The ecological significance of giant clams in coral reef ecosystems

AbstractGiant clams (Hippopus and Tridacna species) are thought to play various ecological roles in coral reef ecosystems, but most of these have not previously been quantified. Using data from the literature and our own studies we elucidate the ecological functions of giant clams. We show how their tissues are food for a wide array of predators and scavengers, while their discharges of live zooxanthellae, faeces, and gametes are eaten by opportunistic feeders. The shells of giant clams provide substrate for colonization by epibionts, while commensal and ectoparasitic organisms live within their mantle cavities. Giant clams increase the topographic heterogeneity of the reef, act as reservoirs of zooxanthellae (Symbiodinium spp.), and also potentially counteract eutrophication via water filtering. Finally, dense populations of giant clams produce large quantities of calcium carbonate shell material that are eventually incorporated into the reef framework. Unfortunately, giant clams are under great pressure from overfishing and extirpations are likely to be detrimental to coral reefs. A greater understanding of the numerous contributions giant clams provide will reinforce the case for their conservation

Recruitment Constraints in Singapore's Fluted Giant Clam (Tridacna squamosa) Populations - A Dispersal Model Approach

10.1371/journal.pone.0058819PLoS ONE83

Molecular Characterization of a Dual Domain Carbonic Anhydrase From the Ctenidium of the Giant Clam, Tridacna squamosa, and Its Expression Levels After Light Exposure, Cellular Localization, and Possible Role in the Uptake of Exogenous Inorganic Carbon

A Dual-Domain Carbonic Anhydrase (DDCA) had been sequenced and characterized from the ctenidia (gills) of the giant clam, Tridacna squamosa, which lives in symbiosis with zooxanthellae. DDCA was expressed predominantly in the ctenidium. The complete cDNA coding sequence of DDCA from T. squamosa comprised 1,803 bp, encoding a protein of 601 amino acids and 66.7 kDa. The deduced DDCA sequence contained two distinct α-CA domains, each with a specific catalytic site. It had a high sequence similarity with tgCA from Tridacna gigas. In T. squamosa, the DDCA was localized apically in certain epithelial cells near the base of the ctenidial filament and the epithelial cells surrounding the tertiary water channels. Due to the presence of two transmembrane regions in the DDCA, one of the Zn2+-containing active sites could be located externally and the other one inside the cell. These results denote that the ctenidial DDCA was positioned to dehydrate HCO3- to CO2 in seawater, and to hydrate the CO2 that had permeated the apical membrane back to HCO3- in the cytoplasm. During insolation, the host clam needs to increase the uptake of inorganic carbon from the ambient seawater to benefit the symbiotic zooxanthellae; only then, can the symbionts conduct photosynthesis and share the photosynthates with the host. Indeed, the transcript and protein levels of DDCA/DDCA in the ctenidium of T. squamosa increased significantly after 6 and 12 h of exposure to light, respectively, denoting that DDCA could participate in the light-enhanced uptake and assimilation of exogenous inorganic carbon

Proportion of larvae settled onto Singapore's coral reefs.

<p>Percent of total number of <i>T. squamosa</i> larvae released from various regional donor reefs that reached recipient reefs around Singapore's Southern Islands.</p

Mortality rates for <i>Tridacna</i> larvae.

<p>Where data for <i>Tridacna squamosa</i> were deficient, larval mortality at 5 larval stages was extrapolated from published and unpublished reports of other giant clam species. Data have been reworked to fit into the model, <i>k</i> = −In(1−p_m)/(<i>D</i>/24) in which <i>D</i> is stage duration and p_m is the proportion of dead larvae.</p

Egg dispersal potential of individual giant clams among the Southern Islands reefs.

<p>Only clams with more than one egg per m² arriving onto a reef within the first 6 hours were considered to constitute successful transport.</p

Contour plots of settler density.

<p>Distribution patterns of giant clam larvae on local coral reefs at the end of transport phase for the three spawning periods: A) 22 January, B) 10 April and C) 18 June 2004.</p

Source-sink dynamics for Singapore's coral reefs.

<p>Summation matrix of settled larvae (per 10,000 m²) showing the potential sources (rows) versus sinks (column) among the Southern Islands coral reefs. Source sites are arranged according to the descending shortest straight-line distance to the mainland.</p

Singapore regional model.

<p>This model is composed of 3 domains. A) The overall outer domain including Peninsular Malaysia and the 8 regional release points (green dots). The red and blue domains represent the refined grid resolutions for Singapore's coastal waters. B) The blue grid encompasses the waters surrounding Singapore's Southern Islands. The red dots represent the 28 release points (i.e. the positions of <i>T. squamosa</i> in Singapore).</p