MEPS 215:251-260 (2001)  -  doi:10.3354/meps215251

ABSTRACT: It is usually considered that reinvasion of the estuarine habitat by crab larvae that develop in the sea involves 2 different steps, which are controlled by different factors: transport of larvae from the shelf towards the coast and upstream movement until appropriate settlement habitats are found. In order to investigate reinvasion of the Saco mangrove creek (Inhaca Island, Mozambique) by crab megalopae, the timing and spatial distribution of settlement were related to major environmental forcing factors. Several sets of experiments were performed, with different specific objectives, using megalopal collectors. The total supply of brachyuran megalopae to the mangrove creek over a 2 mo period responded to 2 forcing factors, the first related to cycles of tidal amplitude and the second to offshore wind stress. The response of settlement on the collectors to the tidal amplitude cycle was reflected by clear maxima recorded on every spring tide. This contrasts with the results of previous studies analysing single species recruitment processes, which found a much more irregular pattern. This result was interpreted as a consequence of 2 factors: (1) the use of a cumulative number of total Brachyura, which increases the probability of detecting supply pulses, and (2) a larger tidal range at Inhaca than at the locations where most previous studies took place. The results of this experiment suggest interaction of the stochastic effect of offshore wind stress, which maximises transport to the coastal area, with the deterministic effect of the tide and spring-neap cycles, which operate at the nearshore level to carry megalopae to specific mangrove areas. Settlement was also studied by deploying collectors in 3 different strata of the mangroves, and the results showed a differential settlement distribution in favour of lower areas in the vicinity of the mangroves. This suggests subsequent juvenile migration to suitable areas following first moult. Different settlement stimuli were also tested in the field by enclosing in the collectors a mixture of mangrove crabs and mangrove vegetal material. The results were inconclusive, as no significant differences were found; this was attributed to the effects of interference of stimuli between adjacent collectors and of proximity to mangroves.

KEY WORDS: Settlement · Megalopae · Larval transport · Mangrove · Inhaca island · Mozambique