PRODUCTION OF SYMBIOSIS-SPECIFIC FATTY ACIDS BY DIFFERENT SYMBIODINIUM CLADES ASSOCIATED WITH TRIDACNA CROCEA LARVAE  

Zooxanthellae (Symbiodinium) are found in an endosymbiotic association with several metazoan phyla, includind giant clams. These symbiotic dinoflagellates are classified in nine different clades (A-I) and supply their hosts with essential ω3 fatty acids, some of which are specific to the symbiont. While much information has been produced on animal-dinoflagellate symbioses, few experiments were performed on zooxanthellae while incubated in host larvae. Facing such information, we intended to investigate which symbiont clade produces more symbiont-specific ω3 fatty acids when incubated in giant clam larvae. For this experiment, ten adult specimens of the Boring Giant Clam Tridacna crocea (mean shell length 7.6 ± 0.9 cm) were acquired and induced spawning was performed by means of an intragonadal injection of 1.0 mL of a 1.0 g L^-1 solution of serotonin. Gametes were collected and fertilized and larvae cultured until 48 h post-fertilization, at veliger stage. At this point larvae were stocked in 21 plankton kreisels at 1.0 veliger larva mL^-1. Six groups of three kreisels received one of six Symbiodinium clades (A-F) at a final concentration of 10⁴ cells mL^-1. A control group was maintained, to which no symbionts were offered. At 0, 12, 24, 48 and 72 h after symbiont addition, samples of 50 larvae were taken from each kreisel. All the water from the kreisels was filtered prior to the 12 h sample, in order to prevent symbiont re-acquisition during the time-course experiment. Total lipids were extracted from the samples, using the Bligh & Dyer (1959) protocol, with minor modifications. Extracts were taken to a gas chromatograph equipped with flame ionization detection. The concentration of three symbiont-specific ω3 fatty acids (SDA - 18:4ω3, DPA - 22:5ω3 and DHA - 22:6ω3) was determined. Veliger larvae acquired 25.5 ± 5.9 cells; the larvae exposed to clade B acquired the highest number of 33.5 ± 18.7 cells while larvae exposed to clade C acquired only 18.1 ± 3.2 cells. Gas chromatography showed that the amount of SDA, DPA and DHA increased over time for larvae exposed to three different clades, while larvae exposed to the other clades presented lower increase rate or stagnant levels of these fatty acids. These results suggest that some clades may engage in a stronger association with giant clam veliger larvae. These findings create new perspectives on coral reef plankton ecology and dynamics, apart from being useful in optimizing growth and survival in giant clam aquaculture protocols.