Grapsid crabs can play a considerable role in the predation of mangrove propagules and possibly are a threat to the regeneration of mangroves, whether natural or artificial. Experiments consisting of artificial plantation of mangrove juveniles were set up in Gazi Bay (Kenya). Grapsid crabs, particularly Neosarmatium meinerti in the most landward fringe and N. smithii and Sesarma guttatum in the middle fringe of the mangrove forest, were found to be a regeneration-limiting factor. Almost 100% (n = 40) of the landward plantations were cleared. There was a slight initial preference for Rhizophora propagules lying horizontally on the mangrove floor, which simulates the position of a stranding propagule. In Mida Creek (Kenya) the snail Terebralia palustris was observed predating mangrove propagules. This study shows that crabs may affect the regeneration potential of mangroves, and snails might also be a factor in predation. A need to actively search for ways to protect re-afforestation plots from predators of mangrove tree juveniles is necessary.

The potential long-term effects of altered salinity regimes on the bioenergetics of two ecologically important Kenyan mangrove crabs, Neosarmatium meinerti de Man, 1887 and N. smithi H. Milne Edwards, 1853 were investigated in light of recent findings suggesting that groundwater redirection may alter salinity regimes in Kenyan mangroves. Although changes in groundwater may cause only small increases in salinities, these changes would be chronic and may impact crab populations already living above their optimal salinity. To assess potential impacts, fundamental physiological processes and hemolymph components were measured on animals acclimated to 16‰, 32‰, 48‰ and 65‰ for four weeks in a field laboratory. For comparative purposes, crabs were also sampled in the field. N. smithi survived poorly in all salinities except the control (32‰). Although high mortality in N. smithi did not allow for reliable estimations of an energy budget, mortality and osmoregulatory capacity shows that this species can osmoregulate for a limited time in elevated salinities (± 1 week), but cannot withstand long-term hypersaline conditions. In contrast, N. meinerti survived well and was able to osmoregulate for one month in all salinity treatments. Nevertheless, their energy budget, was significantly reduced (to below zero) in the 65‰ treatment. Overall, this study shows that these two congeneric species exhibit different long-term responses to variations in salinity. However, they are both negatively effected by hypersaline conditions, suggesting that long-term alteration of mangrove salinity regimes may be detrimental for these ecologically important mangrove crab populations.

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Abstract. A number of coastal organisms (e.g., bivalves) are known as important consumers of particulate organic carbon (POC) in the sea. In this paper we present new evidence that they are also important consumers of dissolved organic carbon (DOC) in the colloidal size range down to about 0.2 μm in diameter. Those colloids play an important role in the global flux of carbon in the seas. We compare the uptake of colloidal DOC by marine bivalves of the North Sea and of the western Indian ocean. To measure this uptake we made use of colloidal melanin as an alternative way to measure the uptake of colloidal DOC.

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