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)
Distribution of Four Species of Acanthopteura (Polyplacophora: Chitonidae) in Sesoko Island, Okinawa'
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. toochooana. A. tenuispinosa and A. miles, have been investigated. In the horizontal aspect, they have specific distribution in the sheltered-exposed gradation. A. gemmata distributes all rock shores around the island. A. Ioochooana distributes almost al1 around the island except the most exposed area. A. tenuispinosa mainly distributes the most sheltered area. A, miies mainly distributes the most exposed area. In the vertical aspect, their habitats between are mostly overlapping, but their mean height have significant difference except A. tenuispinosa and A. mites. The mean height of A, gemmata is lower, that of A. toochooana is middle, those of A, tenuispinosa and A, miles are higher in their habitat have specific zone. Putting the horizontal and vertical a ¢ counts together, those four species `space sharing' in the twe dimensions although their habitat area and range overlap in
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 Okinawa and its neighboring islands. In addition, Saito and Yoshioka (1993) redescribed A. tenuispinosa (Fig 1-C) and revealed that its distribution spreads from Amami and Okina- Wa to the Gulf of Thailand and Vietnarn. Sesoko Island is located 600 m distance from Okinawa Island, and surrounded by rock shores. Those four species of Acanthopleura, in fact, inhabit the upper-intertidal rock 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 Research Center, 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 distributio' n: On the daytime low tide on 2, 3 and 4-May 1995, vertical distri- 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 to southernmost for 1,200 m, and it occurred densely on the north-east and south-east rock shore. A. tenuispinosa appeared densely on the north-east rock shore, and fewer individuals occurred on the north, east and south rock shore, but never occurred on the west rock shore. A. mites occurred mainly on the south-west rock shore, and fewer individu- als occurred on the north-west, south-east and southernmost rock shore. Coexistence of
A. tenuispinosa and A. miles was rarely observed. Jt:ertieal dtstribution: The occurrence and numbers of these four species in O.25 m zone of upper intertidal wall at the 6 points (A-F) were shown in Fig. 4. A. gemmata and A. Ioochooana occurred in all 6 points, A. tenuispinosa occurred in 4 points and A. miles occurred in 3 points. All chitons appeared in the zone from MTL+ 1.25 m to
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 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. Yamazato et al. (1974) sur- veyed the 13 transections of coral reefs around Sesoko Island. They found that there were gradual changes in the reef structure and coral fauna from exposed to sheltered shores. The most exposed shore was on the south-west coast facing to East China Sea beyond the short ceral reefs, and the most sheltered shore was on the north-east coast facing to Motobu Peninsula of Okinawa Island beyond the broad lagoon. Coral reefs of the most exposed shore was constructed by the tabular formed Acropora in the outer reef flat and in the upper reef slope. On the other hand, coral reefs of the sheltered shore was construct- ed by the branching formed Aeropora. As well as the distribution of underwater coral reefs, intertidal chitons gradually appeared along the exposed-sheltered gradation as shown in Fig. 3. The commonest species, A. gemmata, occurs in the shore all around the island, and the next commonest, A. Ioochooana, occurs almost all around the island except in the south-west (the most exposed) shore. Dense occurrence of A. tenuispinosa is limited to the north-east (the most sheltered) shore, and dense occurrence of A. miles is limited to the most exposed shore. Nishihira surveyed 94 points around Okinawa Island (Nishihira, 1974b) and 58 points around Yaeyama Islands (Nishihira, 1975), and marked the occurrence of common species in the transection figures of those points, Two names for chiton species were written in `Oni-hizaragai' the above articles. (=Japanese name of A. gemmata) occur at 45 points in Okinawa and at 17 points in Yaeyama. They mainly occur in upper intertidal rock shores, and tend to occur rather in exposed shores than in sheltered shores. Those occur- rences coincide with the occurrences in our survey. `Hizaragai' Another name for chiton species in Nishihira's articles, (=Japanese name of A. japonica) occur at 17 points in Okinawa and at 11 points in Yaeyama. According to Taki (1937), A. japonica distributes from Southern coast of Hokkaido to Yakushima Island at southernmost, and it does not occur in Okinawa and neighboring islands also in our field observations. When Nishihira surveyed Okinawa and Yaeyama, an old article describing Liotophura juponica forme tenuispinosa written by Leloup (1939) (an original description of A. tenuispinosa redescribed by Saito and Yoshioka (1993)) was not a common knowledge in the Japanese biologists. And, apparently A. tenuispinosa resembles Aeantho- `forma' pteura J'aponica, in fact, Leloup identified it as a of Liotophura (= Acanthopieura) `Hizaragai' juponica. As the above-mentiened facts revealed, in Nishihira's articles can be identified as A. tenuispinosa in the present taxonomical understanding. In addition, in the subsidiary observation around Sesoko Island, A. tenuispinosa is observed in several points where Nishihira recorded the occurrence of `Hizaragai'. In Nishihira's articles, `Hizaragai' tend to occur rather in sheltered shores than in exposed shores. Such tendency also coincides with the occurrence of A, tenuispinosa in our observation. These four species of Acanthqpleura were distributed from the range from MTL + 1.25 rn to MTL-O.25 m, for 1.50 m in height. In the case of investigation at Shirahama NII-Electronic Library Service The malacologicalsocietymalacological society of Japan Yoshioka.Nakashima: DistTibution of Chitons in Sesoko Island 47 A.tenuispin(]Sa A loochooana high ,miles A.gemmata low shelLer'ed- exposed `space-sharing' four species of Acanthqpleura Flg. 6 Schematic drawing of of 'zamapiimo' around Sesoko Island. mazanmaWT(fcOAcanthopleuraet ･1 dian oma;kma . in Honshu (Yoshioka, 1983), A. japoniea were distributed from MTL+O.68 m to - `Tide MTL O.86 m, for 1.54 m in height. EHWL predicted in Tables 1994' is MTL + 1.21 m in Naha and MTL + 1.04 m in Tanabe (the nearest point from Shirahama in the table). So, habitat range of chitons in Sesoko is evidently higher than the range in Shirahama by comparing those intertidal zones. Two reasons, specific characteristic and spatial charac- teristic, can be hypothesized to explain the difference of habitat height. In the present study, it is difficult to say which of the two is correct. It will be revealed by investigating of the vertical distribution at the place inhabited by A. joponica and other congeneric chitons. Habitat zones of these 4 speeies are vertically overlapping as shown in Fig. 4. Actually, they are obseryed on a rock and sometimes in a small pit or a narrow gap side by side, In spite of such observation, their mean heights have significant differences except between A. tenuispinosa and A. milas. And, this combination of species is rarely obseryed in a same point. Putting horizontal and vertical accounts above together, they haye specific position in the two dimensional space as shown in Fig. 6. It is a suggestive fact that some interaction `space between these species affect their specific sharing'. In the case between A. gemmata and A. tenuispinosa, the distribution of A, gemmata rnoving down to MTL where these two species coexist (see these distributions shown in Fig. 4. A and C). In the present study, howeyer, observation of interaction between two species was not tried. So, from `space the present knowledge, it is difficult to declare that their specific sharing' is caused by their interactive habitat segregation. We will be able to prove it along some operational research programs, for example, observing the distribution movement of one species after another species has been removed. Randall and Martin (1987) observed the distribution of Chiton squamosus and Acan- thopleura granulata and other chitons in the Caribbean coast of Mexico. They revealed that C. squamosus mainly occurred waye-washed situation well above MTL, and A. granula- ta occurred in more p[otected and lower situation. According to the tables in the article, their segregation was much more obvious than that revealed from the present study. Otaiza and Santelices (1985) described the vertical distribution of chitons in the rocky intertidal zone of central Chile. Their results showed that there is a vertical separation of these NII-Electronic Library Service The malaoologioalmalacological societysooiety of Japan 48 VENUS ： VoL 55， No ．1 （1996） species both on rock walls and in pools and boulder fields． According to the figures in the article ， the separation was more obvious than the present study also ． Comparing the articles with the we preceding present data ， had betteremphasize their coexistence rather than the segregation ． 要 約 の の い ニ ヒ 沖 縄 島瀬 底 島 潮 間帯 上 部 岩 礁 に お て ，オ ザ ラ ガ イ （Acanthopteura gemmata ）， リ ュ ウ キ ュ ウ ピ ザ ラ ガ イ （A ． loochoOana ），キ ク ノ ハ ナ ピ ザ ラ ガ イ （A ． tenuispinosa ），A ．〃miles の 4 種 の 水平 分．布 と 垂 直 分 布 を 調 べ た 。水 平 分 布 で は そ れ ぞ れ の 種 で 以 下 の よ う な 波 当 た り の 強 弱 の 勾配 に 応 じ た 特 徴 が 見 ら れ た 。 オ ニ ヒ ザ ラ ガ イ は 島 の 全 周 に 分 布 し て い る 。 リ ュ ウ キ ュ ウ ピ ザ ラ ガ イ は 最 も 波 当 り の い ’ 強 ． ハ 海 岸 を除 い て ほ ぼ 全周 に 分 布 し て い る c キ ク ノ ナ ヒ ザ ラ ガ イ は お も に 波 当 り の 弱 い 海 岸 に 分 布 し て ・ い る ，A ． m ．itEtSは お も に 波 当 り の 強 い 海 岸 に 分 布 し て い る 。 方 ，垂 直 分．布 は 4 種 と も ほ ぼ 同 じ 高 さ に ． ． み ら れ る が ，分 布 の 平均 高 を統 計的 に 比 較 す る と ，オ ニ ヒ ザ ラ ガ イ が も っ と も低 い 位 置 に ， リ ュ ウ キ ． ’ ュ ウ ピ ザ ラ ガ イ ガ 中 ほ ど の 位置 に ，キ ク ノ ハ ナ ピ ザ ラ ガ イ と A ．miteh が 最 も高 い 位 置 に 分 布 す る。 こ れ 一 ら 4 種 は ，そ れ ぞ れ 相 当部．分重 な り な が ら 分 布 し て い る 方 ，水 平 ・垂 直空 間 に お け る 分 布 で そ れ ぞ れ 独 自の 範囲 を 占 め て い る こ と が わ か っ た 。 Refe 「ences Ferreira − ， A ． J．（1986）． A revision of the genus Acanthb ρleura Guilding ，1829 （Mollusca ： Poly − placophora ）， The レセ”8e厂 28（3）： 221 279 ． Leloup E 1939 ． A amphineures L ’olo hura aponi ‘α Uschke ， （ ） propos des ρ ／ （ ，1873） et L ．gaima 厂di Blainville 1825 ’ − （ ， ）： deux nouveUes formes ． Bul ’． Mus ． r の ． Hist． natur ． Belgique 15（且）： 1 7 ． Morton ，」． and M ． Mnler （1968）． The fVew Zeatand sea shore ， xxx 十 653pp ． Collins， Glasgow， U ． K ． Nakasone Y ． K ． Yamazato M 皿 ， ， 賢 ，Nishihira， S ．Ka aura and Y 。Arimoto （1974）．Preliminary report on the ecological distribution of benIhic animals on the coral reefs of Sesoko Island， Okinawa ． ln Japancse with English summary Ecoi 伽 ゴ ハJat Co 刀 5 Ryukyu Isl 1 − （ ） ．3 ． ． ． ．， 」Report／b’ Specia’ P “ ” 」 厂q／ect 」Research Environment and Hu 〃 lan Su厂 viva ’ 一 ノレfOE ， 213 − 236 ． Nishihira， M ．（1974a ）． Oceanographic conditions around Sesoko Island， Okinawa ．（ln Japanese with summary ’ ハ − EngliSh ） Eco ，Stud ． rat．Cons ．R ア確 yμ Is’．，1 Re ρort プbr Spee ’al p 厂（ijectResea 厂ch ‘‘ ．− En レ’厂onment and Human Surv ’ソa ” MOE ． 195 − 200 ． − Nishihira， M ．（1974b）． Okinawa no chokan ’α ’ 19 74．（lntertidal zone of Okinawa ）（ln Japanese ）262pp ． Ryudai −kaiyo−hozen − kenkyukai． Nishihira M ． 1975 】raeyama no chokantai 一ノ ， （ ）． 975． （lntertidaizone of Yaeyama ） （ln Japanese ） xii ＋ 190pp ． Ryudai − kaiyo− hozen − kenkyukai ， Otaiza ， R ． D ． and Santelices， B ．（1985）． Vertical distribution of chitons （Mollusca ： Po 且yplacophora ） in the rocky intertidal zone of central Chile．ノ． 即 ． Mar ． β ’o ’． Ecoi ． 86 ： 229 −240 ． Randal1 C ． W ． and R F 1987 Distrlbution abundance and movement ， ． ．Martin （ ）， ， ， patterns of shore 且ine chitons of the Caribbean coast of Mexico ． The Naut ’tus 101（2）： 75 − 79 ， Saito H ． and E ． Yoshioka occurrence of tenuispinosa Leloup − ， （1993）．An Aeanthoρ leura （ ， 且939）（Poly placophora ： Chitonidae ）from Amami and Okinawa Islands in reference to taxonomic evaluation ． ’ Bul’・ ハ〜atn ・ Sci． ノし4ius．， ］rヒ）kyo ， St〜厂． ノ4 ． 19（2）： 45 −50． Taki，1．（1937）． Report of the biological survey of Mutsu Bay．31， Studies of chitons of Mutsu Bay with discussion on chitons of ． ． Rep ． Tohoku lmp ．こノhiv． Ser 4 12 general Japan Sci ． ， ． ， ： 323− 423 ，Pls．14 − 34． Tokioka ， T ．， E Harada and S ． Mshimura （1972｝． Umi no seita ’8aku ，（Ecology in the sea ．） （ln − Japanese） 2pls． ＋ vii ＋ 317pp． Tsukiji shokan ， Tekyo ． Yamazato K ．M ．Nishihira ， ， ， S ． Kamura ， Y ． Nakasone and Y ， Aramoto （且974）． Biogeomorphological notes on the Sesoko Island Reef， Okinawa ． （ln Japanese with English summary ）Eco ’．Stud， 一 NII-ElectronicN 工 工 Eleotronio Library Service The malacologicalsocietymalacological society ofJapanof Japan YoshiDka.Nakashima: Distribution of Chitons in Sesoko lsland 49 - "Environment IVOt. Cons. Ryukyu lsL, I Report for Speciat Project Research and Human Survival'' - MOE. 201-212. Yoshioka, E. (1983). Vertical distribution and size eomposition of chiton, Lioiophura J'aponiea (Lischke, 1873). (In Japanese with English abstract) IVbnkiseibutu 25(1): 126-129. [Receiyed: October 30, 199S] NII-Electronic Library Service