Contribution of the symbiotic dinoflagellate Symbiodinium microadriaticum to the nutrition, growth and survival of larval and juvenile tridacnid clams

Abstract

The role of the symbiotic alga Symbiodinium microadriaticum (= zooxanthellae) in growth, development, and survival of larval and juvenile tridacnid clams was investigated. The zooxanthellae were not present in eggs released from adult Tridacna gigas and Hippopus hippopus, but were ingested by veliger larvae after development from the trochophore stage. Growth of veligers of the clam H. hippopus fed freshly-isolated S. microadriaticum was significantly higher than the growth of starved veligers. In contrast, veligers fed cultured strains of S. microadriaticum and 1–5-day-old isolates of S. microadriaticum from H. hippopus had growth and survival rates not significantly different from starved veligers. These results suggest that translocated photosynthetically-fixed carbon from the ingested freshly-isolated algae was responsible for increased veliger growth and survival. The presence of zooxanthellae in the stomachs of veligers before metamorphosis facilitated establishment of a symbiosis with S. microadriaticum and a concurrent increase in growth after metamorphosis. The establishment of the symbiosis with S. microadriaticum entailed passage of the algae from the stomach, through the digestive gland, and into the developing haemal sinuses. The contribution of photosynthetically-fixed carbon from the symbiotic zooxanthellae could theoretically account for the respiratory demand of 6-week-old juveniles of H. hippopus, corroborating positive growth of unfed symbiotic juvenile clams maintained in filtered seawater. When offered a variety of free-living and symbiotic species of algae, juvenile H. hippopus established a symbiosis only with strains of S. microadriaticum. Other species of algae were apparently digested if small enough to be ingested. Less than 7% of the free-living phytoplankton species Isochrysis galbana and Platymonas subcordiformis that were ingested by symbiotic juvenile Tridacna gigas (3–12 months old) were passed intact through the digestive system after 3 days. In contrast, when freshly-isolated zooxanthellae from T. gigas were offered to symbiotic juvenile T. gigas, up to 76% of the zooxanthellae ingested passed through the clam's digestive system and were released in faecal pellets intact. The symbiotic dinoflagellate Amphidinium klebsii and the diatom Phaeodactylum tricornutum were not ingested by veliger or juvenile clams.