Zonation of Intertidal Organisms and Community Structure of Title Small Animals Associated with Patches of the Mussel Mytilus edulis L. along the Rocky Coast of Dinard, Brittany, France Author(s) Tsuchiya, Makoto; Retiere, Christian 琉球大学理学部紀要 = Bulletin of the College of Science. Citation University of the Ryukyus(54): 47-81 Issue Date 1992-10 URL http://hdl.handle.net/20.500.12000/13169 Rights Bull. Coll. Sci., Univ. Ryukyus, No. 54: 47-81 (1992) 47 Zonation of Intertidal Organisms and Community Structure of Small Animals Associated with Patches of the Mussel Mytilus edulis L. along the Rocky Coast of Dinard, Brittany, France Makoto Tsuchiya* and Christian RetiBre Laboratoire Maritime de Dinard, Museum National dUistoire Naturelle, 17 Avenue George V, 35800 Dinard, France Abstract A marked zonation pattern was recognized on the rocky intertidal along the coast of Dinard, Brittany, France. Patella spp., Gibbula spp. and balanoid barnacles showed a habitat segregation in terms of vertical distribution. The community structure of small animals associated with various sized patches of Mytilus edulis ("Mytilus islands") was analyzed at two tidal levels and differences in the species composition were found. At both levels, the species richness and number of individuals increased with patch size, but the species density (number of species/patch area) decreased. On the other hand, the absolute density (number of individuals/patch area) was relatively constant irre spective of patch size. This was due to the great abundance of small barnacles such as Chthamalus monlagui, Balanus balanoides and Elminius modestus, which were attached to the Mytilus shells, and the small bivalve Lasaea rubra. Species diversity (H') did not show any trend with patch size, but equitability (J') decreased. The importance of micro spatial characteristics in discussion of species diversity is emphasized. Introduction It is well known that very large tides, about 13 m in amplitude during spring tides, occur in the Gulf of Saint-Malo, Brittany, France. Although a great number of ecological works on the distribution and community structure of rocky intertidal organisms have been conducted along the coast of Great Britain (see a review by Lewis, 1964), it is surprising that not much comparable information is available for the French coast. Crisp and South ward (1958) reported the distribution of intertidal organisms along the coasts of the English Channel, including the French side, and Richoux (1967) reported the distributional pattern of crevice fauna with reference to the tidal level. The distributional pattern of littorinids has also been studied (Daguzon, 1976). Numerous studies concern the distribution and the zonation of intertidal species along the French coasts of La Manche (Audouin & Milne-Edwards, 1892; Beauchamp, 1914; Fischer-Piette, 1932, 1934, 1936; Fischer-Piette & Gaillard, 1956; Davy de Virville, 1940; Crisp & Fischer-Piette, 1959; Plessis, 1961; Gaillard, 1965; Ancellin et al., 1969) and more precisely of Le Golfe de Saint-Malo (Vaillant, 1870; Beauchamp & Lami, 1921; Beauchamp, 1923; Fisher-Piette, 1926, 1928; Hamel, 1928; Hamel & Lami, 1930; Joubin, 1929; Hatton, 1938). Crevice fauna was also described (Retiere & Richoux, 1973). Nevertheless, current information on the zonation pattern and community structure of intertidal organisms is generally lacking. The senior author had an opportunity to visit rocky shores in Brittany as an exchange Accepted : August 4, 1992. •Permanent address: Department of Biology, University of the Ryukyus, Nishihara, Okinawa 903-01, Japan 48 Makoto TSUCHIYA • Christian RETIERE researcher in a cooperative research project between the Japan Society for the Promotion of Science (JSPS) and the Centre National de la Recherche Scientifique (CNRS) in 1988 and to study two aspects of their intertidal ecology, i.e. 1) to describe the zonation pattern of rocky intertidal organisms, mainly around Dinard, and 2) to analyze the community structure of small animals associated with patches of the mussel Mylilus edulis. M. edulis has been the subject of various biological investigations (see Bayne, 1976). It has been known that mussel beds harbour a variety of small animals (Hewatt, 1935), but no attention has been paid to the mussel bed community (Seed, 1976). Recently, mussel bed communities have been subjects of several workers (Bellan-Santini, 1963; Suchanek, 1979, 1985; Tsuchiya & Nishihira, 1985, 1986; Tsuchiya et al., 1989b; Tsuchiya & Bellan- Santini, 1989). The structure of the mussel community (mussels plus associated animals), which is designated as "Mytilus island" (Tsuchiya & Nishihira, 1985), has not been well studied and studies on the dynamic processes within the community are completely lacking. Because mussel beds develop all over the world, comparative studies on this community should be useful for discussions not only of intertidal community ecology but also commu nity ecology in general. Secondary space created by animals or plants plays an important role in increasing species diversity by supporting environmental heterogeneity. In this sense, the mussel bed system is a good model for the analysis of community organization processes because Mytilus is easy to handle and the degree of heterogeneity can be manipulated. Communities developing on algal growths have been investigated (Prenant, 1923; Benard, 1960; L'Hardy, 1962; Herberts, 1964; Haage & Jansson, 1970; Hazlett & Seed, 1976; Gunnill, 1982, 1983). Although we found interesting subjects of the communities on the rocky shore of Dinard, we had not enough time to study them. In the present study, therefore, discussion is focussed mainly on the zonation pattern of rocky intertidal organisms and structure of mussel bed communities. Study area and Methods Nine stations (Sts. 1 — 9) were selected for a study of the vertical distribution of in tertidal shore organisms along the rocky coast of Dinard, Brittany, France (Fig. 1). The survey was conducted from the middle of July to late August, 1988. In order to determine the effect of shore elevation, expressed as the vertical distance from mean low water of spring tides (M.L.W.S.), on the vertical distribution pattern of the intertidal organisms, the density of macrobenthic animals and algal cover within 25 cm x 25 cm or 10 cm x 10 cm quadrats was surveyed along a transect line placed perpendicularly to the shore line at each station. The smaller quadrat was used only for counting the small barnacles such as Chthamalus spp. Balanus balanoides* and Elminius modestus. The survey was conducted at intervals of several meters, and the quadrats were placed on both sides of each study point along the transect line. Sea level was recorded several times at each station during the study period and then M.L.W.S. was determined with reference to a tide table. At Sts. 2 and 4, many patches or dense beds of Mytilus edulis were found. At St. 4, 'Newman & Ross (1976) proposed a new taxonomic system for the balanomorph barnacles. They separated Balanus balanoides from other Balanus species and placed in the genus Semibaianus. Zonation and Mytilus Islands on Rocky Intertidal of Dinard 49 A^ Sti Laboratoire Maritime de Dinard Primel-Tregastel _A „ , / I StMalo Fig. 1. Map of study site. 60 patches of various sizes, 30 from around mean tide level (M.T.L.) and 30 lower ones from 5m above M.L.W.S., were measured for length, width and height and then scraped off fully with a spatula during the daytime low tide. The size of mussels was recorded and the associated animals were identified and counted in each patch. The size of the crab Carcinus maenas, which occurred in many patches, and the amount of sediment in each patch were also measured. The rock surfaces from which the patches of mussels had been removed were surveyed again about one month later and any newly invading animals were counted. Qualitative observations on the intertidal organisms were also made at several other places in Brittany, including Roscoff, Primel-Tregastel, Saint-Malo, Saint-Lunaire (La Garde Guerin), Carnac and Vannes (Fig. 1). Results 1. Tide and temperature conditions According to the tide tables of 1988, a maximum of about 13 m tidal amplitude was shown in March, August and September, while the tidal amplitude was lower in May, June, December and January. A marked semi-diurnal tide was recognized. During the study period (July 11 —Aug.30,1988), a minimum tidal amplitude of 4.5 m was recorded on July 24 and it increased rapidly to 12 m on August 1. The range between the twice daily high and low waters decreases on passing from spring to neap tides. Water and air temperatures, which have been measured in every Monday morning by staffs in the "Laboratoire Maritime de Dinard", fluctuate 7 — 18*0 and 3 — 19*C, respec- 50 Makoto TsucHIYA * Christian Retiere tively (Unpublished data). 2. Vertical distribution of intertidal organisms St. 1 (Fig. 2): This station, with a lower intertidal boulder shore, was situated on a relatively exposed shore at Saint-Enogat. Among the 9 stations surveyed, this was the only one with an entirely rocky intertidal zone from the littoral fringe to around extreme low water of spring tides (E.L.W.S.); sandy bottoms were present around the low water mark at the other stations. CZZ] 0 50 100 °/o cover 16 0 50 100 Na/0.125m2 * 0 100 200 No./100cm2 K MHWS.- ~ 10 E - MIL- M.LWS- 0 Fig. 2. Vertical distribution of rock}' intertidal organisms at St. 1. The coverage of the yellow lichen Xanlhoria parielina was not high and it had a patchy distribution about 15 m above M.L.W.S. Light green Enteromorpha sp. and the black lichen Verrucaria maura occurred above the mean high water of spring tide (M.H.W. S.). The zonation pattern of seaweeds was remarkable, i.e. Enteromorpha sp., Pelvetia canaliculata, Lichina pygtnaea, Fucus spiralis, Laurencia sp., Fucus vesiculosus, Fucus serratus, Sargassum muticum and Laminaria saccharina were seen sequentially from the high to low intertidal.