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

Abstract

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.

Modelling mangrove propagule dispersal trajectories using high‐resolution estimates of ocean surface winds and currents

“…Regional and global scale studies in other marine systems have been presented recently (Treml et al 2008, Kool et al 2011, Neo et al 2013, Paris et al 2013, Hellweger et al 2014, Simpson et al 2014, Wood et al 2014. Wood et al (2014), for example, used a Lagrangian global dispersal model (the Connectivity Modelling System; Paris et al 2013) for a generic broadcast spawning coral, and compared the outcome with connectivity estimates derived from available genetic studies (global potential connectivity matrix).…”

Status of giant clam resources around Okinawa‐jima Island, Ryukyu Archipelago, Japan

“…In this study, the low density of adult giant clams (i.e. relatively small abundances and sparsely distributed) around Okinawa‐jima Island could affect reproduction in two ways: (a) individuals are unlikely to reproduce if there are no mature clams to trigger the cascade of synchronous broadcast spawning; and (b) it lowers the probability of gametes meeting for fertilization (Neo et al, ). These populations likely experience component Allee effects, where the individual's reproductive success has a positive density‐dependence at low relative population density (Stephens & Sutherland, ).…”

Larval ecology of the fluted giant clam, Tridacna squamosa, and its potential effects on dispersal models

“…Neo et al (2009) also observed reduced effects of CCA on settlement for older T. squamosa larvae and this probably reflects a change in settlement requirements (such as seeking protectionproviding habitats). Larval settlement behaviour (gregarious and/or associative) is usually incorporated into biophysical models to 'mimic' settlement (e.g., Bolle et al, 2009;Neo et al, 2013;Tay et al, 2012) to realistically direct larvae to appropriate habitats within time-limited dispersal (i.e., PLD). However, accurate knowledge regarding species-specific behaviour is often absent and hence assumptions are frequently drawn from other taxa.…”

“…Field observations of larval dispersal are inherently difficult, hence biophysical models (in either two or three dimensions) are increasingly being used to visualise and quantify larval transport and population connectivity (Cowen and Sponaugle, 2009;Levin, 2006;Metaxas and Saunders, 2009). Researchers have also inferred "source" and "sink" sites from their models (e.g., Lipcius et al, 2008;Neo et al, 2013;Tay et al, 2012), where sources may become optimal restocking areas and sinks targeted for conservation (Pulliam, 1988). Understanding source-sink dynamics in restocking and restoration of marine populations can also assist in preventing or mitigating potential anthropogenic impacts such as near-shore development (Lipcius et al, 2008).…”

“…While the physical parameters largely determine the hydrodynamic transport functions (Huret et al, 2010;van Maren and Gerritsen, 2012), including biological data is also important (as reviewed by Metaxas and Saunders, 2009). Critical factors such as planktonic larval duration (PLD: Shanks, 2009;Gilbert et al, 2010;Wolanski and Kingsford, 2014), timing of gamete or larval release (Edwards et al, 2007;Reitzel et al, 2004), swimming speeds Dekshenieks et al, 1996), diel vertical migration Edwards et al, 2007), mortality rates (Neo et al, 2013), and behaviour Neo et al, 2013) have been included into previous modelling studies for sessile marine invertebrates and fish. Duration of larval cycle, mortality rates, and larval behaviour, in particular, are known to significantly affect the success of larval dispersal, settlement and recruitment (Metaxas and Saunders, 2009).…”