Larval dispersal in marine invertebrates plays a significant role in the maintenance and conservation of adult populations. Reef-building corals spawn eggs and sperm, or brood larvae (planulae) and shed them directly into the water. Larval dispersal is physically determined by ocean currents and biologically by egg and larval behaviour, buoyancy in the water column, and the period during which larvae can settle after spawning or release (settlement-competency periods). In addition, the settlement-competency periods may be affected by the available energy source during the planktonic phase. However, little is known about how these factors determine larval dispersal ranges. Here, I review larval dispersal processes of corals based on our researches of the two different dispersal types. The planula larvae which have mainly narrow dispersal ranges (fine scale, shorter than 1 km) occurred throughout the water column and have shorter settlement-competency periods. This is due to a lower lipid content and the influence by weaker tidal currents. In contrast, the planula larvae which have mainly wider dispersal ranges (40 km or more) are characterized by positive buoyancy and longer settlement-competency periods. This is due to the energy from a higher lipid content; especially wax esters. In addition, the symbiotic dinoflagellates in some larvae provide additional energetic resources which may allow longer larval dispersal.

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The mussel (Mytilus galloprovincialis) is an important seafood species and is cultivated in many countries. It is also considered a major fouling organism because it colonizes artificial structures causing economic problems. Many studies have demonstrated that M. galloprovincialis larval settlement is influenced by various substrata, i.e., macroalgae, conspecifics and biofilms. This review summarizes the roles of substrata on the larval settlement, with reference to the involvement of chemical cue(s). Literature on M. edulis larval settlement is also included. Larvae of M. galloprovincialis settle on specific filamentous algae in response to chemical cue(s), and not to the algal morphology or associated organisms. Evidence of the gregariousness in this species has also been demonstrated in the laboratory. Biofilms strongly induce the larval settlement and their activities are modulated by immersion periods and months (seasons). Bacteria in biofilms are directly involved in the induction of larval settlement and the viability of bacteria is a requirement for the production of chemical cue(s). Evidence also suggests that two chemical cues derived from bacteria, i.e., waterborne and surface-bound cues, act synergistically on the larval settlement. Further research on the identification of chemical cues is necessary to clarify the mechanism of mussel larval settlement.

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Both intact thalli of Sargassum fusiforme and those without holdfasts were aquacultured in Okirai Bay, Northeast Japan, to establish them as a multi-harvestable aquaculture resource. Elongated main branches (edible part) of the intact thalli were harvested twice (August 2005 and June 2006) during 1.5 years of aquaculture (November 2004-June 2006). However, two fouling organisms, Laminaria japonica and Mytilus galloprovincialis, also occurred on the aquaculture rope. As a fouling countermeasure, S. fusiforme thalli without holdfasts were aquacultured for 6 months (February-August 2005) with freshwater treatment (biweekly 2-min exposures). Their stipes regenerated holdfasts, and the attached holdfasts elongated around the aquaculture rope; moreover, the new holdfasts gave rise to a number of stipes, which became the bases of new main branches. The elongated main branches were harvested after 4 months of aquaculture. Thalli without holdfasts collected from the rope were aquacultured again for ca. 1 year (October 2005-August 2006) with the same freshwater treatment. After 8 months, elongated main branches were harvested from these thalli with their new regenerated holdfasts. The freshwater treatment was effective to remove L. japonica, but it had no effect on fouling by M. galloprovincialis. Therefore, the two successive harvests were conducted before the mass occurrence of M. galloprovincialis in the summer.

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Larval settlement of the barnacle Amphibalanus amphitrite on coal-flyash concrete plates was investigated in the laboratory using a water-flow assay. Settlement was evaluated in comparison to that on ordinary concrete plates. A choice test was conducted using coal-flyash concrete and ordinary concrete plates, and no significant difference in settlement between the two kinds of plate was found, whether or not the plates had been treated with crude extract from conspecific adults. When larvae were exposed to only one type of plate in each assay, again no difference in larval settlement was observed between the two types. The number of settled barnacles on the plates could be raised by mixing crushed barnacles into the coal-flyash concrete. These results indicate that coal-flyash concrete can be used without problem as a substitute for concrete to make artificial fishing reefs.

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Microbial biofilms were allowed to develop on glass slides immersed 1.5 m below the sea surface in Tachibana Bay, Nagasaki, Japan, for different periods of time from May, 1999, to March, 2001. Effects of age or bacterial and diatom densities of the biofilms on the settlement of cyprids of the barnacle Amphibalanus amphitrite were investigated in the laboratory. Furthermore, biofilms were subjected to various treatments in order to investigate the nature of the chemical cue for settlement. Cyprids of A. amphitrite responded differently to various biofilms. Of the 58 biofilms tested, 3 induced settlement of A. amphitrite larvae, while 4 significantly inhibited it. The other 51 films had no effect on larval settlement. No positive correlation was observed between age or bacterial and diatom densities of biofilms and their respective capacities to induce larval settlement. Inactivation of biofilms by ethanol and heat treatments resulted in the induction of larval settlement, suggesting that the treatments made a potential settlement cue in the films available to A. amphitrite larvae. Moreover, LCA (lentil agglutinin), WGA (wheat germ agglutinin), and H₅IO₆ treatment of active biofilms resulted in the reduction their settlement-inducing activity. This finding suggested that the settlement-inducing compound in certain marine biofilms contains specific sugar chains that have similarities to the SIPC (Settlement-Inducing Protein Complex) from adult barnacle shells.

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A flow-through microcosm experiment was conducted using filter-feedering bivalves and ascideans together with intact sediment cores collected from near a seawall and from the natural sandy seashore and an artificial sandy tidal flat in Hamana Bay. The objective of this experiment was to examine individual specimens’ ability to remove particulate organic matter and release nitrogen occurring in the different environments of the seawall and the shore. In consideration of apparent seasonal changes observed in Hamana Bay, summer- and fall- based models were used. Mytilus galloprovincialis (summer seawall model), Molgula manhattensis (fall seawall model), and Ruditapes philippinarum (summer and fall seashore models) served as the filter feeders in the experiments, and ¹⁵N-labeled Chaetoceros calcitrans was used as the nitrogen source. In both the seawall and seashore models, 30% of PON was decomposed into NH₄⁺ in the fall, while only a few percent was decomposed in the seawall model in summer due to inactivation of the macrobenthic metabolism by hypoxia. In the latter model, Mytilus galloprovincialis and the sediment both acted as sinks to accumulate most of the nitrogen, thus causing a de-acceleration in the nitrogen cycling rate and, consequently, summer eutrophication. In contrast the nitrogen cycling rate of the seawall model in fall was as high as that of the seashore model. These results suggest that the sandy seashore and tidal flat region may be playing a significant role in eutrophication improvement in Hamana Bay.

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