Recruitment limitation and competent settlement of sesarmid crab larvae within East African mangrove forests

The functioning of many ecosystems is highly dependent on their faunal communities, dominated by brachyuran crabs. Key to the maintenance of crab populations are the factors that influence their recruitment into the system, particularly the settlement of their megalopae. While spawning is normally highly synchronised, settlement is often not, implying temporal disruption between spawning and settlement. We examined the roles of several key factors (vegetation type, day/night cycle, and lunar phase) on settlement, testing for generality by working in mangroves at 2 sites in Kenya and South Africa. At each site, larval settlement was quantified using artificial collectors placed in 2 vegetation zones for 12 h periods throughout the part of the neap-spring tidal cycle when tide was sufficient to cover the collectors. This continued for 6 mo in Kenya and 11 mo in South Africa, giving the first long-term settlement dataset for east African mangroves. Moon phase proved to have a major role in megalopal settlement, which demonstrated a high degree of spatial specificity. Settlement was correlated across species with tidal amplitude on both short (monthly) and long time scales and focussed on the vegetation zones occupied by conspecific adults. As the distribution of mangrove species is tightly related to local hydrology, this implies a feedback situation with the conservation of mangroves requiring the maintenance of patterns of water flow within the system to support both the vegetation and the associated fauna.