Role of early post-settlement mortality in recruitment of benthic marine invertebrates

Newly settled invertebrates usually are subject to high rates of mortality (Type ill survivorship). Therefore, knowledge of early post-settlement events is critical in determining if and when patterns of abundance and distribution of juveniles reflect settlement patterns. Causes of mortality of early juvenile invertebrates include delay of metamorphosis, biological disturbance, physical disturbance and hydrodynamics, physiological stress, predation, and competition. Predation is the best documented cause of early mortality, particularly for mobile species. Other possible causes which have not yet been investigated are developmental abnormalities, insufficient energy reserves, disease and. parasitism. In most studies of sessile invertebrates, early post-settlement mortality did not obscure the relationship between recruit and settler abundance. This relationship appears to be more variable among mobile species for which migration also can modify the distribution of settlers. There is still insufficient data to support general conclusions about the conditions under which recruitment rate can be predicted from settlement rate. Studies have found evidence of the effects of both settlement and early post-settlement mortality on the distribution of some sessile species at small spatial scales, but mortality appears to have less influence at larger scales. Much of the present knowledge of the early postsettlement period has come from studies of barnacles and ascidians and more information is needed for other groups of benthic marine invertebrates, particularly mobile species. The relative importance of mortality during the early post-settlement period compared to other life history stages can only be determined in studies which examine several stages.