Drivers of the Abundance of Tridacna spp. Giant Clams in the Red Sea

Abstract

Giant clams (Subfamily Tridacninae), are important members of Indo-Pacific coral reefs, playing multiple roles in the framework of these communities. Although they are prominent species in Red Sea reefs, data on their distribution and densities in the region are scarce. The present study provides the first large-scale survey of Red Sea Tridacna spp. densities, where we examined a large proportion of the Saudi Arabian Red Sea coast (1,300 km; from 18° to 29°N). Overall, Tridacninae were found at densities of 0.19 ± 0.43 individuals m–2 (±SD). Out of the total 4,002 observed clams, the majority (89%) were Tridacna maxima, with 0.17 ± 0.37 individuals m–2, while only 11% were Tridacna squamosa clams with 0.02 ± 0.07 individuals m–2. We also report on a few (total 6) Tridacna squamosina specimens, found at a single reef. We identified different geographical parameters (i.e., latitude and distance to shore) and local environmental factors (i.e., depth and reef zone) as the main drivers for local Tridacna spp. densities. Our results show that the drivers influencing the densities of Red Sea giant clams are complex due to their co-occurrence and that this complexity might explain the high variation in Tridacninae abundances across the Indo-Pacific, but also within a given reef. We also estimate that giant clam calcification likely contributes to an average of 0.7%, but potentially up to 9%, of the overall mean calcium carbonate budget of Red Sea coral reef communities.

Phylogeographic Analysis Suggests a Recent Population Bottleneck in the Rare Red Sea Tridacna squamosina

“…Overharvesting of Red Sea giant clams was probably one of the main drivers for their historical population decline (Gladstone, 2000;Ashworth et al, 2004). Nowadays, fishing pressure seems to have a rather negligible impact on the present giant clams stocks in the region (Rossbach et al, 2020). The particularly sharp decline in T. squamosina densities, however, may be caused by its weak attachment to the substratum by its byssus threads, making it more vulnerable to collection by hand than the other Tridacninae species in the same region.…”

“…To date, at least 31 reported sightings of T. squamosina have been documented from the Red Sea since their first description during the Pola Red Sea Expeditions 1895/96 and 1897/98 (Huber and Eschner, 2011). The observations from the literature include 16 individuals from Roa-Quiaoit (2005) and Richter et al (2008), seven individuals from Huber and Eschner (2011), two individuals from Fauvelot et al (2020) and six individuals from Rossbach et al (2020). The past and present known distribution of T. squamosina are Egypt, Jordan, Yemen, Saudi Arabia, and possible sightings in Israel based on photographic anecdotes (Richter et al, 2008;Huber and Eschner, 2011;Neo et al, 2017;Fauvelot et al, 2020;Rossbach et al, 2020).…”

“…The observations from the literature include 16 individuals from Roa-Quiaoit (2005) and Richter et al (2008), seven individuals from Huber and Eschner (2011), two individuals from Fauvelot et al (2020) and six individuals from Rossbach et al (2020). The past and present known distribution of T. squamosina are Egypt, Jordan, Yemen, Saudi Arabia, and possible sightings in Israel based on photographic anecdotes (Richter et al, 2008;Huber and Eschner, 2011;Neo et al, 2017;Fauvelot et al, 2020;Rossbach et al, 2020). Although a recently published analysis using genome skimming has provided highly credible phylogenetic relationship between T. squamosina and the other Tridacninae (Tan et al, 2021), information about their population genetic structure across the Red Sea is still lacking.…”

Conserving threatened species during rapid environmental change: using biological responses to inform management strategies of giant clams

“…Within coral reef ecosystems, giant clams are known to make important contributions such as enhancing the net primary productivity of coral reefs by mixotrophy, nutrient recycling, provisioning of nurseries and shelters for other reef animals and serving as reservoirs of viable symbionts ( Neo et al , 2015 ; Umeki et al , 2020 ). Giant clam calcification also contributes to carbonate budgets and is estimated to contribute 0.7–9.0% of the mean calcium carbonate budget of Red Sea coral reef communities ( Rossbach et al ., 2021 ). The provision of structural refugia in particular scores bivalves, including giant clams, highly for their functional importance on coral reefs ( Wolfe et al , 2020 ).…”

Genetic Recombination of the Mantle Color Pattern of Two Boring Giant Clam (Tridacna crocea) Strains

“…Giant clams are often covered with unique patterns and various colors. For a long time, why they look as fancy has been a mystery (Fatherree, 2006;Neo et al, 2015;Rossbach et al, 2021). In the aquarium market, the price of giant clam is partially determined according to the different mantle colors.…”