Benthos Research Vol.52,No.2:89-102(1997) BENTHOS RESEARCH The Japanese Association of Benthology Distribution of Intertidal Macrobenthos in 1993 around Hatakejima Island, Central Japan, Compared with 1969 and 1983-84 Shun-ichi Ohgaki,Ryohei Yamanishi Yasunobu Nabeshima and Keiji Wada2 ) Osaka Museum of Natural History, 3)Osaka Prefectural Fisheries Experimental Station , 4)Department of BiologicalS cience,Faculty of Science, Nara Women's University Abstract: The intertidal biota of Hatakejima Island in Tanabe Bay , central Japan, was in- vestigated in May, 1993. A survey of selected animal species around the entire coast of the island revealed different patterns of distribution in relation to wave exposure and substra- tum type. Comparison between the present data and those obtained in 1983 shows that sev- eral species obviously extended or contracted their distribution ranges around the island . In the survey of a selected area on the south coast, 173 macro-benthic species including 112 animal and 61 plant species were recorded. Mollusca and Crustacea were dominant among the animals, and Rhodophyta among the plants, in terms of number of species . Comparison between the present data and those obtained in 1969 and 1984 on the south coast reveals that the total number of animal species and their distribution ranges decreased from 1969 to 1984 and increased from 1984 to 1993. The species with a southern geographical distribu- tion (<35•‹N) contracted and the indicator species of eutrophic water expanded their ranges from 1969 to 1984, and the reverse changes occurred from 1984 to 1993 . Photographs show drastic changes in the shore scene: obvious sedimentation in 1984 and alternation of the dominant sessile bivalves between 1969 and 1993 . The fluctuation in abundance of the species with a southern geographical distribution paralleled changes in the water tempera- ture, and the fluctuation in abundance of the indicator species of eutrophic water paralleled changes in the indices of nutrient level. An influence of these environmental factors on the f aunal changes of the intertidal Hatakejima Island during the last 25 years is inferred . Key words: intertidal fauna, Hatakejima Island ,faunal change,temperature, eutrophication ple, on coral reefs (Dahl & Lamberts 1977; INTRODUCTION Davis 1982; Done 1992) and concerning subtidal soft-bottom benthos (Pearson et al. 1985; Long-term ecological studies of marine benthic Josef son 1990; etc.). However, information on communities have been undertaken, for exam- rocky-shore communities is still scarce, al- though several studies have followed the popu- Received September 30,1996 : Accepted July 25,1997 lation fluctuations of individual species for 89 Vol.52,No.2 Benthos Research December,1997 more than a decade (Wilson 1971; Pearse & tribution of selected animals around the entire Hines 1987; Ohgaki & Tanase 1987). coast. In this report, the results are presented It has been noted that the rocky-shore biota and compared with the previous data. Possible of Hatakejima Island in Tanabe Bay, central factors for the changes detected in species com- Japan, have experienced marked changes in the position are explored with reference to last ca. 30 years; many intertidal species have oceanographical data from Tanabe Bay. declined and a few species have increased or ex- hibited population outbreaks. Water pollution and temperature conditions in Tanabe Bay have METHODS been suggested as causes of such changes, Entire coast survey based mostly on qualitative information (Ohgaki 1989). On the other hand, quantitative Hatakejima Island (33•‹42'N,135•‹22'E) is monitoring on Hatakejima Island has been car- situated in Tanabe Bay in the southwestern ried out sporadically since the late 1960s, in- part of the Kit Peninsula, Japan (Fig. 1a). Its cluding surveys of the intertidal fauna and west and north coasts face the mouth of the flora on the south coast in 1969 and 1984 bay, which opens to the Pacific Ocean, and they (Ohgaki 1984) and of the distribution of se- are exposed to active wave action. The east and lected animal species around the entire coast in south coasts face the innermost part of the hay 1983 (Ohgaki et al. 1985). and are comparatively sheltered. The intertidal In 1993 we resurveved the fauna and flora on zone of the island covers 6.0 ha and comprises the south coast of the island as well as the dis- various kinds of substratum: rocky shore, Fig.1. Study sites. In "a", points for measuring water temperature (asterisk) and air temperature (double circles) are shown. In "b", areas surrounded by solid lines are rocky shores, by small circles boulder beaches, and by dotted lines sandy beaches or sand-mud flats. The shaded area is land. Hatched parts in "c" indicate the study area for the south coast survey. Numerals indicate the codes of survey plots. 90 Long-term change in intertidal fauna boulder beach, sand beach, and sand-mud flat. plots (Fig.1c). Of these, only Plot 1 faced open The distribution of selected animal species water and was more exposed to wave action was surveyed around the entire coast of the is- than the others. Four survey teams of two to land on May 9, 1993. Eighty species were se- four members each investigated the following lected because of their large size and ease of four taxonomic groups: molluscs, crustaceans, identification in the field. Six members of a sur- other animals, and algae. They recorded all vey team participated and each of them species of the four groups found in each plot. searched for 10-15 of the 80 species. They evalu- During the survey, photographs were taken at ated the maximum density (no./m2) of each the same points where they had been taken in species within each of the 43 survey plots shown previous investigations, and compared with the in Fig.1b and recorded it as one of the follow- earlier photographs. ing five classes: none (0), r (1-9), + (10-99) , c (100-999) and cc (>1000). The abundance of each species was compared Comparison with previous data between the sheltered area (east and south The results of the entire-coast survey in 1993 coasts: Plots 1-25 in Fig.1b) and the exposed were compared with those obtained in 1983. In area (west and north coasts: Plots 26-43) of the 1983, 56 animal species had been sought in 31 island. The significance of differences was as- rocky-shore plots, and 45 species in 12 boulder- sessed by the Mann-Whitney U-test, based on or sandy-shore plots (Ohgaki et al., 1985) . The the number of plots in which each density class abundance of each species was assessed quali- (0, r, +, c, or cc) was recorded. Species.signif i- tatively as 'none', 'rare', 'common', or 'abun- cantly more abundant (P<0.05) in Plots 1-25 dant'. Fifty-five of the same rocky shore spe- were classified as having a 'sheltered-coast dis- cies were investigated again in 1993, and the tribution', and species significantly more abun- abundance of each of the 55 in 1983 was com- dant in Plots 26-43 as having an 'exposed-coast pared between the sheltered area (east and distribution'. Species found in more than half of south coasts: Plots 1-9, 11, 14, 15, 17, 23-25 in all the 43 plots (>22) and showing no signifi- Fig.1b) and the exposed area (west and north cant difference between the two areas were coasts: Plots 26-35, 39-43). The significance of classified as having an 'entire-coast distribu- differences was assessed by the Mann-Whitney tion'. The abundance of each species was com- U -test, and each species was classified as hav- pared among the boulder-shore plots (Plots 19- ing 'sheltered-coast distribution', 'exposed- 22, 36-38 in Fig.1b), the sandy-shore plots shore distribution', or 'entire-coast distribu- (Plots 10, 12, 13, 16, 18), and the rocky-shore tion' as before. Twenty-one species showed one plots (the other Plots) by the Mann-Whitney U- or another of these three distribution patterns test, based on the number of plots in which each in both 1983 and 1993, and for each such species density class was recorded. Species limited to the distribution pattern was compared between the rocky-shore plots, or significantly more the two years. With the 55 species investigated abundant (P<0.05) there than in either the in both 1983 and 1993, the number of rocky plots boulder-shore plots or the sandy-shore plots, where each species occurred was counted. The were classified as having a 'rocky-shore distri- sum of the numbers for all the species (the 'cu- bution'. Species with a 'boulder-shore distribu- mulative number of plots') was calculated and tion' or a 'sandy-shore distribution' were deter- the values were compared between the two mined in the same way. years. The results of the south-coast survey in 1993 South coast survey were compared with those in 1969 and 1984 ob- tained by the same methods. The total number The rocky-shore fauna and flora were inves- of species, the number of plots in which each tigated in a selected area of the south coast of species was found, and the cumulative number the island on May 8, 1993. The area extended ca. of plots of all the species were compared 300 m along the coastline, and the rocky sub- among the three years. The geographical distri- stratum within it was divided into 16 survey butions of the recorded species were 91 Vol.52,No.2 Benthos Research December, 1997 determined from the literature (Higo & Goto and increasing or decreasing trends in these en- 1993; Nishimura 1992, 1995), and those with a vironmental variables were identified.