Yoshimasa Aonuma

INDIRECT CONTRAMENSALISM THROUGH TROPHIC CASCADES BETWEEN TWO OMNIVOROUS FISHES

The impact of trophic cascades on the interspecific relationship between two omnivorous stream fish, ayu (Plecoglossus altivelis altivelis) and Japanese dace (Tribolodon hakonensis), were analyzed in experimental pools. Benthic algae grew on the pool beds, and macroinvertebrates colonized the pools. Ayu predominantly grazed the algae, occasionally forming territories. Japanese dace fed on both benthic algae and invertebrates when ayu were absent, and predominantly on invertebrates when ayu were present. The presence of Japanese dace reduced the number of invertebrates and increased algal biomass (chlorophyll a) through trophic cascades, but the effects of respiration and excretion of dace did not notably increase algal biomass probably because the water continuously pumped into the pools was already nutrient rich. The growth rate of ayu was enhanced in the presence of Japanese dace due to increased algal biomass. In contrast, as the number of ayu increased, the growth rate of Japanese dace was reduced bec...

Experimental analysis of the territorial establishment of ayu, Plecoglossus altivelis

Ayu mainly grazes benthic algae on river beds. To clarify the effects of individual behavioral differences on the territory establishment of ayu, we first recorded the number of active movements, aggression towards a wooden ayu model, and scraping feeding activities for 40 ayu, each in an aquarium with a ceramic plate with benthic algae. We then released 10 randomly chosen ayu and other cyprinids, 10 Tribolodon hakonensis and 10 Zacco platypus, into each of the four ponds with recirculating water. Four to six ayu established a territory for 1–20 of the 21 days observed in each pond. Territories were unstable in the first half period but became stable in the latter period. Territorial period was positively correlated with the number of active movements in aquaria and negatively with the number of conspecific invasions. The percentage of attacks against other species was positively correlated with the number of scraping behavior in aquaria and negatively with standard length of ayu. The growth rate of non-territorial ayu was large in case the ayu scraped more than half of ceramic plates in aquaria. In contrast, the growth rate of territorial ayu was correlated negatively with the standard length of ayu, and the percentage of attacks against other species, indicating that individual behavioral differences and the defending cost were both important in territory establishment and growth.

Reproductive activity in the checkered snapper, Lutjanus decussatus, off Ishigaki Island, Okinawa

The checkered snapper, Lutjanus decussatus, is an important species for fisheries in the Okinawan region. This study estimated the reproduction of this lutjanid species in the waters around Ishigaki Island. The main spawning season was estimated to be between June and October, since oocytes at the maturation stage and/or postovulatory follicles were found during the study period. In the main spawning season, high gonadosomatic index values were found around the time of the last quarter moon for each month from June to September. It is suggested that L. decussatus is a lunar-synchronized spawner off Ishigaki Island.

Difference in response by two cyprinid species to predatory threat from the nocturnal catfish Silurus asotus

Response to predators may not be identical between different prey species with different life histories and body sizes, particularly when the threat of predation is not great. To clarify this hypothesis, we introduced two prey species (10 Japanese dace, Tribolodon hakonensis, and 10 pale chub, Zacco platypus) into each experimental pond (in total, 8 ponds × 4 trials) in which benthic algae had been allowed to grow. The presence or absence of Far Eastern catfish, Silurus asotus, and a refuge for prey fish was used to produce four treatments. The presence of catfish and/or a refuge did not affect either the feeding behavior or growth rate of Japanese dace. In contrast, when catfish were present and no refuge was available, the incidence of bottom feeding for pale chub greatly decreased. Pale chub growth rate was low when catfish were present and a refuge was available, indicating that pale chub spent more of their time in the refuge and lost opportunities of acquiring food. Japanese dace can reach a threshold size at which the prey are safe from predation, but pale chub cannot, and this may explain the differences in response to predators of the two species.

Two new species of the genusAmblyeleotris (Pisces: Gobiidae) from the Ryukyu Islands, Japan

Two new shrimp-associated gobies,Amblyeleotris yanoi sp. nov. andA. masuii sp. nov. are described on the basis of specimens from Iriomote-jima Island and Okinawa-jima Islands, Okinawa Prefecture, Japan.A. yanoi is distinguished from other members of the genus by the combination of the following characters: 14 anal fin soft rays, 19 pectoral fin rays, 97–103 longitudinal scales, a candle flame-shaped marking on the caudal fin, a very low membrane connecting the pelvic fins and absence of a ventral frenum.A. masuii differs from all other congeners by having 92–97 longitudinal scales, the length of the interpelvic connecting membrane relative to the longest pelvic fin ray (0.43–0.66), black blotches on the sides of the chin, and blue spots on the opercle and preopercle.

A new species of the genus amblyeleotris (pisces: Gobiidae) from Japan

A new shrimp-associated goby,Amblyeleotris melanocephala, is described on the basis of specimens from Okinoshima Island. Kochi Prefecture, and Okinawa Island, Okinawa Prefecture, Japan. The species is distinguished from other members of the genus by the following combination of characters: head dark brown, a few yellow spots on pectoral fin base and opercular margin, 13 second dorsal and 13 anal fin soft rays, 20 pectoral fin rays, longitudinal scales 92–101, proportional length of interpelvic connecting membrane relative to longest pelvic fin ray (CM-value) 0.46–0.55, presence of a ventral frenum, midline of nape naked, sides scaled above midpoint between preopercle and opercle.

First record of the gobiid fish,Priolepis fallacincta, from japan

T he gobiid genus Priolepis comprises small fishes inhabiting rocky and coral reef areas and is found mainly in the Indo-Pacific region. About 30 species of the genus are known, having been divided into three groups (two grades and one clade) by the transverse pattern of sensory papillae on the cheeks and presence or absence of predorsal scales (Winterbottom and Burridge, 1993). In Japanese waters, six species have been reported by Akihito et al. (1984), Winterbottom and Burridge (1992), Winterbottom and Burridge (1993), Akihito, Iwata et al. (1993), Akihito, Sakamoto et al. (1993), and Masuda and Kobayashi (1994); P cincta (Regan, 1908) and P inhaca (Smith, 1949) of/?. cincta grade, P borea (Snyder, 1909), P latifascima Winterbottom & Burridge, 1993 and P semidoliata (Valenciennes in Cuvier and Valenciennes, 1837) of P semidoliata clade and Priolepis. sp. (undescribed species, figured in Akihito et al., 1984: pl. 239D) o f P profunda grade. During a study on the gobiid fishes of the Ryukyu Islands, some examples of P fallacincta Winterbottom & Burridge (P. profunda grade) were collected at Iriomote-Jima Island, Kakeroma Island and Amami-Oshima Island, Ryukyu Islands. This is the northernmost record of the species, being the seventh species of Priolepis recorded from Japan thus far. Methods for counts and measurements fol-