PHOTOADAPTATIONS OF THE CARIBBEAN COLONIAL ASCIDIAN-CYANOPHYTE SYMBIOSISTRIDIDEMNUM SOLIDUM

Abstract

Photosynthetic organisms show a number of photoadaptations which enable them to acclimate to the light regime in which they grow. In the case of invertebrate-algal symbioses, the responses can occur in both the algae and the invertebrate. Colonies of the colonial ascidian-cyanophyte symbiosis Trididemnum solidum at Galeta, panama, show morphological variation relative to the light regime in which they live. Colonies growing in full sunlight are white, thicker, more heavily calcified, and distribute their cyanophytes more uniformly throughout the depth of the colony than do colonies in lower light regimes. Shaded colonies are purple, thinner, have fewer spicules at the colony surface, concentrate their algae near the surface, and have a greater amount of phycoerythrin relative to phycocyanin in their symbiotic algae. The purple coloration of low light colonies appears to be due primarily to the phycobilin pigment of the algae. Experimental shading of colonies in the lowest light areas in which they are found at Galeta, resulted in significantly higher mortality than colonies beneath clear control roofs. The ascidian host appears to be physiologically dependent upon its symbionts and capable of changing its morphology in response to ambient light levels.