A COMPARISON OF THE PHYSIOLOGY AND ECOLOGY OF THE ESTUARINE ISOPOD CYATHURA POLITA IN MASSACHUSETTS AND GEORGIA

Abstract

1. Aspects of the physiology and ecology of two populations of the estuarine isopod Cyathura polita have been studied. One of the populations was located in a beach on Saelo Island, Georgia; the other, in an estuary in Marshfield, Massachusetts. The ecological aspects of the study included habitat observations; analyses of the density, animal size, dispersion and sex ratio of the populations; and investigation of the faunal associations. The physiological aspects included studies of osmoregulatory behavior and oxygen consumption rates. 2. Ecologically, the two populations were similar in a number of respects. They were both found in habitats which had sandy sediments, well aerated water, and wide fluctuations of temperature and salinity. The Sapelo Island population was less dense, and was niade up of smaller animals than the Marshfield population, but they both had clumped dispersion patterns and sex ratios which changed seasonally. The faunal associations were liniited, and although Cyathura polita, and Scoloplos fragilis, were the only species found in both areas, others were closely related. C. polita was one of the niost coninion animals in both associations. 3. The physiological studies showed that cyathurans from the two populations have similar osmoregulatory behavior and oxygen consumption rates. Cyathura polita maintains a body fluid concentration which is hypertonic to its environment at salinities between 1 and 28%o, but which becomes isosmotic with the environment at salinities between 28 and 42%o. The oxygen consumption rate of Cyathura polita is similar to that of other isopods. The rate increases at higher temperatures, but does not change significantly over a salinity range from 1 to 37%o. 4. The physiological similarity of the two populations seems to indicate that distinct physiological races have not developed within the species. This physiological homogeneity may be an example of the evolutionary conservatism of estuarine organisms.