Chronic and subchronic toxicity following exposure to the DSP (Diarrhetic shellfish poisoning) toxin okadaic acid (OA) is receiving increasing attention as a public human health biohazard. However information on ecological impacts induced by proliferation of the OA producing dinoflagellate Prorocentrum is scarce. In order to analyse the toxicity of these substances, in vivo experiments were conducted on medaka fish (Oryzias latipes) embryos used as an experimental model. The study was focused on two strains of benthic Prorocentrum species, P. arenarium and P. emarginatum, naturally found in the Indian Ocean. Sample extracts (crude extracts, CE) were obtained from algal cultures and their toxic potential was explored. Their OA (and derivatives) content was evaluated by two methods: one based on chemical analysis using HPLC-MS, the other based on screening the inhibiting effect on protein phosphatase PP2A. P. arenarium extracts inhibit PP2A and the active toxin was confirmed as being OA by HPLC-MS. In contrast, P. emarginatum showed negative results regardless of the method used. The development of medaka fish embryos kept in medium containing pure OA or Prorocentrum CE was examined. Survival rates were reduced up to 100% depending on the concentrations used of both OA and CE of P. arenarium, while no effect was observed with CE of P. emarginatum. Anatomopathological studies of surviving embryos indicate that OA treatment resulted in significant increases in liver and digestive tract areas compared to controls. P. arenarium treated surviving embryos exhibited significant quantitative increases of global body and vitellus areas. Together, our results indicate that the toxic effects to medaka embryos development of pure OA and P. arenarium extracts containing OA are distinguishable. The differences may indicate the presence of additional toxic substance(s) (or molecules able to modulate OA impact) in the P. arenarium CE that probably are not present in P. emarginatum.