- Elcosanoids are thought to play a role in the regulation of invertebrate reproduction, as they do in vertebrate systems. This was investigated using the previtellogenic ovary of the freshwater prawn Macyobrachium rosenbergii as a biological model. Concentrations of prostaglandin E 2 (PGE 2 ), assessed by means of radioimmunoassay, in the previtellogenic ovary (oocyte diameter 20-40 μm) were 32.4 ± 14.1 pg/mg ovary. Preincubation of the ovary with indomethacin (10 μ M ) inhibited PGE 2 synthesis by 43%. In addition, if indomethacin was added to the culture medium. cAMP levels decreased by 48%. When previtellogenic ovaries were incubated in vitro with PGE 2 (0.05 μg/ml medium and up), cAMP levels in the tissue homogenate sharply increased. The levels of cAMP rose most significantly (up to 10-fold) when 1-10 μg PGE 2 /ml medium was applied. These results suggest that PGE 2 , and possibly other prostaglandins, may play a role in the endocrine regulation of crustacean reproduction.