Elucidation of reproductive biological aspects of the deep-water velvet shrimp Metapenaeopsis kyushuensis (Yokoya 1933) (Crustacea, Decapoda, Penaeidae)

This study offers novel insights into oocyte development, ovarian maturation, size at sexual maturity, and the spawning season of the deep-water penaeid shrimp, Metapenaeopsis kyushuensis, collected from deep-water Kagoshima Bay, southern Japan. Additionally, through histological observation, we determined the most efficacious ovarian maturity indicator for this species among the gonadosomatic index, oocyte diameter, and the ovary-to-body width ratio. Histological analysis enabled us to categorize oocyte development in M. kyushuensis into seven stages and confirmed the absence of cortical crypts. We defined females as mature when their oocytes displayed signs of germinal vesicle breakdown or a migrating nucleus. The ratio of ovary-to-body width was identified as the most efficient index for evaluating maturity status. The minimum carapace length (CL) of the histologically confirmed mature females was 11.5 mm, whereas the CL at which 50% of females attained maturity was 13.7 mm. M. kyushuensis is a multiple spawner, with spawning occurring from April to January, peaking between June and September. The prolonged spawning season of M. kyushuensis was presumably influenced by deep-water environments and comparatively higher water temperatures year-round.