Nobuyuki Azuma

Food sources for the bivalve Corbicula japonica in the foremost fishing lakes estimated from stable isotope analysis

Carbon and nitrogen stable isotope ratios in tissue of the bivalve corbicula Corbicula japonica and particulate organic matter (POM) were measured in Lake Jusan, Lake Ogawara and Lake Shinji, which are the foremost fishing grounds for the corbicula in Japan, to determine their food sources. The bivalves in Lake Ogawara and Lake Shinji showed enriched isotope composition, while those in Lake Jusan were depleted. In addition, the difference in the isotope ratios between the sampling sites was remarkable in Lake Jusan. Chlorophyll concentrations were significantly higher in Lake Ogawara and Lake Shinji than those in the inflow rivers, although that in Lake Jusan was equivalent to that in the river. Residence time of river water was estimated at 1 day, 455 days and 88 days in Lake Ogawara and Lake Shinji assimilate autochthonous phytoplankton, while those in Lake Jusan assimilate terrestrial matter in the upper reaches and marine phytoplankton in the lower reaches because of low production in the lake.

A New Population of the Amphioxus (Branchiostoma belcheri) in the Enshu-Nada Sea in Japan

Abstract The amphioxus, Branchiostoma belcheri, is currently listed in the registry of “Endangered Animals of Japanese Marine and Fresh Water Organisms” issued by the Japan Fisheries Resource Conservation Association. We surveyed a new population of this species in an area near the Irago Channel and the Enshu-Nada Sea in Aichi Prefecture, Japan. This population was originally discovered by a research group of the Aichi Fisheries Research Institute when they were collecting Japanese sand lances (Ammodytes personatus) in this area. Every month from June to November of 1995 with the exception of October, we collected amphioxi and sand lances in an area of about 1 km2 to study the population profile of the amphioxus and the relation between these two animals. The newly discovered population of the amphioxus was suspected to be larger than any other Japanese populations of this species. Animals of this population could be divided into two groups according to body size distribution: those of approximately 20 mm long on the average (termed small) and those of approximately 50 mm long on the average (termed large). No small amphioxus was found in June; 10% or fewer of the animals were of small size in July and August; and about 40% of the animals were of small size in September and November. The breeding season was estimated to last from June (or earlier) to August based on observation of the gonadal size. Amphioxi share a habitat with sand lances, which stay in the sand during the summer dormant period. The newly discovered amphioxus population may provide an important source of material for modern biological researches on Cephalochordata in Japan, although special care for protection of this population should be taken.

Gonadal state of wild amphioxus populations and spawning success in captive conditions during the breeding period in Japan

Abstract Adult individuals of amphioxus (Branchiostoma belcheri) were collected by dredging from a research vessel at selected stations in two areas off the coast of Japan in July 2000: Deyama and Takamatsu so named by local fishermen in the Enshu Nada Sea. The number of males collected exceeded that of females at all the stations in Takamatsu and at four of five stations in Deyama. The over all sex ratio (males : females) of the collected animals was 1.2 : 1. The animals showed various maturational stages of the gonad, and approximately 70% had mature gonads. However, post-spawning animals were identified only at two stations in Takamatsu. Mature animals were placed in laboratory tanks. These animals remained in good conditions for about two months, and many animals spontaneously spawned in the tanks. This is the first report of spontaneous spawning of B. belcheri in Japan.

Field evidence for bioluminescent signaling in the pony fish, Leiognathus elongatus

Luminescence of the pony fish, Leiognathus elongatus, was observed in the natural environment during nighttime diving. The light was emitted from the lateroventral portion of the body, as bright rectangular-shaped luminescence patches turned on and off periodically. Luminescent fish had a distinct clear patch on the flank through which light was emitted, whereas non-luminous fish did not have such a clear patch. Both luminous and non-luminous fish were found within a shoal, where non-luminous individuals were chased by luminous ones. From previous morphological studies, the luminous and non-luminous individuals are likely to be male and female, respectively. Our observations provide field evidence that the luminescence functions as intraspecific communications in L. elongatus.

Fractal evaluations of fish school movements in simulations and real observations

Fish schools behave like a single organism, and this offers considerable survival advantages. In our simulations, a fish school is well organized, without a leader, and behaves like a single creature depending solely on the interactions among individuals. This kind of system can be said to be typical of “complex systems.” In this article, it is shown that fractal evaluation is useful to understand the features of fish school movements. We make clear the validity of fractal analyses to quantify fish school movements through evaluations of simulated fish school movements andsardine movements. These fractal analyses show that we need two different fractal dimensions (D1,D2) to understand the features of fish school movements:D1 corresponds to thesmaller coarsening levels, andD2 corresponds to thelarge coarsening levels. The linear analyses in log-log space give an excellent fit with both the simulated movements and the sardine school movements. In approaching complex systems or complex behaviors, fractal analyses have attracted wide attention in mathematics, physical sciences, and information science. The fractal evaluations here convince us that we are coming close to understanding the structure of complex movements of animals.

Isotopic fractionation with morphological change and sexual specificity in the lappet moth Euthrix potatoria

The δ15N values of adult holometabolous insects exceed those of larvae, but otherwise little information on terrestrial invertebrates has been obtained in food‐web analyses using stable isotope ratios (δ15N, δ13C). Changes in δ13C during metamorphosis and differences between males and females have not been examined. We collected the larvae and cocoons of Euthrix potatoria (L.) (Lepidoptera: Lasiocampidae) in the field and used them to assess the species’ isotopic fractionation. Each emerged moth was divided into five body parts. We conducted stable N and C isotope analyses for each body part, as well as for cocoons and exuviae, and also compared stable isotope ratios between sexes. We confirmed δ15N enrichment through metamorphosis and estimated that δ15N enrichment is accomplished by the relative concentration of 15N due to the excretion of copious meconium, which contains abundant 14N. We also observed changes in δ13C values through metamorphosis. Both isotope values tended to change more in males than in females. The proportion of the whole‐adult weight represented by meconium was higher in males than in females, suggesting that high meconium secretion in males contributes to the sexual difference in δ15N. These phenomena may be common in Holometabola, which require a pupal stage. For more accurate food‐web assessments, it is important to consider stable isotope changes during different life cycles, as well as sexual differences.