Effects of Symbiodiniaceae Phylotypes in Clades A–E on Progeny Performance of Two Giant Clams (Tridacna squamosa and T. crocea) During Early History Life Stages in the South China Sea

Chao Long,Jun Li,Yunqing Li,Haitao Ma,Zohaib Noor,Yinyin Zhou,Yanping Qin,Zihua Zhou,Xingyou Li,Jinkuan Wei,Yuehuan Zhang,Ziniu Yu,Lijuan Long

doi:10.3389/fmars.2021.633761

Abstract

Unlike most bivalves, giant clams (tridacnids) harbor symbiotic microalgae (zooxanthellae) in their other fleshy bodies. The effects of mixed populations of zooxanthellae on larval metamorphosis has been reported in several papers, but there have been very few studies on the effects of single zooxanthella species on the establishment of symbiosis in giant clams. In this study, we obtained five pure zooxanthella species (clades A3, B1, C1, D1, E1) from antler coral by molecular identification, and analyzed their effects on the larval metamorphosis and progeny performance of two giant clams, Tridacna squamosa and T. crocea, in the South China Sea. Clam larvae with all five zooxanthella species underwent larval settlement and metamorphosis, and formed the zooxanthellal tubular system. There was some variation in metamorphic rate and time to metamorphosis between clams with different zooxanthella species, but no significant differences in size at metamorphosis. After metamorphosis, larvae with all zooxanthella types continued to develop normally. Mantle color was consistent within clam species and zooxanthella species had no effect on mantle color. However, clam progeny with clade E1 zooxanthellae were smaller than progeny with the other four zooxanthella clades (A3, B1, C1, and D1). Survival rate was over 90% for all progeny and there were no significant differences in survival between progeny with Symbiodinium clades A–E during the entire culture process. Two-way ANOVA analysis revealed that giant clam species was the main factor influencing progeny growth, with some variation in growth attributable to zooxanthella type. Our results provide new information on both the symbiotic relationship between giant clams and zooxanthellae and the mantle coloration of giant clams, and will be useful in giant clam seed production and aquaculture.

Highlights

Zooxanthellae are found in several marine invertebrates, including corals, sea anemones, molluscs and other taxa, and are major producers of primary productivity in tropical and subtropical coral reef ecosystems (Neo et al, 2015)
Implantation Rate When giant clam larvae developed to the late-veliger stage (Figure 2A), they were soaked in zooxanthellae solution at a density of 103/mL for 2 h daily from day 6 to day 9
Between the two giant clam species, similar implantation rates were observed for the different zooxanthellae species, and the overall implantation rate of T. squamosa was slightly higher than that of T. crocea

Summary

Introduction

Zooxanthellae are found in several marine invertebrates, including corals, sea anemones, molluscs and other taxa, and are major producers of primary productivity in tropical and subtropical coral reef ecosystems (Neo et al, 2015). Giant clam species are an important part of coral reefs with great ecological and economic value, and are distributed throughout the tropical Indo-Pacific from the East African coast to the Pitcairn Islands in the eastern Pacific, and from Okinawa in Japan to south of the Great Barrier Reef (GBR) in Australia (bin Othman et al, 2010; Neo et al, 2017). It is well-known that giant clams have a mutualistic relationship with dinoflagellates of the genus Symbiodinium, known as zooxanthellae (Hirose et al, 2006; Mies, 2019). The association with Symbiodinium is unique to Tridacninae and Fraginae (Kirkendale and Paulay, 2017; Rossbach et al, 2020)

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