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

Abstract

1. Giant clams provide and support valuable functions to coral reefs, as well as represent a sustainable resource for traditional fisheries throughout the Indo-Pacific region. The Ryukyu Archipelago (southern Japan) is known to be the northern latitudinal limits of giant clam distribution, but there is only limited information in the literature regarding species diversity, status, and distribution in this region.2. In this study, we report findings from a rapid survey in 2016, the first of its kind for the Ryukyu Islands, to determine species distribution and abundance of giant clams (tridacnines) around Okinawa-jima Island. 3. Results indicate the presence of four species with an overall density of 5.03 per 100 m 2 , from most to least abundant: Tridacna crocea, Tridacna maxima, Tridacna squamosa, and Tridacna noae. The previously reported species Tridacna gigas and Hippopus hippopus were both absent from the surveys. The densities and distributions of tridacnines varied among species and sites, which are likely attributable to efforts in replenishing and protecting stocks of selected species. 4. The most abundant species, T. crocea, is an important fishery species in Okinawa, and has been widely cultured and restocked to augment depleted populations. In comparison, restocking efforts for less popular species, such as T. squamosa, have been limited, and their current sizes and numbers suggest recruitment constraints. 5. Given the importance of the giant clam fishery in this region and the current declines of various species (except T. crocea), further regulations should focus on the protection of larger and mature clams that function as broodstock necessary to maintain spawning and natural recruitment.

Population status and genetic diversity of two endangered giant clams (Tridacna squamosa and Tridacna maxima) on the fringing reefs of Perhentian Islands, Malaysia

“…The overall COI haplotype diversity of T. squamosa and T. maxima from PMP was comparable to those reported in the Indo‐Pacific region (Nuryanto & Kochzius, 2009; Neo & Todd, 2012; DeBoer et al, 2014; Hui et al, 2016; Neo et al, 2018), but T. squamosa have lower haplotype diversity compared to T. maxima . This is likely to be the consequence of the reduced population density of T. squamosa across the region (Neo et al, 2017; Neo et al, 2018; Neo et al, 2019; Liu et al, 2020). Giant clams are known to display seasonal synchronous spawning (Heslinga, Perron & Orak, 1984); given the sedentary life and reliance on external fertilization, the reproductive success of giant clams is linked to the density of populations (Downing et al, 1993).…”

“…Illegal poaching of giant clams has been reported as one of the main factors in the decline of giant clam populations in Malaysia (Tan & Yasin, 2001a). In light of that, a sustainable giant clam fishery management plan will have to be implemented in MPAs of Malaysia; for example, residents are allowed to fish giant clams but with certain regulatory measures, such as minimum legal‐size limits, catch quotas, and maintaining certain areas as no‐take marine reserves (Gilbert et al, 2005; Gomez & Mingoa‐Licuanan, 2006; Teitelbaum & Friedman, 2008; Neo et al, 2019; Purcell, Gossuin & Ceccarelli, 2020).…”

“…The giant clam species were identified based on the common morphological characteristics of giant clams, these include the shell architecture, mantle tissue colour patterns, arrangement of hyaline organs or ‘eye spots’ on the outer mantle, and the habitat ecology (Lucas, 1988; Norton & Jones, 1992; Neo et al, 2019). One‐way analysis of variance (ANOVA) was used to test density differences between the two species.…”

Fluted giant clam (Tridacna squamosa) restocking experiment in an urban turbid reef environment

“…Overharvesting is one major cause that has led to giant clams being endangered in most parts of their geographic distribution (Neo et al, 2017; Neo, 2020). Although large‐scale harvesting is currently less prominent than in past decades (Hester & Jones, 1974; Hirschberger, 1980), giant clams remain under pressure from subsistence and semi‐commercial fishers (Neo et al, 2019; Ramah et al, 2019). Environmental impacts, such as habitat loss and climate change‐driven bleaching, have also contributed to their threatened status (Watson et al, 2012; Andréfouët et al, 2013; Watson & Neo, 2021).…”

Diversity of Three Small Type’s Giant Clams and Their Associated Endosymbiotic Symbiodiniaceae at Hainan and Xisha Islands, South China Sea

“…In the world, the Tridacninae subfamily includes two extant genera, Hippopus (two species) and Tridacna (10 extant species), and all these species host symbiotic zooxanthellae in the mantle tissues that plays important ecological roles in the coral reef ecosystem (Liu et al, 2021b). Recently, the natural resources of giant clams sharply decreased in many countries, including Australia, Indonesia, Singapore, Philippines, and Japan because of overfishing, habitat destruction, and global climate change (Copland and Lucas, 1988;Pringgenies et al, 1995;Neo and Todd, 2012;Neo et al, 2019). Therefore, all the giant clam species are listed in Appendix II of the Convention on International Trade in Endangered Species [United Nations Environment Programme-World Conservation Monitoring Center (UNEP-WCMC), 2007] and International Union for Conservation of Nature (IUCN) Red List of Threatened Species (Wells, 1997).…”