Giant Clam Populations Research Articles

Assembly rules of fish communities in Tuamotu archipelago atoll lagoons: The case of Fangatau, a lagoon dominated by giant clam habitats

Assembly rules of fish communities in Tuamotu Archipelago lagoons have been previously investigated but without considering lagoons where shallow habitats are dominated by giant clam populations. We tested if assembly rules for 14 atolls were challenged by such an atoll (Fangatau), and in doing so investigated the robustness of inter-island population patterns to specialized habitats. Fangatau had significantly higher proportions of Pomacentridae and Scaridae, but lower proportions of Labridae, Acanthuridae, Balistidae and Serranidae. Functionally, herbivores, omnivores, species <7 cm or between 50 and 80 cm, and species forming schools were found in higher proportions, while sessile invertebrate feeders, medium-size species (30–50 cm) and solitary species were lower. These differences were rather related to Fangatau isolation, as no other physical or ecological feature could explain them. Total richness, functional richness, and functional redundancy remained correlated mainly to atoll size, without outliers related to high clam abundance and presence of peculiar habitats.

High density of Tridacna crocea in exposed massive corals proximate the Ranger Station of Tubbataha Reefs Natural Park, Cagayancillo, Palawan, Philippines

The crocus clam Tridacna crocea, one of the seven known giant clam species in the Philippines (Poutiers 1998), is widely distributed in the IndoPacific Region but is possibly extinct in Guam and Northern Mariana Islands (IUCN 2014). In the Philippines, the exploitation of all giant clam species is prohibited under Fisheries Administrative Order No. 208, series of 2001 (DA 2001). However, illegal exploitation of giant clams in many parts of the country has greatly reduced natural stock populations (Gomez and MingoaLicuanan 2006; Gonzales et al. 2014; Picardal and Dolorosa 2014), and only in marine protected areas such as Tubbataha Reefs Natural Park (TRNP) where high densities (100-220 ind.100 m-2) of T. crocea (Calumpong and Cadiz 1993; Ozoa 1995; Dolorosa and Schoppe 2005) have been reported. Methods to quantify the abundance of this species involved the use of either belt transect with 1 x 1 m quadrat placed every 10 m or 2 x 100 m belt transect. No data are so far available on the density of T. crocea embedded in coral rocks exposed at low tide. To quantify the abundance of T. crocea in massive corals with heads exposed at low tide, some exposed corals near the vicinity of the Ranger Station of TRNP were randomly sampled in June 2010. The area of coral heads was measured by using a quadrat and then by measuring the shell lengths of T. crocea with calipers (Figure 1). Among the ten coral heads surveyed (area =40.75 m2), a total of 236 T. crocea was noted. The densities of T. crocea ranged from 200-2,200 ind.100 m-2 with an average (±sd) of 767 (±559) ind.100 m-2 (Table 1). The sizes of T. crocea varied between 10-135 mm with an average (±sd) shell length of 67.67 (±32.26) mm (Figure 2). Tridacna crocea can attain 150 mm maximum shell length but commonly 110 mm (Poutiers 1998). There is no available literature on the size at which T. crocea becomes sexually mature, however, if T. crocea begins to reproduce after attaining nearly 40% (60 mm) of its maximum shell length (150 mm) as recorded for Tridacna maxima by Jameson (1976), then at least more than half (57%) are sexually mature. About 14% of samples were very small (10-29 mm) and such may indicate a self seeding reef. These high densities of T. crocea could be due to the manner of survey which only accounted for the coral heads exposed at low tides. In 2009, a survey involving 40 transects (20 x 2 m) in the surrounding reefs within the Ranger Station of TRNP recorded an average density of 39.25 ind.100 m-2 (range: 3.13-126.25 ind.100 m-2 (Dolorosa unpublished data). Previous densities obtained along the transect lines at intertidal (13 ind.100-2m) and 5 m deep (3 ind. 100-2 m) areas of the park (Dolorosa and Schoppe 2005) were lower than the present data. Lower densities (31 ind.100-2 m) were also recorded using transect surveys in Apulit Island, Taytay, Palawan (Gonzales et al. 2014) and many other areas of the Philippines (see bin Othman et al. 2010). Much lower were the densities in Seribu Island (2.80 ind. 100 m-2) and Manado Waters, Indonesia (0.85 ind. 100 m-2) (Yusuf et al.2009), and in intertidal (0.037 ind. 100 m-2) and subtidal (0.027 ind. 100 m-2) reefs of Singapore (Neo and Todd 2012). Giant clams are important component of the reef ecosystems. Their ecological roles have recently been reviewed and quantified which include their capacity to promote a balanced reef ecosystem (Neo et al. 2015). Any reef restoration project should therefore at least include the revival of giant clam populations. The current higher abundance of T. crocea within the Ranger Station of TRNP therefore requires effective protection, as the park represents an important natural genetic bank and seed source for clamdepleted reefs within its vicinity and other reefs in the Sulu Sea.

Rehabilitation of Biological Resources: Coral Reefs and Giant Clam Populations Need to Be Enhanced for a Sustainable Marginal Sea in the Western Pacific

This short article arose from a presentation at the workshop on “Implementing the DOC (Declaration of Conduct) and Cooperation Mechanisms in the South China Sea” held on 14–15 August 2014 in Haikou...

A study on genetic structure of giant clam (Tridacna spp.) (Tridacninae) population in south central and southern Vietnam’s coast

Giant clams have economic value and play an important role in coral reef ecology. However, in recent years, giant clam resources have been declining due to overexploitation. This study investigated the population genetic structure of giant clams (Tridacnae spp.) in South Central and Southern Vietnam’s coast. Analysis of genetic diversity and population structure of two species of giant clams (Tridacna crocea collected in Nha Trang Bay and Con Dao and T. squamosa collected in Nha Trang Bay and Phu Quoc Island) was done based on CO1 mitochondrial DNA. The results showed that populations of T. crocea represented the average diversity with sequence differences between individuals 0-3.5%, 10 haplotypes/30 individuals (haplotype diversity= 0.846) in Nha Trang Bay and sequence differences 0-9.7%, 16 halotypes/28 individuals (haplotype diversity=0.934) in Con Dao. Populations of T. squamosa expressed the low diversity with the genetic sequence differences 0-2.1%, 7 haplotypes/19 individuals (haplotype diversity=0.468) in Nha Trang Bay and the genetic sequence differences 0-1.72%, 5 halotypes/18 individuals in Phu Quoc (haplotype diversity=0.314). Phylogenetic tree did not show the specific population structure of two species from South Central and Southern coast of Vietnam. Simultaneously, populations of two species of giant clams in Vietnam had close relationship with the genetics of giant clam populations collected in some areas of Southeast Asian coast.

Recruitment constraints in Singapore's fluted giant clam (Tridacna squamosa) population--a dispersal model approach.

Recruitment constraints on Singapore's dwindling fluted giant clam, Tridacna squamosa, population were studied by modelling fertilisation, larval transport, and settlement using real-time hydrodynamic forcing combined with knowledge of spawning characteristics, larval development, behaviour, and settlement cues. Larval transport was simulated using a finite-volume advection-diffusion model coupled to a three-dimensional hydrodynamic model. Three recruitment constraint hypotheses were tested: 1) there is limited connectivity between Singapore's reefs and other reefs in the region, 2) there is limited exchange within Singapore's Southern Islands, and 3) there exist low-density constraints to fertilisation efficacy (component Allee effects). Results showed that connectivity among giant clam populations was primarily determined by residual hydrodynamic flows and spawning time, with greatest chances of successful settlement occurring when spawning and subsequent larval dispersal coincided with the period of lowest residual flow. Simulations suggested poor larval transport from reefs located along the Peninsular Malaysia to Singapore, probably due to strong surface currents between the Andaman Sea and South China Sea combined with a major land barrier disrupting larval movement among reefs. The model, however, predicted offshore coral reefs to the southeast of Singapore (Bintan and Batam) may represent a significant source of larvae. Larval exchange within Singapore's Southern Islands varied substantially depending on the locations of source and sink reefs as well as spawning time; but all simulations resulted in low settler densities (2.1–68.6 settled individuals per 10,000 m2). Poor fertilisation rates predicted by the model indicate that the low density and scattered distribution of the remaining T. squamosa in Singapore are likely to significantly inhibit any natural recovery of local stocks.

Isolation and characterisation of nine microsatellite markers in the boring giant clam (Tridacna crocea) and cross-amplification in five other tridacnid species

Nine novel polymorphic microsatellite markers were isolated in the boring giant clam (Tridacna crocea). The number of alleles ranged from 7 to 21. The observed and expected heterozygosity varied from 0.400 to 1.000 and 0.727 to 0.932, respectively. No significant linkage disequilibrium between pairs of loci was found and eight of nine loci were in Hardy-Weinberg proportions. Cross-amplification in other tridacnid species showed that all loci could be successfully amplified in Tridacna maxima, five in T. squamosa, three in T. derasa, three in T. gigas, and three in T. costata. These markers are therefore potentially useful for studies on the genetic diversity and connectivity of giant clam populations in order to facilitate the spatial arrangement of marine protected areas (MPAs) and fisheries management for these species.

Ecological determinants and stochastic fluctuations of Tridacna maxima survival rate in Lakshadweep Archipelago

Compounding effects of slow life-history traits and adult-specific overharvesting have caused a range-wide decline of several giant clam populations. Erratic changes in these populations reported at isolated reefs, are attributable to stochastic fluctuations in recruitment and survival rates. These population rate parameters may be adversely influenced by the considerable and increasing human–reef economic interactions in tropical waters. The unsubstantiated knowledge on such influences hinders conservation management of species-habitat linkages particularly for species like the small giant clam (Tridacna maxima). Our study reports small giant clam survival rate, its ecological determinants and stochastic fluctuations from shallow lagoon waters of Lakshadweep Archipelago; a natural habitat not subjected to commercial giant clam harvesting. Survival rate was estimated by monitoring the status of identified individuals in belt-transects through successive years 2005–2009. The role of different factors in explaining the variation in survival rates was investigated at relevant scales: anthropogenic pressure at the population level in the reef, clam density at sub-population level in the belt-transects, and variables related to clams at the individual level. Effects were quantified through univariate regression techniques and Kaplan–Meier estimator coupled with generalized linear (logit link) models. Small giant clam survival rate in the reef decreased and its spatio-temporal fluctuations increased along the increasing gradient of human populations. Survival was density-independent. Size/age specific survival followed Siler distribution characterized by very low early-age survival, increased adult-age survival, and low older-age survival. Anchorage in coral substrate exerted a curvilinear effect, where survival of moderately anchored clams > loosely anchored clams >> deeply embedded clams. Our study recommends regular population monitoring in densely inhabited islands to detect incipient changes, which can subsequently be confronted by scientific lagoon bed management.

A comparison of two surveys of invertebrates at Pacific Ocean islands: the giant clam at Raivavae Island, Australes Archipelago, French Polynesia

Abstract Andréfouët, S., Friedman, K., Gilbert, A., and Remoissenet, G. 2009. A comparison of two surveys of invertebrates at Pacific Ocean islands: the giant clam at Raivavae Island, Australes Archipelago, French Polynesia. – ICES Journal of Marine Science, 66: 1825–1836. An assessment of invertebrate fisheries is currently taking place at several Pacific Ocean islands. The objectives are to obtain either detailed information on certain stocks at limited sites or to assess more broadly a variety of benthic resources across different islands. In French Polynesia, giant clam (Tridacna maxima) populations were surveyed by Service de la Pêche and Institut de Recherche pour le Développement (SPE/IRD). Sampling was optimized to determine stock abundance as a tool to enhance management of the clam fishery. Currently, the Secretariat of the Pacific Community (SPC) is investigating throughout the Pacific the status of invertebrate resources; a large-scale study not necessarily establishing a precise stock estimate for resources such as clams, but comparing resource status for several target species using coverage, density, and size measures. Raivavae Island (French Polynesia) was investigated by both programmes and offered an opportunity to verify whether the different sampling schedules provided consistent perspectives of the status of the T. maxima resource. The different strategies that SPE/IRD and SPC adopted resulted in no direct spatial overlap between the locations investigated: nevertheless, the ranges of densities and clam sizes recorded were generally consistent between surveys, and both programmes described similar spatial variation in clam presence at an island scale. SPE/IRD provided a detailed map of clam densities per habitat using a high-resolution satellite image, which yielded an estimated standing stock of 8.16 ± 0.91 million clams, representing a flesh biomass of 354 ± 41 t. SPC's study delivered coverage, density, and clam length, but no stock estimate. Unavailable from SPE/IRD, SPC also described the status of a variety of important invertebrate species targeted by fishers in the Pacific. Both programmes independently made similar fishery management recommendations. The relative merits and complementarities of the two approaches in the context of Pacific Ocean Island resource management are discussed.

Can giant clam (Tridacna squamosa) populations be restored on Singapore's heavily impacted coral reefs? James R. Guest, Peter A. Todd, Eugene Goh, Balasubramaniam S. Sivalonganathan and Konda. P Reddy. Aquatic Conservation: Marine and Freshwater Ecosystems. Published online 10 August 2007 DOI: 10.1002/aqc888

Can giant clam (Tridacna squamosa) populations be restored on Singapore's heavily impacted coral reefs? James R. Guest, Peter A. Todd, Eugene Goh, Balasubramaniam S. Sivalonganathan and Konda. P Reddy. Aquatic Conservation: Marine and Freshwater Ecosystems. Published online 10 August 2007 DOI: 10.1002/aqc888

Can giant clam (Tridacna squamosa) populations be restored on Singapore's heavily impacted coral reefs?

1. Giant clams have been a sustainable resource for millennia, but unregulated harvesting has led to local extinctions within the Indo-Pacific region. Giant clam mariculture can produce large numbers of juveniles for restocking wild populations where natural recruitment is low or absent. 2. Singapore is surrounded by more than 60 small islands, many with fringing reefs. These reefs, however, experience increased turbidity and sedimentation resulting from massive coastal development projects and regular dredging of shipping lanes. 3. Seven reefs off Singapore's southern islands were surveyed (9670 m2) for giant clams. Also, an experiment was conducted to determine the growth of Tridacna squamosa reared in aquaria under three light treatments: ∼50% ambient photosynthetically active radiation (PAR); ∼25% ambient PAR; and ∼12% ambient PAR. Finally, 144 clams (T. squamosa) were transplanted to four reefs around Singapore to study survival and growth in a heavily impacted environment. 4. A total of 23 adult clams from three species were found during the survey, representing a mean density of 0.24 per 100 m2. Most clams were found at Raffles Lighthouse, Singapore's ‘best’ reef. No juvenile clams were encountered. In the aquarium experiment, clam growth was significantly different among the three light treatments, with growth greatest in the ∼50% ambient PAR treatment. Of the 144 transplanted clams, 116 (80.6%) were recovered after 7 months. All specimens had increased in size, with growth rates among reefs ranging from 3.3 mm month−1 (SD=1.3 mm) to 4.8 mm month−1 (SD=1.6 mm). 5. Results suggest that, despite high levels of sedimentation and turbidity on Singapore's reefs, giant clams can survive and grow well. Restocking efforts using maricultured clams may be effective in enhancing the dwindling local populations. It is not clear, however, whether a self-sustaining community can be established as high sedimentation may hinder larval settlement and survival. Copyright © 2007 John Wiley & Sons, Ltd.

Genetic variability in the Indonesian giant clam (Tridacna crocea and Tridacna maxima) population:implications for mariculture and restocking program

Tridacna crocea and T. maxima are relatively abundant in the Indonesian coral reef. These two species are, however, under high presure due to exploitation for food, industry, and aquarium trade. It is, therefore, necessary to understand their biology, such as genetic variability within and between populations, before utilizing them for strain improvement and restocking, prior to the extinction of the populations of T. crocea and T. maxima. Here we amplified a length of 456 bp of the mitochondrial DNA cytochrome c oxidase I gene from Tridacna crocea and of 484 bp from T. maxima to asses the genetic variability within and between populations of both species. The results showed that both species have high genetic diversity and polymorphism within each local population. This provides a sufficient basis for selection of improved strain of T. crocea and T. maxima for mariculture. However, if the genetic variation led to genetic differentiation among populations due to the result of evolutionary adaptation, mixing genetically different populations may result in the break up of co-adaptation gene complexes. This might result in the loss of the physiological capacities of the parental populations.

Molecular Genetic Analysis of Giant Clam (Tridacna sp.) Populations in the Northern Red Sea

Molecular Genetic Analysis of Giant Clam (Tridacna sp.) Populations in the Northern Red Sea

High variability of genetic pattern in giant clam (Tridacna maxima) populations within French Polynesia

Nine populations of giant clams, Tridacna maxima, from six islands of French Polynesia were screened for allozyme variation at ten polymorphic loci. The genetic structure of populations of T. maxima were studied at different spatial scales: within an island, between islands of the same archipelago and between archipelagos. Significant genetic differences were observed only between populations from different archipelagos, and genetic differentiation was correlated with geographical separation. However, these results were only supported by a single locus, PEP * and all other loci were homogeneous between studied populations. According to Lewontin & Krakauer’s model, the genetic structure can be explained by selection. The selective factors most likely depend on the respective habitat of each archipelago. We also studied genotype‐phenotype correlation using the colour of the clam mantle, and did not find any relationship between the mantle colour and the genetic structure of the individuals. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 221‐231.

Sequence variation in the ribosomal DNA internal transcribed spacer of Tridacna crocea.

DNA-based genetic markers are needed to augment existing allozyme markers in the assessment of genetic diversity of wild giant clam populations. The dearth of polymorphic mitochondrial DNA regions amplified from known universal polymerase chain reaction (PCR) primers has led us to search other regions of the genome for viable sources of DNA polymorphism. We have designed tridacnid-specific PCR primers for the amplification of internal transcribed spacer regions. Sequences of the first internal transcribed spacer segment (ITS-1) revealed very high polymorphism, showing 29% variation arising from base substitutions alone. Preliminary restriction analysis of the ITS regions using 8 restriction enzymes revealed cryptic changes in the DNA sequence. These mutations are promising as marker tools for differentiating geographically separated populations. Such variation in the ITS region can possibly be used for population genetic analysis.

Genetic Structure of Giant Clam (Tridacna maxima) Populations in the West Pacific is Not Consistent with Dispersal by Present-Day Ocean Currents

Genetic Structure of Giant Clam (Tridacna maxima) Populations in the West Pacific is Not Consistent with Dispersal by Present-Day Ocean Currents

Genetic structure of giant clam (Tridacna maxima) populations from reefs in the Western Coral Sea

Allozyme variation at six polymorphic loci was examined in 10 populations of Tridacna maxima from reefs in the Western Coral Sea, to test whether patterns of relatedness previously reported for foraminiferan populations reflected a fundamental structuring of the fauna in the region. Genetic distances (Nei's D) among populations of T. maxima ranged from 0–0.065 and increased with increasing geographical separation. No significant differences in gene frequencies were observed among populations within two groups of reefs identified by cluster analysis: the Great Barrier Reef (GBR), and among the offshore reefs excluding Lihou and Osprey. Significant genetic differences among these groups and the outliers Lihou and Osprey were consistent with the greater geographical separation of populations between areas than within areas. There was no evidence of differentiation along a north-south axis as reported for the foraminiferan Marginopora vertebralis, nor did populations from offshore reefs on the Queensland Plateau form a well-defined group that was genetically distinct from the GBR. The patterns observed for M. vertebralis do not appear to reflect a fundamental structuring of biota in the region. The differences in the pattern of genetic variation for M. vertebralis as compared with those for T. maxima may be due to several differences in the biological characteristics of the two species. The time of breeding in particular may influence the extent to which the divergence of the East Australian Current restricts larval dispersal among reefs in the central Queensland Plateau.

No genetic differentiation of giant clam (Tridacna gigas) populations in the Great Barrier Reef, Australia

Six Tridacna gigas populations were sampled in 1990 from locations throughout the central and northern Great Barrier Reef (GBR). Despite separations in excess of 1000 km, mean Nei's unbiased genetic distances among the populations was 0.0007. The complete lack of spatial variation observed among populations did not results from lack of genetic variability. Genetic variation within populations was high, with mean heterozygosities from 0.221 to 0.250. Gene frequencies were consistent with expectations under conditions of Hardy-Weinberg equilibrium. These data suggest panmixis, or random mating, throughout the highly connected reef system of the central and northern GBR. The large gene exchange among the giant clam populations has important implications for conservation management of one of the few large populations of T. gigas in the world. Small local effects are likely to be overcome in time by inputs from other sources. However, large genetic perturbations, particularly from up-current sources, may spread rapidly through the population.

Genetic structure of giant clam (Tridacna derasa) populations from reefs in the Indo-Pacific

Large genetic differences were observed among the Great Barrier Reef (GBR), Fiji and Philippine populations of Tridacna derasa (Roding) sampled in 1989 and 1990 (Nei's unbiased genetic distance, D,=0.137 to 0.341). This result contrasted strongly with the low genetic distance (D=0.032) reported previously for the giant clam T. maxima over similar geographical scales. No significant genetic differentiation was observed among most populations from the GBR (mean D=0.007), consistent with the high gene exchange expected in this highly connected reef system. However, significant differentiation resulting from differences in the frequencies of less common alleles between the North-Central GBR and South GBR (Swain region) were observed. Historical isolation of blocks of the Central Indo-West Pacific from the GBR and present-day restrictions to gene exchange between the GBR, Fiji and the Philippines as a result of oceanographic current patterns, were thought to be responsible for the high degree of genetic differentiation of T. derasa populations. The relevance of these findings to clam mariculture and reef restocking are briefly discussed.

Conservation and cultivation of giant clams in the tropical Pacific

Giant clams (Tridacnidae) are slow-growing and long-lived animals on the coral reefs of the Indo-West Pacific faunal region. There is a peak period, either summer or winter, in breeding activity of four giant clam species studied. Fertilised eggs develop into planktotrophic veliger larvae which settle and metamorphose into benthic juveniles about ten days after spawning. Although short pelagic larval life favours cultivation of the clams, spat handling is difficult because of the small size in postmetamorphosis juveniles of about 0.2 mm shell length. Colonising and maintenance strategies of giant clam populations resemble those of forest trees, indicating that giant clam resource management might parallel that of forestry.