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Polyplacophora: the Sesoko Gtation The malacological society of Japan VEN US Jour. MHIac.} Hn (Jap, - Vol. 5S, Nu. 1 c1996}/ 41 49 -vs mazanog= u 7 ti l K 4 twoi)ts - itimaptL ptideptkA ･ ( pan-E11i!;( ftcma j(4}!: J,!UKJI(A7mpt}ttpt) of Distribution of Four Species Acanthopteura in Sesoko Island, Okinawa' (Polyplacophora:Chitonidae) Eiji YosHIoKA and Yasuhiro NAKAsHIMA (Kobe Yamate Women's College, Nakayamate-dori 6, Kobe 650 and Faculty of Science, Kyoto University, Kyoto 606-Ol) Abstract: Horizontal and vertical distribution of chitons, Acanthopleura gemmata. A. have been investigated. In the horizontal aspect, toochooana.A. tenuispinosa and A. miles, in the sheltered-exposed A. distributes they have specific distribution gradation. gemmata all rock shores around the island. A. Ioochooana distributes almost al1 around the island area. A. tenuispinosa mainly distributes the most sheltered area. except the most exposed In the vertical aspect, their A, miies mainly distributes the most exposed area. habitats significant except between are mostly overlapping, but their mean height have difference is lower, that of A. A. tenuispinosa and A. mites. The mean height of A, gemmata toochooana is middle, those of A, tenuispinosa and A, miles are higher in their habitat counts together, those species have specific zone. Puttingthe horizontal and vertical a ¢ four `space sharing' the dimensions although their habitat area and range overlap in in twe the most cases. Introduction Ferreira (1986) reviewed the genus Acanthopleura elaborately, and surnmarized the distri- bution of Acanthqpleura spp. According to his article, the distribution of three species, A. gemrnata (Fig. 1-A), A. Ioochooana (Fig. 1-B) and A. mites (Fig. 1-D) spread to and redescribed Okinawa and its neighboring islands. In addition, Saito Yoshioka (1993) and revealed spreads from Amami and Okina- A. tenuispinosa (Fig1-C) that itsdistribution Wa to the Gulf of Thailand and Vietnarn. Sesoko Island is located 600 m distance from Okinawa Island, and surrounded by rock rock shores. Those four species of Acanthopleura, in fact, inhabit the upper-intertidal shore of the island and compose main part of its fauna. To date, some articles dealing have with fauna around Sesoko Island (Yamazato et al., 1974; Nakasone et al., 1974) ' Contribution No. 284 of the Sesoko gtation,Tropical Biosphere ResearchCenter, University of the Ryukyus NII-Electronic Library Service The malaoologioalmalacological societysooiety of Japan 42 VENUS ： Vo1 ．55， No ．1 （1996 ） Fig．1 Four species of Acanthopleura collected from Sesoko Island． ： A Agemmata ， B ： A ． looehooana ： ： miles ＝ ， C A ．tenuiSpinosa，D A ． ．（Scale bar 10 mm ） で 翩 仂 ’8 脚 ニ ヒ 瀬 底 島 採 集 され た Ac ρ 属 4 補 ．A ： オ ザ ラ ガ イ ，B ： リ ュ ウ キ ュ ウ ピ ザ ラ ガ イ ．C ： キ ク ノ ハ ナ ピ ザ ラ ガ イ ，D ： A ． miies ， ゴ ’ 127 52 E ．， で 26 chore ＼ 1 ＼ ｝ ＼ ノ Fig．2 Location of Sesoko Island and circumstances of study area ． − A F ： Points for investigation of vertical distribution，瀬底 島 の 位 − 置 と 沿 岸 地 形 の 概 略 ．A F： 垂 直 分 布 の 調 査 場 所 ． 一 NII-ElectronicN 工 工 Eleotronio Library Service The malacologicalsocietymalacological society of Japan Yoshioka.Nakashima: Distribution of Chitons in Sesoko Island 43 been written, but they paid little attention to upper intertidal animals. The aim of the present article is to reveal the detail distribution of them around Sesoko Island, and to compare their characteristics on horizontal and vertical distributions. Study area and methods Location of stucly area: The location and circumstances of study area are shown in Fig. 2. The island is mainly surrounded by rock shores and partially long and short sand beaches. The longest beach of the island (800 m) is in the north-west area. The island is surrounded by coral reefs all around the coast off shore. Distance from rock shore or sand beach to reef edge is the shortest at south-west (20 m) and the longest at southern- most (5oo m), and the distance is relatively longer at north rock shore and at north-west sand beach. There is no muddy tidal land and pebble beach around the island. Most chitons inhabit upper intertidal zone of upright rock walls on which rock shelf overhang. Herizontai distribution: On the daytime low tide on 18 and 19-August 1994, all inter- tidal rock shores around the island were observed. At 68 points of chiton's habitat, the number of individuals of each species in 2 m width of rock wall was recorded. The width of the rock walls was measured by a straight 1 m bar. So, small concaye and convex upright, were ignored from the wall width, Because the rock walls of these points stand the densities (nurnber per 2 m of the wall width) were not affected by the difference of the angle. vertical distri- Vertical distributio' n: On the daytime low tide on 2, 3 and 4-May 1995, bution of each species at 6 points around the island (Fig. 2. A-F) was recorded. Ten-meter string was strained at O.5 m height from mean tidal level. And the height of all chitons over and under the line was marked. The strict position of chiton was identified as the center of the 5th shell. The height of the line was yerified twice by measuring and compar- ing it with the height from the actual water level. Nishihira (1974a) confirmed that the tide-table at Naha can be used at Sesoko Island. Results Horizontal distribution: The occurrence and density of the four species around the island were shown in Fig. 3. A. gemmata occurred in high density at almost all the points searched. A. Ioochooana occurred around the island except on the south-west rock shore and occurred on the north-east and south-east to southernmost for 1,200 m, it densely rock shore. A. tenuispinosa appeared densely on the north-east rock shore, and fewer the individuals occurred on the north, east and south rock shore, but never occurred on west rock shore. A. mites occurred mainly on the south-west rock shore, and fewer individu- of als occurred on the north-west, south-east and southernmost rock shore. Coexistence A. tenuispinosa and A. miles was rarely observed. Jt:ertieal dtstribution: The occurrence and numbers of these four species in O.25 m in Fig. 4. A. zone of upper intertidal wall at the 6 points (A-F) were shown gemmata and A. Ioochooana occurred in all 6 points, A. tenuispinosa occurred in 4 points and All chitons appeared in the zone from MTL+ 1.25 m to A. miles occurred in 3 points. NII-Electronic Library Service The malacologicalsocietymalacological society ofJapanof Japan 44 VENUS: Vol. 55, No.1(1996) y"tX"IZ?;?... "gtpmZ/ez. A.gemmata A.Ioochoonna ''- "tl"rtW/ey.... "t"1".z A.tenujspinosa A.mi1es '･NJC' //X O 1-4 5--19 20- indiv, f2rm Fig. 3. Herizontal distribution and density of four species of Acanthopleura around Sesoko Island. vaasnJaelU!cDAcanthopleuva tt 4 mba)alg"l'-fi6' . MTL-O.25 m, and habiting zone of each species was mostly overlapping. These occur- rences and their densities fairly reflect the horizontal distribution which revealed in Fig. 3. The number of each species in each zone was totaled and shown in Fig. 5, and differ- ences of mean height between all species ¢ ombinations were examined by t-test. Mean height of A. gemmata was significantly lower than that of the other 3 species (p<O,Ol, p<O.Ol, p<O.Ol). Mean height of A. Ioochooana is significantly higher than that of A. NII-Electronic Library Service The malaoologioalmalacological societysooiety of Japan Yoshioka ・Nakashima ： Distribution of Chitons in Sesoko Island 45 A ．gemma ta ＋ ・m A B C F 肌 TL 巳o 3B 55 26 20 A ．loochooana ＋lm E 卜lTL 16 20 A tenu ユsp ．ヱnosa ・ ＋ lm A B C D E 馨 諍 ．． … M ［1． ＝ ，．．L ＿ ■ ・ 一 一 L …．∴ 20 A ． miles A B C E ー F 例 ー l 1 1 ー ユ ・ITLIL 皇．L − ＿ LL − ． L ー一 20 Sesoko Fig ，4 Vertical distribution of four species of Acantho ρleura in the selected 6 points around histogram ． The location of the Island． Total number of each species is shown in the each （ ． points A −F are shown in Fig ．2 ．） 瀬 底 島 周 辺 6 地 点 で の Acanthopieura 属 4 種 の 垂 直 分 布 ， ・ マ． 呼 ’ 4 ． ！OQ 称 、 吻 ％ 呪 ％ （b 丶 ％ 十 lm 齢 撫暦 ｝ITL L ＊ ＊ ＝ ガ ＊ ＊ − − Fig．5 The total nurnber （A −F）of each species in each zone ． Results of t test with which the differ the ence of mean height between an spe ¢ ies combinations was examined ， were shown at 榊 bottom ．6 地 点 を 合計 し た 垂 【A：分布 と 平均 高 の 差 の 検 定 結 果 ． （ ： p ＜ 0．01， NS ： p ＞ 05 ） 垂 ． 〃 〃 significantly lower than that of the other two species ＜ 0，01 ， ＜ 0．0 ） ge 1 lata ， and （p p No significance was detected （p ＞ 0．5）between mean height of the higher two species ，、4． tenu ’spin osa and ／ 4． mi ’es ． 一 NII-ElectronicN 工 工 Eleotronio Library Service The malacologicalsocietymalacological society of Japan VENUS: 46 Vol. 5S, No. 1 (1996) Discussion It is the general view that sea shore takes on gradual aspect according to the strength of surf (Morton and Miller, 1968; Tokioka et al. 1972), and some shore can be set in a situation in the gradation from exposed to sheltered areas.
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