DEMOGRAPHIC-STUDY OF A SUBTIDAL POPULATION OF THE NEW-ZEALAND ARTICULATE BRACHIOPOD TEREBRATELLA-INCONSPICUA

Size-frequency distributions from subtidal populations of Terebratella inconspicua (Sowerby, 1846) consistently display prominent modes at 18 to 22 mm shell length, representing many years accumulation of non-growing adults. In addition, these distributions contain seasonally variable modes made up of new recruits which are present in very high numbers towards the end of the breeding season, but are reduced to very low numbers by the end of their first year of life. Growth of recruits follows a sigmoid curve with considerable variability between individuals. Some adults survive at least 14 years. Females are 2 years old before they breed. Larvae are brooded internally by the parent and spawned in autumn and spring. The density of recruitment is positively correlated with the density of residents. Larvae commonly settle on the shells of large brachiopods; these recruits can survive to breed, but their survivorship as adults is lower than that of conspecifics attached to rock. Total mortality in the population is size (=age)-dependent; the mean life expectancy of a new recruit is 0.54 years compared with 2.57 years for a 1 year-old. Sources of mortality are considered. © 1979 Springer-Verlag.