A natural history study of Encope grandis and Mellita grantii, two sand dollars in the northern gulf of California, Mexico

Abstract

Populations of the sand dollars Encope grandis and Mellita grantii were studied at Playa Hermosa in the northern Gulf of California, Mexico. Intertidal distribution and abundance were estimated for the two species from 1969 to 1972. They occurred in the lower intertidal (-0.4 to -1.3 m) and on into the subtidal. Density of E. grandis varied from a high of 380/m2 in the spring of 1970 to a low of 0.1/m2 in the fall of 1971. Density of M. grantii ranged from a high of 56/m2 in 1970 to a low of 0.8/m2 in 1971. Growth curves were constructed based on shifts in the modes of the size distributions, and rates of mortality were estimated from the growth parameters and size distributions. It is estimated that E. grandis would attain 95% of its maximum size (74 mm) in about 6 years with an annual mortality rate of 18%. M. grantii would reach 95% of its maximum size (38 mm) in about 5 years with an annual mortality rate of 58%. The populations of these sand dollars appear to be limited by the physical environment.