The Small Giant Clam, Tridacna maxima Exhibits Minimal Population Genetic Structure in the Red Sea and Genetic Differentiation From the Gulf of Aden

Abstract

The Red Sea serves as a natural laboratory to investigate mechanisms of genetic differentiation and population dynamics of reef organisms due to its high species endemism. Giant clams, important yet understudied coral reef engineering species, are ideal candidates for such study in this region. This paper presents the first population genetics study of giant clams covering the entire East coast of the Red Sea. Our study aimed to investigate the population structure of the small giant clam, Tridacna maxima, based on 501-bp fragment of the cytochrome c oxidase I gene from 194 individuals (126 new sequences from this study plus 68 sequences from GenBank), collected from 14 locations in the Red Sea and Gulf of Aden (RSGA). For the genetic analysis, each sampling site was treated as a population. T. maxima showed high genetic diversity, with high gene flow in almost all sampling sites. The insignificant global φ ST-value of 0.02 (p > 0.05) suggests the presence of one large, panmictic population across a wide range of temperature and salinity gradients in the RSGA. Despite this, the population in Djibouti was genetically differentiated from the other 11 populations in the Red Sea, suggesting a connectivity break between the Red Sea and the Gulf of Aden. These results could be explained by the oceanographic features facilitating wide larval transport inside the Red Sea, and creating a dispersal barrier to the Gulf of Aden. Besides larval dispersal by currents, apparent successful establishment following dispersal is probably facilitated by the mode and time of reproduction as well as the ability of T. maxima to achieve high fitness in the highly variable environmental conditions of the Red Sea.

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

“…Occasional confusion of T. maxima and T. squamosa have been also previously suspected for the Red Sea (Pappas et al, 2017). However, as the same observers as in the present study, reached a very high and genetically confirmed accuracy in previous species-targeted sampling (Lim et al, 2020), and since the presented ratio of 89% T. maxima and 11% T. squamosa clams corresponds to past reports on species allocation in the region (Jantzen et al, 2008;Pappas et al, 2017), we are confident that presented numbers actually reflect what can be found in Red Sea coral reefs.…”

“…For the Red Sea, three species of giant clams have been previously reported (e.g., by Roa-Quiaoit, 2005;Richter et al, 2008;Huber and Eschner, 2010;Ullmann, 2013), namely Tridacna squamosa Lamarck, 1819, Tridacna squamosina Sturany, 1899 (previously described as T. costata by Richter et al, 2008) and Tridacna maxima Röding, 1789. The latter, also called the "small giant clam, " is assumed to be the most abundant giant clam species in the Red Sea (Jantzen et al, 2008;Pappas et al, 2017;Lim et al, 2020). Previously reported densities of T. maxima vary substantially, with typical averages ranging between 10 −4 and 10 −5 individuals per meter square (m 2 ), in different regions of the Indo-Pacific .…”

Flexibility in Red Sea Tridacna maxima‐Symbiodiniaceae associations supports environmental niche adaptation

“…The observed site‐specific structure of the Red Sea giant clam‐Symbiodiniaceae associations contrasts with the recently reported population structure of the T. maxima host. Current evidence points toward a genetic disparity between the Red Sea T. maxima clams and other populations from the West Indian Ocean (Fauvelot et al., 2020), but that the population inside the Red Sea is characterized by high gene flow among regions and panmixia (Lim et al., 2020). Yet, Lim et al, also observed a high level of host haplotypic diversity within the Red Sea population of T. maxima (i.e., a number of haplotypes at each site), which contrasts with the observed homogeneity of the associated Symbiodiniaceae assemblages that we identified here, especially in the more southern sites.…”

“…In contrast to corals, where more fine‐scale resolutions increasingly reveal a relatively specific host genotype‐determined algal assemblage, such flexibility in clams represents, if confirmed, a mechanism conferring giant clams an additional resilience to warming. Indeed, although coral reefs in the Red Sea, particularly those in the southern Red Sea, have experienced intense warming‐induced bleaching in the past, such mass bleaching events have not been reported for Red Sea T. maxima populations (Lim et al., 2020). Testing this hypothesis would require experimental assessment of thermal performance of T. maxima under concurrent manipulation of their symbionts.…”

“…The observed site-specific structure of the Red Sea giant clam-Symbiodiniaceae associations contrasts with the recently reported population structure of the T. maxima host. Current evidence points toward a genetic disparity between the Red Sea T. maxima clams and other populations from the West Indian Ocean (Fauvelot et al, 2020), but that the population inside the Red Sea is characterized by high gene flow among regions and panmixia (Lim et al, 2020).…”