Equatorial Submergence in a Solitary Coral, Balanophyllia elegans, and the Critical Life Stage Excluding the Species from Shallow Water in the South

Abstract

The geographic and bathymetric distributions of a solitary coral, Balanophyllia elegans, were studied, especially in the southern part of its range. This species is an example of equatorial submergence, occurring intertidally and subtidally in the north (Vancouver Island, Canada, to Point Conception, California), but subtidally only in the south (Point Conception to northern Baja California, Mexico). The absence of B. elegans from depths less than 10 m in the south was investigated to identify the life history stages most sensitive to environmental factors, particularly temperature, in shallow water. Newly settled and adult individuals were transplanted to buoys in the top few meters of water. The transplanted adults grew more slowly than controls, but survived well and reproduced; moreover, planula larvae released by the transplanted adults successfully settled nearby under shallow-water conditions. Thus neither very early nor relatively late stages in the life history appeared critical in excluding B. elegans from shallow water. The critical period was the juvenile stage, since newly settled individuals showed poor survival after transplantation to buoys in shallow water. Distribution and mortality patterns were correlated with the temperature of the seawater. However, it is difficult to establish a simple causal relationship between high temperature and equatorial submergence, since subtidal temperature patterns are complex and since temperature may have many indirect but important effects.