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

Giant clams (Tridacninae) are important members of Indo-Pacific coral
reefs and among the few bivalve groups that live in a symbiosis with
unicellular algae (Symbiodiniaceae). Despite the importance of these
endosymbiotic dinoflagellates for clam ecology, the diversity and
specificity of these associations remains 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
gradients. 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, i.e. Cladocopium, Durusdinium, and Symbiodinium,
whilst 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 a flexibility in T. maxima-Symbiodiniaceae
associations that may explain its relatively high environmental plasticity
and offers a mechanism for environmental niche adaptation.