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 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 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, 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 ~

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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. All the areas with abundant sea-stars are located at the center of wide sandy flats. In area IV with the widest sandy flat in the bay-head, the sea-stars were common but the density was not so high. As several streams flow into this area, the salinity may fall at low tide. The distribution of the sea-stars was not dramatically affected, 370 BULLETIN OF MARINE SCIENCE, VOL. 38, NO.2, 1986

Table I. Contents of HCl-soluble matter in sediment samples of Kabira Cove. (Values in weight percent; for station locations see Fig. 1)

Area Station Contents Area Station Contents II 1 96.7% VI 32 8.1 2 98.4 34 16.9 3 96.9 VII 35 34.5 4 93.6 36 64.9 5 95.7 37 68.2 6 95.5 38 70.7 III 7 35.3 39 74.8 8 28.5 40 75.4 9 19.0 41 83.6 10 23.8 42 89.5 11 6.0 VIII 43 82.9 IV 12 13.2 44 80.4 13 10.7 45 82.6 14 4.0 46 80.3 15 6.7 47 78.3 16 8.2 IX 48 77.3 17 41.3 49 75.3 V 19 44.5 50 75.9 20 39.9 51 73.1 21 36.1 52 75.4 22 30.3 53 82.3 VI 23 33.3 54 82.0 25 4.7 55 89.1 26 1.6 X 56 91.9 27 15.9 57 90.9 28 32.9 58 91.5 29 10.0 VIII 59 88.7 30 5.6 60 93.4 31 7.4 II 61 91.1

except near the mouths of the streams. Area VII, with a narrow sandy beach, includes some stations where the sea-stars were dense. Most of the sea-stars in the area in March 1974 were small, and were assumed to have settled in 1973 (Mukai et a1., unpub1.). To identify the factors influencing the distributional pattern observed, we tried a multiple regression analysis, taking nine physical factors as the explicative vari- ates; i.e. the pebble component of the sediment larger than 2 mm; the amount of coarse sand (2-0.5 mm); medium sand (0.5-0.25 mm); fine sand (0.25-0.062 mm); and the silt-clay component smaller than 0.062 mm; the proportion in weight of Hel-soluble matter of the sediment; the shore angle (measured clock- wise) from the NE which is the prevailing wind direction in Ishigaki Island (90 means the most exposed to wind and waves and 270 means the most calm); the degree of influence of the open ocean or degree of embayment, expressed by the distance (km) from the sampling point to the mouth of the bay; the width (m) of the intertidal sandy flat inhabited by the sea-stars. The results obtained show that no single factor or any combination of factors could explain successfully the abundance of the sea-star. However, shore angle against wind direction and width of sandy flat were relatively important in de- termining the abundance of the sea-star. Vertical Distribution. -It was confirmed by preliminary diving observation that MUKAI ET AL.: DISTRIBUTION AND ABUNDANCE OF SEA-STARS 371

March 1975

Legend )eNol\\,'

• 1 • 5 .20• 10 .ao.40 o .160

Figure 3. Density distribution of Archaster typicus along the shoreline of Kabira Cove in March 1974 and 1975. the sea-stars occurred neither on hard bottom nor soft bottom in the subtidal zone below the maximum depth of our stations. Therefore, six transect lines were set up on intertidal sandy flats (Fig. 1). Not all parts of the intertidal sandy flats support sea-stars. Actual range of the distribution on the traverse depends upon the topography. Sediment grains were relatively fine on the wide sandy flats, whereas sediments of their fringes were composed mainly of poorly-sorted coarse sands. Table 2 shows the characteristics of sediments along six transect lines in March 1978. In the upper intertidal area that inclines steeply to the flat, coarse sediments are deposited, and the deeper the depth or the more inner the location in the cove, the finer the sediments. In Figure 4, the abundance of the sea-star is shown on the profile of each transect. The vertical extent of the distribution ranged from + 120 cm to the 0 or -20 cm level. On the transects having a steep inclination such as Transects A and D, the sea-stars was distributed in a narrower zone centered around just below MSL (110 cm level). In contrast, the sea-star was spread over a wider vertical range on Transects E and F. These observations suggest that the distri- bution of the sea-star is not decided by depth alone and that the depth at which the maximum abundance occurs is not the same at all transects (maxima ranged between +60 cm and + 110 cm). Even at the census points A-35, C-50, C~85, D-60 and E-80 (numerals indicate distance in meters between the uppermost point and the census point), all in the range of the abundant zone, no sea-stars occurred. There are some other factors apart from depth which affect the distri- bution and abundance of the sea-star. Because the sea-star is a deposit-feeder, 372 BULLETIN OF MARINE SCIENCE, VOL. 38, NO.2, 1986

Table 2. The median and the mean particle size of the sediment along transect lines. (Numerals in station name mean distance in meters from high tide point on the transect line)

Sill-day Station Mdo contents

A-05 0.61 0.37 0.04 A-15 0.70 0.56 0.07 A-25 -0.Q1 -0.08 0.04 A-35 -0.70 -0.68 0.04 A-50 0.14 0.19 0.24 B-15 0.27 0.39 0.49 B-20 0.87 0.73 0.43 B-40 0.65 0.47 0.10 B-60 0.55 0.37 0.09 B-80 -0.15 -0.17 0.16 B-100 0.43 0.28 0.12 ColO 1.87 0.44 0.94 C-25 -0.41 -0.43 0.23 C-35 -0.58 -0.23 0.33 C-50 -0.16 -0.08 0.45 C-70 -0.10 -0.04 0.49 C-85 0.30 0.19 0.31 C-95 -0.02 0.03 0.68 C-125 0.95 0.52 0.89 C-150 1.01 0.46 1.36 C-170 1.21 0.93 1.34 C-190 1.12 0.82 1.16 C-200 1.13 0.89 1.00 C-21O 1.34 1.05 1.05 D-IO 0.39 -0.68 0.08 D-20 1.73 1.37 1.49 D-30 1.52 1.30 1.72 D-40 0.97 0.59 1.36 D-60 1.06 0.74 1.36 D-85 1.15 0.70 1.55 E-IO 0.20 0.25 0.56 E-15 0.57 0.51 0.42 E-40 -0.20 -0.09 0.49 E-60 0.38 0.29 0.03 E-80 0.49 0.05 0.21 E-120 0.28 0.Q7 0.50 E-150 0.64 0.44 0.71 E-200 0.28 0.05 0.51 E-240 0.58 0.38 0.53 F-25 -0.63 -0.45 0.39 F-50 -0.81 -0.55 0.26 F-75 0.67 0.57 0.80 F-100 0.01 0.17 0.87

taking organic detritus in sediments, the abundance may be related to the amount of organic detritus. As there have been many reports of a positive correlation between detritus and silt-clay content of the sediments (Sanders, 1956), the pro- portion of silt and clay fragments of sediments was used in place of organic detritus itself. Figure 5 shows a range of silt-clay content of the sediment related to the abundance of the sea-star for the transect censuses. The points where the sea-star was abundant appear to be clustered. The sea-star did not occur on the coarse sediment having silt-clay fragments less than 0.2%. MUKAI ET AL.: DISTRIBUTION AND ABUNDANCE OF SEA-STARS 373 ~.~.\. \0-\ '--O~ -'\:::0'\·------LHHw 100 \ ----IS~O •••••em ------\:0-;:::8------~10-1~------LLHW § ---_\ -"-'~,~-.-----~:-:'--:'~--- E ~ ~~ ~ 0 LHLW 0 :;; 0 •••••••••••••••••••••••••••••••••••••• -.- ••.••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••. c 6 ~

:0-••••

~ \\ ~~ ---~\,::;~~~------:: ~o J 36 IJ~

______~----_- ~34~_;__~~~-MLLW

o F I~ I· \------LHLW--- 0 4_ o ...... •....•.•..•.••.•....•.•...... ••...•..•...••....•.....••....•.....•.•.•...... '~b'~'''''''''~

0,

Figure 4. Vertical distribution of density of Archaster typicus (number/m2) in six transect censuses. Numerals on the profile show the density at the point.

Biological Factors Affecting Distribution. - The influence of bivalve distribution and other potential prey on carnivorous sea-stars such as , and others (Feder and Christensen, 1966; Nojima, 1979) is not considered here, because Archaster typicus is a detritus feeder (our pers. obs.). Predators are not thought to affect the distribution of the sea-star, although some xanthid crabs were observed gnawing at the tips of sea-star arms. There remains a possibility that the predator(s) affect the mortality of juvenile sea-stars just after settlement. We did not conduct any study concerning this possibility. Thus, only intraspecific interaction was examined. For this, a uniform environment with flat sandy bottom was selected at three points, E'7', C9 and B6 (Figs. 1 and 2). A total of 282 sea-stars were collected (including 4 individuals larger than 40 mm radius) at E'7', 88 (including 5 larger inds.) at C9, and 88 at B6; the average density was 3.5, 35.2 and 12.0·m-2, respectively (Table 3). The change of variance was analyzed by changing the size of a quadrat from 0.5 m to 3.0 m at intervals of 0.5 m. Then the statistical index of Moris ita (1959), lA, was calculated for each (Table 3). Table 3 shows: 1) that the coefficient of variation (CV) was smallest around the quadrat-size of 2 m x 2 m in an three cases, and the smaner the quadrat, the larger the CV; 2) at B6, indices of uniformity, S2!Xand lA' showed contagious distribution in all quadrat-sizes (F > F(n, 00, 0.01)). 374 BULLETIN OF MARINE SCIENCE, VOL. 38, NO.2, 1986

-.150 Legend E X X u 2 -.....J x x No.lm W x x X 0 > W X • 2 .....J • • it ~100::J • 6 • 10 ~ o .e x .14 w 20 X e o> • CO <{ 50 • .30 l- • • I X X ~ " W I x o

X . .• • ,. • I , -20 ... 0.05 0.1 0.2 0.5 1.0 2.0 5.0 5 ILT- CLAY CO NTENTS (°/0) Figure 5. Density of Archaster typicus in relation to height above datum line and silt-clay content of the sediment in the transect censuses.

At C9, significant aggregation was seen in quadrat-sizes of 1.5 m x 1.5 m and smaller at the 1% or 5% level. Contagious distribution was not recognized in 2 m x 2 m quadrat-size. Aggregation at E'7' was significant only in 0.5 m x 0.5 m quadrat-size. Morisita (1959) proposed a method to reveal not only the degree of uniformity in individual dispersion, but also intra patch distribution of individuals and patch sizes in a population, using the I~-quadrat size relations. Figure 6a shows I~- quadrat size curves, and the ratios between I~ values of the quadrat area of Sand 2S, i.e. I~(S)/I~(2S), are shown in Figure 6b. The dispersion pattern is different from point to point. The sea-stars at B6 (March 1978) had 2 m x 2 m-sized aggregations within which they were dispersed randomly, with no smaller aggregations. All individuals collected there had a radius of about 60 mm. If they spread over the sea-floor without any overlapping disks with a radius of 60 mm, the expected maximum density would be about 64·m-2• The observed maximum at B6 was 48·m-2, which was already beyond half of the expected limit. At C9 (September 1974), although there was a slightly contagious distribution in the 1.5 m x 1.5 m size, the degree of contagiousness was generally weak. Sea- MUKAI ET AL.: DISTRIBUTION AND ABUNDANCE OF SEA-STARS 375

Table 3. Morisita's index of dispersion of Archaster typicus at three points at the head of Kabira Cove

Quadrat Size Density Unbiased Coefficient Point No. (m) (x) variance of variation I. F

B6 192 0.5 X 0.5 12.0 24.4 0.82 1.348 2.045** 48 1.0 X 1.0 12.0 54.7 0.61 1.292 4.563** 20 1.5 X 1.5 11.7 93.3 0.54 1.253 7.981** 12 2.0 X 2.0 12.0 115.8 0.43 1.167 9.681** 6 2.5 X 2.5 11.4 189.0 0.44 1.183 16.594** 4 3.0 X 3.0 11.1 329.1 0.47 1.216 29.690**

C9 64 0.5 X 0.5 35.2 46.5 0.38 1.036 1.321* 16 1.0 X 1.0 35.2 68.0 0.23 1.025 1.933* 4 1.5 X 1.5 34.2 163.9 0.22 1.037 4.788** 4 2.0 X 2.0 35.2 72.2 0.10 1.006 2.053

E'7' 64 0.5 X 0.5 3.5 5.6 1.35 1.704 1.615** 16 1.0 X 1.0 3.5 4.3 0.57 1.060 1.219 4 1.5 X 1.5 3.4 1.3 0.19 0.938 0.376 4 2.0 X 2.0 3.5 3.8 0.24 1.005 1.095

stars were those of a year class showing a Gaussian distribution with a mode at 20 mm radius with the exception of several larger individuals. The maximum density was 68·m-2• The point E'7' showed a different pattern from the previous two stations. Con- tagious distribution occurred only in the 0.5 m x 0.5 m quadrat-size. There were no aggregations in the larger quadrat-sizes because of the random distribution of these small aggregations. All sea-stars were of a year class having a mode of 6-8 mm radius and most were smaller than 15 mm. The maximum density was 20· m -2. It is not clear whether the small individuals occurring at C9 and E'7' belong to the same year class, because the average length of the radius was significantly different between them. But, provided the breeding season lasts for some 3 months and larval settling occurs in aggregates, it may be possible that they are of the same year class. In any case, the above results seem to suggest that the dispersion pattern of the sea-stars changes with growth from random distribution of small aggregations to contagious distribution at the 2 m x 2 m level, and also that larval settling occurs in small-scale clusters. Temporal Change oj Distributional Pattern. -Intensive studies on distribution of A. typicus were carried out at 282 stations which covered area VI (Fig. 2). Results were shown in Figure 7. Comparison with an iso-bath map (Fig. 2), shows agree- ment with abundance of the sea-star. A time series of results showed that the distribution did not change seasonally (Fig. 7). Size-frequency distribution of the sea-star population showed that more than one age class were present as similar as in Hongkong (Morton, 1979) (Fig. 8). In order to verify existence of a size- or age-specific distribution, the pattern of each age class was examined. We were able to distinguish the age-class of 1973, which had a mean arm radius of 26.0 mm in July 1974. Temporal change in distribution of the 1973 cohort could be followed throughout the growth of this age-class. The 1973 cohort had a critical change of its distributional pattern between March and July 1975 (Fig. 9). Before 1975, the cohort had inhabited a particular depth in a center of distribution, but it had a wider range towards the shoreline after July 1975. 376 BULLETIN OF MARINE SCIENCE, VOL. 38, NO.2, 1986 a

~ . ----~------.--.-. \-~ -~-.:. ~.beI,.-1'. -_.._~~_._------1.0 --- (" - -

• 1/4 1 2 4 8 16m2 b 1.5 16(5)/16(25)

~ 1.0 --_.g ~-----

. . . 1/4 1/2 1 2 4 8 m2

Figure 6. Change of 16 (a) values and 16(5)/16(25) (b) by enlarging one quadrat area. Open circles indicate value at sampling point B6, open circles with a dot at point E'7', and solid circles at point C9.

The breeding season of A. typicus is assumed to be in June-August (our pers. obs.), and the critical change in distribution relates to the first maturation of the cohort. The regional distribution of the A. typicus population in Ishigaki Island is shown in Figure 1. All localities with many sea-stars are located on the side of the island sheltered from the prevailing wind, or in enclosed bays having a wide sandy intertidal flat. These observations suggest that the effects of wind and wave have a bearing on the regional distribution.

DISCUSSION Of the many attempts to explain the distribution of a single benthic species on the basis of physical factors, such as size-composition of sediments, few have been successful (Hansen, 1965). A clear-cut explanation is hardly to be expected, due to the complicating effects of many factors. The issue is further complicated by larval settlings at different places in habitats of adult populations. It was difficult to identify overwhelming factor(s) affecting the abundance of A. typicus population along the whole shore ofKabira Cove. Although fine sediments usually support a large detrital component which may act positively as a food MUKAI ET AL.: DISTRIBUTION AND ABUNDANCE OF SEA-STARS 377

1975

",.DE,C 1975 ~ '::'::':'::::':'n :',:. ~ :::;.V(). D~r ~ (f

JULY 1975 o

::::1-9 :::::::10-29 lIIIIII30-49 BIIIl50-99 -100- 2 Legends of density (No·m- ) Figure 7. Change of density distribution pattern of Archaster typicus population in bay-head area of Kabira Cove, VI, from July 1974 to February 1977,

source for detrital feeding by A. typicus, the abundance of the sea-star could not be predicted on the basis of this component, and feeding requirement does not seem to play an important role, because food is not limiting. We frequently saw abundant sea-stars on fine sandy sediment and even on a sand-pebble mixture. 378 BULLETIN OF MARINE SCIENCE, VOL. 38, NO.2, 1986

20 1974.3.24

N=605 10

N I E c::: 0 w m ~ 1975.3.20 :::J10 Z N=S8

00 to 20 30 40 50 60 RADIAL LENGTH OF Archaster typicus (mm) Figure 8. Comparison of the size-frequency distributions of Archaster typicus collected in bay-head area of Kabira Cove, area VI, in March 1974 and 1975.

Intuitively we felt that the sea-star was not restricted to a bottom with a specific grain size, but occurred over a wide range of sediment types. There is a suggestion that the distribution of adult sea-stars in 1975 was cor- related to the degree of exposure to wind and the width of sandy flats. Espina (1971) who studied an A. typicus population in Silut Bay of Cebu Island, Phil- ippines, reported that strong currents influenced the spatial distribution of the sea-star. Briefobservations of very dense temporary aggregations of the sea-stars on the sandy flat in the innermost part of the cove were made on 28-29 August 1974 when a strong northerly wind was blowing at low tide (Fig. 10). In all cases observed, most individuals in the aggregation were large, having a 50-60 mm radius. Aggregations were so dense that specimens overlapped one another. Al- most all these temporary aggregations were made in the troughs between sand bars facing the windward direction. This shows that aggregation occurred as a result of passive movement, and that the sea-stars were transported by wind- generated water currents. The population sustained high densities on the sandy flats sheltered from the prevailing wind direction, because the sea-star cannot easily be carried away by wind-generated currents. The fact that temporal aggregation depends upon the micro-topography of the sand flat and on water movement, or more specifically upon wind and wave action, strongly suggests that the spatial distribution in the whole of Kabira Cove can be explained by the combination of the degree of exposure and topography, and probably by passive movement not an active habitat-preference of the sea- star. The distributional pattern of March 1975 implies this idea. Colman (1933) identified three critical tidal levels for vertical distribution of intertidal organisms based on periods of immersion by the tide. Doty (1946) MUKAI ET AL.: DISTRIBUTION AND ABUNDANCE OF SEA-STARS 379

SE PT 1975

NOV 1974

~ ~ o ~ .. ~

MAR 1975

:::: 1-9 ;:;::::10-29 mm30-49 -50-99 -100-

Legends of density (No· m-2) Figure 9. Change of density distribution pattern of I973-cohort of Archaster typicus population in bay-head area of Kabira Cove, VI, from July 1974 to February 1977.

identified seven critical levels, due to semi-diurnal inequality of the tides, i.e., HHHW (highest higher high water), LHHW (lowest higher high water), LLHW (lowest lower high water), HHL W (highest higher low water), LHLW (lowest higher low water), MLL W (mean lower low water) and LLL W (lowest lower low water). Many transect censuses from different types of shores have shown that the level of vertical distribution changed with localities or with species (Underwood, 1978). 380 BULLETIN OF MARINE SCIENCE, VOL. 38, NO.2, 1986

wind * direction • •

* o Figure 10. An observed example of temporary aggregations of Archaster typicus in August 1975. Stars show sampling points of the intensive sampling (Fig. 2).

Even if such critical levels do not exist, the A. typicus population has vertical distributional boundaries related to tidal behavior. Tidal levels at Ishigaki Island in 1974 and 1975 years were as shown in Table 4. As LHHW level was similar with HHL W level at Ishigaki Island in both years, six water levels and six zones were discriminated. The transect censuses at several points in Kabira Cove suggest vertical distri- bution limits (Fig. 4). The upper limit was the LHHW (or HHLW) level with a single exception (lower distribution along transect E). In the case of Kabira bay- head area VI, 96.3% (July 1974) and 96.2% (March 1975) of all sea-stars were found in the MLLW-LHHW (HHLW) zone (Table 5). Reese (1966) contended that aggregation of is a simple passive response of individuals to the physical environment and not a result of social behavior. Tyler and Banner (1977) explained distribution of ophiuroid popula- tions by the relations between coastal hydrodynamics and larval settlements. Aggregated settling of larvae may be suggested in the present case, as shown by

Table 4. Tidal levels (in cm) at Ishigaki Island in 1974 and 1975. (Data from Tide Tables for the Year 1974 and 1975 published by Japan Meteorological Agency)

Year HHHW HHLW LHHW LLHW MLLW LHLW LLLW

1974 207 124 116 86 31.5 II -31 1975 206 116 116 91 32.3 8 -25 Mean 206 120 116 88 31.9 9 -27 MUKAI ET AL.: DISTRIBUTION AND ABUNDANCE OF SEA-STARS 381

~ ~ ::e 8 ::e >- ::e j:Q ~ ~ '" 00"" 00"" i 0... o~ "" ON. oN J ::::."" ::::.'" i N N ~ "" N '-0 ::eX~~ ~'" 5; -a '"d) .d ~ ~ ••..•0 N ~ o. r-- •••.• .c'" . "" t"'-ioO ~~"" t!-"" .s 0 0.. .S '" N"" "" '"d) ~ !a= ...l5 ~ ';:1 ~ ~ .•... 0 N 0\ .•....•... '=~ "'....: .,.;.,..; ~ t:."" ~r-- "" :a 00 .S '"00, 00""

o~ ~ ~ ...l t; ::< ~~ ~ ~ ~ 0\ r-- ~~ -0 ~ ""':0 '-0 --~"" ~'" 8' '" "" e '" .S ~ ef :5 '"::l 0..~ ~ :a •.•.•N "" e o-i o-i'" r0- ~N ~"" = 0.. r-- e'"d) -a fil g ~ ~ ~ '" 0 ~ 00'" 0 -a o' 8- ~'"_N :a'> 0 oS '-..0 .8 ZIlC:: ZIlC:: ::le

=d) .d !-o .d .,; 0 ~ ,>- d) an'" a is r-- ::l :0 0\"'-••.••s;::; !-o'" 382 BULLETIN OF MARINE SCIENCE, VOL. 38, NO.2, 1986 the results of I,:,-analysis on the dispersion pattern of the immature sea-stars. In an Echinus esculentus population (Larsson, 1968) and two sand dollar (Encope grandis and Mellita grantii) populations (Ebert and Dexter, 1975), the center of vertical distribution shifts downwards with increase of size or age. Recruitment of the sea-star occurred always offshore, deeper than the center of adult distri- bution. Mean arm radius lengths in each tidal zone are shown in Table 5, based upon the data obtained by the intensive researches in Kabira bay-head of 1974 and 1975. The sea-stars that inhabited higher zones are larger, probably due to great tolerance oflow salinity, desiccation, wave action, etc. The higher intertidal zone may critically affect juvenile sea-stars sensitive to lower salinity, summer heat and winter cold. Aggregation behavior of echinoderms in relation to reproduction is known in several species (Tripneustes esculentus (=T. ventricosus) in Lewis, 1958; Archaster typicus and A. angulatus in Mortensen, 1931). In the present investigation carried out in the reproductive season (June-August), the distribution patterns described here may be affected by behavior directly related to reproduction. The adult subpopu1ation showed a different pattern whose distribution center shifted from a deeper level to a shallower one and from a concentrated to a dispersed one. The distribution of the A. typicus population is summarized as follows from the results presented here: A. typicus has a restricted distribution on intertidal sand flats. Individuals form small and relatively loose patches during the early stages of their life, probably because of aggregated settling of larvae. The patches are enlarged to about 2 m x 2 m size and the density becomes slightly lower with growth of the sea-star. Within a patch, distribution may be random. In the summer season following larval settlement, sea-stars mature and disperse to shallower, wide sandy flats. The pattern of dispersion is affected by many physical factors such as topography, wave exposure and size composition of the sediment, etc., among which wind exposure and topographical characteristics are thought to be most important for adult sea-stars. Accordingly, the population of A. typicus in Kabira Cove may be abundant on the extended sandy flats which are relatively unaffected by the prevailing wind in winter and have a gentle slope with many micro-depressions. Such factors may also be important in the regional distribution of the sea-star. Behavior of larvae during the pelagic stage and settlement in relation to the characteristics of the environment of Kabira Cove is an essential aspect of future studies.

ACKNOWLEDGMENTS

We thank Dr. G. Yamamoto, Tokai University, for giving us the opportunity to make this study, and also Dr. S. Tanaka and Dr. D. D. Swinbanks, Ocean Research Institute, University of Tokyo, for critically reviewing the manuscript. We are indebted to the staff of the Yaeyama Branch of Okinawa Prefecture Fisheries Experimental Station, for kind assistance and for permitting free use of facilities, and students of the Department of Biology, University of the Ryukyus, who assisted with the field sampling. Thanks are also due to the members of the joint survey team of Kabira Cove area coral reefs. This work was partly supported by a grant (No. 910313) from the Ministry of Education, Science and Culture, Japan.

LITERATURE CITED

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DATEACCEPTED: January 2, 1985.

ADDRESSES:(H.M.) Ocean Research Institute. University of Tokyo. Nakano. Tokyo 164. Japan; (M.N.) Department of Biology, University of the Ryukyus. Nishihara. Okinawa 903-01, Japan; (H.K. and Y.F.) Okinawa Prefecture Fisheries Experimental Station. Kabira, Ishigaki-City, Okinawa 907-04, Japan.