The malacological society of Japan

flre VENUS CJap. Jour. Malac.) Vol. 43, No. 4 (19S4): 315-330

-? v* on v s"? Di 8 v-? trl vx pt' t/otsSaifkts=JE-4*

wt pm k =-

oA',e, /lt･# fi]n uatuI-k wt"r)

Sexually Dimorphie Radulae in Cronia mavgariticola and MoT2Llct musiva (: )'

\oshimi FUJIOKA

(Mukaishima Marine Biological Station of Hiroshima University,

Onomiehi P.O., Hiroshima Pref. 722)

Abstract: The radulae of C･ronia wzaryaenlticolct (Broderip) and Monela musiva (Kiener) were examined on the specimens celleeted from southern Japanege waters. Although the radulae of these have been already reportecl on by sorne authors, the present study Teveals existenee of sexual

dimorphism and continuous ehanges with age in radula of both species. The

length of radular ribbons and the breadth of rachidian teeth relative te shell

length inerease with growth. In the adult stage larger than 16mm in shell

length, the raehidian and lateral teeth of males of these species were usually larger and grew more rapidly than those of females. Thus, the male radular

ebaraeters may represcnt the seeondary sexual chaTacters. The aetual number

of radular rows is not sexually diTnorphie and does not reveal differences

between young and adult speeimens. The raehidian teeth exhibit remarkable

ehanges in shape as well as size with growth. As a rule, the raehidian teeth

found in the young stage are of a complieated form with rnarked exterior

(euter) dentieles and marked marginal humps or cusps, and by gradual trans-

formation. ehange to the simple form found in the adult stage. The rachidian

teeth of fully grown males are extrernely simplified due to sueh ehanges as

well as sexual dirnorphism. Investigation into previeusly published works eembined with the known influenee ef sexual dimorphism, it ean be shown

that the raehidian teeth of Morztla are eharaeterized by atTophied exteTior

dentie]es and marginal eusps, whieh are intermediate in radular eharaeters

between Croiiia and Thais.

Introduction

The radula has been frequently investigated because of its importance as a t tt ' tttt ttt * Contribution from the Mukaishima Marine Biological Station. No. 214. Received July 5, 1984.

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316 VENUS: VoL 43, Ne.4(1984)

tool in supraspeeific systematies and as an oceasional aid in the diagnosis of

speeies. Numerous studies have been done dealing with the deseription and

iilustration of radulae, but these have usuaily been based on examination of a

small number of speeimens. However, the radular teeth are not, as is generally

supposed, always of the same size and shape in a given speeies, It is pointed

out by some authors that the variation of the radulae is due to ontogenetic

"metamerphosis" changes such as a (Sterki, 1893) or growth changes (Carriker,

1943; Fujioka, 1984; etc.), to differenees between the sexes sueh as sexual

dimorphism (Arakawa, 1958a, 1958b; Maes, 1966; Robertson, 1971; Cernohorsky,

1971; Fujioka, 1982) and secondary sexual characters (Fujioka, 1982), in

addition to individual variations or radular anomaly (Carriker, 1975; Fujioka,

unpublished).

In the present study, the radulae of two eommon muricid gastropods, C7"onia ･ma,rga,rit'icoea 'musivna (Broderip) and Mo7'uLa (Kiener), were examined in detail,

and it was found that they exhibited sexual dimerphism, whieh was restricted

to the adult stage. The radula of Cronia･ 7nargaT'it'icola･ has been alTeady

described and figured by several authors. At first, Cooke (1919) deseribed the

radula of specimens from Bombay and Karaehi. Subsequently, Kinoshita and

Kinoshita (1931) gave a figure of the radula of this speeies and referred to

the speeies predation on oysters. Arakawa (1965), in his study on the radulae

of Japanese Muricidae, deseribed and figured radulae on the basis of speeimens

from Yoron and Haehijo Islands. Furthermore, Wu (1965) and Cernehorsky

(1969) clescribed and figured radulae of specimens from Taiwan and the Fiji

Islands, respe ¢ tively. The raclula of Moruta･ 7n.usivnt has been deseribed and

figured by Arakawa (1965) and illustrated by Kinoshita and Kinoshita (1931) and Cernohorsky (1982b). However, as their results were based upon the

examination of a few specimens, variations in radula due to sexual differences

with grewth were not mentioned.

Materials and Methods

Forty specimens of Cronia 7nargariticola used for the present study were

seleeted among the various sizes of populations eollected from some loealities

along the shoreline of the southwestern Japanese coast during the years 1981-84.

The localities and the number of specimens examined are as follows : Yaku-shima

Island (N=:15), Tanega-shima Island (N=4), Amami-oshima Island (.N=3) (all Kagoshima Prefecture), Sesoko Island (N=14), Kuro-shima Is]and (N=1), Okinawa Island (N=1) (all Okinawa Prefecture), and Shirahama (N=2) (Waka-

yama Prefeeture). The forty specimens of Moruta musiva were eollected from

the intertidal zone of Shirahama, Wakayarna Prefeeture, in February 1984. The

speeimens were fixed and preserved in a 10% neutralized forrnalin.

After the shell length was measured, the shell was erushed by a viee to

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317 Fujioka : Sexually Dimorphic?Radulae in Muricidae

in the shell, the extract the soft parts. Although there are some variations among measure- shell length appears to be the most eonvenient and reliable index ments of various parts of the shell. The radular ribbon was removed from the

and measured under a stereoscopie disseeting mieroscope. Measurernents

ef the teeth was done with an oeular mierorneter in a compound light miero-

scope. As the teeth aTe somewhat different in size between the anterior and the posterior parts of a radular ribbon, the dimension was represented by the mode of a radula. The number of radular rows were eounted longitudinally ineluding

the nescents (newly formed rows) at the posterior part of the ribbon. For detailed morphologieal studies of the teeth, the radulae of twenty speeimens of

eaeh species were prepared for observation under a scanning electron microscope. The discrimination between adult and young specimens were based on the features of outer lip of shell, gonad, and the charaeters of radular teeth.

Although there are large differenees among individuals, it was appropriate to

discriminate at a size level of 16 rnm in both species.

The presenee of a penis and the reproductive organs, especially prostate gland of the male and capsule gland of the female, aided in diseriminating male from female. In the case of C7'onin marga7'iticota, it is possible to decide the

sex on the basis of the presenee of a penis beeause the female does not have a pseudopenis. On the other hand, identifieation of males by the presenee of a penis was not possible in ･ musiva beeause of the variable size of the vestigial penis in females, whieh is sometimes slender, sometimes small like

a fieshy nodule, and sometimes absent. The male of M. 7nusiva exelusively has

a large and thiek penis.

Results

Externa･l fecttures and ta:ronomic eomments: The trivial name fiscettzem Gmelin, 1971 has been currently used for a speeies

of the Mzarieod7"upa "rhich has squarish, window-like depressions on the

"basket shell and is usually known as the drupa". Emerson and D'Attilio (1981)

mentioned that Gmelin's fiscettza7n based on the drawing of Chemnitz is not

referable to the basket dr'upa but indicative of the common and wide-ranged

Indo-Paeifie speeies subsequently deseribed as Mureft: maTgaTiticola Broderip,

1833. IIowever, Ceriiohorsky (1982a) reviewed and elarified the eontusing

taxonomic problem involving these two species. Aeeording to him, the well-

known name ma･rga･riticoea Broderip, is a valid name and not a junior synonym of lltlurieodrupa fiscettum (Gmelin). The shells of Cronia 7na-rgariticola from

Japanese waters are shown in Fig. 1. The shell is seulptured by many fine

spiral eoi-ds with the white or violet aperture.

The second species used in the present study is MoTuta mus-iva (Kiener, 1835). Some authors assigned this species to the genus Azumamorula Emerson,

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318 VENUS: VoL 43, No.4 (1984)

t tt tttt ttt/tt.tt tttt /t tt ttt t tt/t tttt tt - t ttt..tttttttttt/ tt /it/tt;tttt/tt t ,t,tt. tt,t. t ttttt tt tt ,,t/ t t

t/tA".,e.t･,,-t/,b/.-.tt/ttt

. ..-v ..nttiL..Xvs-...ulth/' t'/"/'r{Sigi･lg/･,g,//･,/･//,},/."･, ' Fig. 1. The shells of Cronia margariticoZa from Japan･ i)*vlys v)e Left: 24.8mm, from Kuroshima Island･ re.xkree Center : 19. 7mm, from Yakushima Island. nl

Fig. 2. The shells of Moruta musiva from Shiraharna, Japan･ )"'? tzi V t;f'rt V. From left to right: 21. Imm, 19. 3mm, 16. 7mm･ eutrk.

1968, which is a new name for Morz{lina Dall, 1923 (not Borner, 1906), but it

is eurrently assigned to the genus Moruta. The shells of Morula musiva (Kiener)

from Shirahama are shown in Fig. 2. The shell is charaeterized by two different

colored nodules arranged alternatively forming spiral ribs. The nodules are

frequently eroded especially in the full-grown speeimens.

The size of 7'aduta :

The greater the shell-length, the longer the radular ribbon and the greater

the size of radular teeth, as the size of the radula steadily increases ontogeneti-

eally. In studying 40 radulae of Cr'onia margariticoLa and Mo7"uta musiva, the

length of radular ribbons in relation to shell length is Tepresented in Fig. 3A

and Fig. 4A, respectively. Separating the sexes into male and female, a positive

correlation is obtainable for each of both speeies. In the adult larger than 16 mm

in shell length, the male radular ribbons are longer than the famale ones, with

a few exceptions. While the four specimens of young males up to 16mm do

not have the longer ribbon than females of the equivalent size. This relativity

is summarized in Table 1 and Table 2 fer C. margaTiticota and M. musiva,

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Fujioka ： Sexually Dimorphic Radulae in Muricidae 319

（ E 8 ♂ ε ● ） A 仁 ’ 一 ・ ・ ￥ ・ ・ ・ 8n 6 ・ （ ジ． レ ／ ／ 罎 ● CD OO 」 O FEMALES 轟 σ 4 著 ● MALES 麟 〃 ／ ／ ．° 8 e 。 ／ も 2 彡／ qD £ 2 0

（ρ 0．3 ヒ 一F ）

匸 一σ 伽η 0。2 ‘ り O ¢

学 O 0．1 ‘ 一 〇 〇 〇 」 m o

σ 39 200 C 配 ￥

ち し

い 100 一 濁 麟 ♂ ≧ 飜 O O FEMA し ES 匡 お ● MALES ． OZ LOO 10 20 30 SHELL LENGTH （mm ） − Fig ．3 ． Relationships between shell length and length of radular ribbon （A ）， rachidain tooh B and no of rows of radu ！a the breadth of （ ）， ． （C ）， in Cronia margariticola ． Sexual dimorphism is observable respectively ， in the first two relationships ， The coe 伍 cient of correlation for each ： ＝ ・− 8351 rBa ＝0 8917 regression line is as follows rA ♂ 0．8628，rAg O． ， ． ， ＝ ； rB ♀ ；o ．9021 （all significant ）， rc ♂ 0．3184 （not significant ）， rc ♀ 0．3915 （not significant ）・ ・．一 一 一 ウ ネ レ イ シ ダ マ シ の 殻 長 歯 舌 紐 長 （A ）， 中 歯 ll畠（B ）， 歯 列 数 （C ）の

二 め る 関 係 。 （A ）お よ び （B ）の 関 係 で 性 的 形 が 認 られ 。

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320 ： 43 4 VENUS Vo1． ， No ． （1984 ）

（ E 8 ε co ρ 6 £ 僅

」 05 4 ℃ σ ¢ ち 2 ‘ 一 〇 匚 彑 0

（ 」匚 一F ♂ ｝ B CO 一 0 ．2 〇 ・ 一 t ノ ・ ． te ‘ り O FEM 圦LES σ ヨ 配． ● MA しES イ 噛 ．1 凄 o 一 6魎蠡 O ＿ ヌ ＿ つ ‘一 PU 僧 O 0

O 言 η O 匡 2 ち

≧ O ¢ 1 ち ． OZ

SHELL LENGTH （mm ） − Fig ．4 。 Relationships between shel1 length and length of radular ribbon （A ）， the breadth ef rachidian tooth （B ）， and no ， of rows Qf raduia （C ）， respectively ， in Moruta musiva ． Sexual dimorphisln is observable in the first two relationships ． The coe 伍 cient ef correlation for each ＝ regression line is as follows： rA ♂ 0．5732 ， rA 早 ＝ 0．5243 ， rB ♂ ＝0．7100 ， rB ♀ ＝0 ．5442 all significant rc ；− 0 1872 not （ ）， ♂ ． （ significant ）， rG ？ ＝ 0．3915 （not significant ）・

シ マ レ イ シ ダ マ シ の 一 A 一 一 殻 長 歯 舌 紐 長 （ ）， 中 歯 幅 （B ）， 歯 列 数 （C ）の

A お よ び B の 関 係 。 （ ） （ ） 関 係 で 性 的 二 形 が 認 め られ る 。

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321 Fujioka:Sexually Dimorphic Radulae in Muricidae

Table 1. Comparative measurements of sexually dimorphic radulae of Cronia margariticola. ,)*tr(vffvyovaa:-&it . .-

of of Shell Length Breadth No. of G.r,O.:8h i(e(."s:.t),h,ibr,(a..d.".tiaL)rR)MS,.L./LRsDi."ih ± SD sex ± gp.Ne.cii,il;.l.Oe[n,s ,hilgtba,")Mg.L.#rsDr.o.w,s.f.,fMean ttt ttt t ttt tt tt ..ttm.. . ..- ttttt ttttt tttt 118 15.5 Ad[lt Female 16 17.e-24.7 2.8-6.14.57 ± O.77 ll5-195 130 ± 11.0 89-143 ± 9.6 Male 14 16.1-27.8 4.7-7.63.54 ± O.41 165-280 98 ± 10.7 96-131 114 ± Differences between Significant Significant Not Ol significant sex (t-test) p

Table 2. Comparative rneasurements of sexually dimorphic radulae of Morula messiva.

vi v! pt"yvodwfi;X.i- v-? .. . Breadth of Shell Length of No. of No. of rachidian SL/Br G.rtO.}gh i(e(."sg.tt))h,ibrb(a..d.".tiaL)rR)MgaLn/LRsDtee(tpth.(gr)Mean ± SD sex 2p.e.tt?.;.e.nds ± ± sDr,o.wds.io.fMea" tt ' --t t ttt ± 181 ± 27.9 Adult Female I4' 16.7-22.2 :g.o-6.o 3.7o ± o.so 9o-13s 163 2o.s lol-21s Male 19 16.0-21.0 3.8--6.93.15 ± O.46 95-185 121 ± 20.3123-198 164 ± 20.5 Differences between Significant Significant Not -test) significant sex (t O. Ol

of radular ribbons to respectively. The mean value of the ratio of the length of shell length ranges from 4.36-4.60 in adult females and in young speeimens C. margariticoLa, and from 3.52-3.70 in those of M. musiva･, but adult males show considerably lew values within both species. The difference between adult females and adult males is significant under the 98% confidence limit. Fig. 3B and Fig. 4B indicate the relativity between shell length and the breadth of the raehidian teeth of Cronia margariticola and Morula musiva･, to the case of respectively. The breadth of raehidians shows a similar pattern the length of radular ribbon. Sexual dimorphism is clearly reeognizable, but is restricted to the adult specirnens larger than 16mm in shell length. The rachidian teeth of adult males are prominently larger than those of females, and the rate of increase in breadth of the rachidians relative to shell length is much higher in males than females. It is also clear from the evidence that the difference in the size of rachidian teeth between the anterior, or oldest, and is the posterior, or more recently formed, parts in a given radular ribbon

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322 VENUS: Vol. 43, No.4(1984)

large in the male and small in the female. The difference of the mean value

of the ratio of the breadth of rachidians to shell length is highly significant between females and males (Table 1 and Table 2). The radula is of the rachigrossate type usually formulated as 1-C-1 (or 1-1-1), in which a transvcrse radular row contains a single raehidian tooth and a lateral Cor margina]) tooth en either side of the r'achidian. Because the

laterals grows in unison with the rachidian, the sexual dimorphism is observable in the lateral as well as the rachidian teeth.

The number of rows of the radula or the number of longitudinal rows of CTonia･ marga'riticota and Morula･ mus･iva relative to shell length is shown in Figs. 3C and 4C, and eompared in Tables 1 and 2, respectively. There is no

correlation in the number of rows arnong speeimens within both females and males, and the variation among individuals is very large. In the ease of C. ?nargaTiticota, as the length of the shell inereases, the number of rows go slightly, up in both females and males (Fig. 3C). The mean value of the number of rows in adult females is 118 ± 15.5, which is the largest of all stages and sexes. The number of rows of the adult males is slightly fewer than that of females, but the difference between sexes is not significant (Table 1). The number of radular rows of the female ef Morzala musiva･ also tends to increase slightly with the growth ef the animal. However, the males do not show such a tendency. This may be eaused by the fact that individual variation of the males is very Iarge and that the smaller specimens up to 13.9 mm shell length

were not available for radular examination.

Morphotogy of the ra･duta: In addition to the sexual dimorphism in size, the shape of the radular teeth undergoes sexual dimorphism in the adult stage. Furthermore, there are more or less differeiices between the young and the adult stages, and the radulae of young anirnals do not differ between sexes. Although there are some individual

variations among the specimens, the following eharactei-s are typieal in the radulae of the young, of the female adult, and of the male adult stages, respectively.

Cronia marga7"iticola (Broderip, 1833) (Pl. 1, figs. 1-8)

Young stage (Pl. 1, figs. 7-8): The rachidian tooth is trieuspidate. The central cusp is IQng and slender and sharply pointed at the tip, whieh reaches approximately 1.5 times the length of the lateral cusps. The base of the lateral cusps is broad, and the tip is pointed and eurved outward. There is a marked inner denticle isolated between the central and the lateral cusps. Immediately outside of each Iateral cusp is a series of two to four small and equal sized outer denticles or merely wrinkles, the tips of which are rounded. No marginal cusps present, but both ends are elevated to form nodule-like marginal humps.

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Fujioka:Sexually Dimorphic Radulae in Muricidae 323

The anterior rnargin of the base is concave, but project strongly at the median, thus forming two waves. The posterior basal line is concave.

Adult females (Pl. 1, figs. 3, 6): The general plan of the rachidian tooth

is the same as that of t.he young stage exeept for some minor points. The

outside region of the lateral eusps is relatively broader than that of the young

stage, and the surface is usually smooth without outer dentieles or wrinkles.

The marginal humps feund in the young stage semetimes disappear. The central and the lateral eusps bend strongly to the pesterior {Pl. 1, fig. 4).

Adult males (Pl. 1, figs. 1, 5): The base of the rachidian tooth is extremely

broad and thiek (Pl. 1, fig. 2). The central eusp is long and sometimes extr'ernely,

slender. The lateral cusps become short and bend to the posterior, and are

straight, not curved outward as that of the young er the adu]t female. The

inner dentieles aTe very sma]1 or beeome atrophied. The eutside ef the lateral

cusps is broad and smoothly surfaeed without wrinkles or the marginal humps.

The ]ateral teeth are typically, siekle-shaped with a thick t{nd broad base,

and is not morphologically different throughout the growth stages as well as

between females and males. The radula is translucent in the anterior part and amber in color in the pesterior part of the ribben. In the radula of the

fully grown animuls, especiall}, in the males, the posterior part of the ribbon

is sometimes coloi'ed brown.

Mo･iJztla mztsiva (Kiener, 1835) (Pl. 2, figs. 1-7)

Young stage (}'l. 2, figs. 5-7): The rachidian tooth is basically pentacus-

pidate. The central cusp is long and slender, and point,ed at the t,ip, which

reaches approximately 1.6-2.0 times the length of the Iateral eusps. The trigonal

lateral cusps are positioned a short, distance from the central cusp. The tip of

the lateral cusps is pointed and s.ometimeg curved outward. Between each

latei'al and the central cusp a single inner denticle is evident,. There are one or

two small outer denticles between the lateral and the marginal cusps. The

marginal cusps are small and usually rounded with blunt, tips. The small knobs

projeet in the transverse direction from both sides of the base. The anterior

basal Iine is more or less waved, and the I)osterior one is eoncave,

Adult female (Pl. 2, figs. 3-4): The raehidian tooth is similar to that of

the young stage, but the lateral eusps are relatively short. Moreover, the outer

denticles and marginal eusps become mueh smaller and b]unter, In eontrast, the

exterior projeetions from both sides of the base tend to become large.

Adult male (Pl. 2, figs. 1-2) : The rachidian tooth is basieally tricuspidate.

The central cuesp is relatively long and very slender. The small lateral eusps

fiank at a good distance from t,he eentral cusp. [Dhe inner and outer denticles

are apparently atrophied and sometimes become vestigial wrinkles. The marginal

eusps are blunt and knoblike. The exterior projections from both sides of the

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324 VENUS: Vol. 43, No.4(1984)

base are stronger than those ef female rachidians.

The lateral teeth are siekle-shaped as in Cronia ?narga?aiticoLa, and do not exhibit the morpholegical difference among the young, the female adult, and

male adult . The radula is exclusively transparent in eolor.

Discussion

Cooke's (1919) description of the radula of Cronia 7na･rgar･iticola frorn Bombay

"Central and Karaehi is as follows: eusp normal; side cusps small, sharp,

slightly, inclined inwards; inside, a very small denticle, whieh is sometimes far

removed from the cusp, sometimes adjacent, and sometimes absent altogether;

basal projection very marked". Judging frorn his description, especially from

"side "a that of the part of cusps small" and very srnall dentiele", it is presumed

that t,he radula examined by him is of the adult male. The deseriptions and

the figur-es of Arakawa (1965) and Wu (1965) appear to eoineide with the

result of Cooke's observation. It is believed that they also figured the i'adula

on the basis of either a male or a fairly large female. Whereas the radula

figured by Cernohorsky (1969) is considered to be based on that of a female

because the rachidian is relatively small (about 135 "m in breadth) and shows

the marginal humps.

Arakawa (1965) described the radulae of Morula mu,sion from Japanese

"The waters as follows: rachidian tooth is trieuspid with a broad base......

The inside of the cusp is not usually, toothed, but sometimes decerated with a

few wrinkles. On the outside, there is a bare open space...... " His descrip-

tion and the aecompanying figure agree well with the features of the rachidian

of a full.v adult male. He examined the radula of twe specimens. It is surmised

from the results of the present study that he described the radula on the basis

"74" of the specimen o'f number collected from Mage Island, and that he did

"117" TioL observe in detail the other specimen numbered eollected at ]N{anazuru.

Cernohorsky <1982b) noted that t,he radula is of the C?"onia-type. It is believed

that he also figured Lhe radule on the basis of a full grown male.

The rachidian toot,h of Morula･ uva･ (Rbding, 1798) (syn. mo7iu･s Lamarck,

1822), which is the type-speeies of this genus, has a single central and two lateral

cusps, but laeks the apparent marginal cusps (Cooke, 1919; Cernohorsky, 1969;

Radwin and D'Attilio, 1972). The ends of the tooth-base are blunt and knob-

like, and the euter denticles aTe relatively small. As shown in the present study,

the shape of the marginal cusps and the outer denticles exhibit continuous

changes with age, and tend to be atrophied in full-grown adult specimens. It

can be eonsidered that the raehidian teeth of the adult stage of Moru･la are

eharaeterized by atrophy in both outer denticles and marginal cusps. Such

radular character is intermediate between those of CTonia and Thais. The

Tadula of M. 7nzLsiva agrees with these features.

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325 Fujioka : Sexually Dimorphic Radulae in Muricidae

also eorn- The characters observed in the radulae of the genus Mo･ruta are patible with those of Morula granztla･ta (Duclos, 1832) reported by ArakaxN:a (1965). He established the new genus, Tenguellnc, on the basis of sueh ies of this characters ef radula, and designated M. granulata as the type spe ¢ genus. Wu (1965) also examined the radula of sorne morulid species, and I agree believed that M. g?v(znuLata should be retained in the genus MoruZa. as as with Wu's opinion because TenguetLa is a superfluous genus, far the

radular criterion is concerned. The radulae of Cronia ma･rgncriticota･ and Meru･ea･ mu,s･ivct exhibit the eame pattern of sexual dimorphism. Since the radulae of young specimens are not aequired sexually dimorphic, it is believed that the male radular charaeters are

as the secondary sexual eharacters at certain growth stages, as has been reported The sexual in the radulae of some species of the genus Drzzpella (Fujioka, 1982). dimorphism of the radula of Drupetea is limited to the raehidian teeth, in to that of the which the gr'owth rate of the raehidians is quite independent laterals. The lateral teeth of C. ma,rycar'iticola and M. musiva･, however, shos-red that sexual dimorphism is restr-icted to the size of teeth, suggesting that the lateral teeth of these speeies are secr:eted and replaced synehronously with the

rachidian teeth of the same transverse rows. The raehidian teeth of these

species exhibit sexual dimorphism in shape as well as in size. The length of t,he radular ribbon and the size of rachidian teeth are sexually dimorphic, and steadily increase with growth in females and rapidly in males.

The length ef the radular ribbon is determined by the size of the teeth, more

strictly by the thiekness of the teeth arranged along the longitudinal axis of

the ribbon, and by the number of radular rews. Sinee the number of rows does not increase with growth, it is considered that the sexual dimorphism feund in the length of radular ribbon is largely dependent on the gTowth of individual

teeth themselves.

The remarkable sexually dimorphic radulae of murieid gastropods have

been reperted in the four speeies of Dru･pelta (Arakawa, 1958a; Fujioka, 1982), in Pttrpura echinulat(t (Arakawa, 1958b), and in tvvo speeies of Nassa (Maes, 1966). In all cases including the present two species, the radulaT teeth of rnales

are larger than those of females. Although the biological significance must remain purely speculative for the time being, it is of interest to consider that the sexual dimorphism is related to the difference of feeding habits between females and males. Robertson (1971) believed that sexual dimorphism of the radula found in the archaeogastropods, H-iLoa･, may be seleetively advantageeus

for their differ-ent food requirements.

Apart from the sexual dimorphism, the shape of raehidians ef Cronia margariticota and Moruta musiva is somewhat different in the various stages of life even in females as well as males. The raehidian teeth observed in the

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326 VENUS ： Vol ．43 ， No ．4 （1984 ）

stage are of a complex form with marked marginal humps young ， or cusps and ・ marked exterior denticles， and by gradual t1ansformation they give place to the

simple form observed in the adult stage ． The continuous change of radula with

advancing age is observed not only in the present species but also many other species of muricid gastropods （Fujioka ， unpublished ） including the example 圏’ already reported in Dv MpetLa ． minuta ． （Fujioka ，1984 ）． ・ It was generatly accepted that radular characters a1 e constant among 七he species or that individual variation does not exceed the difference between

species （Fretter ＆ Graham ， 1962 ； Abbott ， 1974 ）． It became clear by the critical observations on murieid gastropods including this study that the charac −

ters of radular teeth are however not − ， ， always a reliable cri 七erion in the dis

crimination of species beeause the striking differences in shape and size of

七he radula are frequently sexual ontogenetic due to dimorphism ， change ，and

other forlns of variation ． Further study of eomparative radular morphology ，

taking aceount of such variatiolls ， are necessary to provide the information

reciuired to explain the difference between species and between other taxa ． − Acknowtedgements ： The au 七hor would Iike to express his grati 七ude to Dr ． Walter O ．

Cernohorsky Auckland Institute and Museum 七ani − ， ， and Dr ．Takashi Oku ， Tokyo Uni versity of Fisheries for revising manuscript ． ， the and to Dr ． Akihiko Inaba Mukai ， ， shima Marine Biological Station of Hiroshima University ， for his encouragement throughout this study ． His thanks are also due to Mr ． Susumu Ohtsuka ， Seto Marine

Biological Laboratory of Kyoto University for − ， collecting some snails from Shira

hama for this study ．

要 約

ウ ネ レ イ シ ダ マ シ （Fig ．1 ） と シ マ レ イ シ ダ マ シ （Fig ．2 ） の 歯 舌 は す で に 報 告 さ れ て い る が ，本 ’ 研 究 で は こ れ ら が 性 的 二 形 を 示 す こ と と に て に て い こ ， 成 長 伴 っ 形 態 的 変 化 し く と を 明 ら か に し た 。

殻 長 に 対 す る 歯 舌 紐 の 長 さ （Fig．3A ， Flg ．4A ） や 中 歯 の 幅 （Fig．3B ， Fig ．4B ） は 成 長 と 共 に 増 加 し ，殻 長 16mm を 越 え る で は に の 一 成 貝 阿種 共 雄 歯 舌 が 急 速 に 人 ぎ く な っ て い く。 し た が ． て ，雄 一 の 成 只 で み ら れ る は ： と て 一 歯 舌 次 性 徴 し 獲 得 さ れ た も の と 考 え ら れ る 。 方 ，歯 列 数 は 幼 貝 よ り 成 貝 が ．雄 よ り 雌 が わ ず か に 多 い 傾 向 を 示 す が ， そ の 差 は 有 意 で は な い （Fig ．3C ， Fig ．4C ）。

側 に み ら れ る 二 は の の に は に お い て 歯 性 的 形 大 ぎ さ 点 に 限 ら れ る 対 し ， 中 歯 大 ぎ さ も形 態 的 に も 性

的 ．二 形 が 発現 し ， し か も 成 長 と 共 に し て い Plate P 】ate 2 変 化 く （ L ）。 概 し て ， 幼 期 で み られ る 中 歯 は 顕 著 な 外小 歯 や 縁 歯尖 を 備 え た 複 雑 な 形 態 を 示 し ， こ れ が 徐 々 に 成 期 で み られ る 単 純 な 中 歯 と 置 き ・ 換 え ら れ て い く こ の 変 化 は 二次 性 微 を 示 の で は に の 。 ． ， す 雄 中 歯 層 顕 著 認 め ら れ る 。 以 上 知 見 に 基 づ い て 過 去 の 報 告 を 再 検 討 し た 結 果 ， Moruta 属 の 成 具 の 中 歯 が 退 化 的 な 外 小 歯 や 退 化 的 な 縁 歯尖 な

で づ こ ど 特 徴 け ら れ る と を 明 ら か に し た 。 References

Abbott R ． T 1974 Amerioan α second edition ， 。 ． Se shells （ ）． Van Nostrand Reinhold

Company ．Y ， N ．663 pp ．

“ ” Arakawa ， K 。 Y ．1958a （ 1957 ）． On the remarkable sexual dimorphism of the radula

of Drtepellα ．Venus 19 ： 206 −214 ． ． ， ， p1 6

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Fujioka:Sexually Dimorphic Radulae in Muricidae 327

Arakawa, K.Y. 1958b. Some netes on the radula of Pttrpura･ echinulata Larnarek.

Venus, 20: 69-75. Arakawa, K.Y. 1965. A study on the radulae of Japanese Murieidae <3), Vew'tts, 24:

113-126, pls. 13-14. Carriker, M. R. 1943. Variability, developmental changes, and denticle-replaeement in

the radula of Lywznaea stay"agis appressec Say. Nattt･ikts, 57: 52-59. Carriker, M.R. 1975. Radular anomaly in Urosalpinx cine･rea-

dae). Naut･iltts, 89: 91-94. Cernohorsky, W.O. 1969. The Muricidae of Fiji. PaTt II-Subfamily Thaidinae.

Velige･r, 11: 293-315, pls. 47-49. Cernohorsky, W.O. 1971. Indo-Paeifie Pisaniinae (: Gastropoda) and related

bueeinid genera. Rec. A7Lekland Inst. Mt{s., 8: 137-167. Cernohorsky, W. O. 1982a. The taxonornie status of CToavia fiscetla

Inst. Mus., 19: 113-124. Rec. Cernohersky, W. O. 1982b. The of some Indo-Paeifie Mollusca. Part 10.

Aztckland hzst. Mus., 19: 125-147. Cooke, A. H. 1919. The radula in Thads, Drupa, Montla, Concholepa･s, Cronia. Iopas,

and the allied genera. Proc. malaeol. Soc. Lond., 13: 90-110. Emerson, W. K. 1968. Azumamorula･, new name for Morzalina Dall, 1923, not Boerner, 1906 (Gastropoda: Muricacea). Natttitzts, 81: 125-127. Emerson, W.K. and D'Attilio, A. 1981. RemaTks on Mur･icoclrz{pu Ireclale, 1918 Nautilus, 95: (Muricidae: Thaidinae), with the deseription of a new spccies. 77m82.

Fretter, V. and Graham, A. 1962, B'rittsh Prosobra･nch Molluscs. Their fzt."ct･ional a7ztztomy and eeology. Ray Soeiety, London, 755 pp. found in the dimorphie radula Fujioka, Y. 1982. 0n the secondary sexual eharaeters ef Pr?.tpella (Gastropoda: Muricidae) with referenee to its taxonomie revision. ' Venus, 40: 203-223. Venzts, Fujioka, Y. 1984. Remarks on two speeies of the genus Drz{petta- (Muricidae).

43: 44-54. to the Kinoshita, T. and Kinoshita, S. 1931. 0n some muricid shells as enemies oyster. Venus,2: 190-199. Nassa. Maes, V.O. 1966. Sexual dimorphism in the radula of the muricid genus

Nautil-us, 79: 73-80. Radwin, G.E. and D'Attilio, A. 1972. The systematics of some new world murieid and t･wo species (Mollusca, Gastropoda), with descriptions of two new genera new speeies. Proc. Biol. Soc. Wash., 85: 323-352. Robertson, R. 1971. Sexually dimorphic archaeogastropods and radulae. A7tn. Rep. 1970 Amer. malacol. Un･i., 75-78. Sterki, V. 1898. Grewth changes of the radula in land-mollusks. Proc. Aead. 2Vat. Sci. Philad., 1893: 388-400, pls. 10-11. Wu, S"K. 1965. Studies of the radulae of Taiwan muricid gastropods, Bull. Inst. Zool. Academ'ia Sinica, 4: 95-106.

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328 VENUS ： Vol．43， No ．4 （1984 ）

Explanation of Plates 1冒2．

Plate 1． Scanning electron micrographs of the radulae of Cronia margariticola ． （All scale barsr100 m ） ’ μ ウ ネ レ イ シ ダ マ シ の の 歯 舌 走 査 電 顕 写 貞 。 ． 1 526478AduIt Inale an anterodorsal view ， ．成 厂1，雄 ， 前 背 而 図

ditto a side ． ， view lp亅E ， 側 面 図 「 D Adult female view ， an anteroClorsal ，成 貝 ，雌 ，前 背 面 図

a ditto， side view ．同 上 ， 側 面 図

Young male an anterodorsal view ． ， 幼 貝 ， 雄 ，前 背 面 図 Young female ， an antercdorsal view ．幼 貝 ，雌 ，前 背 面 図

Plate 2． Scanning elec 亡ron micrographs of an antercdorsal view of the radulae of ＝ Morula musiva ．（All scale bars 100 μ m ） シ マ レ イ シ ダ マ シ の 歯 舌 の 走 査 竃 顕 写 真 。 − 1 2． Adult male ．成 只 ，雄 ，前 背 而 図 − 3 4 ． Adult female ．成 只 ，雌 ，前背 面 図 5 ． Young male ．幼 貝 ，雄 ，前 背 面 図

： 67 ． YOung female ．幼 只，雌， ij i∫ H 面 図

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Fujioka: SexuallyDimorphic Radulaein Muricidae 329

Plate 1

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330 VENUS:Vol. 43, No. 4 (1984)

PIate 2

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