Clam Populations Research Articles

Evolution, diversity, distribution and the endangered future of the giant clam–Symbiodiniaceae association

Giant clams are found in a mutualistic association with Symbiodiniaceae dinoflagellates. While clams are economically important for fisheries, Symbiodiniaceae are responsible for most of the primary production in coral reefs. This review addresses key issues regarding the giant clam–Symbiodiniaceae holobiont: diversity and distribution; functional traits; evolution and coevolution; and consequences of climate change. Findings show that holobiont distribution is partitioned by host species and geography. So far, giant clams have been reported in association with only 30 Symbiodiniaceae phylotypes, all within genera Symbiodinium, Cladocopium and Durusdinium. Species and functional diversities are highest in the Coral Triangle. The association evolved in the Eocene–Oligocene transition; there is evidence for coevolution, including host organs developed for housing symbionts, change in host habitat and multiple symbiont phylotypes found exclusively in giant clams. Clam bleaching events have been recorded throughout the Indo-Pacific. These clams may be particularly vulnerable, as they rarely associate with stress-tolerant symbionts and are heavily targeted by fishing. The Great Barrier Reef is particularly at risk, as clams in that area do not associate with thermally tolerant, low-irradiance-adapted and opportunistic symbiont phylotypes. Further reduction in giant clam populations may serve as an important indicator of the increasing decline of coral reef biodiversity.

Impact of sea surface temperature anomalies on giant clam population dynamics in Lakshadweep reefs: Inferences from a fourteen years study

Abstract Frequent sea surface temperature (SST) anomalies may have global consequences on marine biodiversity conservation through their potential impacts on fragile coral reefs and associated organisms such as the giant clams. However, effects of SST anomalies on giant clam population dynamics is not clearly understood as their slow life-history traits require long-term population studies for unravelling these patterns. Here, we have examined the effects of SST anomalies on Tridacna maxima by monitoring their populations in Lakshadweep archipelago across a 14 year period (2004–2017). We monitored T. maxima density annually in three reefs (Agatti, Bangaram-Tinakkara and Kavaratti) using 100 m × 8 m belt transects. The annual patterns of density change rate (λt) against time since bleaching were modelled and used to develop stochastic population growth models under different scenarios of bleaching incidence. Our study was preceded by one and intercepted by two El Nino events (1998, 2010 and 2016). Mean summer SST > 30 °C triggered T. maxima bleaching. Population density showed large inter-annual variations that were synchronous between reefs, showing gradual increase in density during 2004–2010, and decrease during 2010–2017. SST anomalies did not cause immediate clam mortality. However, T. maxima population change rate (λ) was 0.91 ± 0.05SE immediately after bleaching and increased by 0.02 ± 0.01SE per year, resulting in positive growth only after five years since bleaching. Time-series analysis of SST data (1965–2016) indicated increasing incidence of anomalous temperature (β = 0.09 ± 0.04SE). Population growth models showed stable density trend after 50 years under past bleaching incidence (20% yr−1), but declining density (R = −0.05 ± 0.02), high risk of quasi-extinction (0.31–0.61 across reefs), and low density (geometric mean: 9–15 individuals ha−1 across reefs) under the prevailing high bleaching incidence (40% yr−1). Thus, climate change induced frequent SST anomalies are likely to compromise the viability of giant clam populations.

Dominant Bivalve in an Exposed Sandy Beach Regulates Community Structure Through Spatial Competition

Fluctuations in abundance of dominant species can cause competitive release of resources with consequences on community structure and functioning. In the present study, changes in the intertidal macroinfauna community of an exposed sandy beach were evaluated during two contrasting periods characterized by low and high densities of the yellow clam Amarilladesma mactroides. The increase in clam abundance and biomass was associated with a significant decrease in abundance of the rest of the community. In particular, a decline was observed for the pea crab Austinixa patagoniensis, a commensal species that lives in the burrows of the shrimp Sergio mirim. Our study demonstrates that fluctuations in clam abundance lead to long-term changes in community structure, suggesting the presence of competitive interactions. The environmental stability over the two periods strengthens the hypothesis that the competition between species is crucial for shaping the ecological community. Stable isotope analysis allows discarding trophic competition as mechanism of exclusion. Image maps reveal complementary distribution of species, showing the relevance of the spatial competition, which is mediated by changes in abundance of a third species. Indeed, high densities of A. mactroides reduce the available area for the establishment of the S. mirim burrows, limiting the foraging behavior of its commensal, the pea crab. Such an interaction drives density-dependent exclusion of the pea crab from the intertidal zone following the establishment of the yellow clam population. This study illustrates that spatial competition triggered by the increase of a bed-forming species can have community-wide consequences in exposed sandy beaches.

Tolerance, Growth, and Physiological Responses of the Juvenile Razor Clam (Sinonovacula constricta) to Environmental Ca2+ and Mg2+ Concentrations.

To facilitate transplanting razor clam (Sinonovacula constricta) populations to inland saline-alkaline waters (ISWs), we evaluated the tolerance of juvenile S. constricta (JSC) to Ca2+ and Mg2+ concentrations, and determined the effects of these ions on JSC growth and physiological parameters. After 30 days stress, the tolerable ranges of JSC to Ca2+ and Mg2+ were determined to be 0.19 mmol⋅L-1–19.46 mmol⋅L-1 and 0 mmol⋅L-1–29.54 mmol⋅L-1, respectively. The concentrations of Ca2+ (less than 0.65 mmol⋅L-1 or more than 3.24 mmol⋅L-1) and Mg2+ (less than 0.37 mmol⋅L-1 or more than 14.17 mmol⋅L-1) significantly inhibit JSC growth. Physiological enzyme activity no significant response when the concentrations range of Ca2+ and Mg2+ are 0.93 mmol⋅L-1–6.49 mmol⋅L-1 and 0.37 mmol⋅L-1–14.77 mmol⋅L-1, respectively. For transplantation practice, these data indicate that only high concentrations of Ca2+ (3.24–6.825 mmol⋅L-1) and Mg2+ (14.77–33.69 mmol⋅L-1) in target inland saline-alkaline water had significantly impact on growth and physiological response. In addition, present study suggests that the increase in Ca2+ and Mg2+ ion concentrations caused by ocean acidification will not affect the survival, growth and physiology of S. constricta. Current research suggests that S. constricta can adapt to extreme changes in the marine environment (Ca2+ and Mg2+) and may be an excellent candidate for inland saline-alkaline water transplantation practice.

Clam Spawning & Red Tide: Helping Students Learn the Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle and associated calculations are often challenging for students to learn for three reasons. First, several assumptions need to be understood to correctly apply the principle. Second, a series of calculations are required for proper application. Third, the principle, assumptions, and calculations are often taught separately from students observing population changes over time. We describe a classroom activity in which students investigate how the allele frequencies of soft-shell clam (Mya arenaria) populations change over time as a result of environmental disturbances by simulating the effects on population data. These classroom population changes are then compared to authentic research data collected on populations of M. arenaria. This activity was implemented at three institutions in introductory- and senior-level biology courses. Students reported that the activity helped them better understand and apply both the principle and its calculations.

Vulnerability of juvenile clams to predation by shore crabs

Predation, especially by crabs, is a common source of mortality for natural and farmed populations of intertidal clams. Consumption of juvenile “seed clams” forces aquaculturists to try to exclude predators and/or raise juveniles in hatcheries until they can reach a size refuge. We ran a variety of lab experiments assessing vulnerabilities of juvenile clams to small, common shore crabs (Hemigrapsus spp.). Crabs <1 cm width can consume hardshell Manila clams larger than those normally used for “seed”, and can readily crush even larger softshell Mya clams. We suggest that using netting to prevent consumption by shore crabs is not practical given that smaller individuals can fit through mesh openings. Raising seed in hatcheries until they are 10–15 mm will provide a size refuge from shore crabs, but not larger cancrid crabs. Farming on beaches with little habitat (e.g., cobbles) for shore crabs can likely reduce juvenile clam mortality. A better understanding of predation threats to commercially important clams is critical, especially as the invasion of the green crab Carcinus to Washington shorelines further threatens survival of juvenile clams.

Distribution and abundance of the giant clams (Cardiidae: Bivalvia) on Kei Islands, Maluku, Indonesia

Abstract. Triandiza T, Zamani NP, Madduppa H, Hernawan UE. 2019. Distribution and abundance of the giant clams (Cardiidae: Bivalvia) on Kei Islands, Maluku, Indonesia. Biodiversitas 20: 884-892. Giant clams, ecologically important bivalves in coral reefs, are under anthropogenic pressures in most parts of their range, necessitating the study of population status for conservation management. Here, we assessed species composition, distribution, size density, and habitat condition of giant clams in Kei Islands. A total of 9 reefs around the islands (Dar 1, Dar 2, Pulau Kur, Pulau Tanimbar Kei, Pulau Adranan, Pulau Dullah Laut, Difur, Labetawi, dan Ohoidertoom), were surveyed using quadrat-transect line in September-December 2017. We found five species of giant clams, i.e., Tridacna crocea, T. maxima, T. squamosa, T. noae, and Hippopus hippopus. Of these species, we report a new record for T. noae which was previously not known to be present in the region. The overall density was recorded at 0.0428 individual/m2. Juveniles clams were fewer than the adults clam, this may indicate that the survival rate of recruit is low. Most individuals were found living on dead coral algae substrate (75 %). Based on the findings, this study suggests that giant clam population in Kei Islands are imperiled, indicated by low population density and local extinction for species T. gigas and T. derasa. It is recommended to establish a protected area and restocking of giant clams by through implementing sasi laut (traditional law) with a minimum period of 5 years.

Clam population dynamics as an indicator of beach urbanization impacts

Modification of natural environments due to human activities has become an issue of great concern in the last years. In this study, beach urbanization impact was assessed through population dynamics of two dominant infaunal clam species: Amarilladesma mactroides and Donax hanleyanus. The beaches used as sampling sites were characterized by an urbanization index, which exhibited three levels of urbanization. Recruits and juveniles of both studied species were collected monthly during a two years sampling. Results showed that beach urbanization reduces A. mactroides recruits and juveniles’ abundances, whereas the abundance of D. hanleyanus juveniles increased with higher urbanization degree. Also, the observed effects were limited to summer and autumn, coinciding with the time of higher impact due to recreation activities. The interspecific differences found and the importance of the sampling period when studying anthropic impacts over beaches, are discussed.

Understanding consequences of adaptive monitoring protocols on data consistency: application to the monitoring of giant clam densities impacted by massive mortalities in Tuamotu atolls, French Polynesia

Abstract During long-term monitoring, protocols suitable in the initial context may have to change afterward because of unforeseen events. The outcome for management can be important if the consequences of changing protocols are not understood. In Tuamotu Archipelago atolls, French Polynesia, the density of giant clams (Tridacna maxima) has been monitored for 12 years, but massive mortalities and collapsing densities forced to shift from a line-intercept transects and quadrats (LIT-Q) method to a belt-transect (BT) method. We investigated with a simulation approach the conditions (density, size structure, aggregation of giant clam populations) under which the two methods provided different results. A statistical model relating the BT density to the LIT-Q density was validated using new field data acquired on the same sites with both protocols, on two atolls. The BT method usually provided higher estimates of density than the LIT-Q method, but the opposite was found for very high densities. The shape of the relationship between measurements depended on population size structure and on aggregation. Revisiting with the model the historical LIT-Q densities suggested that densities have been underestimated in the past but previously detected trends in population trajectories remained valid. The implication of these findings for management are discussed.

The prevalence of parasitic pea crab Afropinnotheres monodi in mussels depending on the degree of exposure of habitats: implications for mussel culture

XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019.

Adapting fisheries management to the spatial variability in the reproductive cycle: the case of the sword razor clam in Galicia (NW Spain)

Adapting fisheries management to the spatial variability in the reproductive cycle: the case of the sword razor clam in Galicia (NW Spain)

Spatial distribution of Donax trunculus (Linnaeus, 1758) populations along the intertidal area of the Gulf of Cádiz (SW, Spain)

Spatial distribution of Donax trunculus (Linnaeus, 1758) populations along the intertidal area of the Gulf of Cádiz (SW, Spain)

Intertidal clams exhibit population synchrony across spatial and temporal scales.

Long‐term datasets can be particularly useful for parsing out factors influencing populations, yet few studies have utilized continuous datasets to quantify population dynamics in bivalve molluscs. We used dynamic factor analysis on a clam biomass dataset spanning 28 yr and five distinct regions in the southern Salish Sea to determine (1) if native intertidal clam populations exhibit synchrony and (2) what environmental covariates may be correlated with these population trends. Once covariates were accounted for, the model with the most data support included three predominant trends to describe multidecadal change in clam biomass. Intraspecific synchrony was highest among Saxidomus gigantea and Leukoma staminea populations, with no clear evidence of covariance in Clinocardium nuttallii. Specifically, we quantified a pronounced decadal decline in L. staminea and an increase in S. gigantea biomass on most beaches. No beaches showed synchrony in trends across all three species, indicating that species‐specific trends (regardless of location) were more common than beach‐specific trends (regardless of species). Seven environmental covariates were evaluated in their capacity to explain variability in annual mean biomass. Of these, the North Pacific Gyre Oscillation lagged 4 yr prior to the observation year was most supported by the data in the best fitting model, implying that 4 yr old clam biomass is partially determined by oceanographic processes affecting larval clams. Although results suggest large‐scale density‐independent factors play a role in venerid clam population dynamics, it is also likely local factors account for variability not explained by our model.

Implications for Fisheries Management: Sex Ratio, Size at Sexual Maturity, and Reproductive Cycle of Saxidomus purpuratus (Bivalvia: Veneridae) in Korea

This study was performed to obtain information on the sex ratio, size at sexual maturity, and main spawning period of the butter clam Saxidomus purpuratus in Korea to contribute to the management of fisheries resources of this species. The age-specific sex ratios (female:male) of S. purpuratus were 1:2.20 (31.3% female) and 1:1.03 (49.2% female) in the 0+ and 1+ y classes, respectively, illustrating the initially higher percentage of males in the butter clam population. From the 2+ y class onward, however, the percentage of females in the population tended to increase with age. The shell length at sexual maturity was analyzed using a logistic regression model, and was determined to be 54.5 and 45.5mmfor females and males, respectively. The gonad index displayed a definitive pattern of seasonal changes, with high values in the spring and fall and low values in the summer and winter for both males and females. The butter clam S. purpuratus was found to have two spawning seasons per year, with the main spawning period being in May–June.

The ability of macroalgae to mitigate the negative effects of ocean acidification on four species of North Atlantic bivalve

Abstract. Coastal ecosystems can experience acidification via upwelling, eutrophication, riverine discharge, and climate change. While the resulting increases in pCO2 can have deleterious effects on calcifying animals, this change in carbonate chemistry may benefit some marine autotrophs. Here, we report on experiments performed with North Atlantic populations of hard clams (Mercenaria mercenaria), eastern oysters (Crassostrea virginica), bay scallops (Argopecten irradians), and blue mussels (Mytilus edulis) grown with and without North Atlantic populations of the green macroalgae, Ulva. In six of seven experiments, exposure to elevated pCO2 levels (∼1700 µatm) resulted in depressed shell- and/or tissue-based growth rates of bivalves compared to control conditions, whereas rates were significantly higher in the presence of Ulva in all experiments. In many cases, the co-exposure to elevated pCO2 levels and Ulva had an antagonistic effect on bivalve growth rates whereby the presence of Ulva under elevated pCO2 levels significantly improved their performance compared to the acidification-only treatment. Saturation states for calcium carbonate (Ω) were significantly higher in the presence of Ulva under both ambient and elevated CO2 delivery rates, and growth rates of bivalves were significantly correlated with Ω in six of seven experiments. Collectively, the results suggest that photosynthesis and/or nitrate assimilation by Ulva increased alkalinity, fostering a carbonate chemistry regime more suitable for optimal growth of calcifying bivalves. This suggests that large natural and/or aquacultured collections of macroalgae in acidified environments could serve as a refuge for calcifying animals that may otherwise be negatively impacted by elevated pCO2 levels and depressed Ω.

Seasonal changes in short-neck clam (&lt;i&gt;Ruditapes philippinarum&lt;/i&gt;) population and its controlling factors on Sashiki Tidal Flats

Seasonal changes in short-neck clam (&lt;i&gt;Ruditapes philippinarum&lt;/i&gt;) population and its controlling factors on Sashiki Tidal Flats

Biogeography of resistance to paralytic shellfish toxins in softshell clam, Mya arenaria (L.), populations along the Atlantic coast of North America

Blooms of Alexandrium spp., the causative agent of paralytic shellfish poisoning (PSP), recur with varying frequency and intensity on the Northwest Atlantic coast of North America, from New York, USA, to northern Canadian waters. Along this latitudinal range blooms co-occur with abundant, intertidal populations of softshell clams, Mya arenaria. Prior work identified a naturally-occurring genetic mutation in Domain II α-subunit of the clams’ voltage-gated sodium channels (NaV), which significantly reduces the binding affinity of the paralytic shellfish toxin, saxitoxin (STX). This mutation provides clams with resistance to the deleterious effects of STX, allowing them to continue feeding during Alexandrium spp. blooms and attain very high tissue toxicities. This study used genetic sequencing of the NaV mutation locus in clams from four coastal regions of the Bay of Fundy-Gulf of Maine and the mid-Atlantic to determine the percentage of clams in each region that possess the resistant NaV mutation. The genotype composition was related to the occurrence and magnitude of PSP outbreaks based on shellfish toxicity, primarily that of mussels, Mytilus edulis, used as a proxy for the prevalence and severity of Alexandrium blooms in each region. As hypothesized, the proportion of clams bearing the resistant mutation generally matched up well with the historical incidence and intensity of Alexandrium spp. blooms. The highest percentage of homozygote resistant clams (RR = 70.0%), and the lowest percentage of sensitive clams (SS = 4.5%) were found in eastern Gulf of Maine populations. Exceptions at a few sites where anomalously high numbers of M. arenaria with the resistant mutation were found despite the absence of blooms, may be attributable to larval gene flow. There was no evidence that Alexandrium blooms occurring in Northport Harbor, Long Island, have resulted in a shift in genotypic composition of the local clam population, presumably due to their low cell toxicity. Seasonal mismatch of highly vulnerable M. arenaria postset with toxic blooms at this latitude may also partly explain this result. This study provides strong supporting evidence that Alexandrium blooms can select for resistance to PSP-toxins in M. arenaria populations and proposes a mechanism for the persistence of the sensitive allele throughout the region. Implications for clam aquaculture (seeding) efforts, as well as for shellfish toxicity monitoring are discussed.

A lakewide survey of Asian clam (Corbicula fluminea) distribution at warmwater discharges in Lake Michigan

The Asian clam Corbicula fluminea is among the most prolific aquatic invaders in the world; but in colder mid-latitude areas, like the Laurentian Great Lakes, their population expansion has likely been limited by poor overwinter survival. In these areas, Asian clams are typically found in thermal refugia like warmwater discharges from industrial facilities. We sought to identify the current extent of Asian clam populations in Lake Michigan and waters immediately adjacent to it, specifically at locations most likely to harbor overwintering populations – industrial warmwater discharges. During April–May 2017, we surveyed 17 locations around Lake Michigan. Evidence of Asian clam populations was found at four sites, though live specimens (n = 3) were only found at the Indiana Harbor Ship Canal in East Chicago, IN. Shells or fragments of shells were found at Green Bay, WI, Waukegan, IL, and Port Sheldon, MI. Our findings indicate that although Asian clams are present in Lake Michigan, they are relatively rare, and remain isolated to a few small pockets of over-wintering habitat.

Genetic Variations of Intra- and between-razor Clam Solen corneus Population Identified by PCR Analysis.

The author undertook PCR-founded genetic platform to investigate the hierarchical dendrogram of Euclidean genetic distances of one razor clam population, particularly for Solen corneus, which was further associated with those of the other clam population, by engaging with the precisely designed oligonucleotide primer sets. Seven oligonucleotides primers were used producing a total of 639 counted bands in population A and 595 in population B, respectively, ranging in size of DNA fragments from larger than approximately 50 bp to less than 1,100 bp. Their primers generated 39 specific fragments (6.10%) in population A and 47 (7.90%) in population B, respectively Comparatively, individuals of one razor clam population were fairly related to that of the other clam population, as shown in the hierarchical dendrogram of Euclidean genetic distances. The analysis of genetic variation between razor clam populations could offer important statistics for fisheries and mariculture. Generally the results showed specific and/or conserved genetic loci between razor clam populations. Specific markers established by the author will be valuable for the genetic analysis, species protection and increase of razor clam individuals in coastal region of the Korean Peninsula.

The structured demography of open populations in fluctuating environments

Abstract At the spatial scale relevant to many field studies and management policies, populations may experience more external recruitment than internal recruitment. These sources of recruitment, as well as local demography, are often subject to stochastic fluctuations in environmental conditions. Here, we introduce a class of stochastic models accounting for these complexities, provide analytic methods for understanding their long‐term behaviour and illustrate the application of methods to two marine populations. The population state n(x) of these stochastic models is a function or vector keeping track of densities of individuals with continuous (e.g. size) or discrete (e.g. age) traits x taking values in a compact metric space. This state variable is updated by a stochastic affine equation nt+1 = At+1nt + bt+1 where is At+1 is a time varying operator (e.g. an integral operator or a matrix) that updates the local demography and bt+1 is a time varying function or vector representing external recruitment. When the realized per‐capita growth rate of the local demography is negative, we show that all initial conditions converge to the same time‐varying trajectory. Furthermore, when A1,A2,… and b1,b2,… are stationary sequences, this limiting behaviour is determined by a unique stationary distribution. When the stationary sequences are periodic, uncorrelated or a mixture of these two types of stationarity, we derive explicit formulas for the mean, within‐year covariance and autocovariance of the stationary distribution. Sensitivity formulas for these statistical features are also given. The analytic methods are illustrated with applications to discrete size‐structured models of space‐limited coral populations and continuously size‐structured models of giant clam populations.