I review the distribution of microphytobenthic biomass and production in the intertidal, shallow subtidal (<5 m), and offshore (5-30 m) zones of temperate coastal regions by compiling previous studies, with special emphasis on the light regime as a driving force behind the observed patterns. In the intertidal zone, which is periodically exposed to direct sunlight, the mean biomass and production of microphytobenthos reach 84±61 mg Chl-a m⁻² and 121±100 g C m⁻² yr⁻¹, respectively. The biomass and production of intertidal microphytobenthos generally exceed those of phytoplankton in the overlying water column on an areal basis. Although subtidal microphytobenthos communities are never exposed to direct sunlight, both the microphytobenthic biomass and production of the shallow subtidal zone, mainly in microtidal areas, are similar to those of the intertidal zone. In contrast to intertidal habitats, however, the benthic microalgal productivity in the shallow subtidal zone is often less than that of the overlying phytoplankton. The dynamics of offshore microphytobenthos are probably affected by phytoplankton in the overlying water column due to a shading effect. The magnitude of both the biomass and the production of microphytobenthos in the offshore zone is relatively low, but microphytobenthic production can be nearly equivalent to the phytoplanktonic production in certain locations where the waters are characterized by low densities of phytoplankton and consequently by high transparency. Changes in the distribution of microphytobenthic biomass and production, and in the relative importance of the microphytobenthos and phytoplankton, are likely to affect the availability of food for the macrobenthos, with implications for the entire benthic food web. To achieve a better understanding of the benthic ecosystem in coastal regions, it is therefore necessary to conduct further quantitative studies of the microphytobenthos, including the development of suitable methodology.

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The abundance and size structure of a population of an endangered brackish-water snail, Iravadia (Fairbankia) sakaguchii, was surveyed annually in August and September in the Waka River Estuary, Wakayama, middle Japan. The number of snails at the three surveyed stations, comprising a total area of ca. 9.5 m², was greatest in 2000 (1738 snails) and decreased in most successive years to 2009 (7 snails), but increased slightly in 2003 and again in 2010 (27 snails). Few or no small-sized snails were present in any year, which suggests that little or no recruitment of planktonic larvae was taking place. When the population density increased, large-sized snails were responsible, reflecting the immigration of snails from other nearby populations. This phenomenon indicates the importance of maintaining a patchy population distribution for the conservation of this species.

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Official documents and archives suggest that the alien crayfish Pacifastacus leniusculus (Dana, 1852) was introduced into Japan from North America on five occasions from 1926 to 1930, and populations have survived in Hokkaido, Nagano, and Shiga Prefectures. The chemical conditions and biological environment of an established habitat near their original a release point in Akashina, Azumino City, Nagano Prefecture, were surveyed on four occasions in 1998 and 1999. Water quality (pH, DO, BOD) and WT were recorded seasonally at each of eight stations, and crayfish and other aquatic macro-organisms were collected. Some predatory fishes (Oncorhynchus mykiss, Anguilla japonica, and Lepomis macrochirus) that might actively eat juveniles of P. leniusculus were recorded. Individual density of P. leniusculus was relatively low compared to its other habitats in Japan. Only 52 crayfish were collected during this study, on 19 of 32 sampling occasions (eight sites, four times each). The shape of the acumen and the species composition of ectosymbiotic crayfish worms (Annelida: Clitellata: Branchiobdellidae) can be used as tools to trace the origin of introduced of P. leniusculus in Japan. The present crayfish from Nagano Prefecture have a relatively short acumen, different from those of specimens from Shiga and Hokkaido Prefectures. Furthermore, the crayfish worm Xironogiton victoriensis occurs only on crayfish from Akashina, Nagano Prefecture. The dates of introduction of P. leniusculus in Nagano (1926, 1929) also differ from those for Hokkaido (1930) and Shiga Prefecture (1926). These findings suggest that the regional population of P. leniusculus in Nagano has a different origin than those in Shiga and Hokkaido, having originated from crayfish that were introduced independently from North America.

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In semi-quantitative methods used in ecological field censuses, a specified area is surveyed, often within a fixed period of time, and the abundance of a species is evaluated as one of several density classes. It has been suggested that this method can cover a variety of habitats and takes less time compared to widely used quantitative methods involving quadrats and transects. However, low standardization of semi-quantitative surveys often causes fluctuations in survey effort, and this makes it difficult to compare the results from different localities or occasions. In the present study, the distribution of intertidal molluscs was investigated both by semi-quantitative and quantitative methods in the same plot. The time needed to complete the semi-quantitative survey was a third or a fourth of that required for the quantitative investigation, yet the two approaches yielded near-identical results in terms of species richness, diversity, and relative abundances of molluscan functional groups. A theoretical analysis suggests that relative abundance is robust against fluctuation in survey effort, and the present field investigation supported this viewpoint. That is, the number and cumulative density of species comprising a particular biogeographic sub-set of molluscs increased with increasing survey effort while the proportions of these among the total recorded species remained comparatively constant. The present results suggest that semi-quantitative methods can allocate finite human resources to a wider area or for a longer period than quantitative methods. This can be done without decreasing accuracy, provided relative abundance is used as the basis for calculating assemblage parameters.

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We determined the stable isotope ratios of carbon and nitrogen of three decapods, Ilyoplax pusilla, Macrophthalmus japonicus, and Helice tridens, that dominate the Tamagawa Estuary as well as those of other coexisting benthic animals, benthic diatoms, terrestrial plants, surface sediment, and suspended particulate organic matter present in the estuary water. The results suggest that I. pusilla feeds on benthic diatoms as its primary food source; H. tridens feeds on other benthic animals in addition to organic matter in the bottom sediment, while M. japonicus feeds on benthic diatoms and other animals. The values of total organic carbon (TOC), total nitrogen (TN), and stable isotope ratios of sediment samples from the burrow walls and mud around the burrow entrances of M. japonicus were similar to those of surface sediment samples. However, sediment samples from the burrow wall and mud balls of I. pusilla showed lower TOC, TN, and δ¹⁵N values than the surface sediment samples, which suggests that I. pusilla uses mud devoid of organic materials to construct the burrow wall.

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The sea urchin, Anthocidaris crassispina, is a common herbivore on shallow rocky coasts in the warm temperate region of Japan. Although, this sea urchin is known as the cause of continuing “barren sea urchin grounds” in the situation of high densities, there are limited data about its population dynamics. To understand the population dynamics, especially the causes for it reaching high densities, monitoring of A. crassispina was carried out at a boulder coast on Tachibana Bay, Nagasaki Prefecture, Japan, over a fourteen-year period. The densities of A. crassispina varied from 0.03 to 3.8 ind./m², Their densities dramatically decreased in the years 1991, 1995, and 2003, and the causes must be the disturbances due to typhoons. After those decreases, their densities rapidly increased by the large recruitment and remained high as long as a typhoon did not strike. From these results, temporally occurring large recruitments and high survival rates except for typhoon were the causes to make A. crassispina high densities. Because the high recruitments only occurred in the years when a typhoon struck, the disturbance by the typhoon must have some positive effects on the recruitment of A. crassispina.

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