Population dynamics of an ectoparasitic gastropod, Hypermastus sp. (Eulimidae), on the sand dollar, Arachnoides placenta (Echinoidea)

Abstract

The association between the sand dollar, Arachnoides placenta, and the eulimid gastropod, Hypermastus sp., was investigated at Pallarenda Beach, Townsville, Queensland. Hypermastus sp. is a temporary ectoparasite of A. placenta that penetrates the latter's test to ingest gonad tissue and coelomic fluid. The infestation rate normally varies between 8 and 15% but increases in winter to 26%. This seasonal increase in the infestation rate appears to be due to an increase in the duration of attachment of female parasites. Hypermastus sp. is sexually dimorphic, with males being, on average, 0.6 times the size of females. Individuals may be protandric sequential hermaphrodites.