Mei Lin Neo

Recruitment constraints in Singapore's fluted giant clam (Tridacna squamosa) population—A dispersal model approach

Recruitment constraints on Singapores 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 Singapores reefs and other reefs in the region, 2) there is limited exchange within Singapores 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 Singapores 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.

Quantification of water squirting by juvenile fluted giant clams (Tridacna squamosa L.)

The giant clam species Tridacna crocea, T. deresa, T. gigas, T. maxima and Hippopus hippopus are known to exhibit squirting behaviour, ejecting a stream of water either from their exhalant or inhalant siphon. Here, for the first time, squirting in juvenile fluted giant clams, T. squamosa, was measured. By analysing stills from video recordings it was possible to determine the horizontal and vertical distances travelled by each squirt above the water line, the cross-sectional area of the water jet, and the angle of squirt perpendicular to the horizontal axis of the giant clam. The weight of each “aerial squirt” was measured by collecting the displaced water. Using these parameters, the initial velocity, force and pressure exerted by each squirt on an object was calculated. Strong positive correlations were observed between shell length and weight of seawater and force exerted by aerial squirts. We also modelled the pressures that would be experienced by underwater targets. The simulated “underwater squirts” indicate the pressure produced rapidly decreases with distance from the clam.

First observations of Tridacna noae (Röding, 1798) (Bivalvia: Heterodonta: Cardiidae) in Christmas Island (Indian Ocean)

We report these first in situ observations of the Noah’s giant clams, Tridacna noae from Christmas Island. On two separate occasions during diving in Christmas Island, we found five unique individuals of T. noae, which were first identified by their mantle ornamentation patterns. These preliminary observations expand geographic range westwards from South China Sea to East Indian Ocean, and are compared with known information on current T. noae geographic distribution.

Quantification of coral sperm collected during a synchronous spawning event

Most studies of coral reproductive biology to date have focused on oocyte numbers and sizes. Only one (ex situ) study has enumerated sperm numbers, even though these data have multiple potential applications. We quantified total coral sperm and eggs per gamete bundle collected from six species in situ during a synchronous spawning event in Singapore. Egg-sperm bundles were captured midwater as they floated towards the surface after being released by the colony. For each sample, a semi-transparent soft plastic bottle was squeezed and released to create a small suction force that was used to ‘catch’ the bundles. This technique provided several advantages over traditional methods, including low cost, ease of use, no diving prior to the night of collection needed, and the ability to target specific areas of the colony. The six species sampled were Echinophyllia aspera, Favites abdita, F. chinensis, Merulina ampliata, M. scabricula and Platygyra pini. The mean number of sperm packaged within one egg-sperm bundle ranged from 2.04 × 106 to 1.93 × 107. The mean number of eggs per egg-sperm bundle ranged from 26.67 (SE ± 3.27) to 85.33 (SE ± 17.79). These data are critical for fertilisation success models, but the collection technique described could also be applied to studies requiring in situ spawning data at the polyp level.