Abstract
Giant clams are found in a mutualistic association with Symbiodiniaceae dinoflagellates, however, the diversity of the giant clams, as well as the diversity and distribution of Symbiodiniaceae in different Tridacnine species remain relatively poorly studied in the South China Sea. In this study, a total of 100 giant clams belonging to small type’s giant clams, Tridacna maxima, T. crocea, and T. noae, were collected from Hainan and Xisha Islands. Based on mtDNA cytochrome c oxidase subunit 1 gene (COI) and 16S rRNA fragments, T. maxima and T. crocea showed a closer phylogenetic relationship than T. noae. All the three species of giant clams hosted Symbiodiniaceae including genera Symbiodinium (formerly Clade A) and Cladocopium (formerly Clade C). Geographically, symbionts in Cladocopium are restricted to Xisha Islands, probably because Cladocopium prefers to inhabit in waters with higher mean temperatures. The endosymbiont specificity among the three giant clam species was also detected. T. noae and T. crocea are found to harbor Symbiodinium preferentially, compared with Cladocopium. These results could provide important information to understand various endosymbionts occurring in giant clams in the South China Sea.
Highlights
Giant clams (Tridacninae) inhabit tropical coral reefs throughout the Indo-Pacific oceans (Lucas, 2014)
Both the c oxidase subunit gene (COI) and 16S sequences supported the monophyly of T. crocea, high-intraspecific morphological diversity was found within this group, such as the values of shell width/shell length, byssal opening length/shell length, byssal opening width/shell length and number of hinge tooth (Supplementary Table 2)
The reconstructed phylogeny using the COI and 16S fragments indicated a relationship shown as T. noae + (T. maxima and T. crocea) and supported the validity of T. noae
Summary
Giant clams (Tridacninae) inhabit tropical coral reefs throughout the Indo-Pacific oceans (Lucas, 2014). The Tridacninae subfamily includes two extant genera, Hippopus (two species) and Tridacna (10 extant species), and all these species host symbiotic zooxanthellae in the mantle tissues that plays important ecological roles in the coral reef ecosystem (Liu et al, 2021b). Eight giant clams have been reported in the South China Sea, including two large species Tridacna gigas and T. derasa; three middle species T. squamosa, H. hippopus, and H. porcellanus; and three small species T. maxima, T. crocea, and T. noae (Neo et al, 2017; Liu J. et al, 2020). The genetic and phenotypic variation in several giant clam species has been reported in many geographical locations (Nuryanto and Kochzius, 2009; Hui et al, 2016; Pappas et al, 2017), while there were only a few reports on the diversity of giant clams in the South China Sea (Lim et al, 2019; Liu J. et al, 2020)
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