Light-dependent calcification in Red Sea giant clam <i>Tridacna maxima</i>

Abstract

Abstract. Tropical giant clams of the subfamily Tridacninae, including the species Tridacna maxima, are unique among bivalves as they live in a symbiotic relationship with unicellular algae and generally function as net photoautotrophs. Light is therefore crucial for these species to thrive. Here we examine the light dependency of calcification rates of T. maxima in the central Red Sea as well as the patterns of its abundance with depth in the field. Red Sea T. maxima show the highest densities at a depth of 3 m with 0.82±0.21 and 0.11±0.03 individuals m−2 (mean ± SE) at sheltered and exposed sites, respectively. Experimental assessment of net calcification (µmol CaCO3 cm−2 h−1) and gross primary production (µmol O2 cm−2 h−1) under seven light levels (1061, 959, 561, 530, 358, 244, and 197 µmol quanta m−2 s−1) showed net calcification rates to be significantly enhanced under light intensities corresponding to a water depth of 4 m (0.65±0.03 µmol CaCO3 cm−2 h−1; mean ± SE), while gross primary production was 2.06±0.24 µmol O2 cm−2 h−1 (mean ± SE). We found a quadratic relationship between net calcification and tissue dry mass (DM in gram), with clams of an intermediate size (about 15 g DM) showing the highest calcification. Our results show that the Red Sea giant clam T. maxima stands out among bivalves as a remarkable calcifier, displaying calcification rates comparable to other tropical photosymbiotic reef organisms such as corals.

The Small Giant Clam, Tridacna maxima Exhibits Minimal Population Genetic Structure in the Red Sea and Genetic Differentiation From the Gulf of Aden

“…Giant clams, are flagship coral reef species (Soo and Todd, 2014), where they play important ecological roles. These large mollusks act as ecosystem-engineers in coral reefs by supporting high calcification rates (Rossbach et al, 2019) and helping to maintain the overall reef biodiversity and functionality (Neo et al, 2015). Giant clams are generally found on shallow reefs, where light is essential for the survival, growth, and reproduction (van Wynsberge et al, 2017) because the mantle tissue is packed with extracellular single-celled algae of the Symbiodiniaceae family (Taylor, 1969;Yonge, 1975).…”

Tissue-Specific Microbiomes of the Red Sea Giant Clam Tridacna maxima Highlight Differential Abundance of Endozoicomonadaceae

“…Giant clams provide important ecosystem functions in coral reef ecosystems (Neo et al, 2015), including providing a food source for a number of predators and scavengers (Alcazar, 1986), shelter for commensal organisms (De Grave, 1999), or substrate for epibionts (Vicentuan-Cabaitan et al, 2014). Further, through their high photosynthetic (Jantzen et al, 2008; Rossbach et al, 2019) and calcification rates (Rossbach et al, 2019) they contribute to the primary production and spatial framework of the reef structure.…”

Drivers of the Abundance of Tridacna spp. Giant Clams in the Red Sea

“…Existing studies on densities of giant clams across depth report the highest numbers of Tridacninae per m 2 in shallow reefs (<10 m), principally reef flats and the reef edge (Van Wynsberge et al, 2016;Rossbach et al, 2019). The recently described, strong light-dependency of calcification in Red Sea giant clams (Rossbach et al, 2019), may therefore be among the main drivers which restrict the depth-distribution of Tridacninae within the reef.…”

“…Existing studies on densities of giant clams across depth report the highest numbers of Tridacninae per m 2 in shallow reefs (<10 m), principally reef flats and the reef edge (Van Wynsberge et al, 2016;Rossbach et al, 2019). The recently described, strong light-dependency of calcification in Red Sea giant clams (Rossbach et al, 2019), may therefore be among the main drivers which restrict the depth-distribution of Tridacninae within the reef. In addition to the availability of light, sea temperatures (Apte et al, 2019) and wave exposure are also important environmental factors for symbiotic benthic organisms (Williams et al, 2013), and are potentially impacting the overall abundances of Tridacninae.…”

“…Due to their photosymbiotic relationship, and the resulting mixotrophic feeding strategy (Jantzen et al, 2008), Tridacninae can even reach a net photoautotrophic metabolism (Klumpp et al, 1992;Klumpp and Griffiths, 1994). Processes of light-enhanced calcification are presumed to be the main reason why Tridacninae show high calcification rates (Ip et al, 2017b(Ip et al, , 2018Boo et al, 2019;Chew et al, 2019;Rossbach et al, 2019) and their subsequent impressive sizes. The requirement for photosynthetically active radiation (PAR, 400-700 nm) of their algal symbionts restricts giant clams to sunlit, shallow waters, where they are -at the same timeexposed to potentially high levels of ultraviolet (UV) radiation (Overmans and Agustí, 2020;Rossbach et al, 2020b) and high water temperatures (Andréfouët et al, 2013).…”