The majority of the marine benthos is pelagic in their larval stage. The Manila clam Ruditapes philippinarum, is widely distributed on sandy tidal flats across Japan and a commercially important shellfish. The clam has a planktonic larval stage, which lasts 2 to 3 weeks after hatching. In this stage, the most clam larvae are able to disperse over wide areas with the currents and then settle on new habitats on other shores. Many scientists have recently focused to these processes as the “supply-side ecology”, which includes the dispersal and transport of planktonic larvae, their retention at or return to adult habitats, settlement at tidal-flat and recruitment to their populations. Such factors play an important role in the community and population dyanamics of Manila clam in tidal-flat. This article briefly summarizes the studies about these factors of the clam.

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One of the most distinctive and unique characteristics of the early life history of flatfishes is the “metamorphosis” and “settlement” from pelagic, symmetrical larvae to benthic, asymmetrical juveniles, while undergoing profound internal and external changes represented by eye migration, and a habitat shift from spawning to nursery areas. The nursery habitats of flatfishes range from shallow freshwater areas to deepwater areas, and the ontogenetic migrations to specific nurseries vary widely among species. Settlement to the optimal nurseries is critical for flatfishes because the recruitment variability appears to be determined during the pelagic dispersal phase through density-independent mortality processes. First, information on the metamorphosis, settlement and ontogenetic migration processes of flatfishes, mainly Pleuronectidae and Paralichthydae, is reviewed from the points of biological characteristics of each species (e.g. spawning strategy, metamorphic size, vertical movement, lateralized swimming behavior and settlement) and physical features of each locality (e.g. currents, tide, temperature and salinity). Second, the variability on the ontogenetic migrations and recruitment processes of five pleuronectid species, which settle on the different nurseries from rivers to offshore areas, is discussed from the point of the ontogenetic developments of morphology and low-salinity adaptability. This paper reveals the variability and flexibility of recruitment strategies of flatfishes.

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Professor Takeo Okuda (1929-2005) studied egg release and settlement of germling in Japanese Sargassaceae. Some Sargassum species were found to release eggs at non-periodic intervals, and this finding was different from that in previous studies. Germlings remained on the surface of receptacles for periods of one to eleven days. The period where germlings remained on the surface of receptacles varied with species. Settling velocities ranged from 0.12 to 0.56cm/s, with bigger germlings settling at higher velocities. Germlings of intertidal species attached to the substrate immediately, while those of subtidal species required undisturbed condition for settlement.

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Abalone are dioecious broadcast spawners. The eggs hatch into planktonic larvae which develop and swim for a few days to a few weeks before becoming competent to settle. Competent larvae undergo settlement after contacting an appropriate chemical cue. In nature, abalone larvae settle primarily on crustose coralline algae (CCA) and remain there for a few months. The settlement process is triggered by external chemical cues that require contact from the larva, and two putative inducer molecules have been identified. The question of whether the inducers are produced by CCA or by their surface microbes has not been unequivocally resolved, but the weight of evidence supports CCA as the likely source. The recent progress in understanding feeding ecology of post-larval and early juvenile abalone has indicated that CCA provide sufficient food sources for abalone at least until several mm in shell length.

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Some small invertebrates use other larger sessile organisms as hosts that have defenses against predators and other natural enemies. Patterns of host use are quite different between marine and terrestrial systems. The difference is expected to indicate differences of mechanisms in the evolution of host use between those systems. In this review, major similarities and dissimilarities in evolutional and ecological factors of host use are discussed. Requirements for host as a food source have long been considered as an important factor of the evolution of host use, such as chemical coevolution theory. On the other hand, tri-trophic interaction involving impacts by predators has recently been recognized as ecological factors of host use range. Additionally, host range is affected by availability of host and dispersal ability. Differences between marine and terrestrial host use mechanisms are found in the all factors above, particularly in the availability of food source and taxonomical characteristics.

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Most coral reef fishes pass through a pelagic larval stage, varying from a few weeks to months duration. During this period, some larvae travel considerable distances in the pelagic environment, others remaining at their natal reefs. Following the pelagic stage, individuals settle into coral reef and/or surrounding habitats, thereafter entering the benthic juvenile stage. One of the great challenges in coral reef fish ecology is how the young, pelagic stages locate the habitat they settle. At large spatial scales, some larvae detect their settlement reefs by chemical signals of the reefs and/or reef noise, and the good swimming abilities in some reef fish taxa facilitate their navigation to suitable post-larval habitats through long distances. After arriving a reef, individuals make a choice for settlement habitat on the presence of specific benthic substrates, conspecifies and/or heterospecifies. Such fishes may either attempt to settle or reject the habitat and move away in search of a more suitable settlement site, with the habitat choice which strongly affects subsequent growth and survival of individuals. Many studies have indicated that settlement strategies could be a prominent factor determining the distribution of some fish species, as well as post-settlement processes including predation and subsequent stage movements.

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Barnacles are prominent sessile organisms in marine ecosystem. Larval settlement in sessile organisms is a critical developmental stage for their survival and reproduction. The gregarious settlement of barnacles and its chemical basis was first described in the middle of the 20^th century. An overview of studies on barnacle settlement pheromones, especially recent molecular approach, is described in this review. An adult glycoprotein, purified from Amphibalanus amphitrite and named as the settlement-inducing protein complex (SIPC), induces settlement of conspecifc cypris larvae. SIPC is active when adsorbed on the surface of substratum, and is thought to be a leading molecule for the gregarious settlement in barnacles. From immunological studies, it was shown that SIPC is a barnacle specific protein and is involved in both adult-larva and larva-larva interactions. Recent cDNA cloning revealed that SIPC shares a 30% sequence homology with α2-macroglobulins and the cDNA encodes 1, 547 amino acids with a 17-residue signal peptide region. The SIPC may have evolved from an ancestral α2-macroglobulin gene.

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Ascidians have the most simple chordate body plan. Ascidians share one curious characteristic in that they produce cellulose. Cellulose is a major component of tunic that surrounds the whole body of ascidians. Recent genome project and phylogenetic analyses of the ascidian, Ciona intestinalis, suggest that (1) ascidians have a gene encoding cellulose synthase, and (2) ascidian may have acquired cellulose synthase through lateral transfer from bacteria. Developmental genetics of Ciona intestinalis with the transposon Minos has generated one mutant for cellulose synthase. This mutant shows abnormal metamorphosis and settlement, suggesting that cellulose is required in the sessile life of this ascidian.

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Macrofouling on ship hulls and man-made surfaces submerged in seawater causes technical and economic problem worldwide. Paints containing organotin compounds, such as TBT and TBTO, and cuprous oxide compounds have been commonly used as antifouling measures. However, the use of these metal-based compounds has been brought to public attention due to reports of environmental contamination. As such, antifouling technologies that are not only effective but also environmentally benign are urgently needed.
In the search for effective and “environmentally benign” antifouling methods, various efforts are being made to develop novel non-toxic antifouling materials and chemicals based on studies on settlement mechanisms of sessile organisms. The barnacle is one of the most commonly used species in fouling control experiments. This paper describes recent antifouling studies, such as surface wettability and microtopography and marine natural products, which are applications of findings from barnacle settlement mechanism studies.

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Coal-flyash concrete and ordinary concrete plates were immersed in the sea near Shin-Nagasaki fishing port, Nagasaki, Japan, for different periods from October, 2004, to January, 2006. The marine sessile organisms that settled on these plates were studied. Throughout the study period, barnacles (Amphibalanus amphitrite, B. trigonus), bryozoans (Bugula neritina), ascidians (Styela plicata), some species of polychaetes, and algae were observed settling on both kinds of plate. Wet weights of organisms on the two types of plate were the same. These results indicate that coal-flyash concrete is suitable for use in constructing artificial fishing reefs, as a substitute for usual concrete.

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We have developed a large-scale oligo DNA microarray of the ascidian, Ciona intestinalis to investigate global gene expressions of the ascidians. Using this oligo DNA microarray, we obtained the expression profiles of 10, 415 genes of C. intestinalis and categorized them into 5 large clusters; embryonic gene cluster A, embryonic and adult gene cluster B, adult gene cluster C, stably-expressed gene cluster D and maternal gene cluster E, and 49 sub-clusters based on expression patterns during the life cycle. Each subcluster contained functionally related genes. We also used DNA microarrays to detect up- or down-regulated genes in ascidians after exposure to organotin compounds, a serious marine pollutant. We found that organotin compounds up- or down-regulated more than 300 genes. We also found that many of the down-regulated genes belonged to Clusters B and C, especially the c1 (immune-related genes) and c4 (juvenile-specific genes) sub-clusters. As well, most of the up-regulated genes also belonged to Clusters B and C. From the array results, we estimated that organotin compounds have effects on ascidian embryogenesis, metamorphosis and immunity. Our experimental results confirmed that organotin compounds inhibited embryogenesis and morphogenesis of ascidians. These findings demonstrated that the combination of our analyses of genes up- or down-regulated by chemicals and the categorization of those genes by our classification method is a valuable method for estimating the effects of chemical pollutants on ascidians. Such applications provide a unique method for monitoring and for making risk assessments of marine chemical pollutants.

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Five specimens of Ostrea fluctigera Lamy, 1925 were collected from the Sagami Bay, adhering by their left valves to the lowest body whorl (near the aperture) of empty shells of several gastropod species, including those used by hermit crabs. In each case, the curved edge of the left valve precisely matched the curvature of the lowest body whorl of the snail shell, and both the left and right valves grew as an extension of the snail shell's aperture. The possibility of a mutualistic symbiosis between O. fluctigera and hermit crabs is discussed.

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