SEASONAL BREEDING CYCLES AND SEX RATIOS OF EIGHT SPECIES OF CRABS FROM MGAZANA, A MANGROVE ESTUARY IN TRANSKEI, SOUTHERN AFRICA

Abstract

Three breeding trends were observed in mangrove grapsids and ocypodids from Mgazana. First, significant changes were found from continuous breeding in the low shore (Macrophthalmus grandidieri) to seasonal breeding on the middle to high shore (Uca and Sesarma spp.). The higher on the intertidal a species occurred, the larger the crab and the more defined the breeding season (Sesarma meinerti). Small species (M. grandidieri) have adopted a strategy of continuous breeding to maximize reproductive output. The second pattern was a staggering of spawning peaks among seasonal breeders with increasingly later peaks the higher the crabs occurred on the shore. Sesarma catenata commenced breeding in August, while the high intertidal species Uca lactea annulipes was ovigerous in March. The third trend was a sharp decline in breeding activity in autumn (March) when temperatures dropped. For the grapsids and Uca vocans hesperiae, sex ratio peaks declined during ovigerous periods. With the exception of M. grandidieri and S. meinerti, there were significantly more males than females. Breeding periodicity may be controlled by a combination of factors, including latitude, temperature, food availability (both adult and larval), and intertidal zonation. Larval interspecific competition in coexisting guilds (Sesarma and Uca) could be minimized by staggering egg-bearing peaks, with breeding delayed and more defined higher on the intertidal.