The decorator crab, Microphrys bicornutus, is a common inhabitant of the turtle grass Thalassia testudinum. In order to establish the behaviour of a population of this species in an extreme shallow Thalassia complex (maximum depth less than 50 cm, and often exposed to the air), size distribution, population dynamics, reproductive effort and egg size were studied in Buchuaco—Venezuela. Monthly sampling was undertaken between June 1988 and December 1990. A total of 1403 specimens of M. bicornutus were captured. In each of the sampled years, size distribution was strongly skewed to the predominance of small size crabs. Females were significantly larger than males in all the three years. Small size and large size crabs showed periodic annual fluctuations in abundance, with peaks during the last months of the year for the first, and peaks between April and June for the latter. Medium size crabs showed no periodicity at all. Number of eggs per female ranged between 40 and 4305 (mean = 1067.4), and was positively correlated with female body size. Eggs increased their size gradually during their development, and differences in egg size between development stages were significant. The biological parameters found in this study are compared with other populations of this crab, found in Venezuela and Brazil, in different ecosystems. Results acquired in this work add new insights to the behaviour of this decorator crab, specifically on density variation and egg size (among others), living in an environment that is constantly exposed to air and to other physicochemical variations.