Ovarian development and size at maturity of the mud crab Scylla olivacea in Pak Phanang mangrove swamps, Thailand

Abstract

Ovarian development and size at maturity of the mud crab Scylla olivacea from Pak Phanang mangrove swamps, Thailand, were investigated. Samples were taken from local traders between June 2006 and January 2008. Ovarian development was determined based on both external appearance and histological observation and was classified into five stages: immature (stage I), previtellogenesis (II), primary vitellogenesis (III), secondary vitellogenesis (IV) and tertiary vitellogenesis (V). Size of oocytes and volume of ovary increased as ovarian developmental stage progressed. External and histological observation of the ovaries revealed that initiation of vitellogenesis could be identified by external observation of the ovary. However, ovarian maturity could not be determined precisely by morphological observation of the abdomen. Absence of post-spawned females and small sample size of mature females in near-shore mangroves implies the occurrence of spawning migration toward offshore regions. The minimum size at which females attain ovarian maturity was 89.8 mm internal carapace width (ICW), and ICW at which 50% of females attain ovarian maturity was estimated to be 99.1 mm.