Atsushi Nanami

The structures and dynamics of fish communities in an Okinawan coral reef : effects of coral-based habitat structures at sites with rocky and sandy sea bottoms

Although there have been many studies on ecological factors responsible for the organization of reef fish communities, most of the studies have focused on isolated habitats. However, findings from isolated habitats cannot necessarily be applied to fish communities in other habitats (e.g., a continuous habitat). In this study, therefore, we examined the structures of fish communities (abundance, species richness and species composition), and the dynamics of fish communities (seasonal changes in abundance, species richness and species composition) over a 2-year period in two different habitats (continuous habitat and isolated habitat) in an Okinawan coral reef. We established eight permanent quadrats (8 m × 8 m) on a rocky reef flat (continuous habitat) and rock reef patches surrounded by a sandy sea bottom (isolated habitat). The abundance and species richness of fishes such as pomacentrids, labrids, chaetodontids and acanthurids were greater in quadrats located in the continuous habitat, whereas those of blenniids, gobiids and mullids were greater in quadrats located in the isolated habitat. This caused marked differences between the fish community structure at the two sites. Seasonal and annual changes in fish community structure were relatively small at the continuous habitat site (>0.7 similarity based on Cπ index) but were large at the isolated habitat site (Cπ < 0.4), indicating that the fish community structure was relatively stable at the continuous habitat site but unstable at the isolated habitat site throughout the study period. Spatial differences between the fish community structures in the four quadrats at the continuous habitat site were small (Cπ > 0.65 in most sites), but these differences were large at the isolated habitat site (Cπ < 0.4). Our findings suggest that habitat structure (spatial arrangement of habitats) affects both spatial and seasonal differences in the reef fish community structure. The results also suggest that the main mechanisms underlying organization of reef fish communities in continuous and isolated habitats are different.

Species-specific habitat distribution of coral reef fish assemblages in relation to habitat characteristics in an Okinawan coral reef

We determined the species-specific habitat associations of coral reef fishes and environmental characteristics in an Okinawan coral reef in Japan. We focused on three families (Pomacentridae, Gobiidae and Labridae) and attempted to determine differences in habitat utilization. We selected six sites along the coast of Amitori Bay, from the entrance to the innermost part, in order to cover a wide range of habitat characteristics (exposed habitat, semi-exposed habitat and sheltered habitat). The species diversity of coral assemblages was greater at the exposed and semi-exposed habitats, whereas branching coral mostly covered the sheltered habitat. The environmental factors that determine the species-specific spatial association in fishes differed among families. Both biological characteristics (coral morphology and coral species diversity) and physical characteristics (water depth and wave exposure) affected the spatial association of pomacentrids and gobiids. In contrast, physical characteristics such as substrate complexity and water depth affected the species-specific spatial association of labrid species. Further study is needed to determine the ecological factors that regulate the species-specific habitat preference in Okinawan coral reefs.

Seasonal dynamics of fish assemblage structures in a surf zone on an exposed sandy beach in Japan

Seasonal dynamics in fish assemblage structure, comprising postlarval to early adult stages, in the surf zone on an exposed sandy beach on the Kashimanada coast were studied over a 2-year period. A total of 32 species and two higher group taxa were found. In terms of abundance, Salangichthys ishikawae, Mugil cephalus cephalus, Plecoglossus altivelis altivelis, and Engraulis japonicus were the four most dominant species. The results of cluster analysis and canonical correspondence analysis using 12 dominant species in terms of abundance revealed that water temperature and wind factor (wind speed × wind direction) had significant independent associations on species occurrence patterns for 11 species; that is, occurrence of 11 species was significantly related to water temperature and occurrence of 7 species was related to wind factor. Two species (S. ishikawae and P. altivelis altivelis) showed clear monthly increases in size, consequently suggesting their use of the surf zone as a nursery area. In contrast, M. cephalus cephalus and E. japonicus showed few increases in standard length, indicating their use of the surf zone only in their postlarval and/or juvenile stage. It is suggested that numerous fish species use the surf zone as shelter and/or a nursery area, even in the harsh conditions of an exposed sandy beach with great wave action.

Population Dynamics and Spatial Distribution of Coral Reef Fishes: Comparison Between Continuous and Isolated Habitats

Recent studies have shown that there are high degrees of spatial and temporal stability in coral reef fish assemblage structures in a continuous habitat, in contrast to results of observations in isolated habitats. In order to determine the reason for the difference in temporal stability of fish assemblage structures in a continuous habitat site and an isolated habitat site, population dynamics and spatial distributions of coral reef fishes (six species of pomacentrids and two species of apogonids) in the two habitat site were investigated over a 2-year period in an Okinawan coral reef. The population densities of pomacentrid and apogonid species increased in juvenile settlement periods at both sites, but the magnitude of seasonal fluctuation in population density was significantly greater at the isolated habitat site, indicating that the rate of juvenile settlement and mortality rate in the isolated habitat were greater than those in the continuous habitat. The magnitude of aggregation of fishes, which affects density-dependent biological interactions that modify population density such as competition and predation, was also significantly greater at the isolated habitat site, especially in the juvenile settlement season. Most of the fishes at the isolated habitat site exhibited more generalized patterns of microhabitat selection because of less coral coverage and diversity. The seasonal stability in the species composition of fishes was greater at the continuous habitat site than that at the isolated habitat. Our findings suggest that the relative importance of various ecological factors responsible for regulation of the population density of coral reef fishes (e.g., competition, predation, microhabitat selection and post-settlement movement) in a continuous habitat site and the isolated habitat site are different.

Preliminary observations of spawning behavior of white-streaked grouper (Epinephelus ongus) in an Okinawan coral reef

Epinephelus ongus forms spawning aggregations at specific reef sites at Okinawa. The aim of the present study was to describe the spawning behavior of E. ongus based upon underwater video imagery. Epinephelus ongus spawning is usually paired with courting males and females rushing upwards towards the surface during the spawning ascent, with a subsequent release of eggs at the apex of the ascent. In two of four spawning events we observed, however, the ascending pair was joined by additional males. Fish eggs collected at the site were identified as those of E. ongus and confirmed that this species spawns mainly at night during a slack tide. This study is the first documentation of E. ongus spawning behavior in the field.

Microhabitat association and temporal stability in reef fish assemblages on massive Porites microatolls

A massive Porites microatoll generally has three types of microhabitat at the top, side, and base of the microatoll. The purpose of the present study was to analyze microhabitat associations of reef fish on microatolls to determine whether habitat characteristics play an important role in the structuring of reef fish assemblages in a patchy habitat. We also investigated temporal stability of reef fish assemblage structures over a period of 17 months to determine whether fish assemblage structures vary in a random manner. The results of correspondence analysis indicated species-specific habitat associations for pomacentrids (five species) and labrids (seven species). The degree of temporal stability of fish assemblage structures, calculated by Pianka’s Α index, was relatively high in a large-sized microatoll (0.503–0.831: ≫3.6 m in diameter), in which microhabitat associations of fishes were clearly observed. The present study suggests that a microhabitat association is one of the important factors responsible for organization of reef fish assemblages in a microatoll.

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.

Nest construction by the labrid fish Choerodon jordani (Snyder 1908)

Nest construction by the labrid fish Choerodon jordani for a sleeping site was observed on the sandy bottom near Sesoko Island, Okinawa, Japan (Fig. 1a, Fig. 1b). Most labrid fishes become inactive after sunset and sleep buried in sand or at the base of coral colonies. In contrast, adult C. jordani construct a “nest” for its sleeping site. This novel behavior is the first such report in labrid fishes. Some coral reef fishes, including pomacentrids and gobies, construct nests for refuge space and/or breeding (Thresher 1984), but C. jordani constructs nests only for sleeping. C. jordani constructs its nest as follows: First, the fish thrusts its head under a plate of coral rubble and repeatedly scrapes out sand to make one or two entrances. The fish then picks up dead branching coral skeletons and lays them at the entrances. It often patrols the nests within its territory and tidies the nest in the daytime. Just after sunset, the fish enters its nest to sleep (Fig. 1c). Like most labrid fishes (Thresher 1984), C. jordani forms a harem and spawns around sunset, but spawning does not take place in the nest. Competitive interactions for sleeping sites have been observed in diurnal coral reef fish (Robertson and Sheldon 1979). The present findings suggest that C. jordani avoids such competition by making nests. Further detailed investigation should lead to new insights into the function of nest-building in this species.

Effects of Habitat Connectivity on the Abundance and Species Richness of Coral Reef Fishes: Comparison of an Experimental Habitat Established at a Rocky Reef Flat and at a Sandy Sea Bottom

Coral reef fish assemblages are widely recognized for the coexistence of numerous species, which are likely governed by both coral diversity and substratum complexity. However, since coral reefs provide diverse habitats due to their physical structure and different spatial arrangements of coral, findings obtained from an isolated habitat cannot necessarily be applied to fish assemblages in other habitats (e.g. continuous habitats). The aim of this study, therefore, was to determine by a field experiment whether habitat connectivity (spatial arrangement of coral colonies) affects abundance and species richness of fishes in an Okinawan coral reef. The experiment consisted of transplanted branching coral colonies at a 4 m×8 m quadrat at both a rocky reef flat and sandy sea bottom. Generally, the abundance of fishes was greater at the sandy sea bottom, especially for three species of pomacentrids, one species of labrids, one species of chaetodontids and two species of apogonids. Species–area curves showed that the species richness of fishes was significantly greater in the quadrat at the sandy sea bottom at 3, 6 and 9 months after the start of the experiment. The rate of increase in abundance of fishes per area was significantly greater in the quadrat at the sandy sea bottom over the study period. The results of rarefaction analyses showed that the rate of increase in species richness per abundance was significantly higher in the quadrat at the sandy sea bottom in the juvenile settlement period, indicating that the magnitude of dominance by particular species was greater at the sandy sea bottom habitat. Our findings suggest that habitat connectivity affects the abundance and species richness of coral reef fishes, i.e. the isolated habitat was significantly more attractive for fishes than was the continuous habitat. Our findings also suggest that the main ecological factors responsible for organization of fish assemblage at a continuous habitat and at an isolated habitat are different.

Interspecific differences in prey items in relation to morphological characteristics among four lutjanid species (Lutjanus decussatus, L. fulviflamma, L. fulvus and L. gibbus)

Clarifying interspecific differences in prey items in relation to morphological characteristics is a fundamental aspect to understand the mechanism enabling the diversity of feeding ecology of fishes. The aim of the present study was to clarify the relationship between prey items and body shape variation, teeth and mandible characteristics for four lutjanid species: Lutjanus decussatus, L. fulviflamma, L. fulvus and L. gibbus. Stomach contents analysis revealed that the main prey items of L. decussatus were fishes, L. fulviflamma were crabs and fishes, and L. fulvus and L. gibbus was crabs. Body shape analysis revealed that L. decussatus and L. fulviflamma had a shallower body depth whereas L. fulvus and L. gibbus had a deeper body depth. The two species with a shallower body had long teeth whereas the other two species with a deeper body depth had shorter teeth. The jaw-lever mechanics were compared and L. decussatus and L. fulviflamma have a faster mouth opening–closing mechanism. Canonical correspondence analysis revealed that fishes was the major prey item for the species having a shallower body depth, higher teeth length and lower ratios of in-lever to out-lever of mandibles, whereas crabs was the major prey item for the species having a deeper body depth, lower teeth length and higher ratios of in-lever to out-lever of mandibles. It is suggested that the interspecific differences in main prey items among the four species are directly related to behavioral differences based on body shape, teeth characteristics and jaw-lever mechanics.