The influence of methanol extracts of Hypnea musciformis (Rish odophyceae) and Ulva fasciata (Chlorophyceae) on the development and behavioral changes in the rock boring sea urchin larvae Erhinonletra mathaei was evaluated. H. musciformis extract at 0.095 and 0.0095 rng/ml suppressed the development of the sea urchin larvae and retained them in the prism stage itself. In the short-term exposure, larvae up to 5^th day were only affected. The extracts of U. fasciata did not influence the larval developmental stages at lower concentrations.