Spatial distributions, feeding ecologies, and behavioral interactions of four rabbitfish species (Siganus unimaculatus, S. virgatus, S. corallinus, and S. puellus)

Broad-scale spatial distribution, microhabitat association and habitat partitioning of damselfishes (family Pomacentridae) on an Okinawan coral reef

Spatial distribution of coral reef fishes is related to diverse environmental variables. This study aimed to elucidate the (1) broad-scale spatial distribution, (2) microhabitat-scale substrate association, (3) degree of dependence on live corals and (4) habitat partitioning of 26 damselfish species on an Okinawan coral reef. Broad-scale analysis revealed that fish assemblages could be divided into three groups in relation to the degree of wave exposure, and the coverage of live corals as well as non-coralline substrates: (1) 11 species that were found in exposed reefs with greater coverage of rock; (2) nine species that that were found in inner reefs with greater coverage of live corals, dead corals and macroalgae; and (3) six species that were found in inner reefs with a greater coverage of sand. Microhabitat-scale analysis revealed that fish assemblages could be divided into six groups in relation to diverse microhabitat availability: (1) 12 species showed significant positive associations with rock; (2) two species showed significant positive associations with coral rubble; (3) two species showed significant positive associations with corymbose Acropora, Pocillopora and branching corals; (4) three species showed significant positive associations with branching Acropora; (5) three species showed significant positive associations with branching Acropora, branching Isopora and branching Porites; and (6) two species showed significant positive associations with staghorn Acropora and branching Millepora. The microhabitat-scale analysis also revealed that Pomacentrus amboinensis showed a significant positive association with branching Millepora, whereas Neopomacentrus anabatoides showed significant positive associations with branching Porites, foliose coral and dead branching Porites. Among the 26 species, nine species were categorized as obligate coral dwellers (>80% of the individuals were associated with live corals), and three species showed a greater degree of dependence on acroporid corals (>60% individuals were associated with acroporid coral). Niche overlap analysis revealed that 14 species showed relatively greater habitat partitioning with other species, whereas the remaining 12 species showed greater habitat overlaps among some species. These results suggest that broad-scale and microhabitat-scale habitat partitioning is one of the factors supporting coexistence in at least 14 species among the 26 species, and the effects of habitat diversity on the species coexistence of damselfishes should be incorporated to establish effective marine protected areas to preserve damselfish species diversity.

Biogeographical diet variation within and between the rabbitfishes Siganus corallinus, Siganus doliatus, Siganus trispilos and Siganus virgatus

Feeding habits of herbivorous fishes play an important role in shaping the form and function of coastal marine ecosystems. Rabbitfishes (Siganidae) are important consumers of macroalgae on Indo-West Pacific coral reefs. However, it is unclear how their diet varies among and within species at biogeographical scales, casting doubt on their precise functional roles across different regions. The present study assessed the inter- and intra-specific diet variation of four rabbitfishes (Siganus trispilos, Siganus corallinus, Siganus virgatus and Siganus doliatus) factored by morphological relatedness among populations from Ningaloo Reef (western Australia), the Great Barrier Reef (GBR, eastern Australia) and the Yaeyama Islands (Okinawa Prefecture, Japan). Results showed that the region had a strong effect on diet, effectively reducing the expected effect of morphologic similitude. While intra-specific differences were only significant when populations inhabited different regions; interspecific differences were not as predicted, with different morphotypes having similar diets when populations inhabited the same regions. Rabbitfishes consumed more corticated and filamentous macroalgae on the GBR, more foliose and membranous macroalgae at the Yaeyama Islands, and more leathery macroalgae at Ningaloo Reef. The findings indicate that rabbitfishes have high diet plasticity, and hence their functional role as mediators of competition between macroalgae and corals can change across biogeographic regions. Local context is therefore important when assessing the diet and functional role of herbivorous fishes. As climate change unfolds, shifts in the distribution, trophic behaviour and function of species are expected, making the study of trophic plasticity more important.

Feeding habits of four-finger threadfin fish, Eleutheronema tetradactylum, and its diet interaction with co-existing fish species in the coastal waters of Thailand

This study assessed the feeding habits of four-finger threadfin fish, Eleutheronema tetradactylum, and its diet relationship with other fish species in the tropics. Fish samples were collected from four locations along the coastal regions of Thailand. A whole year field sampling event was conducted to investigate the diet relationship of threadfin fish with other ten co-existing fish species in Pattani Bay during January 2021 and January 2022. E. tetradactylum was an active and specific predator with significant diet shift during ontogeny. Specifically, the juvenile fish fed largely on zooplankton especially Acetes/shrimp postlarvae, and small sized-fish fed on penaeid shrimps, while medium and large-sized fish shifted their diets to a combination of penaeid shrimp, fish and squid. Size and sex of fish as well as site of collection significantly affected gut fullness index and average number of food type (p < 0.05). Transitional sex fish predated almost entirely on other fishes (87.2%), whereas male and female fish fed mainly on penaeid shrimp (66.5%) and other fish (51.3%), respectively. Fish size and mouth opening controlled the size of prey, with the larger fish with larger mouth-opening fed primarily on the larger size of prey. Moreover, E. tetradactylum shared its diets inclusively with Epinephelus coioides, Johnius belangerii, Scomberomorus commerson, Scomberoides lysan, Otolithes ruber and Lutjanus russelli. Penaeid shrimp and teleost fish were the main food types shared by these fishes. This study provided important information on the feeding habits of E. tetradactylum and its diet relationship with other co-existing fish species living in the same habitat of a tropical coastal region.

Spatial distribution of parrotfishes and groupers in an Okinawan coral reef: size-related associations in relation to habitat characteristics

Parrotfishes (Labridae: Scarini) and groupers (Epinephelidae) are important fish groups that are regarded as the fisheries targets of primary importance in coral reefs. In order to establish ecosystem-based management of these two fish groups, clarifying the spatial distribution relative to habitat characteristics is of central importance. The present study investigated the spatial distributions of 12 parrotfishes species and seven groupers species in relation to environmental characteristics in an Okinawan coral reef. Ten out of the 12 parrotfish species and all seven grouper species showed species-specific spatial distributions. Four substrate types in the inner reefs (branching Acropora, bottlebrush Acropora, dead branching Acropora, and dead bottlebrush Acropora), three substrate types in the exposed reefs (massive coral, other coral, and calcium carbonate substratum), and water depth showed significant associations with the spatial distribution of fishes. Among the 12 parrotfish species, two species (Scarus spinus and S. forsteni) and four species (S. psittacus, S. hypselopterus, S. dimidiatus and S. ghobban) were primarily found in exposed reefs and inner reefs, respectively. Among the seven grouper species, two species (Cephalopholis argus and C. urodeta) and two other species (C. miniata and Epinephelus ongus) were primarily found in exposed reefs and inner reefs, respectively. Size-related spatial distribution was also found for three parrotfish species (Chlorurus microrhinos, Scarus rivulatus and S. hypselopterus), indicating that smaller-sized and larger-sized individuals were respectively found at sites with greater coverage of substrates with fine structure (live bottlebrush Acropora and dead bottlebrush Acropora) and coarse structure (live branching Acropora, dead branching Acropora and calcium carbonate substratum). The present study suggested that the spatial distribution of parrotfishes and groupers is not necessarily associated with the higher coverage of living corals, but positively associated with high substrate complexity. Thus, actual spatial distributional patterns of species should be considered to select candidate sites for protection and conservation for the two fish groups.

Spatial distribution and feeding substrate of butterflyfishes (family Chaetodontidae) on an Okinawan coral reef

Coral reefs support diverse communities, and relationships among organisms within these communities are quite complex. Among the relationships, clarifying the habitat association and foraging substrate selection relative to habitat characteristics is of central importance to ecology since these two aspects are the fundamentals for survival and growth of organisms. The aims of the present study were to investigate the spatial distribution and feeding substrate selection of 14 species of butterflyfishes on an Okinawan coral reef in Japan. Species-specific spatial distributions varied with habitat characteristics (e.g., encrusting corals, massive corals, branching Acropora and rock). For feeding substrates, seven species of obligate coral polyp feeders exhibited significant positive selectivity for tabular Acropora, corymbose Acropora, encrusting corals and massive corals but significant negative selectivity for dead corals, coral rubble and rock. Among six species of facultative coral polyp feeders, two species exhibited significant positive selectivity for encrusting corals and massive corals, and one species showed significant positive selectivity for dead corals as feeding substrates. In contrast, three species exhibited no significant positive selectivity for any feeding substrates. A similar result was observed for one non-coralline invertebrate feeder. Among the 14 species, 12 species showed a relatively close relationship between spatial distribution and feeding substrates but the remaining two species did not. The present study is the first study to elucidate species-specific spatial distributions and feeding substrate selection of butterflyfishes on an Okinawan coral reef. The results of the present study suggest that diverse substrates, including various types of living corals (especially encrusting corals, massive corals, tabular Acropora, corymbose Acropora and branching Acropora) and non-coralline substrates (rock) are the primary determinants of spatial distributions and feeding sites. Thus, diverse substrates are important for maintaining high species diversity of butterflyfishes and changes of the substrates would likely change the spatial patterns and foraging behavior, although species-specific responses may be found, depending on their species-specific dependence on vulnerable substrates.