Sayaka Ito

Mate choice and cannibalism in a natural population of a stream goby,Rhinogobius sp.

Breeding ecology of the stream goby,Rhinogobius sp. LD (Large Dark), was investigated under natural conditions. Males selectively courted females of similar size to lead them to the nests, whereas females followed courting males preferentially when the relative male size was greater. Male-male competition for a female was relatively infrequent and not severe. Developmental stages of eggs and egg numbers in one nest indicated that males receive 1–3 clutches during one breeding cycle. Males guarding multiple clutches frequently ate some of the eggs, but those guarding single clutches rarely did so. Gravid females in the nest also frequently cannibalized eggs laid by a previous female, probably in order to extend the area available for egg deposition. Mate choice in this species is discussed in relation to paternal ability, limitation of available spawning area and the female-biased sex ratio.

Mate Choice and Mating Pattern in a Stream Goby of the Genus Rhinogobius

The mate choice and mating pattern of a benthic goby Rhinogobius sp. CB (cross band type) were investigated in the Kamo River, Shikoku, Japan. During the breeding season, gravid females assumed a nuptial color and either males or females initiated a courtship display. Males preferentially courted a female of similar size to lead her to his nest, whereas females courted more frequently when they encountered a large male. Eggs in any one nest were always at the same developmental stage. Sampling data of nesting males and females indicated that, in more than half the nests, males gathered more than one female before spawning. In some nests with eggs, two or three females had spent ovaries, indicating that the eggs were laid by multiple females within a short span of time. However, a comparison between the total number of eggs which females would spawn in one nest and the number of eggs actually deposited suggested that eggs were contributed by one female in most nests. This low level of polygyny in spite of multiple female availability is attributed to a limited available spawning area of the nest.

Female spawning strategy in Rhinogobius sp. OR : how do females deposit their eggs in the nest?

Abstract We tried to elucidate how females of a paternal nest brooding goby Rhinogohius sp. OR deposit their eggs in a nest, using a marking technique for live eggs under laboratory conditions in which male somatic condition, nest space, and mating pattern (monogamous or bigamous) were controlled. Whether females rejected mating was independent of either male quality, such as body size and somatic condition, or nest space. In a situation in which two females were allowed to spawn sequentially with a male, however, females rejected mating at a higher rate when they were the first to spawn than when they were the second to spawn; this is because eggs from first females were more vulnerable to cannibalism by parental males and second females. Even when nest space was limited and thus was occupied by eggs from the first females, second females could deposit all their eggs in the nest by using the minute interspace of existing eggs. In the presence of the parental male, such a female seemed less likely to suffer a cost from increased egg mortality due to crowdedness, still holding the advantage of being the second spawner. Finally, we extrapolated the field breeding ecology of this fish from the laboratory data. It was suggested that a single monolayer brood of the same age usually consisted of eggs from multiple females and thus the mating pattern would be more polygynous than previously expected.

MATE AVAILABILITY AND SOMATIC CONDITION AFFECT FILIAL CANNIBALISM IN A PATERNAL BROODING GOBY

Summary We conducted a laboratory experiment to test two major predictions from a game-theoretical model for the evolution of filial cannibalism in species with paternal care - that parental males are more cannibalistic when their energy reserve is low and that filial cannibalism increases when mate availability is high. We used a freshwater goby, Rhinogobius sp. OR, in which males care for eggs from multiple females in a nest. For each breeding male, we manipulated food ration, sex ratio and nest space: the former to control its energy reserve and the latter two to control its mate availability. The ANOVA showed that all three factors had a significant effect on filial cannibalism, which was facilitated when male somatic condition was poor, the sex ratio was female-biased and the nest space was small. Furthermore, filial cannibalism became more intense with increasing brood size and with decreasing female body size. In conclusion, the parental energy reserve and the quality and availability of mates are important factors affecting filial cannibalism in Rhinogobius sp. OR.

Habitat Use of the Grazing Goby (Sicyopterus japonicus) in Response to Spatial Heterogeneity in Riparian Shade

ABSTRACT The local density of the grazing goby (Sicyopterus japonicus) and benthic algal production were investigated in a 720 m reach of the Sate River in the northern part of Okinawa Island, Japan. The reach was spatially heterogenous in riparian shade and was divided into three segments (open-canopy, shaded and patchy segment) based on vegetation cover. In the reach, the goby was more abundant in the open-canopy segment and open-canopy patches than in the adjacent shaded segment and shaded patches. Between the open-canopy and shaded segments there was no significant difference in the algal biomass; however, the algal growth rate was greater in the open-canopy segment than in the shaded segment. These results indicate that the spatial heterogeneity in riparian shade controls the habitat use of the goby, with open-canopy areas with high algal renewal rates being preferred.

Changes in male physiological condition during brooding activities in a natural population of a stream goby, Rhinogobius sp.

We investigated the change in physical condition of males during paternal care in a natural population of a stream goby Rhinogobius sp. LD. Hepatosomatic index (HSI) and Condition factor (K) of males guarding an egg mass were higher than those of non-breeding males. In the first brooding cycle, the HSI of males decreased with the progress of embryo development. Moreover, in the second brooding cycle, both the HSI and K decreased, suggesting that the energy reserves in the somatic tissue were used when the liver reserves were exhausted as a source of energy for paternal activities. After paternal care, males showed reduced K and were unable to recover HSI immediately. Comparisons of the physical condition between non-guarding males after the first brooding cycle and males guarding an early stage clutch in the second brooding cycle indicated that males which could recover HSI and K were able to have a second brooding cycle. This suggested that the physical condition determines participation in breeding activity and the number of brooding cycles, greatly influencing male reproductive success.

Female mimicry in a freshwater goby Rhinogobius sp. OR

Abstract We found female-mimic males in a freshwater goby, Rhinogobius sp. OR, from Kamo River, Ehime, Japan. Of 171 adult males collected, three had a female appearace. A discriminant analysis showed that they had a spectrum of morphology from a wholly female morph, through a neutral morph, to a somewhat malelike morph. These three males made an extremely higher gametic investment than did the normal males, suggesting that they have the potential for sneaking fertilizations. Among Rhinogobius species, female mimicry might be more common than previously known.

Comparison of current-velocity tolerance among six stream gobies of the genus Rhinogobius

Tolerance to current velocity was compared among six stream gobies of the genus Rhinogobius (cross-band, dark, large-dark, cobalt, and orange types, and R. flumineus). Each individual of the six gobies was exposed to incremental increases in current velocity within a laboratory flume. The current velocity at which each individual was dislodged was recorded and compared by species. Results from comparisons showed that the tolerance of gobies declined in the following order: cobalt, large-dark, dark, cross-band, R. flumineus, and orange type. This interspecific difference in velocity tolerance is consistent with their habitat segregation patterns as previously reported by field observations.

Feeding ecology of two planktonic sculpins, Comephorus baicalensis and Comephorus dybowskii (Comephoridae), in Lake Baikal

Planktonic sculpins (Comephorus baicalensis and Comephorus dybowskii) are endemic to Lake Baikal, and their migration behavior is characterized by their strange movements, which occur only vertically. We investigated their detailed food sources at each growth stage by stable isotope analyses in combination with stomach content analysis. At all growth stages, both fishes and amphipods were the main diet for C. baicalensis, whereas amphipods alone were the main diet for C. dybowskii. Our results indicate that the relationship between the two species shifts from a competitive interaction to a predator–prey interaction with the growth stages of C. baicalensis.

A marking technique for live fish eggs and larvae

Abstract A new marking technique for live fish eggs and larvae was proposed to elucidate the larval biology and adult breeding ecology of wild fish. In the laboratory, females of a freshwater goby Rhinogobius sp. OR were abdominally injected with one of three coloring agents—brilliant blue FCF, rose Bengal, or β-carotene—before their oviposition. The rose Bengal proved lethal to adult fish. The other two dyes had little effect on adult mortality. With these two treatments, there were negative effects on neither fecundity nor egg mortality, resulting in normally developed larvae. The brilliant blue FCF stained eggs and larvae greenish blue whereas the staining effect of β-carotene was unclear. The timing of injection was important in effective staining of eggs and reducing the risk of miscarriage. In conclusion, the brilliant blue FCF was the more useful marker. We discuss what this method can show us about the ecology of wild fish and how this method can be applied to field study.