阿嘉島のさんご礁域に出現する浮遊性カイアシ類の種組成，個体数密度および季節変動

<p>Species composition and abundance of planktonic copepods in the Nansei Islands, including Okinawa, have not been well studied. We undertook a study in 1995, investigating the occurrence of copepods at the AMSL station (26°10.48′N, 127°16.21′E) located in a channel just outside the fringing reef between Akajima Island and Gerumajima Island, Kerama waters in Okinawa. Samples were collected monthly by vertical net haul using a NORPAC net with a 100 µm mesh aperture. 103 species of adult copepods were identified. Together with an additional 7 species collected from a nearby coral reef lagoon, a total of 110 species are distributed in Kerama waters. Species of the genera Clausocalanus, Paracalanus, Parvocalanus, Bestiolina, Oithona, Oncaea and Microsetella were found to be the dominant taxa within the samples. Abundances of the adult copepods varied from 58 to 962 inds. m^-3, with an average of 336 inds. m^-3. Differences in the number of species, Shannon-Wiener species diversity index and Sheldon’s equitability index were not statistically significant (t-test p > 0.05) between the hot period from July to October (surface temperature > 27˚C) and the cold period from January to March (surface temperature < 22˚C). Species such as Calocalanus pavo, Parvocalanus elegans, Acrocalanus monachus, Oithona rigida, and Farranula gibbula occurred in the hot period but not in the cold period. Cosmocalanus darwini, Clausocalanus arcuicornis, C. pergens, Ctenocalanus vanus, Lucicutia flavicornis, Pleuromamma gracilis, Oithona decipiens, and Agetus limbatus occurred in the cold period but not in the hot period. Bestiolina similis, Farranula concinna and Microsetella norvegica occurred more abundantly in the hot period than the cold period, whereas Paracalanus aculeatus appeared in significantly greater numbers in the cold period than in the hot period (p < 0.05). Total number of species, species diversity and equitability were highest in February, indicating inflow and mixing of copepod assemblage from the surrounding waters when wind force is strongest and the surface temperature is lowest.</p><p>In order to compare the specific composition of adult copepods in the AMSL station with a nearby station in the reef lagoon, samples were collected in the reef lagoon at Hizushi Beach (26°11.22′N 127°16.27′E) in October of 1998 using a hand net with a 100 µm mesh aperture. The net was hauled by a SCUBA diver at high and low tides during both night and day. Species that have been reported as abundant in closed lagoons were very few or did not appear at all at the Hizushi station. Despite the close proximity, however, the composition varied. Daytime species composition at the AMSL station, in both September and October, showed the greatest difference when compared to night samples at the Hizushi station. Among the samples at Hizushi station, species composition was most similar between the day/high tide and day/low tide, whereas it differed considerably between the day and night/high tide, indicating stronger influences of light conditions than tidal conditions.</p>