Zooxanthellal genetic varieties in giant clams are partially determined by species-intrinsic and growth-related characteristics

Abstract

Giant clams (tridacnine shellfishes) are large bivalves that inhabit tropical and subtropical waters and harbor the symbiotic microalgae zooxanthellae, which consist of diverse phylotypes (clades). Each clade exhibits unique physiological characteristics, and the cladal composition may influence the host's survival and its ability to tolerate environmental changes. Using quantitative PCR (qPCR) assays, we investigated the zooxanthellal genetic clades in Tridacna crocea (n = 93) and Tridacna squamosa (n = 93). These two clam species were artificially bred and maintained for an extended time period under an equivalent environment in an outdoor pond. Results showed that T. crocea had a simpler cladal composition and with an apparent dominance of clade A, whereas multiple clades were present in T. squamosa. The zooxanthellae clade A is known to occur in other zooxanthellae-bearing animals that inhabit shallow waters, which is consistent to the shallow water habitat preference of T. crocea. Interestingly, in larger individuals of T. squamosa, the main zooxanthellal clade was C rather than A. The mechanism underlying the dominance of clade C in the larger T. squamosa has not yet been clarified. However, the additional photosynthates supplied by clade C may be preferable for growing clams, as is observed in corals. The cladal composition of giant clams has previously been reported to be primarily controlled by environmental factors. However, our experiments subjected different clam species to the same environmental conditions, and our results suggested that species-intrinsic and/or growth-related processes may also influence the cladal composition.

Effects of temperature andpCO2on the respiration, biomineralization and photophysiology of the giant clamTridacna maxima

“…Giant clams live in obligatory symbiosis with photosynthetic dinoflagellates of the family Symbiodiniaceae (Holt et al, 2014; LaJeunesse et al, 2018). Different giant clam species, i.e., Tridacna maxima, Tridacna crocea, Tridacna noae and Tridacna squamosa , have been found to associate with different Symbiodiniaceae genera, i.e., Symbiodinium (formerly clade A, LaJeunesse et al, 2018), Cladocopium (formerly clade C, LaJeunesse et al, 2018) and Durusdinium (formerly clade D, LaJeunesse et al, 2018) (Pinzón et al, 2011; DeBoer et al, 2012; Ikeda et al, 2017; Lim et al, 2019). Hereafter, the symbiotic Symbiodiniaceae are named “symbionts”.…”

Polymorphic adaptations in metazoans to establish and maintain photosymbioses

“…This bivalve family encompasses large species that can reach up to 1 m in diameter, like Tridacna , and are occasionally referred to as giant clams (Vermeij, ). Their exceptionally large sizes are thought to result from the establishment of symbioses (Griffiths & Klumpp, ) often with several clades of Symbiodinium (Baillie, Belda‐Baillie & Maruyama, ; DeBoer et al, ; Ikeda et al, ). The major characteristic of cardiids is an enlarged and colourful mantle, in which Symbiodinium is hosted in extracellular spaces (Hernawan, ).…”

Status of giant clam resources around Okinawa‐jima Island, Ryukyu Archipelago, Japan

“…Previous studies of giant clams in Japan have focused heavily on clam seed production and techniques (Kawaguti, ; Kurihara, Fuseya, Katoh, & Inoue, ; Kurihara, Yamada, & Inoue, ; Kurihara, Yamada, Inoue, Iwai, & Hatta, ; Murakoshi, ), as well as the physiological interactions between endosymbionts and host (Hirose, Iwai, & Maruyama, ; Ikeda et al, ; Kurihara et al, ; Masuda, Miyachi, & Maruyama, ). This research has contributed towards the Okinawan conservation effort being one of the most established and longest running giant clam mariculture programmes in the Indo‐Pacific region (Neo et al, ).…”