VENUS 61 (1-2): 25-39, 2002

Shell Shell Morphology of Larvae and Post-Larvae of the Two Tropical Modiolus Modiolus Species, Modiolus auriculatus and Modiolu~ philippinarum (Bivalvia: (Bivalvia: Mytilidae)

Hiroyuki Ozawa and Hideo Sekiguchi Facul 砂 of Bioresources, Mie Universi η，1515 Kamihama-cho, Tsu, Mie 514-850 スJapan; [email protected]. αc.jp

Abstract: Abstract: In order to aid specific identification of th 巴 larva 巴 CD-shaped and urnbo larvae) and and post-larvae of the two tropical Modiolus species M. auriculatus and M. philippinarum cornrnon cornrnon in shallow waters around Okinawa, southern Japan, we describe and compare the the morphological features of their shells based on larvae and post-larvae reared in the laboratory laboratory observed with optical microscope and SEM. The urnbo larva 巴 of M. philippinarum are are distinguishable from those of M. auriculatus by differences in hinge morphology, shell shell color, and the size of th 巴 shell at th 巴 time when eyespots appear. The post-larvae of M. phil伊•pinarum are also distinguishable from those of M. auriculatus by differences in hinge hinge morphology, shell color and the size of shell at the time when primary lateral t巴巴 th and and periostracal hairs appear. The umbo larvae of Mndiolus species (M. auriculatus, M. philippinarum philippinarum and M. modiolus ）紅 E not distinguishabl 巴 from thos 巴 of other genera in the Mytilidae Mytilidae described in past studies, except that the umbo larvae of Perna poss 巴ss primary lateral lateral teeth. The post-larvae of the abov 巴 Modiolus species are distinguishable from those of other other genera in the Mytilidae described to date by a combination of the three types of lateral teeth, teeth, bearing only primary lateral teeth.

Keywords: Modiolus auriculatus, Modiolus philippinarum, larvae, post-larvae, shell morphology morphology

Introduction

Most of the species in the family Mytilidae inhabit marine and brackish waters. Som 巴 species form conspicuous clusters on hard or soft substrata, and 紅巳 considered to be ecologically important. important. Ninety-six species of the Mytilidae have been recorded in Japanese waters (Higo et al., 1999). 1999). About twenty-five are known to occur in Okinawan waters, belonging to eight genera, i.e. Botula, Botula, Brachidontes, Hormomya, Lithoph α ga, Musculus, Modiolus, Perna, and Septifer (Kubo & Kurozumi, Kurozumi, 1995). In In our preliminary survey, four mytilid species (Modiolus auriculatus, Modiolus flavidus, Modilus Modilus philippinarum, Musculus nanus) were found in and around the seagrass bed in Kin Bay, Okinawa Island, southwestern Japan. M. phi ！伊•pinarum was dominant th 巴re, comprising about 90 % of the total mytilid specimens examined. On the other hand, M. auriculatus was dominant on intertidal reef shores in Okinawa (Kubo & Kurozumi, 1995), including areas adjacent to the the seagrass bed in Kin Bay. In 1998, we started research into the larval recruitment of M. philippinarum philippinarum in 白e seagrass bed at the above locality. In the course of these studies, we found it it necessary to distinguish the larvae and post-larvae of M. philippinarum quickly from those of other other mytilids, especially M. αuriculatus, in zooplankton and sediment samples. In surv 巴ying the larval larval recruitment process 回， it is nec 回 sary to identify numerous larval and post-larval specimens as as quickly as possible by timesaving methods, for instance with a light microscope. 26 26 H. Ozawa & H. Sekiguchi

Identification Identification of larval mytilids has usually been based on hinge mo 叩hology (Chanley & Andrews, Andrews, 1971; Le Pennec, 1980; Lutz & Kennish, 1992). SEM observation has made it possible to to clarify the detailed mo 中hological features of the hinge. Based on these features, identification of of 20 larval to post-larval mytilids is possible (Table 1). However, there has to date been no information information regarding the larval and post-larval shell mo 中hology of mytilids in Okinawan waters, with with the sole exception of the green mussel, Perna viridis (Siddall, 1980; Hanyu et al., 2001). In In the present study, based on larvae and post-larvae reared in the laboratory, we examine the the mo 中hological features of the shells of M. auriculatus and M. phil伊•pinαrum with a light

microscope microscope and SEM, in order to distinguish the larval to post 司 larval shells of both species from each each other.

Materials and Methods

Mature Mature specimens of M. auriculatus and M. phil 伊tpinarum were collected at two localities in in October 1998; the former were collected on the intertidal reef shore of Sesoko Island (26'38 〆 N, N, 127 。52'E), and the latter in seagrass beds within Kin Bay (26°20 ’ N, 12T54'E). Spawning of these these mytilids was artificially induced by immersing them in ammoniated seawater. Twenty to 42 mature specimens of both species were put into separate tanks of natural seawater containing O.OOlN of NH40H. The fertilized eggs of both species were kept at 30 ±1 ℃ in separate tanks (10 1) containing filtered filtered natural seawater (salinity 35 PSU). Hatched larvae were transferred into a larger tank (30 (30 1) with an initial concentration of 10 larvae/ml. All the water in the tank was renewed every day, day, and the microalga Pavlova lutheri was supplied every day (1.0 × 104 cells/ml). Then, after most of 白e larvae had settled on the bottom or walls of the tank, two-thirds of the water volume in in the tank was changed twice or three times a week. In the course of culturing the larvae and post-1 紅 vae, five to ten specimens in each tank were sampled every two or three days. Sampled larvae larvae and post-larvae were fixed in small vials with 80% alcohol and deposited in a refrigerator at at 5 ℃ until they were used for mo 中hological observations. Morphological features of shells of both both species were observed with a light microscope and SEM (S-4000 Hitachi Seisakusho Ltd.) following following the methods of Sakai & Sekiguchi (1990, 1992). Because there 訂 e no morphological differences differences between the right and left valves in each species even in the hinge, the shell morphology of only the left valve is described in this paper. In In the present study, terminology is defined as follows: larvae are at the planktonic stage, including including both D-shaped and umbo larvae. ‘ D-shaped larvae' are individuals that have not yet formed formed the umbo. 'Umbo larvae' are individuals that have di 旺erentiated the umbo before they settle settle onto the substrate. Post-larvae are at the stage shortly after settlement onto the substrate. Terminology Terminology for hinge and teeth on larval and post-larval shells follows Fuller & Lutz (1989) (Fig. 1). 1). Shell length is the greatest dimension along any axis of the shell.

Results Results

Modiolus auriculatus Larval Larval development (Fig. 2) Eggs are spherical in shape with a diameter of 74 ± 2 μm. It takes 16 hours from fertilization for for D-shaped larvae to develop, with shell lengths of 115 ±4 μm. D-shaped larvae metamorphose into into umbo larvae with shell lengths of 180 μm or more 5 days after fertilization. Eyespots differentiate differentiate 11 days after fertilization with a shell length of 251 μm. Settlement of larvae occurs 13 13 days after fertilization with shell lengths of 289 μm or more. Larval Larval shell morphology of tropical Mndiolus 27

L

p a

rv rv d

v

t

pl pl

Larva Larva Post-larva

Fig. Fig. 1. Terminology for umbo to post-larval shells and hinges in the Mytilidae (modified in p紅 t from Kimura Kimura & Sekiguchi, 1994). a －組terior, d- dorsal, dy - dysodont teeth, L- shell length, p- posterior, pl - primary ligament,

pt pt -prim 訂 y lateral teeth, rv -right valve, sl - secondary ligament, st 『 secondary lateral teeth, t- teeth, teeth, u- umbo, v - ventral.

Observations Observations by optical microscope

Shell Shell shape: In the D 問 shaped larvae with shell lengths of 115-180 μm, the anterior margin of the the shell gradually protrudes with the growth of shell. Umbo and post-larvae, with shell lengths of of 180-290 μm and 290-500 μm, respectively, are oval in shape with pointed anterior and round posterior posterior margins. The umbo is located at the middle of the valve. The shell then grows postero ・ ventrally ventrally to become triangular in shape with shell lengths of 500 ヴ00 μm. In post-larvae with shell shell lengths of 700 μm or more, the shell grows antero-ventrally, and the postero-dorsal margin gradually gradually slopes postero-ventrally. Shell Shell color and periostracal hairs: The shell of the D-shaped larva is colorless, while that

of of the umbo larva is brown. Post-larval shells of ca. 290 μm or more are pale brown. Scale 幽 like perios 住acal hairs occur on the posterior margin of post-larval shells with shell lengths of ca. 520 μm or more (Fig. 3). Hinge morphology (Fig. 4): D-shaped larvae bear numerous teeth in their hinge, of which 28 H. Ozawa & H. Sekiguchi

800 …0 ・ー M. auricula 伽 ー←－ M.phi ゆ，'Pin arum （E 600 ミ）岡山恒国国

附 両示届主！竺 T：〆イ：コ－ r：工 i：二 f圃，： ω 阿国 ω 岡山由 去ニ L壱r 、~. 200 ーー・・・・ー陪－－－－・：；一．．．．．．

品。 由。 5 10 15 20 25 30 Days after fertilization

Fig. Fig. 2. Larval and post-larval growth of Modiolus auriculatus and M. philippinarum. Open and solid circles circles indicate mean shell lengths of M. αuriculatus and M. philippinarum, respectively. Bars indicate indicate standard deviation.

A N=21

Presence Presence －－－－・ 4・・・・・－・・・－・－－・－－．－－・－－－－ー・・・・・・事・ーーー・・ーー

Absence 嗣．．．．．．－－－ー・・ーー・・・ー・・“・・・－－－－－－－－－－－－－－－ーーーーー・・ 』咽儒 a －帽 ω 岡崎』帽曲。明』曲向同

B N=21 Presence Presence －－－－－ー・ーー・司・・・－－ーー・・・ーーー・・・...... 司 4 ・・ー・幽ー－－－－－－－

Absence 園刷・・・・－－－－－－－－－－－－・・・・・....・~ .. －・ーーー・”・・・・・・ー・・

400 500 600 700 800 900 1000 1100 Shell Shell length ( μm)

Fig. Fig. 3. Development of periostracal hairs in relation to shell lengths for Modiolus aur 的 tlatus (A) and M. philippinarum philippinarum (B). Larval Larval shell morphology of tropical Modiolus 29

a t

170 170 p 242 300 Modiolus auriculatus

p~

180 180 226 Modiolus phil 伊']Jinarum 300

Fig. Fig. 4. Dorsal views of larval and post-larval shells of Modiolus auriculatus and M. philippinarum. Numerals Numerals indicate shell l巴 (μm). ngths See Fig. 1 for letters. Scale bar: 100 μm.

those those in the middle portion are smaller than the others. The primary ligament is observed in a post-larva post-larva with a shell length of 347 μm. In post-larvae with shell lengths of 350 μm or more, it is is di 質icult to observe the hinge du 巴to the increase in shell thickness.

Observations Observations by SEM Hinge morphology (Figs. 5 & 6): A D-shaped larva with a shell length of 118 μm possesses poor poor dentition around the middle portion of the hinge, one of 148 μm bears 18 teeth. The teeth become smaller with growth of the middle portion. Both umbo larvae and post-larvae with shell lengths lengths of 260 and 382 μm respectively bear 23 teeth on each valve. In a post-1 紅 va with a shell length length of 512 μm, the number of the tee 出 has decreased due to fusing with adjoining teeth in the middle middle portion, leaving 14 teeth on each valve. Lateral Lateral teeth (Fig. 5): Primary lateral teeth are observed in post-larvae with shell lengths of 700 μm or more, while secondaηlateral and dysodont teeth are not observed in the larvae and post-larvae. post-larvae. The number of teeth gradually increases with growth. A post-larva with a shell length of of 700 μm bears 3 teeth, and a specimen with a shell length of 1125 μm bears 8 te 巴th.

Ligament Ligament (Figs. 5& 6): D ” shaped larvae lack a ligament. A well-developed primary ligament is is observed in post larvae with shell lengths of 382 μm or more. A secondary ligament occurs in post-larvae post-larvae with shell lengths of 700 μm or more, in front of and extending toward the primary lateral lateral teeth. 30 H . Ozawa & H . Sekiguchi

Fig. Fig. 5. Scanning electron micrographs of the left valve of larval and post-larval shells of Modiolus auriculatus . Numerals indicate shell lengths (μm) . See Fig . 1 for letters.

Modiolus philippinarum Larval Larval development 2) (Fig.

Eggs 訂 e spherical in shape with a diameter of 78 ±3 μm. It takes 16 hours from fertilization for for D-shaped larvae to d巴velop with shell lengths of 116 ±4 μm. D-shaped larvae metamorphose into into umbo larvae with shell lengths of 180 μm or more 4 days after fertilization. Eyespots differentiate differentiate 11 day s after fertilization with shell lengths of 216 μm or more. The settling of larva bearing bearing eyespots, velum and a functional foot with a shell length of 238 μm takes place 13 days Larval Larval shell morphology of tropical Modiolus 31

Fig. Fig. 6. Scanning electron micrographs of hing 巴s of the left valve of larval and post -larval shells of Modiolus Modiolus auriculatus. Numerals indicate shell lengths (um). See Fig. 1 for lett 巴rs.

after after fertilization . In the same day , post-larvae with shell lengths of ca. 240 μm or more shed the velum. velum.

Observations Observations by optical microscope Shell Shell sh α！pe : In D-shaped larvae with shell lengths of 116-150 μm, the anterior margin of the 32 32 H .Ozawa & H. Sekiguchi

Fig. Fig. 7. Scanning electron micrographs of the left valve of larval and post-larval shells of Modiolus philippinarum . Numerals indicate sh 巴： 11 lengths (μm) . See Fig. 1 for l巴tters .

shell shell gradually protrud 巴S with the growth of the shell. Umbo larvae and post-larvae, with shell lengths lengths of 150-240 μm and 240-400 μm respectively, ar 巴 oval in shap 巴 with pointed anterior and round round posterior margins. The umbo is located at the middle of the valv 巴. The shell then grows postero-ventrally postero-ventrally to become triangular in shape with shell lengths of 400-500 μm. In post-larvae with with shell lengths of 500 μm or more, the shell extends antero-ventrally and the postero -dorsal margin margin gradually declines postero-ventrally . Larval Larval shell morphology of tropical Modiolus 33

Fig. Fig. 8. Scanning 巴lectron micrographs of hinges of the left valve of larval and post-larval shells of Modiolus Modiolus philippinarum. Numerals indicate sh 巴 11 lengths (urn). See Fig. 1 for letters.

Shell Shell color and periostracal hairs: Shells of D-shaped larvae are colorless, whereas those of umbo larvae and post-larvae are pale yellow in color. Scale-lik 巴 perios むacal hairs occur on the posterior posterior margin of post-larval shells with shell lengths of ca. 820 μm or more (Fig. 3). Hinge morphology (Fig. 4): D-shaped larvae bear numerous teeth in their hinge, of which 34 34 H. Ozawa & H. Sekiguchi those those in the middle portion are smaller than the others. It is difficult to observe using an optical

microscope microscope the teeth in the middle portion of D-shaped larvae and post 輸 larvae. Early umbo larvae lack lack a primary ligament, but it is present in umbo larvae with shell lengths of 226 μm or more. In umbo larvae with shell lengths of 350 μm or more, it is difficult to observe the hinge due to the increase increase in shell thickness.

Observations Observations by SEM Hinge Hinge morphology (Figs. 7 & 8): A D-shaped larva with a shell length of 114 μm possesses poor poor dentition around the middle portion of the hinge, while one with An shell length of 145 μm bears bears 22 teeth. The teeth become smaller with growth of the middle portion. An umbo larva with a shell length of 190 μm bears 22 teeth, the same as in D-shaped larvae. However, a post-larva with with a shell length of 287 μm bears 12 teeth on each shell, due to fusion with adjoining teeth in the the middle portion. Lateral Lateral teeth (Fig. 7): Primary lateral teeth are observed in post-larvae with shell lengths of 537 μm or more, while secondary lateral and dysodont teeth are not observed in larvae or post- larvae. larvae. The number of teeth gradually increases with growth. A post-1 紅 va with a shell length of 537 μm bears 2 teeth on each shell, and a specimen with a shell length of 720 μm bears 4 teeth. Ligament Ligament (Figs. 7 & 8): D-shaped larvae lack a ligament. The primary ligament is observed in in umbo larvae and post-larvae with shell lengths of 226 μm or more, and it is well-developed in post-larvae post-larvae with shell lengths of 383 μm or more. The secondary ligament occurs in post-larvae with with shell lengths of 720 μm or more, and it develops in front of and extends towards the prim 紅 y lateral lateral teeth.

Discussion Discussion

Morphological Morphological differences between larval and post-larval shells of Modiolus auriculatus and M. philippinarum Based on the presence or absence of lateral tee 由加d ligaments, it is difficult to distinguish D-shaped D-shaped larvae of M. philippinarum from those of M. auriculatus (Table 1). On the other hand, umbo larvae of M. phi ！伊ipinarum are easily distinguishable from those of M. auricul α tus by the the following features (Table 2). 1) A prim 訂 y ligament is present in M. philippinarum but not

in in M. auriculatus (Fig. 4). 2）百ie middle teeth of M. phi ！伊ipinarum 紅 e relatively degenerative compared with those of M. auriculatus (Figs. 4, 6& 8). 3) The shell color of M. ～auriculatus is brown while that of M. philippinarum is pale yellow. 4) Eyespots in M. phi! 伊ipinarum appear in umbo larvae with shell lengths of 216 μm or more, while those of M. auriculatus appear in umbo larvae larvae with shell lengths of 251 μm or more. Post-larvae Post-larvae of M. philippinarum are distinguishable from those of M. αuriculatus by the following following features (Table 2). 1) Post-larvae with shell lengths less than ca. 350 μm have the primary primary ligament only in M. philippinarum. 2) The shell color of M. auriculatus is pale brown while while that of M. philippinarum is pale yellow. Furthermore, in the case of M. auriculatus, the color color of the shell in the umbo larval stage is clearly di 百erent from that of post-larvae; the umbo larval larval shell is brown while the post-larval shell is pale brown. 3) Primary lateral teeth and periostracal periostracal hairs in M. phi ！伊ipinarum develop at shell lengths of 537 μm or more and 820 μm or more respectively, while those of M. αuriculatus develop at shell lengths of 700 μm or more and 520 μm or more respectively. As shown in Table 1, among other Modiolus species only the larval and post-larval shell morphology of Modiolus modiolus inhabiting the north-western Atlantic has hitherto been reported reported (Fuller & Lutz, 1989). Larval and post-1 紅 val shell mo 中hology toge 出er with the present Larval Larval shell morphology of tropical Modiolus 35

Table Table 1. Presence or absence of lateral teeth and ligament in larvae and post-larvae of 出e Mytilidae (modified (modified in part 仕om Hanyu et al., 2001).

Lateral Lateral teeth Ligaments Species Species Stage References pt pt st dy pl sl D-shaped D-shaped Amygdalum papyrium Umbo Post-larval* Post-larval* ＋ ＋ D-shaped D-shaped Aulacomya α fer Umbo 2 Post-larval* Post-larval* ？ ？ ＋ D-shaped D-shaped Br αchidontes exustus Umbo Post-larval Post-larval ＋ ＋ ＋ ＋ ＋ D-shaped D-shaped Brachidontes Brachidontes granulata Umbo 2 ？ Post 司 larval ＋ ？ ？ ＋ D-shaped D-shaped Choromytilus Choromytilus chorus Umbo 2 Post-larval Post-larval ＋ ＋ ＋ ？

D 幽 shaped Geukensia Geukensia demissa Umbo Post-larval* Post-larval* ＋ ＋ D-shaped D-shaped lschadium lschadium recurvum Umbo Post-larval* Post-larval* ＋ ＋ ＋ D-shaped D-shaped Modiolarca Modiolarca impacta Umbo ＋ 9 Post-larval Post-larval ？ ？ ？ ？ D-shaped D-shaped Modiolus Modiolus auriculatus Umbo II Post ”larval ＋ ＋ ＋ D-shaped D-shaped Modiolus Modiolus modiolus Umbo 1,3 Post-larval* Post-larval* ＋ ＋ ＋ D-shaped D-shaped Modiolus Modiolus philippinarum Umbo ＋ II Post-larval Post-larval ＋ ＋ ＋ D-shaped D-shaped Musculista Musculista senhousia Umbo ＋ 7 Post-larval Post-larval ＋ ＋ ＋ ＋ ＋ D-shaped D-shaped Mytilus Mytilus chilensis Umbo ＋ 2 Post-larval Post-larval ＋ ＋ ＋ ？ D-shaped D-shaped Mytilus Mytilus edulis Umbo ー，（＋）料＊ 1,3,4 Post-larval* Post-larval* ＋ ＋ ＋ ＋ D-shaped D-shaped Aそyti/us galloprovincialis Umbo ＋ 4,5,6

Post 司 larval ＋ ＋ ＋ ＋ D-shaped D-shaped Perumytilus Perumytilus purpuratus Umbo ＋ 2 Post-larval Post-larval ＋ ＋ ？ ＋ ？

observation observation of two species show that umbo larvae of M. philippinarum are distinguishable from those those of 出e other two species by the presence of a primary ligament.

Morphological differences between the larval and post-larval shells of Modiolus and other 36 H. Ozawa & H. Sekiguchi

Table Table 1. continued. D-shaped D-shaped Perna Perna canaliculus Umbo ＋ ＋ 8,9 Post-larval** Post-larval** ＋ ＋ ＋ ＋ ＋ D-shaped D-shaped Pernapern α Umbo ＋ ＋ 8 Post-larval** Post-larval** ＋ ＋ ＋ ＋ ＋ D-shaped D-shaped Perna Perna viridis Umbo ＋ ＋ 8,10 Post-larval Post-larval ＋ ＋ ＋ ＋ ＋ D-shaped D-shaped Semimytilus α／ gos us Umbo 2 Post-larval Post-larval ＋ ＋ ？ ＋ ？ D-shaped D-shaped Xenostrobus Xenostrobus pulex Umbo ＋ 9 Post-larval Post-larval ？ ？ ？ ？ D-shaped D-shaped Xenostrobus Xenostrobus secur1s Umbo ＋ 7 Post-larval Post-larval ＋ ＋

* Definitions 紅 e problematic, because beginning of the post-larval stage, i. e. just after met 出 norphosis, is defind defind as the occurence of primary ligament in Fuller & Luts (1989) and Le Pennec (1980). **Defined **Defined as plantigrades in Siddall (1980). *** *** Data for Mytilus edulis aoteanus in New Zealand waters (Redfearn et al., 1986). References. References. 1: Fuller & Lutz (1989); 2: Ramorino & Campos (1983); 3: Luts & Hidu (1979); 4: Le Pennec

(1980); (1980); 5 ：・ Sak む＆ Sekiguchi (1992); 6: Le Pennec & Masson (1976); 7 ：・ Kimura & Sekiguchi (1994;

reported reported as Limnopernafortunei kikuchii); 8: Siddall (1980); 9：・ Redfearn et al. (1986); 10: Hanyu et al. al. (2001); The present study. See See in Fig. 1 for letters.

Aの1tilids As shown in Table 1, the D-shaped larvae of Modiolus species are not distinguishable from those those of species in other genera in the Mytilidae. Furthermore, umbo larvae of Modiolus species are are not distinguishable from those of other genera in the Mytilidae, except those of Perna, which possess primary lateral teeth. On the other hand, post-larvae of Modiolus species are clearly clearly distinguishable from those of other genera in Mytilidae, such as Amygdalum, Aulacomya, Choromytilus, Choromytilus, Geukensia, Ischadium, Modiolarca, Mytilus and Xenostrobus, by the pr ，回 ence of primary lateral teeth, and also from Brachidontes (B. exustus), Musculista, Perumytilus, Perna and Semimytilus Semimytilus by the absence of secondary lateral and dysodont teeth, though no information on secondary lateral and dysodont teeth for post-larvae of Brachidontes granulata has been available.

Acknowledgements

We wish to express our sincere thanks to Profs. Minoru Murai and Kazunori Takano of the Sesoko Station, Tropical Biosphere Research Center (TBRC), University of the Ryukyus, for making facilities facilities available for laboratory experiments. Thanks are due to the staff and stud 巴nts of TBRC for encouragement throughout the present study. We owe many thanks to Mr. Kazuhiro Toyama, Ocean Rebirth Rebirth Center Co. Ltd., for assisting in culturing mussel larvae and phytoplankton.

References

Bayne, Bayne, B. L. 1976. The biology of mussel larvae. In: Bayne, B. L. （巴 dふMarine Mussels. ・Their Ecology and Physiology, pp. 81-120. Cambridge University Press, Cambridge. Chanley, Chanley, P. & Andrews, J. D. 1971. Aids for identification of bivalve larvae of Virginia. Malacologia 11: Table 2. Summary of morphological features of larval and post-I 訂 val shells of Modiolus auriculatus and M. phil 伊ipinarum.

D-shaped larval stage Teeth es SC ph pt pl sl 円相当己目 M auricu/atus (115 ー 180μm) lower height in median teeth absent absent absent absent absent absent M philippinarum (116 ー150μm) lower height in median teeth ホ absent absent absent absent absent absent vo －－ Umbo larval stage B 。同対日 M auriculatus (180-290μm) lower height in median teeth >251 μm brown absent absent absent** absent 5 M philippinarum (150-240μm) fused and then disappeared in median teeth >216μm ocher absent absent >226μm absent －。問。片片岡。｝

Post-larval Post-larval stage UR M auricu/atus (>290μm) fused and then disappeared in median teeth developed pale brown >520μm >700μm >347 μm ＊事＊ >700μm 包ミ。忌ミ M philippinarum (>240μm) disappeared in median teeth developed ocher >820μm >537 μm developed >720μm

E 事 Median teeth of M philippi ，叫，伊 ・um bee 温me degenerated as comp 釘 ・ed with those of M au1 ＊$ Not found using optical microscope and SEM. ＊ホ＊ Smallest shell length in optical microscope observation. es es -eyespot, ph -periostracal hair, pl - primary ligament, pt - primary late1 叫 tee 出， sc -shell color, sl ・secondary ligament.

(.;) (.;) 、j 38 38 H. Ozawa & H. Sek:iguchi

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(Receiv 巴d July 28, 2001 I Accepted December 28, 2001) Larval Larval shell mo 中hology of tropical Modiolus 39

熱帯性ヒバリガイ属 2 種における浮瀞幼生及び初期稚貝の殻形態

小津宏之・関口秀夫

要約

沖縄の浅海域に生息するリュウキュウヒバリガイ Modiolus auriculatus とホソスジヒバリガイ M. philippinarum の浮遊幼生（D shaped larva, Umbo larva ）及び初期稚貝（Post-larva ）の種判別を目的とし て，殻の形態について人工飼育より得られた 2 種の標本を実体顕微鏡及び電子顕微鏡を用いて観察した。 本研究結果から，これら 2 種の殻頂期幼生（Umbo larva ）は，交装の構造，殻色及び眼点出現時期の殻長 等の差違から明確に識別することができた。また，両種の初期稚貝は，交装の構造，殻色，第一側歯と 殻皮毛の出現殻長の差違から明確に識別された。 本研究結果を含めこれまでに幼生の形態が明らかになっているイガイ科全 22 種の， 3 種類の側歯（第 一側歯，第二側歯，前側歯）及び 2 種類の靭帯（第一戦帯，第二鞍帝）の有無について比較した。殻頂 期幼生において，本研究対象種を含みミドリイガイ属 （Perna ）以外の種では何れの側歯も保持しないが， ミドリイガイ属の種では第一側歯を保持することが明らかになった。また，ヒバリガイ属の初期稚貝は， イガイ科他種と 3 種類の側歯の有無により識別された。