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Sexual maturity, larval release and reproductive output of two brachyuran crabs from a rocky intertidal area in central Portugal

Summary Sexual maturity, larval release and reproductive output were examined and compared in the intertidal crabs Xantho incisus (Brachyura: Xanthidae) and Pachygrapsus marmoratus (Brachyura: Grapsidae). Categorising females into predefined abdominal morphotypes proved to be the most realistic procedure to estimate size at 50% sexual maturity (i.e., the size at which 50% of the individuals are mature) in both species. In males, maturity estimates based on the proportion of individuals bearing mature gonads suggested that complementary allometric techniques using the relative growth of gonopods are adequate for both P. marmoratus and X. incisus. During the first half of the breeding season, both species breed intensively producing multiple broods within a single intermoult stage. Clutch age is positively correlated to stage of gonad development, and a new egg batch can be extruded within a few days of larval release. After July, ovigerous females with advanced broods were commonly found with spent gonads indicating a gradual decrease of breeding activity. In contrast to most brachyurans studied so far, asynchronous patterns of larval release were recorded for both species. It is suggested that the lack of an entrained lunar cycle may enable breeding females to produce successive egg batches within a shorter period, thus compensating for an increased larval mortality due to sub-optimal timing of larval release. The production of offspring can be adequately estimated by egg counts in P. marmoratus, but brood loss is significant in X. incisus, female size having no effect on this feature. Differences of brood volume, controlled for variation in crab size, suggested a slightly higher reproductive effort per batch in X. incisus. The reproductive variables examined will be used in future research to generate egg production profiles for the studied populations.