Akio Tamaki

Competitive Bioturbators on Intertidal Sand Flats in the European Wadden Sea and Ariake Sound in Japan

Distribution patterns and changes in abundance of macrozoobenthic species on intertidal sand flats have often been interpreted in terms of the physical environment, such as tidal level, sediment composition and salinity. More recently, however, biotic interactions have been recognised to be a significant structuring force for intertidal sand flat communities (Reise 1985; Posey 1987). There are several kinds of biotic interactions (Arthur and Mitchell 1989). The most frequently studied is competition (mainly for food), a - - interaction where there are effects on both species. The reverse of competition is mutualism, in which both species profit from the interaction (+ +); this is, however, not often observed on intertidal sand flats. The other often observed interaction is predation, which is a + - interaction, termed contramensalism by Arthur and Mitchell (1989). In this case, only one species benefits. In addition to these interactions in which both species are influenced, there are also interactions in which only one species is influenced. This could be either commensalism (+ 0 interaction) or amensalism (- 0 interaction). For this type of interaction the size of the animals often determines the outcome of the interaction (Wilson 1981). An example of this is the mobility-mode hypothesis of Posey (1987), which states that dense aggregations of large organisms or those with strong sedimentary effects may exclude smaller species through modifications related to their mobility type.

Egg size and clutch size in three species of Nihonotrypaea (Decapoda: Thalassinidea: Callianassidae) from western Kyushu, Japan

Three species of the callianassid genus Nihonotrypaea occur in the Ariake Sound estuarine system, southern Japan; they consist of two tidal-flat species ( N. harmandi ; N. japonica ) and one boulder-beach species ( N. petalura ), with maximum population densities of 1440, 343, and 12 ind m −2 , respectively. Nihonotrypaea harmandi and N. petalura are distributed along the coastline from the outermost part of the sound to the open sea, while N. japonica occurs in the middle part of the sound. Nihonotrypaea japonica has an extended reproductive period from late winter to autumn, while those of the other species are from late spring or summer to autumn. Interspecific comparisons were made for recently laid egg size (as volume) and clutch size (as number of eggs per female). Only in N. japonica was a seasonal egg size variation observed, being significantly larger in winter to spring (mean=0.106 mm 3 ) than in summer (0.080 mm 3 ). By contrast, clutch size was significantly smaller in winter to spring, resulting in nearly the same clutch volume per female (product of the mean egg volume and clutch size) between the seasons. Among the three species, the egg size was ordered as N. japonica (overall mean volume through the seasons=0.092 mm 3 )>> N. petalura (0.057 mm 3 )> N. harmandi (0.054 mm 3 ). The clutch size was ordered as N. harmandi > N. petalura ≈ N. japonica . The clutch volume was ordered as N. japonica ≈ N. harmandi > N. petalura . The smallest clutch volume value for N. petalura female showed an opposite trend to the relative size of the major cheliped found in a previous study.

Description of a new species of Armandia (Polychaeta: Opheliidae) from western Kyushu, Japan, with character variations

Armandia amakusaensis sp. nov. (Polychaeta: Opheliidae) is described from an intertidal sandflat in western Kyushu, Japan. The holotype measures 13.14 mm in total body length, and has 32 setigers, with branchiae on setigers 2–31. Eleven pairs of lateral eyes are present on setigers 7–17. The anal funnel opens dorsally, fringed with 11 papillae, its length being slightly shorter than the length of the last three setigers. A long unpaired cirrus originates mid-ventrally from the inside of the anal funnel. Variation in these diagnostic characters is described. The new species is most similar to A. leptocirris (Grube, 1878) and A. intermedia Fauvel, 1902. However, it is distinguished from A. leptocirris in the distribution patterns of the branchiae and the lateral eyes, and from A. intermedia in the distribution pattern of the lateral eyes, the ratio of anal funnel length to total body length, and the number of anal-funnel papillae.