Studies of the relationship between the ascidian Diplosoma virens and its associated microscopic algae. II. Aspects of the ecology of the animal host

Abstract

The numbers of colonies of the ascidian D. virens (Hartmeyer) occurring on a reef flat in the vicinity of Townsville, undergo pronounced seasonal fluctuations. The maximum colony numbers were recorded in October-November, with a smaller peak in May; colonies were almost entirely absent from the reef flat during the summer months.

Observations of the rate of increase of colony numbers, both on the reef flat and on artificial substrates, showed that the organism proliferates by repeated division of the colonies, accompanied by movement of the daughter colonies along the substrate. In the laboratory, colonies of D. virens move at rates of 4.3-8.0 mm per day. The direction of movement is strongly influenced by light.

Observation of the life span of the colonies on the reef flat, supported by measurement of the photosynthetic activity of the contained symbiotic algae, give some indication of the probable major factors influencing colonization of the reef flat by D. virens.