CORAL REEF PAPER BULLETIN OF MARINE SCIENCE, 38(2): 366-383, 1986 DISTRIBUTION AND ABUNDANCE OF THE SEA-STAR ARCH ASTER TYPICUS IN KABIRA COVE, ISHIGAKI ISLAND, OKINAWA Hiroshi Mukai, Moritaka Nishihira, Hiroshi Kamisato and Yutaka Fujimoto ABSTRACT The tropical sea-star Archaster typicus has a high population density on protected sandy flats in the Yaeyama Islands, Okinawa. Intensive studies of horizontal and vertical distribution patterns show that within small areas the young sea-stars show a random dispersal. With growth they shift their distribution pattern to a contagious one with patches of about 2 m x 2 m size. In the middle-scale spatial distribution, they change their center of distribution from deep to shallow intertidal bottoms with growth. Although factors responsible for the macro-spatial distribution remained obscure, aspects of the shore in relation to wind direction and life-size topography were thought to be important. The distribution of a single species may be perceived as a series of different scales of distribution ranging from regional to microscale. This relates to the gradation of abiotic environments. Some biological factors also affect distribution at each level of the environmental gradient. Both intraspecific and interspecific interactions influence distribution. Therefore, it is necessary to get detailed in- formation on the distribution at all levels for a total understanding of the distri- bution of a particular species. We have been studying populations of the sea-star, Archaster typicus Muller and Troschel; this species occurs in tropical and subtropical coral reef areas in the Indo-West Pacific (Clark and Rowe, 1971). According to Yamaguchi (1977), its distribution is restricted to the littoral region of continents and continental islands, and the species is absent on oceanic islands. A. typicus is famous for its pairing behavior (Boschma, 1924; Ohshima and Ikeda, 1934a). In Japan, it is very abundant at some localities in the southernmost islands, the Yaeyama Island Group, especially Ishigaki and Iriomote Islands, and the existence ofa high density population has long been known (Ohshima and Ikeda, 1934a~). Our study, on this sea-star, aims to clarify how a high-density population is maintained and describes the distribution of A. typicus, and shows a shift of distributional pattern accompanied with growth. STUDY AREA Ishigaki Island (Fig. I; 24°30'N, 124°IO/E) lies in the Yaeyama Island Group within the Kuroshio Warm-Current, and is surrounded by well-developed fringing reefs. The coastline is irregular, especially in the western part of the island, and there are several inlets where A. typicus occur. Kabira Cove, located at the northwestern part ofIshigaki Island, faces the northern open ocean, and is well protected by Kojima Islet on the outermost part of the cove. The cove is about 2 km in length and about I km in the maximum width and 1.5 km2 in total area, of which about one-third emerges at low spring tides (Fig. I). There are many small limestone rock islets and well-developed fringing reefs at the mouth of the cove, and exchange of cove water occurs through four narrow channels, of which three are so shallow that they dry up at low tide. There are wide sandy flats along both shores, particularly in the central and innermost part ofthe cove. Corals grow at the outer margin of these flats, and there is a steep drop into the basin of 10-15 m depth. The slope also supports abundant coral growths. 366 MUKAI ET AL.: DISTRIBUTION AND ABUNDANCE OF SEA-STARS 367 30·N • N 20·N -+- 120· E .: 130·E N t 6 5 o Kabira Cove o 200 400m ·V'n .••..•..,.. Coral Qiff -'-'-'-Lowest LowWater Level Figure I. Maps of Ishigaki Island and Kabira Cove. Kabira Cove is divided into 10 areas for convenience, shown by Roman numerals. Border of the areas is shown by dotted lines. Small arabic numbers, 1-61, show stations of the whole area study, and six letters show transect census lines. A large solid arrow indicates the prevailing wind direction in winter, and open ones indicate flowing points of freshwater streams into Kabira Cove. Dark areas in the Ishigaki Island map show the distribution of Archaster typicus population. The areas inhabited by sea-stars are mostly sandy, with scattered cobbles and boulders. The bottom ofthe basin is covered by silt and clay-sized particles. The monthly average surface water temperature recorded in the channel ranges from about 30-31 DC(August) to about 18-20OC (January). Some small freshwater streams flow into the cove (Fig. I); two streams in the innermost part of the cove are relatively large. The geology and general geomorphology and the general outline of the intertidal community of the cove will be described elsewhere by members of the joint survey team of Kabira Cove area coral reefs. 368 BULLETIN OF MARINE SCIENCE, VOL. 38, NO.2, 1986 C) ~ W C 0 a::I <:( I : I i I ~ ___ -:-: ~:. CIO eCL;l J L__.__._.-•....- ~ ___________.!::: d .9•.. e! ::l ~ t:: 8 E •..o -----_.:~ (5 ,0 --------~ 't:I - § .-.-_.C":l-- ;>, 't:I ._----~- B~ .::c-....- .~II.l ~ ___oN t:: - •..II.l t:: - i;: o '"'~ ~ ~ ~ ~ t:: .9 --co- 1;;~ N ----ClCl 00 ....•........... .'.... • , t :! lCIO:a::I:<:(:NI : I • I I ••I ••.r I I -. C 0 -m -<:( N MUKAI ET AL.: DISTRIBUTION AND ABUNDANCE OF SEA-STARS 369 METHODS For Quantitative sampling of A. typicus a hand dredge with an iron frame opening 33 cm wide, 15 cm high and a back-bag of 5-mm mesh was used. Sea-stars collected were counted and their radius was measured to the nearest millimeter. They were then released at the sampling points. In March 1974 and 1975, 75 sampling stations were set up throughout Kabira Cove for the census of the A. typicus population (Fig. I). The sampling points for each station were chosen at a level likely to have a high density of sea-stars in a vertical belt. An additional sample was taken, if necessary, near the original point. In both years, the sampling stations were set at the same locations. Multiple regression analyses were made of physical factors against the abundance of the sea-star. In March 1975 sediment samples were collected at all stations. On 27-29 March 1978, six transect lines on the intertidal sandy flats were set up perpendicular to the shore line at various parts of the cove (Fig. I). The sea-stars were censused along the transect lines using a Quadrat of I m x I m. At the same time, surface sediments were sampled. This census aimed to show vertical distribution range of the sea-star. An intensive study of a temporal change in distribution ofthe sea-star was made at 282 stations in the innermost area of Kabira Cove from July 1974 to Feb. 1977 (Fig. 2). Spatial dispersion was studied by intensive Quadrat sampling at two points, E'7' and C9, on 27 September 1974 and at one point, B6, on 28 March 1978 (Figs. I and 2). These sampling points were selected in homogeneous substrata to avoid as much as possible the influence of abiotic factors on dispersion of the sea-star. Each area of 4 m x 4 m at E'7' and C9, and the area of 6 m x 8 m at B6 were divided into grids with unit areas of 0.5 m x 0.5 m, and each grid was sampled separately. The data obtained were used for analysis of distribution pattern (Id-analysis; Morisita, 1959). Sediment samples were dried at 60·C, 48 h after displacement of sea-water with distilled water, then sieved with a standard set of Taylor sieves, and the fractions weighed. The size-distribution of grains was expressed in percent dry weight. A part of the sediment samples was used for measurement of the proportion of carbonate. The proportion was expressed as percent weight loss after HCI treatment, on a dry weight basis. In addition, careful observations on the processes of formation and break down of temporary aggregations and on behavioral mode of the sea-star were made on the innermost sandy flat on 28- 29 August 1974. All of the censuses and observations were made in daylight. RESULTS For the sake of convenience, the intertidal area of Kabira Cove was divided into 10 areas based on topographical features (Fig. 1). The divisions do not necessarily correspond to the distribution of sediment type; each area includes a wide range of sediments from coarse to fine. To gain an understanding of the substrate characteristics ofKabira Cove, the relationship between size-distribution of the sediments and depth or distance from the shore were studied. Most sediments consisted of calcareous detrital remains of organisms such as corals, forams, molluscs, sea-urchins, etc., although the relative composition dif- fers in various parts of the cove. The relative amount of calcareous material was expressed as the proportion in weight of HCl-soluble matter to total sediment in samples collected in March 1975 (Table 1). The results show a marked tendency for the proportion of calcareous material to be higher in the mouth and on the eastern shore of the cove. In areas II, VII, VIII, IX and X the content of HCl- soluble material was about 80-100%. It was low in areas III, IV, V and VI. Low levels may be due to the influence of the small rivers discharging into these areas (Fig. 1). Horizontal Distribution oj the Sea-star. - The distributional patterns of abun- dance were similar in March 1974 and 1975 (Fig. 3). In areas II, V and IX the sea-stars were rare, whereas in areas III, IV and VIII they were abundant.