The older species name, Tridacna squamosina Sturany, 1899, takes priority over the newer name, Tridacna costata Roa-Quiaoit, Kochzius, Jantzen, Zibdah & Richter, 2008 (Huber and Eschner 2011).

Tridacna squamosina was initially collected during the ‘Pola’ expedition to the Red Sea in the 1890s (Huber and Eschner 2011). Sturany (1899) first published the results of this expedition, which noted the presence of three Tridacna species in the Red Sea: T. maxima, T. squamosa, and a new species, T. elongata var. squamosina. This species was first rediscovered in the late 2000s in the Red Sea region by Richter et al. (2008). The largest individual so far was recorded at 35.9 cm long (J.J. ter Poorten pers. comm. 2024). A morphological comparison of materials revealed that Tridacna costata is identical to T. elongata var. squamosina, hence the former is now considered a junior synonym of T. squamosina.

Tridacna squamosina has a restricted distribution in the Red Sea, where its area of occupancy (AOO) is around 124 km². Despite extensive surveys, it is found in extremely low numbers; out of 1,000 observed tridacnine giant clams from a single site in the Red Sea, only six individuals of T. squamosina were found (i.e., <1% of the total giant clam population). The sites where it has been found represent a fragmented range from a formerly more widespread distribution. Furthermore, it has a restricted vertical range (i.e., it occurs only on reef tops), a narrow reproductive window (i.e., it reproduces only in the spring of the Red Sea), and plankton-dependent larval development, which make it vulnerable to overfishing and climate change.

Although this species currently is not a target of fishing, the present numbers allow inference that subpopulations could not recover their numbers without intervention. Mariculture of this species appears to be ongoing with intention to restock the cultured individuals, although the outcomes have not been reported. Ongoing coastal development and habitat loss in the Red Sea represent a threat to this species. Therefore, population decline of this species is projected to continue in the future as, although the causes of reduction are not well understood they seem not to have ceased.

CITES provides important ongoing protection for this species. Further work on the potential for mariculture to protect or enhance wild populations is needed for future conservation. Giant clams are vulnerable to climate change, as rising temperatures can cause bleaching of their photosymbionts. Ongoing monitoring is important to provide early warning of future declines. 

On the basis of a severely fragmented geographic range and projected ongoing declines, as well as very small populations recorded in each site (< 20 individuals per surveyed region), and as it is inferred to have extremely small total living population size (i.e., <2,500 mature individuals), Tridacna squamosina is assessed as Endangered (EN B2ab(i,ii,iii,v); C2a(i)).

This species is known only from the Red Sea, specifically Egypt, Israel, Jordan, Saudi Arabia, and Yemen (Neo et al. 2017, Rossbach et al. 2021). The population is severely fragmented, now present in disjoint areas of the Red Sea but was apparently previously more widespread (Lim et al. 2021).

Tridacna squamosina appears to be generally rare throughout the coast of the Red Sea. The first survey by Richter et al. (2008) found only 13 individuals during extensive surveys along the Jordanian Red Sea coastline. The most recent survey by Rossbach et al. (2021) also confirmed the species’ rarity, as only six individuals were found at a single reef in the southern Red Sea. Thus fewer than 250 individuals have been directly observed in scientific literature across its known area of occupancy (AOO). The total AOO is well established, as is the rarity of the species within its range. In other giant clam species, the lowest observed population densities are 0.2–0.3 individuals per hectare (Neo et al. 2017); for the AOO of T. squamosina this would indicate a total living population of 2,480–3,720 individuals. Taking a precautionary approach, and since the known numbers of animals in each of the subpopulations is much lower, we are using the lower end of this range to estimate the total extant population for the species.

Genetic evidence indicates that the currently population represents the highly fragmented remnants of a previously more widespread range; the timespan is unclear and may be relatively recent (Lim et al. 2021). The low abundance was postulated to be a result of over-harvesting in the Red Sea, where it formed a vital diet component of early coastal gatherers (>125,000 years ago) (Richter et al. 2008) but may be more recent. Phylogenetic studies of this species revealed that it is most closely related to Tridacna elongatissima, a Western Indian Ocean giant clam species (i.e., lowest genetic distance between both species) (Fauvelot et al. 2020).

Considering its rarity, information on the species’ habitat and ecology is generally limited. Based on the handful of sightings and collections, this species appears to be free-living on the seabed as adults. It occurs in very shallow waters (<2 m), including reef flats, seagrass beds, sandy-rubble flats, or under the branching corals or coral heads (Roa-Quiaoit 2005, Richter et al. 2008).

All species of giant clams are known to be simultaneous hermaphrodites. Preliminary work on the reproductive ecology (n = 5) of T. squamosina was first carried out in Jordan (Roa-Quiaoit 2005), revealing a species-specific reproductive seasonality with a shorter summer spawning (May–July), smaller ova diameter (75 + 2 μm), and earlier ovarian maturation (August) as compared to the other two species in the Red Sea (T. squamosa and T. maxima).

This species was said to form an important diet component of the early coastal gatherers (125,000 years) in the Red Sea (Richter et al. 2008). The current low abundance of T. squamosina has been attributed to past overexploitation.

In general, fishing of giant clams continued until the early 2000s, when the Saudi Arabian Wildlife Commission imposed a ban on the collection of giant clams (Rossbach et al. 2021). Consultation with local fishermen at various sites along the Saudi Arabian coastline indicates that giant clams have not been targeted in local fisheries since the ban (S. Rossbach pers. comm. 2022). Large scale harvesting is now limited by previous population declines and protection from CITES. Still, the populations of this species appear to not have recovered from the past overexploitation, which threatens the survival of this endemic Red Sea species.

The Red Sea coral reefs face numerous anthropogenic threats, including unsustainable tourism, habitat destruction, and overfishing, which could lead to poor habitat quality for giant clams (Rossbach et al. 2021). Moreover, the Red Sea is warming at a rate 2.5 times faster than the global average, according to IPCC reports. This rate of rising temperature and the resulting bleaching may become a threat to the survival of this species.

This species was said to form an important diet component of the early coastal gatherers (125,000 years) in the Red Sea (Richter et al. 2008). In general, fishing of giant clams continued until the early 2000s, until the Saudi Arabian Wildlife Commission imposed a ban on the collection of giant clams (Rossbach et al. 2021). According to local fishermen at various sites along the Saudi Arabian coastline, giant clams have not been targeted in local fisheries since the ban (S. Rossbach, pers. comm. 2022).

All giant clams (subfamily Tridacninae) are listed in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) on the basis of so-called 'look-alike species', i.e., species whose specimens in trade look like those of species listed for conservation reasons (Wells 1997). Thus, CITES regulates the international trade in any of their parts (shells, tissues, alive or dead).

No other known conservation actions are in-place due to a lack of data on the population size, life history and ecology, use and trade, and threats. While it appears that the Hurghada Environmental Preservation and Conservation Association (HEPCA) in Egypt has been culturing giant clams (see website: https://preview.hepca.org/projects/conservation/reef-restoration-program), it is unclear what species have been cultured successfully. Maricultured T. squamosina have appeared in the United States and European aquarium markets in the late 2010s (J.W. Fatherree pers. comm. 2024), although the details of their mariculture are difficult to obtain for assessment on the success of mariculture programmes.

While there is some knowledge of the species’ reproduction ecology (Roa-Quiaoit 2005), it is unclear if the information has been used for conservation purposes. In addition, a key challenge in scaling up this species’ mariculture is the difficulty in obtaining suitable and sufficient parent broodstock (Neo et al. 2017). The general use, trade, and threats of the species are limitedly known, but no management plans are in-place to monitor the trends of harvest and trade levels. Given the general rarity of this species, other anthropogenic activities (such as pollution and habitat destruction) could unknowingly push the species to extinction (partly due to the lack of baseline information on the status of wild populations).

This species has been assessed as a proposed endangered species in a status review for the US Endangered Species Act (NOAA 2024).