortage of Manila clams caused by depleted catches, similar clams were introduced into Japan from China and the Korean peninsula. The introduced clam is an unnamed morph closely related to R. philippinarum that originates from southern China. Its shell is morphologically different from that of the typical Manila clam and there are small but distinct genetic differences. There is evidence to suggest that the Chinese morph may hybridize with the Manila clam in the Ariake Sea, but not in Tokyo Bay. We carried out studies to determine the degree of overlap of the breeding seasons of both taxa by collecting samples of the Manila clam and the alien clam released onto recreational clam-gathering grounds in Tokyo Bay and their gonadal maturity was examined histologically. Spawning-stage individuals of the Manila clam appeared from April to August and those of the southern Chinese morph from July to August, indicating overlapping spawning periods and thus the potential for hybridization. We discussed the forming condition of the hybridized clam population in terms of reproduction and recruitment process of the clams.     

Insights into genetic diversity, parentage, and group composition of Atlantic white-sided dolphins (Lagenorhynchus acutus) off the west of Ireland based on nuclear and mitochondrial genetic markers

The analysis of stranding events and the application of molecular markers can be powerful tools to study cryptic biological aspects of delphinid species that occur mainly in open ocean habitat. In the present study, we investigated nuclear and mitochondrial genetic variability of Atlantic white-sided dolphins that stranded from 1990 to 2006 (n = 42) along the west coast of Ireland, using 8 microsatellite loci and 599 bp of the mitochondrial DNA control region. Results from both classes of markers are concordant with the hypothesis of a large random-mating population of white-sided dolphins along the west coast of Ireland. In addition, the analyses of 2 live mass stranding events (19 and 5 individuals, respectively) revealed that dolphins within each group were mainly unrelated to each other, suggesting dispersal of both sexes from the natal group (i.e., no natal phylopatry). Parentage analyses allowed the identification of mother-offspring pairs but ruled out all adult males as possible fathers. In combination with data on age of individuals, these results confirmed previous knowledge on life-history parameters, with sexually mature females ranging between 11 and 15 years of age and an interbirth interval of at least 2 years. The present study provides novel information on population and group composition of Atlantic white-sided dolphins along the west coast of Ireland, where population and social structure of the species are still poorly understood.     

Physical habitat attribute mediates biotic resistance to non-indigenous species invasion

A soft-shelled non-indigenous clam, Nuttallia obscurata, has invaded coastal soft-sediment habitats of the northeastern Pacific. In a survey of 35 sites within the San Juan Islands, Washington, USA, Nuttallia was found almost exclusively in sandy substrates, higher in the intertidal than most native clams (>1 m above mean lower low water). The distinctive distribution of Nuttallia suggested that tidal height and sediment composition may be important physical factors that control its refuge availability, regulating its exposure to predation and ultimately the success of its invasion. I tethered Nuttallia for 24 h in the high intertidal where it is typically found and in the low intertidal at an elevation where it was never found. Clams restrained to the surface suffered high mortality from crab predation at both tidal heights, whereas control clams with unrestricted burrowing movement exhibited high mortality rates only in the low intertidal. In a second experiment, I transplanted sediment within and between the two intertidal heights to measure effects of tidal height and sediment type on survival and burial depth of Nuttallia. At both tidal heights all clams placed on mud-cobble substrate, naturally common in the low intertidal, suffered high mortality rates (>60% in 24 h). Nuttallia on loosely packed sand substrate, naturally found in the upper intertidal, survived much better, however, because they buried deeper than in the tightly packed mud. Caged control clams at both tidal heights suffered no mortality. Apparently native predators are mitigating community level impacts of an invader by excluding Nuttallia completely from some beaches with improper sediment characteristics or relegating it in others to a zone not often inhabited by native species, thereby reducing potential competitive interactions. These findings show that a physical habitat characteristic can mediate biotic resistance to an invader and thus control invasion success and community-level impacts. Generally, such physical-biological interactions may explain some of the reported site-to-site variability in invasion success, as well as the patchy distribution of many soft-sediment infaunal species.     

Life history and group composition of melon‐headed whales based on mass strandings in Japan

We examined melon‐headed whales that mass‐stranded live in two events in Japan: (1) 171 animals at Tanegashima Island in 2001 and (2) 85 animals at Hasaki in 2002. We report here the results of life history traits and group composition of these strandings, and compare them to another mass stranding with 135 individuals at Aoshima in 1982. In the Hasaki event, most stranded animals, including those released were sexed and measured. The proportion of live males released was much higher than that of females, and larger animals, especially females, were more likely to have died. Females were estimated to attain sexual maturity at around 7 yr and give birth every 3–4 yr. The sex ratio was significantly different between the Hasaki and Aoshima events. Among dead specimens, females of various age classes were included in all strandings, while age distribution of males varied considerably among strandings. This suggests females show group fidelity while males move between groups. Asymptotic body length of females from Hasaki was significantly smaller than that from Tanegashima, suggesting that more than one population of melon‐headed whales exist off Japan.     

The application of GIS and spatiotemporal analyses to investigations of unusual marine mammal strandings and mortality events

In 2006–2007, an unusually high number of harbor porpoises (Phocoena phocoena) stranded along the Washington and Oregon coastlines. Spatiotemporal analyses were used to examine their ability to detect clusters of porpoise strandings during an unusual mortality event (UME) in the Pacific Northwest using stranding location data. Strandings were evaluated as two separate populations, outer coast and inland waters. The presence of global clustering was evaluated using the Knox spatiotemporal test, and the presence of local clusters was investigated using a spatiotemporal scan statistic (space–time permutation). There was evidence of global clustering, but no local clustering, supporting the hypothesis that strandings were due to more varied etiologies instead of localized causes. Further analyses at subregional levels, and concurrently assessing environmental factors, might reveal additional geographic distribution patterns. This article describes the spatial analytical tools applied in this study and how they can help elucidate the spatiotemporal epidemiology of other UMEs and assist in determining their causes. More than one spatial analytical technique should be used if the study objective is to detect and describe clustering in time and space and to generate hypotheses regarding causation of marine mammal disease and stranding events.     

Growth statistics of an exploited razor clam (Ensis siliqua) bed at Gormanstown,Co Meath,Ireland

A clam bed of 21 km² was discovered at Gormanstown on the East coast of Ireland in 1997. Between July 1998 and August 1999, dredge samples were collected from vessels fishing there and these provided material for biological analysis. E. siliqua occurs with an inshore invertebrate fauna; occasional occurrences of large numbers of Donax vittaus, Pharus legumen and Lanice conchilega were associated with razor clams of less than 6 cm long. As harvesting proceeded smaller individuals appeared in the samples; these may have immigrated onto the bed from the periphery of high densities of larger clams. On the clam bed the animals ranged from 0+ to 19+ years old. Ensis siliqua grew at a slower rate on the west coast of the Irish Sea than on the coast of North Wales. Males grew slightly faster and longer than females and males were more numerous in the landings. The clams are estimated to commence maturation at approximately 4 years of age. They have a similar gonadal cycle to those in Portugal and a similar spawning period from mid-May to the end of July or early August. Condition reflects gonad development, reaching a peak in May. Harvesting by hydraulic dredging causes breakages which are higher in quarters 1 and 4 than in quarters 2 and 3. Bruised razor clams do not eliminate sand in desanding tanks and so they are rejected by processors. The total biomass of the Gormanstown bed is estimated at approximately 1500 tonnes of which 2/3 had been removed by July 1999, two years after exploitation commenced.     

Natural stranding of Atlantic sturgeon (Acipenser oxyrinchus Mitchill, 1815) in Scot's Bay,Bay of Fundy,Nova Scotia,from populations of concern in the United States and Canada

Natural mortality of Atlantic sturgeon (Acipenser oxyrinchus) has been determined to be low (M = 0.07). Reported herein is the mortality by beach stranding of 11 Atlantic sturgeon in Scot's Bay, part of the inner Bay of Fundy in Nova Scotia, Canada on 22 June 2014. Genetic analyses, histological analysis and age determination were performed to determine origin, maturity stage and age of the stranded Atlantic sturgeon. Microsatellite and mitochondrial DNA analyses indicated that four of the Atlantic sturgeon (2 males and 2 females) were from the Saint John River, NB population, which was designated as threatened by the Committee on the Status of Endangered Wildlife in Canada. Seven Atlantic sturgeon (1 male, 5 females, 1 unknown) were from the Kennebec River, Maine population, that was listed as threatened under the Endangered Species Act in the U. S. Ageing of A. oxyrinchus Atlantic sturgeon by pectoral fin spine analysis determined that the mean age of the individuals from the Saint John River (x? = 24.25 years, SD = 5.0) and the Kennebec River (x? = 22.7 years, SD = 3.5) were not significantly different. This is the first report of a stranding event of Atlantic sturgeon, and describes a source of natural mortality affecting populations of concern in both Canada and the U. S.     

The susceptibility of Irish-grown and Galician-grown Manila clams, Ruditapes philippinarum, to Vibrio tapetis and Brown Ring Disease

Brown Ring Disease (BRD), which affects the Manila clam in Europe, is caused by the bacterium, Vibrio tapetis. BRD has been diagnosed in Ireland on only one occasion (1997) although the aetiological agent has recently been detected in apparently healthy Manila clams from a number of sites around the Irish coast. The present work investigated the susceptibilities to BRD of two stocks of Manila clams, one from Ireland and the second from Galicia, north-western Spain, where BRD has been reported on a number of occasions. Exposure of the clams was by addition of V. tapetis to the holding waters. Development of BRD was assessed by the appearance of brown ring signs on the host shells, by bacterial isolation and characterization, and by detection of the bacterium by PCR. The pathogen was recovered from infected individuals and confirmed as V. tapetis by biochemical tests and a slide agglutination test. Galician clams experienced significantly higher mortalities, BRD prevalences and V. tapetis levels than Irish clams. Background infection with V. tapetis in the control stocks prevented conclusions being drawn on comparative susceptibility of the two stocks. Irish clams were significantly affected by the experimental challenge, as demonstrated by the development of BRD and an increase in V. tapetis levels. Results illustrate the vulnerability of Irish clams to BRD and have implications for the movement and transfer of clam seed in Ireland.     

Helpful habitant or pernicious passenger: interactions between an infaunal bivalve,an epifaunal hydroid and three potential predators

The importance of positive interspecific interactions within physically stressful habitats has received increased attention from community ecologists. The exposed sandy beach is an example of a physically rigorous environment where biological interactions have long been considered insignificant. We examined the interaction between the infaunal clam, Donax variabilis, and the hydroid, Lovenella gracilis, on exposed sandy beaches in North Carolina. Epibiotic occupation of Donax by hydroids has been repeatedly observed on ocean beaches but rarely investigated. By providing a stable substrate for attachment, the clam facilitates the persistence of the hydroid in the intertidal beach; however, benefits or costs experienced by the host as a result of this association are unknown. By exposing clams with and without hydroid colonies to multiple types of clam predators, we tested the effectiveness of the hydroid, which possesses stinging nematocysts, in defending its host. The hydroid defended the clam against one common predator, the Florida pompano ( Trachinotus carolinus). Against speckled crabs ( Arenaeus cribrarius) and ghost crabs ( Ocypode quadrata), however, the hydroid offered no protection for its host and instead facilitated predation. The epibiotic hydroid, which projects above the surface of the sand, allowed the crabs to more readily detect clams below the surface. In the field, we evaluated the effect of the hydroid on the tidally synchronized migrations and burrowing speed of the clam. The hydroid, which can form large colonies on the posterior end of the clam, had no effect on Donax burrowing speed, but did reduce the speed of transport of clams by wave swash. Depending on relative predation pressure, the occupation of D. variabilis by L. gracilis can alternately be characterized as beneficial or detrimental to the host.     

Strandings provide insight into social group structure of Atlantic white-sided dolphins

Atlantic white-sided dolphins (Lagenorhynchus acutus) are highly social odontocetes with a poorly understood tendency to mass strand. With limited capacity to study social ecology in the open ocean, mass strandings provide an opportunity to improve our understanding of group structure. Our study of 32 mass stranding events that occurred on Cape Cod, Massachusetts, between 1999 and 2009 identifies aspects of social ecology that vary across the year. A greater number of mass stranding events occurred outside of the breeding season and there was evidence of age-structuring during the breeding season. We find generally low average intragroup pairwise relatedness assessed across eight microsatellite loci in a subset of 16 mass stranding events. Mass stranded groups do not show higher than expected relatedness when compared to a baseline estimate derived from single-stranded individuals. Overall, our integration of genetic estimates of relatedness with data on sex and maturity-class from stranded specimens suggests that Atlantic white-sided dolphins fall near the more fluid end of the continuum from short-term, highly fluid social associations to long-term, stable groups represented among the odontocetes. Despite their tendency to mass strand, stable, kin-based associations are not a defining feature of social group structure in this species.