POPULATION ECOLOGY OF THE LOW-SHORE CHITONS ONITHOCHITON-QUERCINUS AND PLAXIPHORA-ALBIDA

Populations of two chitons, Onithochiton quercinus (Gould, 1846) and P laxiphora albida (Blainville, 1825) were sampled at five sites in the low-shore algal community at Cape Banks, New South Wales, Australia, f rom January 1985 to February 1988. Both species were abundant, reachin g densities of 96 and 45 m(-2), respectively and grew to 80 mm in leng th. O. quercinus and P. albida were found on the solitary ascidian Pyu ra stolonifera and on the substratum in amongst the foliose macroalgae which dominate this community. Size-frequency data were used to estim ate the rates of growth, mortality and longevity, and time of recruitm ent. Growth of both species was rapid for the first 3 yr and slowed th ereafter. O. quercinus and Plaxiphora albida had similar longevities a nd were estimated to live for at least 6 yr. Reproductive samples were also collected to identify when sexual maturity occurred, to clarify the reproductive periodicity, and to examine the sex ratios of the two species. Both species spawned over a discrete period in early autumn and there was no evidence of the bi-annual spawning reported in a prev ious study. Recruitment of O. quercinus and P. albida occurred during autumn, but recruits could not be quantified owing to their cryptic na ture. The microhabitats occupied by the recruits of both species inclu ded the empty shells of the barnacle Austromegabalanus nigrescens, the spaces between the calcareous tubes of the polychaete Galeolaria caes pitosa, and the space amongst the algae on Pyura stolonifera. Both spe cies reached reproductive maturity at 2 yr of age. The sex ratios of b oth species were consistent with the results of a previous study: O. q uercinus had a sex ratio of almost 2:1 biased in favour of males, and Plaxiphora albida exhibited a sex ratio of 1:1. The study suggested th at populations of O. quercinus and P. albida could persist in the low- shore algal community in the absence of recruitment. This, together wi th their large size and consumption of macroalgae suggests that they h ave the potential to affect the structure and dynamics of this interti dal community.