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

Abstract Giant clams (Tridacninae) are important members of Indo‐Pacific coral reefs and among the few bivalve groups that live in symbiosis with unicellular algae (Symbiodiniaceae). Despite the importance of these endosymbiotic dinoflagellates for clam ecology, the diversity and specificity of these associations remain relatively poorly studied, especially in the Red Sea. Here, we used the internal transcribed spacer 2 (ITS2) rDNA gene region to investigate Symbiodiniaceae communities associated with Red Sea Tridacna maxima clams. We sampled five sites spanning 1,300 km (10° of latitude, from the Gulf of Aqaba, 29°N, to the Farasan Banks, 18°N) along the Red Sea's North‐South environmental gradient. We detected a diverse and structured assembly of host‐associated algae with communities demonstrating region and site‐specificity. Specimens from the Gulf of Aqaba harbored three genera of Symbiodiniaceae, Cladocopium, Durusdinium, and Symbiodinium, while at all other sites clams associated exclusively with algae from the Symbiodinium genus. Of these exclusively Symbiodinium‐associating sites, the more northern (27° and 22°) and more southern sites (20° and 18°) formed two separate groupings despite site‐specific algal genotypes being resolved at each site. These groupings were congruent with the genetic break seen across multiple marine taxa in the Red Sea at approximately 19°, and along with our documented site‐specificity of algal communities, contrasted the panmictic distribution of the T. maxima host. As such, our findings indicate flexibility in T. maxima‐Symbiodiniaceae associations that may explain its relatively high environmental plasticity and offers a mechanism for environmental niche adaptation.

Tridacninae stand out among other bivalves as they are one of the few molluscan groups that live in a symbiotic relationship with dinoflagellates in the family Symbiodiniaceae (Taylor, 1969;Yonge, 1936). The relationship is comparable to the symbiosis of corals and their associated algae with respect to the symbionts providing a substantial amount of energy in the form of photosynthates for the host. Clam veliger larvae acquire free-living Symbiodiniaceae from the water column (Fitt & Trench, 1981) and harbor their symbionts extracellularly in a tubular system, that originates from the digestive diverticular ducts of the stomach, extending mainly in the outer mantle (Norton et al., 1992). Although Tridacninae have generally been described as mixotrophic (Hawkins & Klumpp, 1995;Klumpp et al., 1992), this photosymbiosis seems to be obligate for the clam host, as previous studies report that Tridacninae often perish in the absence of their algal symbionts (Addessi, 2001;Leggat et al., 2003), for example, following bleaching (i.e., the expulsion of their symbiotic algae; Glynn, 1993).
Importantly, the physiology of the algal symbionts may modulate the phenotype of their marine invertebrate hosts Howells et al., 2020;Rädecker et al., 2015;Rädecker et al., 2021;Silverstein et al., 2015;Terraneo et al., 2019). As an example, many Durusdinium taxa appear to be relatively stress-tolerant , for example, to warm and cold temperature-induced bleaching (Silverstein et al., 2017), and certain specialist Cladocopium taxa are found in the hottest coral-containing waters on Earth (Hume et al., 2016). Characterization of these algal assemblages, including assessing their potential for change, is therefore of particular importance when considering the adaptive potential of the holobiont (the consideration of the animal host and all associating organisms as a single unit). By virtue of their socio-economic and ecological value, these associations have received considerable attention in corals (LaJeunesse et al., 2004(LaJeunesse et al., , 2010LaJeunesse et al., 2014LaJeunesse et al., , 2018Lewis et al., 2019;Pettay et al., 2015;Sampayo et al., 2008;Stat et al., 2009;Thornhill et al., 2006). In contrast, Tridacninae-Symbiodiniaceae associations have received limited attention (as reviewed by Mies, 2019).
Among the markers that are available for assessing Symbiodiniaceae diversity, the internal transcribed spacer 2 (ITS2) of the rRNA gene array is most commonly used Hume, D'Angelo et al., 2018;Hume, Ziegler et al., 2018;LaJeunesse, 2002). This marker is considerably multicopy in nature with a single Symbiodiniaceae cell potentially containing hundreds of copies of the gene (Arif et al., 2014;LaJeunesse, 2002;Thornhill et al., 2007). While this intragenomic character may complicate analyses, profiling approaches such as the SymPortal framework , where sets of sequences may be considered diagnostic of a given genotype, make use of this intragenomic diversity to afford improved resolutions.
Use of the ITS2 marker, and to a lesser extent the full ITS region, is common in the assessment of Tridacninae-Symbiodiniaceae associations (DeBoer et al., 2012;Lim et al., 2019;Pappas et al., 2017;Weber, 2009). The resolutions achieved in these studies varies according to the specific marker (i.e., ITS2 or full ITS region ;Pappas et al., 2017;Weber, 2009), the sequencing technology used (Lim et al., 2019), and the degree to which intragenomic sequence diversity is taken into account (Lim et al., 2019). However, due to analytical (e.g., treatment of all sequence diversity as intergenomic in origin; Lim et al., 2019) and technological (e.g., DGGE;DeBoer et al., 2012) limitations, resolutions in these studies are generally limited to the assessment of the most abundant ITS2 sequence present. This level of resolution masks the majority of ecologically relevant inferences as considerable phenotypic diversity exists between Symbiodiniaceae taxa that share a most abundant ITS2 sequence in common (Arif et al., 2014;Hume et al., 2013;Hume et al., 2015;LaJeunesse et al., 2014;Thornhill et al., 2014).
Characterizations of Tridacna maxima-Symbiodiniaceae associations within the Red Sea are limited to two studies (Pappas et al., 2017;Weber, 2009; hereafter referred to as Weber and Pappas et al., respectively) that cover a limited geographical range.
Pappas et al., investigated 207 samples, from nine sites, all located at 22°N at the eastern coast of the central Red Sea and within 28 km of the "Thuwal" site from this study (given their proximity to each other, these nine sites will hereafter be referred to as a single site, "Thuwal"), while Weber analyzed samples from a total of 20 clams, originating from four sites, of which two were located in the Gulf of Aqaba at 27° (Ras Nasrani) and 28°N (Dahab), and two off the Egyptian coast in the North-western Red Sea at a latitude of 25° (El Qeseir) and 27° (Hurghada), respectively ( Figure 1). Given the extraordinary latitudinal hydrographical gradients that exist in the Red Sea (Agulles et al., 2020;Arz et al., 2003;Berumen et al., 2019;Chaidez et al., 2017), the coverage thus far available provides a limited representation of the region. Here we build on these prior characterizations using the ITS2 marker and the SymPortal framework to conduct a fine-scale characterization of Symbiodiniaceae associations in Red Sea T. maxima giant clams across the Red Sea's North-South gradient (from the Gulf of Aqaba at a latitude of 29°N to the Farasan Banks at 18°N), covering 1,300 km of overwater distance, and environmental differences. F I G U R E 1 Sampling maps, ITS2 sequence haplotype frequencies, and environmental gradients. (a) The five sampling sites of Tridacna maxima for this study along the Saudi Arabian Red Sea coast in the northern (29°-Gulf of Aqaba and 27°-Duba), central (22°-Thuwal), and southern (20°-Al Lith and 18°-Farasan Banks) Red Sea. The site colors correspond to the site colors in Figures 2 and 3. (b) Sampling sites from Weber, 2009 (Dahab, Ras Nasrani, Hurghada and El Qeseir) and Pappas et al. (2017) (9 sampling sites in close proximity on the same reef system referred to as Thuwal). For each sampling location, the number of ITS2 sequences is given as "n = x" and the proportion of the sequences that are one of the three most abundant haplotypes (shades of gray) or some other unclassified sequence (white) are detailed as pie charts. The asterisk at the Dahab site refers to the one sequence from the Weber, 2009 study that contained an ambiguous nucleotide in the exact nucleotide position that differentiates the first most abundant haplotype from the third most abundant nucleotide. For the creation of this figure, this sequence was considered to represent the third most abundant haplotype. Shades of gray refer to the inset ITS2 haplotype network. (b-inset) ITS2 haplotype network. Each node represents a different ITS2 sequence with size proportional to the number of sequences recovered from the Weber and Pappas studies combined. The number of base pairs (bp) different between each of the sequences is denoted by the number of small black nodes on the network edges (one node represents one bp difference).  (Hume & Voolstra, 2021). (b) and (c) were created using Ocean Data Viewer (Schlitzer, 2015; https://odv. awi.de/) and have the five sampling sites of this study overlaid as empty black circles for reference  (Manu & Sone, 1995  cm 2 was cut using a scalpel. Samples were then immediately frozen in seawater, using liquid nitrogen, and transported back to the laboratory where they were kept at −80°C until further analysis. At the two southern Red Sea stations (20° Al Lith and 18° Farasan Banks), seawater samples were collected (one at each site), at the same reefs and depths as the mantle tissue sampling using a 9 L polycarbonate carboy container (Nalgene, Thermo Scientific Fisher).
The containers were kept in a cooling box on ice until arrival at the laboratory, where, for each of the two reefs, 2 L of seawater was immediately filtered through a 0.22 µm hydrophilic polyvinylidene fluoride (GVWP) filter (Millipore, Merck KGaA) using a peristaltic pump.
Filters were instantly frozen at −80°C for further analysis.

| Tissue homogenization and DNA isolation
The frozen T. maxima mantle tissue samples were separated from the frozen seawater and homogenized using a Freezer/Mill ® (Model 6,875, SPEX ® Sample Prep) by grounding up the tissues in liquid nitrogen at a rate of ten impacts per second for a total of 90 s. The Freezer/Mill ® PVC tubes were cleaned thoroughly with 10% bleach between samples and the ground tissues were then dissolved in 5 ml F I G U R E 2 Symbiodiniaceae diversity of Tridacna maxima across the Red Sea. Genus-annotated abundances of ITS2 sequences and predicted ITS2 type profiles (above and below, respectively) arranged by sampling site. For each recovered genus, the 20 most common post-MED ITS2 sequences are plotted with remaining sequences binned into a single "other" category. Predicted profiles are plotted below the sequences. Blue: Symbiodinium; Orange: Cladocopium; Green: Durusdinium. Please refer to Figure S1 for a more thoroughly annotated version of this figure Milli-Q water (sterilized under UV light for 1 hr). The homogenate was transferred to 1.5 ml Eppendorf tubes and samples were frozen at −20°C until further processing.
In total, we extracted DNA from 50 samples, corresponding to 45 T. maxima mantle tissue samples, two water samples, and three negative controls: 1 to assess for DNA extraction kit contamination, 1 to assess for PCR reagent contamination, and 1 to assess for carryover contamination during the tissue homogenization. The carryover contamination negative sample was generated by adding Milli-Q water to the cryotubes after they had been rinsed as part of the rinsing step The PCR negative did not return valid ITS2 sequence reads, and the DNA extraction control returned a greatly reduced number of reads and was dominated by a sequence that was not a defining intragenomic sequence variant (DIV; a sequence that is part of an ITS2 type profile definition; e.g., A1aw, A1em, and A1aw are the three DIVs of the A1-A1aw-A1em profile) in any of the predicted ITS2 type profiles ( Figure S1). By contrast, the negative control for potential carryover contamination during the tissue homogenization returned a predicted profile that matched the profile most dominant at the Duba (27°) site. However, since this profile was only found in five samples that all originated from Duba, and that other profiles were predicted at this site, it can be assumed that there was no bias introduced from putative contaminants in the analysis.

F I G U R E 3
Symbiodinium diversity of Tridacna maxima across sites. Relative abundance of the 20 most common post-MED ITS2 Symbiodinium sequences and predicted ITS2 type profiles (above and below, respectively) arranged by sampling site. Remaining sequences are binned into a single "other" category

| Sequencing of internal transcribed spacer 2 (ITS2)
Symbiodiniaceae communities were characterized using next-  is performed on a per-sample basis before the sequence abundances are used to predict ITS2 type profiles. To plot ITS2 relative sequence abundances, we used the post-MED absolute abundance sequence count tables (as opposed to the pre-MED sequence count tables).

| Analyses using SymPortal
To produce a heat map of between-site average sample dissimilarities, and two PCoA ordinations, we used the Symbiodinium, Bray-Curtis-derived, between-sample dissimilarity matrix (including a square root transformation) that is output by default by SymPortal.
The between-site average dissimilarities were computed using a bespoke Python script and plotted using matplotlib's implementation of imshow. Specifically, for a given pairwise site comparison, for every sample in the first site, the distance to every sample in the second site was collected. A mean average and standard deviation were then computed from these distances. For self-site comparisons (e.g., Duba-Duba), average within-site distances were calculated. PCoAs were computed using Python and the scikit implementation of pcoa and plotted using matplotlib's implementation of scatter. To assess for statistical difference between sites across samples based on Symbiodinium ITS2 sequence assemblages, we ran a one-factor PERMANOVA (Anderson, 2001) using scikit-bio's function PERMANOVA and the Symbiodinium Bray-Curtis square root transformed distances. PERMANOVA is resilient to heteroscedasticity across factor groups when factor groups have equal numbers of samples (i.e., a balanced experimental design; Anderson & Walsh, 2013). Given the balanced design in this study, a PERMDISP2 analysis (Anderson, 2006) was not conducted. To further investigate the sequences driving structure in the Bray-Curtis-derived distances, we conducted a SIMPER analysis (Clarke, 1993) on the post-MED Symbiodinium sequences with site as the grouping factor.
The abundances were square-root transformed (to match the transformation of the abundance matrix used in the Bray-Curtis calculation), and the analysis was conducted in Python using ecopy's simper function.

| Reanalysis of ITS1-5.8S-ITS2 haplotypes from previous characterizations
Two previous giantclam-Symbiodiniaceae characterizations exist from the Red Sea (i.e., Weber, 2009 andPappas et al., 2017). These studies characterized the dominant Symbiodiniaceae genotypes by analyzing the full ITS region (ITS1-5.8S-ITS2) of the rRNA array resolved via Sanger sequencing. To assess for similarity in sampled haplotypes between these studies, we collated all sequences of Red Sea origin from them, computed a multiple sequences alignment using MAFFT (Katoh & Standley, 2013), and cropped at the 5′ and 3′ end of the alignment so that, with the exception of three short sequences that were removed from the alignment; all sequences were represented across the full alignment length. For reference, the A1 ITS2 sequence, as defined in the SymPortal remote database (sympo rtal.org) was included in the alignment (independent of any cropping). The full alignment is provided in Dryad submission https://doi. org/10.5061/dryad.k6djh 9w50.

| Comparison of clam and seawater ITS2 sequence assemblages to support inferences of selectivity
We used the two seawater samples (collected at Al Lith and Farasan Banks) to assess the likelihood that Symbiodiniaceae genotypes (represented in this study by ITS2 type profiles) associating with clams from the three most northern sites (Gulf of Aqaba, Duba, Thuwal) may be physically available for uptake by the clams at the two most southern sites (Al Lith, Farasan Banks). We searched for sequences that were present in the northern site clams and the southern site seawater samples, but not present in the southern site clams.
Our rationale was that while there would likely be a considerably larger richness of ITS2 sequences in the seawater samples compared to the clam samples, much of this richness would be representative of Symbiodiniaceae taxa potentially unable to form associations with T. maxima individuals (e.g., specialist free-living taxa). We therefore constrained our search for sequences in the seawater samples to those that were known to be found in T. maxima individuals (from the northern sites). Finally, we were interested in finding evidence of Symbiodiniaceae diversity that the southern T. maxima individuals could potentially be associated with but had not (evidence of selectivity). As such, we further constrained our search to those sequences not found in the southern T. maxima samples.
Samples were distributed into three groups: clam samples from the three most northern sites, clam samples from the two most southern sites, and the two seawater samples. For each group, we generated a set of sequences that were found at least once in any of the group member samples (i.e., a set of present sequences; using the post-MED abundance count tables output by SymPortal). We then performed union, intersection, and difference operations on these sets to determine unique or shared sequence members for every combination of the groups. The set operations were performed in Python using a bespoke script, and a three-way Venn plot was generated using venn3 from the Python package matplotlib-venn.

| Availability of outputs and scripts used in this study
For outputs used in this study, and the results of the SIMPER analy-

| Tridacna maxima ITS2 haplotypes from the Red Sea
To consolidate the current and previous efforts with regard to Symbiodiniaceae genotyping of T. maxima in the Red Sea, we com-

| ITS2-type profiles of Tridacna maxima across the Red Sea
Out of the 13 predicted distinct T. maxima-associated ITS2 profiles

| Higher Symbiodiniaceae diversity in the Gulf of Aqaba
All non-Symbiodinium profiles were recovered from the Gulf of Aqaba (29°) where several clam samples harbored a mix of Symbiodiniaceae genera. Genera-level profile diversity was therefore highest at this site (three genera) and absent at the four other sites (Figure 2). Clamassociated Cladocopium and Durusdinium sequence diversity was also low outside of the Gulf of Aqaba with only one sample returning one Cladocopium sequence at a relative abundance of 0.00014 (no Durusdinium sequences were returned). While sequences from Symbiodinium were more abundant than those from Cladocopium and Durusdinium (76% ± 24% vs. 22% ± 23% and 3% ± 8%, respectively) in the Gulf of Aqaba, differences in ITS2 array copy number between different Symbiodiniaceae taxa, specifically at the genus level, preclude the accurate inference of the relative abundance of these Symbiodiniaceae genera at this site.

| Symbiodinium diversity and profile distributions across the Red Sea
Given the prominent association of T. maxima with Symbiodinium, we did a dedicated analysis to elucidate fine-scale differences of association. Variation of between-sample dissimilarities within sites was highest in the two northernmost sites, the Gulf of Aqaba (29°) and Duba (27°; Figure 4a). This observation, based on a sequence assemblage metric, is in concordance with the higher number of profiles Discounting the single-DIV A1 profile occurrences, in general, a high site-profile specificity was apparent, with a single specific profile dominant at each site ( Figure 3). This finding is congruent with the significant PERMANOVA results returned from our analysis (pseudo-F = 16.04, p < .001). However, in some cases, these dominant profiles were found at other sites at rarer abundances.

| Availability of northern clam-associated Symbiodiniaceae genotypes to southern clams
We searched for sequences that were present in the northern site clams and the southern site seawater samples, but not present in the southern site clams. We found 18 ITS2 sequences that fit these criteria ( Figure S2) from Symbiodinium, Cladocopium, and Durusdinium.
Although not definitive proof, this result is highly suggestive of the fact that Symbiodiniaceae genotypes, in addition to those recovered in this study, are available for uptake by T. maxima at the southern sites, but that the southern clams are selectively associating with an alternative, relatively small proportion of the available diversity. Our finding, that Symbiodinium seems to be the dominant genus in Red Sea T. maxima clams, is consistent with the few available reports from the region. However, these previous studies, from the Egyptian, North-western Red Sea coast and the Gulf of Aqaba (Weber, 2009), as well as from reefs in the central-eastern Saudi Arabian Red Sea (Pappas et al., 2017), concluded that Red Sea giant clams associate exclusively with Symbiodinium. Yet, our results show that Red Sea T. maxima also associates with Cladocopium and Durusdinium. However, these genera appear to be less dominant than Symbiodinium, and our finding of these genera at a single site in the Gulf of Aqaba, coupled with Weber's finding of only Symbiodinium at two Gulf of Aqaba sites, suggests that their association with Tridacninae is rare and may be restricted to a small sitespecific distribution.

F I G U R E 4 Symbiodinium diversity of
Our finding of a considerable diversity of Symbiodinium populations that have the A1 sequence as their most abundant ITS2 sequence is also in agreement with the multiple distinct ITS haplotypes previously reported from Weber and Pappas et al., (each of which had an exact match to the A1 ITS2 sequence). The resolution afforded by our intragenomic diversity-defined ITS2 profiles and the full ITS region haplotypes may be similar. Of the 9 sites sampled by Pappas et al., a site referred to as "site 1" in their study was closest to our "Thuwal" site (presumably the same sampling site; although all sampling sites were within 28 km distance to our "Thuwal" site).
Three approximately equally dominant haplotypes were recovered from this "site 1." In contrast, we recovered only a single dominant ITS2 profile. However, the single-DIV A1 profile recovered in one of our samples is likely symptomatic of the additional extant diversity at this site. As such, differences in the number of operational taxonomic units recovered may be largely due to sampling methodology (i.e., sampling depths, numbers of samples) rather than a difference in resolution. Finally, our findings of shared profiles be-  Figure 1b). Importantly, the fact that the ITS haplotype samplings were conducted approximately 8 years apart would indicate a significant temporal stability of these clam-Symbiodiniaceae associations and corroborate previous notions of high symbiont fidelity (Howells et al., 2020;Terraneo et al., 2019), even through putative episodes of bleaching . However, and contrary to the temporal stability of such associations, we found largely distinct site associations, as discussed in the following.

| Diversity and regional structuring of Symbiodiniaceae assemblages correlate with regional hydrographic gradients
The Red Sea displays distinct natural latitudinal gradients of temperature (Agulles et al., 2020;Chaidez et al., 2017), which are overall high and increase toward the South, and salinity (Arz et al., 2003;Ngugi et al., 2012), which is high in the North and shows a decrease toward lower latitudes. The antagonistic nature of these two gradients produces a high diversity of prevailing environmental conditions in Red Sea coral reefs, with a strong spatial variance, especially when comparing reefs from the North (i.e., cooler and more saline) to those in the South (i.e., warmer and less saline).
These pronounced latitudinal and environmental gradients have been shown to shape the genetic population structure of a number of marine species, resulting in a distinct genetic break of their populations at a latitude of approximately 19°N (Froukh & Kochzius, 2007;Giles et al., 2015;Nanninga et al., 2014;Shefer et al., 2004). Specifically, they have been reported to shape symbiont associations, for example, in Porites corals, where associated algal symbionts have been shown to shift from a Cladocopium-to a Durusdinium-dominated community, along the North-South gradient of the Red Sea (Terraneo et al., 2019). Our results indicate a regional and site-specific structuring of giant clam-Symbiodiniaceae associations along the latitudinal environmental gradient in the Red Sea. At the regional level, we identified three groupings, based on sequence assemblage dissimilarity, reflecting the latitudinal North-South gradient, as ITS2 assemblages from the Gulf of Aqaba (29°) and the central/northern grouping were more similar to each other than to the southern grouping. However, of particular note is the observed grouping of Duba (27°) and Thuwal (22°) in the northern and central Red Sea, respectively, despite the very large geographical distance (~700 km) between these two sites. However, the Symbiodiniaceae communities at Thuwal (22°) and Al Lith (20°) were considerably dissimilar despite the short geographic distance (~250 km) between these sites. This genetic structuring yet further supports the concept of a genetic discontinuity across Red Sea marine taxa at approximately 19-20°N (Froukh & Kochzius, 2007;Giles et al., 2015;Nanninga et al., 2014;Shefer et al., 2004).
Of the five sites sampled in this study, Symbiodiniaceae communities were more diverse in the northern sites (i.e., Gulf of Aqaba, 29° and Duba, 27°) than in the more southerly sites. Symbiodiniaceae Gulf and the more southerly Red Sea sites, can be hypothesized, therefore, to be the most likely driver of Symbiodiniaceae assemblage diversity.

| Panmictic distribution of Tridacna maxima suggests environmental-rather than host genotypedriven assemblage structuring
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 corals, fine-scale resolutions of Symbiodiniaceae assemblages often strongly correlate to host genotype (Gardner et al., 2019;Howells et al., 2020;Hume, D'Angelo, et al., 2018;Hume et al., 2020). However, if the Red Sea T. maxima populations are assumed to be a single well-connected, yet diverse population, this would suggest that the prevailing environmental conditions, which are also known to strongly influence coral-Symbiodiniaceae associations LaJeunesse et al., 2010;Oliver & Palumbi, 2009;Smith et al., 2020;Terraneo et al., 2019;Varasteh et al., 2017;Voolstra, Valenzuela, et al., 2020;Ziegler et al., 2015), are the driving forces of the observed structure. Whether this relatively high clam-symbiont flexibility is a product of the location in which the algal symbionts reside within the host tissue (i.e., extracellularly in clams vs. intracellularly in corals) remains to be investigated.

| Inter-site flexibility of Tridacna maxima-Symbiodiniaceae assemblages as a putative mechanism for niche adaptation
The finding of multiple T. maxima mtCOI marker haplotypes (distinct from those found in the wider Indian Ocean) at most of the Red Sea sites sampled would suggest that either one or multiple endemic panmictic populations of T. maxima are present along the latitude of the Red Sea. Given the taxonomically broad support of the 19° genetic discontinuity, it would seem exceptional that no such break is seen in T. maxima population(s). Either the host must be considered to possess an exceptional plasticity to survive in such a range of environments, or it must have some other mechanisms by which it is able to niche adapt.
Given the diversity of recovered Symbiodiniaceae genotypes in this study, it would appear that T. maxima-Symbiodiniaceae associations have a degree of flexibility and that there is a relatively high diversity of Symbiodiniaceae with which this host may associate. Our finding that T. maxima in the two most southern reefs associate with a relatively narrow diversity of Symbiodiniaceae, despite the presence of a much wider diversity that likely includes genotypes from the more northern sites, supports this notion of flexibility but also suggests a degree of selectivity by the clam hosts. Given that the high site fidelity seen in this study is most likely environmentally driven, flexibility and selectivity in these associations may offer a mechanism of adaptation for the host.
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.

ACK N OWLED G EM ENTS
We thank Felix Ivo Rossbach and Sebastian Schmidt-Roach for assistance during the sampling of T. maxima specimens, the BioScience Core Lab (BCL) for sequencing assistance, and the Coastal and Marine Resources Core Lab (CMOR) for the support of the diving operations.

CO N FLI C T O F I NTE R E S T
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.