The Ecology of the Tropical Compound Ascidian Trididemnum solidum. III. Symbiotic Association with Unicellular Algae

Abstract

The relation between T. solidum (Van Name), a tropical compound ascidian common on the reefs of Curacao, and its endosymbiotic unicellular algae was studied. Colonies collected from 3 depths (< 10 m, 15-17 m, > 20 m) were simultaneously analyzed for number of algae and amount of chlorophyll [chl.] a g-1 wet wt. Both number of algae (mean 2.78 .times. 106 cells cm-2) and amount of chl. a (mean 150 .mu.g chl. a g-1 wet wt) did not change significantly with depth. T. solidum tadpole larvae contained significantly higher quantities of chl. a. Additional pigment analysis of samples from shallow and deep water (< 10 m, > 20 m) revealed the presence of carotenes and zeaxanthin in quantities that remained the same regardless of depth but the ratio of zeaxanthin to chl. a changed significantly from 0.14 (shallow) to 0.10 (deep). Two-dimensional TLC confirmed the absence of chl. b and c in these algae. The photosynthetic O2 production of colonies from the 3 depth ranges was measured between July-Sept. 1978, mainly in the laboratory. Closed systems under artificial light conditions varying from 0-240 .mu.E m-2s-1 (400-700 nm) were used. Positive production rates were measured from 40-240 .mu.E m-2s-1 (the highest experimental light level). Additional field experiments showed ascidian colonies from the shallowest sample (< 10 m) to consume O2 at light intensities of 600 .mu.E m-2s-1 and more. Maximum measured mean photosynthetic O2 production was .+-. 0.09 mg O2 cm-2h-1 at 240 .mu.E m-2s-1 for colonies of 15-17 m and > 20 m. This O2 production was significantly higher than the mean maximum production of colonies from 10 m (0.04 mg O2 cm-2h-1).