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.
Highlights
Giant clams (Tridacninae subfamily) are prominent members of Indo-Pacific corals reefs, where they play important ecological roles (Neo et al, 2015), by providing a food source for different predators and scavengers (Alcazar, 1986), shelter for commensal organisms (De Grave, 1999), and settling substrate for epibionts (Vicentuan-Cabaitan et al, 2014)
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
Eight of the 18 sequences (A1z, A1g, C1, C3, D1, D4, D6, D4c) were defining intragenomic sequence variant (DIV) from internal transcribed spacer 2 (ITS2) type profiles associated with clams from either the Gulf of Aqaba or Duba. 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
Summary
Giant clams (Tridacninae subfamily) are prominent members of Indo-Pacific corals reefs, where they play important ecological roles (Neo et al, 2015), by providing a food source for different predators and scavengers (Alcazar, 1986), shelter for commensal organisms (De Grave, 1999), and settling substrate for epibionts (Vicentuan-Cabaitan et al, 2014). Many Durusdinium taxa appear to be relatively stress-tolerant (LaJeunesse et al, 2014), 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 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). 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
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