The tolerance of larva at each development stage to organotin antifoulants was studied using Blanus amphitrite DARWIN. The tolerance of nauplius to organotin compounds increased with the larval development. The tolerance of cypris larva was smaller than that of the sixth stage of nauplius larva. These results suggest that the nauplius and the cypris can not be regarded as identical, in physiolgical terms. It has been reminded that the nauplius at the same stage can be replaced by the cypris when comparing the potency of the same compounds, but this substitution is not always possible in the screening test of new antifoulants.

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Grazing behavior of sea urchin, Strongylocentrotus nudus, was observed experimentally on algae, Undaria pinnatifida. The sea urchin made rather active movement in the case without algae compared to the case with algae although the moving was done at random. The sea urchin showed active movement around sunset and sunrise. Daily moving distance was 5-15m and daily moving time was 10-15 hours for the sea urchin. Grazing effect on young Undaria was examind at various densities of sea urchin. In the case of initial density for young Undaria at about 24/m², about 85% Undaria survived after a 20 days period at sea urchin density 0.19/m² but only 2% Undaria survived after a 6 days period at sea urchin density 3.0/m². These results may simply explain occurrence of desert rocks in the northeast Pacific coast of Japan.

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The reproductive conditions of the mussel, Mytilus edulis galloprovincialis, collected from four localities in Japan were studied by histological analyses of their gonads. Reproductive cycle of the mussels was divided into the six developmental stages: resting (a), pre-developmental (b-1), post-developmental (b-2); pre-spawning (c-1), post-spawning (c-2) and degenerative (d) stages. Their spawning seasons and the reproductive patterns were determined by these developmental stages of gonad.
The main spawning season of the mussel was considered longer in the southern areas than in the northern areas: from October to March in Yokosuka Harbor and Otsuchi Bay, from November to February in Urasoko Bay and from October to April in Omura Bay. The shorter spawning season of the Urasoko Bay seems due to the high saline water conditions of this area. In the spawning pattern there were two spawning types: total spawning and partial spawning types. In the former the gonad develops simultaneously and spawning occurs from the whole gonadal tissues. After this total spent gonads develop simultaneously again for the next spawning. In the latter case spawning occurs partially in the gonad tissues, histologically the gonads containing mixed developmental stages. The partial spawnings were more common in the individuals inhabiting the subtidal zone than those in the intertidal zone. In the subtidal zone partial spawning took place much more commonly in the 1-year than 0-year group.
Based on the reproductive patterns, it was suggested that the Japanese mussel is certainly identical to the Mediterranean species, Mytilus galloprovincialis LAMARCK.

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The external morphology of the spermatozoa of Hydroides elegans, H. ezoensis and H. fusicola, common species on the coast of Nagasaki City, was observed using a scanning electron microscope. The head of the spermatozoon of H. elegans consists of a conical acrosome and a pear-shaped nucleus; in H. ezoensis and H. fusicola, it consists of a beret-like acrosome and a globular nucleus, though appreciable differences in size and shape of the acrosome were found between the two species. The middle-piece consists of four globular mitochondria in the three species.

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