Japanese Journal of Ichthyology Vol.32,No.41986

魚 類 学 雑 誌32巻4号1986年

Early Development of Fin-supports and Fin-rays in the Milkfish

Chanos chanos

Yasuhiko Taki,Hiroshi Kohno and Shiro Hara

(Received July 10,1985)

Abstract Development of fin-supports and fin-rays was observed in larval and juvenile Chanos chanos.Chondrification of the caudal complex started at 4.70mm SL.Ossification of the caudal elements started at 7.80mm SL and was nearly completed at about 30mm SL.Carti-laginous fusion of caudal elements,which occurs in hypurals of higher teleostean fishes but is not seen in lower teleosts,was observed between the neural arch of the preural centrum 1 and that of the ural centrum 1 via a small cartilage bridging the distal tips of the two arches.Caudal fin-rays began to develop at 6.60mm SL,and an adult complement ,of principal rays was attained at 7.35mm SL.Dorsal and anal pterygiophore elements were first evident at 6.70mm and 6.65mm SL,respectively.All proximal radials were formed at 8.15mm SL in both fins.Formation of dorsal and anal fin-rays started simultaneously at 8.60mm SL,and adult fin-ray complements were attained at 10.00mm and 10.70mm SL,respectively.In the pectoral fin,the cleithrum,coraco-scapular cartilage and blade-like cartilage(fin plate)had already been formed at 4.65mm SL.The mesocoracoid was observed to originate from the coraco-scapular cartilage and become detached from it in the course of ossification.Pectoral fin-ray formation started at 13.80mm SL and was completed in number of rays at 20.00mm SL.In the pelvic fin,the basipterygium was first evident at 13.00mm SL.Pelvic fin-rays appeared at 13.80mm SL and attained their adult count at 17.15mm SL.

Since Rabor(1938)published a comprehensive

description of the skeletal system of milkfish,

Chanos chanos(Forsskal),detailed information has been presented on vertebrae and associated

bones,caudal skeleton,and cranium(e.g.Rosen and Greenwood,1970;Buri and Motoh,1980).

As to the early development of the skeletal system,however,little has been known despite the recent

success in the spawning of the fish which has led to the elucidation of its early life history(Vanstone

et al.,1977;Chaudhuri et al.,1978;Liao et al.,1979).

In June 1983,milkfish spawners,grown from artificially inseminated eggs from wild-caught adults,spawned spontaneously at the Aquacul-ture Department of the Southeast Asian Fisheries Development Center(SEAFDEC)based in the Philippines and enabled us to observe early osteo-logical development of this species.In this paper we describe the development of fin-supports and fin-rays in larval and juvenile Chanos chanos,as a

part of our study on the functional development of the milkfish.

Material and methods

The study material includes the following lots of specimens of Chanos chanos:1)64 larval speci-mens,4.60-12.10mm in standard length(SL),

grown from eggs spawned spontaneously at the Igang Substation of the SEAFDEC Aquaculture Department,Igang,Iloilo,Philippines,on 12 June 1983,and hatched and reared at the Tigbauan Research Station of the Department in Iloilo,sampled from the second to 21st days after hatching;2)40 larval and juvenile specimens,11.20-31.80mm SL,grown from wild-caught larvae(collected at Tigbauan on 11 June 1983)at the Tigbauan Research Station,sampled from the first to 33rd days of rearing;3)5 juvenile speci-mens,30.00-69.80mm SL,collected from a milk-fish culture pond in Iloilo in 1984;and 4)1 adoles-cent specimen,262.0mm SL,collected at Oton Fish Market,Oton,Iloilo,on 8 March 1985. Of these,78 specimens,4.60-69.80mm SL,were stained with both alcian blue and alizarin red following the method of Dingerkus and Uhler

(1977)for observations on fin-support develop-ment,and 32 specimens,5.45-27.00mm SL and

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魚 類 学雑 誌Japan.J.Ichthyol.32(4),1986

A

B

C

D

E

F

G

Fig.1.Early development of caudal fin-supports in Chanos chanos.A,5.40mm SL;B,6.65mm SL;C,

7.35mm SL;D,8.35mm SL;E,11.00mm-SL;F,13.00mm SL;G,20.00mm SL.Stippled:car-

tilage;red:bone.ep,epural;hl-6,hypurals 1-6;hs,haemal spine;npu 1,neural arch of preural

centrum 1;ns,neural spine;nu 1,neural arch of ural centrum 1;ph,parhypural;rc,radial cartilage;

spc,small piece of cartilage at notochord end;un 1-2,uroneurals 1-2.Scale bars:0.2mm.

262.0mm SL,were stained with alizarin red ac-cording to the procedures described by Taylor

(1967)for observations on fin-ray development and for supplementary observations on bony development.

The SL of all specimens is given as fresh state.The SL of specimens smaller than 15.00mm SL

was measured under a binocular microscope with an ocular micrometer,and that of larger speci-

mens with a dial caliper,read to the nearest 0.05 mm in all cases.Counts for fin-rays of the paired

fins were made on the left side.Drawnings were

prepared with the aid of camera lucida.Ter-minology followed mainly Kohno and Taki(1983).

Observations

Growth discrepancy.As in other fishes,dis-

crepancies in growth has been noticed between wild and reared specimens of Chanos chanos(Liao et al.,1979).Comparative observations were therefore made on the stage of osteological de-velopment between specimens hatched and reared in the laboratory and those reared in the laboratory from wild-caught larvae in a size range from 11.20 mm SL(smallest wild-caught/reared specimen)to 12.10mm SL(largest laboratory-hatched speci-men).In 10 laboratory-hatched and 13 wild-caught/reared specimens falling within this size range,there was no noticeable discrepancy so far as in the characters examined in this study.These two lots of specimens were therefore treated as a continuous series.

Caudal fin-supports and fin-rays.The caudal

complex of Chanos chanos consists of the urostyle

(urostyle proper plus neural arch of the preural

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Taki et al.:Milkfish Fin Development

centrum 1 and that of the ural centrum 1,and uroneural 1),neural spines of the preural centra

2 and 3,epural,uroneural 2,hypurals 1-6,par-hypural,and haemal spines of the preural centra

2 and 3.

The smallest specimen examined in this study,

4.60mm SL,had a notochord straight at its

posterior end with no other element of the caudal

complex.The largest specimen retaining this

state of development was 5.00mm SL.The

notochord began to flex at 6.60mm SL,and

flexion was completed by about 7mm SL.

The first visible element of the caudal complex other than the notochord was a cartilaginous bud

of the hypural 1,which was observed in a 4.70mm

SL specimen.The hypural 2 was first seen as a cartilaginous bud at 5.00mm SL,but it was not

discernible in a 5.45mm SL specimen.A cartila-

ginous bud of the hypural 3 and a weakly stained cartilaginous bud of the parhypural were added at 5.15mm SL.This state of development was

seen up to 6.10mm SL(Fig.1A)except for the

above-mentioned 5.45mm SL specimen and a 5.55mm SL specimen which had the hypurals 1

and 2 only.At 6.25mm SL a cartilaginous bud of the hypural 4 and that of the haemal spine of

future preural centrum 2 became evident.The hypural 5 first appeared in a 6.65mm SL speci-

men,in which buds of the neural spines of future

preural centra 2 and 3,the haemal spine of future

preural centrum 3 and the neural arch of the preural centrum 1(neural arch of the last preural centrum of Nybelin,1973 ;specialized neural

arch of Potthoff,1975)were also observed(Fig.1B).At 7.60mm SL the neural arch of the ural

centrum 1(second neural arch of the first ural centrum of Nybelin,1973;specialized neural arch

of Potthoff,1975)was formed.The epural was first evident as a cylindrical cartilage in another 6.70mm SL specimen.No change in develop-

mental stage was noticed over 7.35mm SL(Fig.

1C).The hypural 6 appeared at 7.45mm SL.

Ossification of caudal skeleton components started at 7.80mm SL in the hypurals 1-3.At 8.15mm SL the parhypural began to ossify,and a small piece of cartilage was formed attaching to the already flexed notochord end.By 9.40mm SL a small cartilage had been formed attaching to the distal tip of the neural arch of the ural centrum 1(Fig.1D).Ossification of the hypurals 4 and 5 was evident at 9.50mm and 11.00mm SL,

respectively.In this 11.00mm SL specimen a radial cartilage appeared at the distal end of the heamal spine of the preural centrum 2(Fig.1 E).At 11.50mm SL the small cartilage attaching to the distal tip of the neural arch of the ural centrum 1 was elongated anteriorly,extending to the neural arch of the preural centrum 1,and fused with the neural arches of the preural centrum 1 and ural centrum 1 forming a short cartilaginous bridge

parallel to the notochord.Specimens up to 12.70 mm SL showed a similar state of development.By 13.00mm SL ossification had started in the neural and haemal spines of the preural centra 2 and 3 and in the small cartilage now bridging the neural arches of the preural centrum 1 and ural centrum 1(Fig.1F).The uroneural 1 was first

perceived at 13.80mm SL as a paired needle-like bone attaching to either side of future urostyle.At 15.70mm SL the hypural 6 and epural began to ossify.At 16.35mm SL the uroneural 2 became discernible,dorsally to the uroneural 1,on each side of the urostyle near its hind margin.The neural arch of the preural centrum 1 and that of the ural centrum 1 began to fuse with the urostyle at 20.00mm SL,at which fusion of the uroneural 1 with the urostyle was also observed(Fig.1G).Between 20.00mm and 30.00mm SL,the radial cartilage of the haemal spine of the preural centrum 2 became enlarged,extending anteriorly to the tip of the haemal spine of the preural centrum 3.In a size range of 30.00-41.60mm SL,ossification was completed in all caudal elements except for the small cartilage attaching to the notochord end and the distal ends of the neural spines,epural,hypurals 1-6,parhypural and haemal spines.With growth the small cartilage at the notochord end reduced its size and,in 68.80mm and 69.80 mm SL specimens,was only palely stained with alcian blue.

Because of poor alizarin red stain of bones,

particularly vertebrae,in the material presently

examined,we refrain from giving descriptions of

developmental process of the centra composing

the caudal complex.Mention is made here only

of our observations that ossification of centra in

the vertebral column started by 8.90mm SL and

all centra were stained by 11.70mm SL.

The first discernible caudal fin-rays were 11

(5+6)principal rays at 6.60mm SL.An adult complement of 10+9 principal rays was attained

at 7.35mm SL.Secondary fin-rays first ap-

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魚 類 学 雑誌Japan.J.Ichthyol.32(4),1986

A B C

D

E F

Fig.2.Early development of dorsal fin-supports in Chanos chanos.A,6.70mm SL;B,7.80mm SL;C, 8.35mm SL;D,8.60mm SL;E,10.50mm SL;F,24.60mm SL (anteriormost pterygiophore).

Stippled:cartilage;red:bone.dr,distal radial;fr,fin-ray;pr,proximal radial;rp,rudimentary

pterygiophore;st,stay.Scale bars:0.1mm.

peared at 10.40mm SL in both lobes of the fin,

and the number of secondary rays increased with

growth.

Dorsal and anal fin-supports and fin-rays.Dorsal

pterygiophore elements first appeared were five cylindrical cartilaginous buds of proximal radials in one of 6.70mm SL specimens(Fig.2A).These

radials were located along the posterior portion of future dorsal fin,but their exact positions relative

to vertebrae were not determinable.At 7.80mm SL proximal radials had increased to nine in num-

ber,situated between the 12th and 15th neural spines from the rear,and weakly stained carti-

laginous balls,i.e.distal radials,were evident above the fourth to eighth proximal radials(Fig.2B).

All proximal radials,11 in number,were formed

A B C

D E

Fig.3.Early development of anal fin-supports in

Chanos chanos.A,6.70mm SL;B,7.80mm

SL;C,8.35mm SL;D,8.60mm SL;E,10.50

mm SL.Stippled:cartilage.dr,distal

radial;pr,proximal radial;st,stay.Scale

bars:0.1mm.

at 8.15mm SL between the 12th and 17th neural spines from the rear,accompanied by a cartilag-

inous distal radial above each of them excepting the anteriormost two(Fig.2C).At 8.60mm SL

a distal radial was formed at the second pterygio-

phore and the stay became evident posterior to the last pterygiophore(Fig.2D).A weakly stained rudimentary cartilage appeared anterior

to the first pterygiophore at 10.50mm SL(Fig.2E).This rudimentary pterygiophore did not

always possess a distal radial and became fused with the first pterygiophore as development went

on(Fig.2F).Ossification of dorsal pterygiophores was first

noticed at 16.35mm SL in all proximal radials at

their central portions and in the stay at its anterior-

most part.Middle radials began to ossify and

became identifiable in posterior four pterygio-

phores at 17.15mm SL and in posterior five at

18.55mm SL.At 30.00mm SL and larger middle

radials developed in posterior eight pterygio-

phores.

Dorsal fin-rays were first evident at 8.60mm SL,

at which seven rays were associated with the fourth to 10th pterygiophores.The last double

fin-ray became perceivable at 9.00mm SL.Adult complement of 13-17 rays(Nelson,1984)were

seen in specimens of 10.00mm SL and larger.Excepting the first pterygiophore which supports

one or more rays,each pterygiophore was as-sociated with a single corresponding fin-ray.

Anal ptergyiophores were perceived earlier than

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Taki et al.:Milkfish Fin Development

A

B

C D

E

Fig.4.Early development of pectoral fin-supports in Chanos chanos.A,4.65mm SL;B,7.35mm SL(ventral view);C,9.00mm SL;D,12.70mm SL;E,20.00mm SL(inner side of a part of left girdle).

Stippled:cartilage;red: bone.ac,actinost(blade-like cartilage);cl,cleithrum;cs,coraco-scapular cartilage;mc,mesocoracoid;pp,propterygium;scb,small cartilaginous block at tips of cleithra;sf,

scapular foramen.In E,outer surface of actinosts is ossifying.Scale bars:0.1mm.

dorsal ones.The first anal components were four cylindrical cartilage,i.e.proximal radials,

located from immediately behind the anus at 6.65mm SL.The number of proximal radials

had increased by one at 6.70mm SL(Fig.3A),at which the five radials were situated between the

ninth and 11th haemal spines from the rear.At 7.80mm SL there were six proximal radials,of

which the second,third and fourth had a distal radial(Fig.3B).Proximal radials reached their

adult count of seven at 8.15mm SL,each of them excepting the first being accompanied by a carti-

laginous distal radial(Fig.3C for a 8.35mm SL specimen).The stay was evident at 8.60mm SL

(Fig.3D).At 10.00mm SL a rudimentary prox-imal radial was added anterior to the first prox-

imal radial.By 10.50mm SL the first distal radial had been formed,and the rudimentary

proximal radial and the first proximal radial had fused together at their proximal ends(Fig.3E).

This fusion was seen constantly in all larger specimens.The rudimentary proximal radial did

not always accompany a distal radial.

All anal proximal radials began to ossify at

16.35mm SL as in the dorsal.Middle radials

became perceivable in posterior two pterygio-

phores at 17.15mm SL and in posterior three at

18.55mm SL.Middle radials were present con-

stantly in posterior four pterygiophores in speci-

mens larger than 35.50mm SL.

Anal fin-rays became discernible simultaneously

with dorsal rays at 8.60mm SL;at this size five rays were formed associated with middle five

pterygiophores.Adult counts of 9-11 rays (Nelson,1984)were observed at 10.70mm SL and larger,two or more rays being associated with the

first pterygiophore.Pectoral fin-supports and fin-rays.The smallest

specimen examined for pectoral fin development,4.65mm SL,possessed a pectoral girdle composed

of a rod-shaped bony cleithrum,a coraco-scapular cartilage with a dorsal and a posterior process,

and a blade-like cartilage(fin plate of Swinnerton,1905)(Fig.4A).A small foramen was present in the lower central part of the coraco-scapular

cartilage.A similar state of development was

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魚 類学 雑 誌Japan.J.Ichthyol,32(4),1986

A B

C

Fig.5.Early development of pelvic fin-supports in Cahnos chano.A,13.00mm SL;B,15.70mm SL;C,18.55mm SL.Stippled:cartilage;red:bone.bp,basipterygium;dr,distal radial.Scale bars:0.05mm (A) and 0.1mm (B and C).

seen over 5.55mm SL.At 5.75mm SL the blade-like cartilage was creviced at its center.With growth the dorsal process of the coraco-scapular cartilage became enlarged both length-wise and crosswise to form a braod plate.A small cartilaginous block was formed at the junc-tion of the ventral tips of the left and right cleithra at 7.35mm SL(Fig.4B).This block remained cartilaginous over 41.60mm SL.At 9.00mm SL another foramen(scapular foramen)appeared in the coraco-scapular cartilage dorsally to the first foramen(Fig.4C).At this stage the crevice in the blade-like cartilage was elongated,but the elongation was not to the extent to cut the prox-imal margin of the cartilage.A second crevice,elliptical in shape and located dorsally to the first crevice,was formed in the blade-like cartilage at 11.00mm SL,and a third crevice,also ellipitcal and situated ventrally to the first,was formed at 11.40mm SL.After this stage,a small projec-tion developed and became separated gradually from the blade-like cartilage(Fig.4D for a 12.70 SL specimen),and at 13.00mm SL,the projec-tion was finally detached completely from the cartilage and formed into the propterygium(Jessen,1972,1973).At 13.80mm SL a cartilaginous swelling (future mesocoracoid)began to develop on the inner surface of the coraco-scapular cartilage.The bony posttemporal and supra-cleithrum were first evident at 14.90mm SL,at which the blade-like cartilage splitted into upper and lower halves which correspond to the actinosts 1+2 and 3+4,respectively.At this stage ossi-fication started in the upper(future scapula)and lower(future coracoid)parts of the coraco-scapular cartilage,the swelling on the inner side of the coraco-scapular cartilage developed into a bridge-

like structure,and a distal radial was formed.The bridge-like structure on the coraco-scapular

cartilage began to ossify to develop into the meso-

coracoid at 15.70mm SL.At 17.15mm SL the upper half of the blade-like cartilage splitted com-

pletely into actinosts 1 and 2,and the actinost 1 began to ossify.There were three distal radials at this stage.Separation of actinosts 3 and 4 was first observed at 18.55mm SL at which ossifica-

tion started in the actinost 2 and there were four distal radials.Inner view of the pectoral girdle

at 20.00mm SL is shown in Fig.4E.The ac-tinosts 3 and 4 began to ossify at 23.20mm SL

and 24.60mm SL,respectively.At 30.00mm SL the coraco-scapular cartilage was separated into the fully ossified coracoid and scapula,and the

ossification of the bridge-shaped mesocoracoid

was completed leaving both ends of the bridge

unossified,resulting in the separation of the mesocoracoid from the coracoid and scapula.The mesocoracoid was now connected with the coracoid and scapula via connective tissues.The

propterygium began to ossify at 51.60mm SL.Pectoral fin-rays,three in number,were first

perceived at 13.80mm SL.Fin-rays developed

from the dorsal part to downward.An adult

complement of 14 rays was attained at 20.00mm

SL.

Pelvic fin-supports and fin-rays.A 13.00mm

SL specimen was the smallest possessing the cartilaginous basipterygium,which was triangular

in shape with an anterior and a posterior process

(Fig.5A).At 14.90mm SL the anterior process was elongated,and two distal radials were formed

on the posterior process.A small foramen de-veloped in the posterior process at 15.70mm SL

(Fig.5B).Ossification of the basipterygium

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Taki et al.:Milkfish Fin Development

started in its center at 17.15 mm SL.At 18.55 mm SL and larger there was a third,relatively large distal radial at the tip of the posterior process

(Fig.5C).Pelvic fin-rays,four in number,were first evident

at 13.80 mm SL.An adult complement of nine

rays was reached at 17.15 mm SL.

Discussion

Cartilaginous fusion is not seen in the caudal skeleton of lower teleostean fishes such as clupeids

(Houde et al.,1974)and engraulids(Kohno and Taki,1983).In higher teleosts,it occurs in hypurals in various ways(cf.Kohno and Taki,

1983;Kohno et al.,1983,1984).During the

development of the caudal fin-supports of Chanos chanos,we observed cartilaginous fusion not in

the hypurals but between the neural arch of the

preural centrum 1 and that of the ural centrum 1 via a small cartilage bridging the two arches.Further comparative observations in other groups

of fishes are required to determine the phylo-

genetic significance of the cartilaginous fusion in Chanos chanos.

In the dorsal and anal fin-supports,the forma-tion of proximal radials preceded that of the corresponding distal radials,and the buds of

proximal and distal radials were rather widely separated from each other.This observation is in support of Kohno and Taki's(1983)view that the distal and proximal radials of dorsal and anal

pterygiophores have their own cartilaginous centers.The anteriormost dorsal and anal ptery-

giophores,which support two or more rays(in secondary association),each originated from two

pieces of cartilage as indicated by Kendall(1976).The stay was observed to develop from its own cartilaginous center,not from the last proximal radial.This is in agreement with Kohno and Taki(1983),though whether or not the stay is a vestigial pterygiophore was not determinable in this study.

The propterygium of the pectoral girdle was

observed to be derived from the blade-like cartilage

as in engraulids and salmonids(cf.Kohno and Taki,1983).It was also revealed that the meso-

coracoid originates from the coraco-scapular

cartilage and becomes independent of it in the course of ossification.

Acknowledgments

We wish to thank Mrs.Clarissa L.Marte,SEAFDEC Aquaculture Department,Philippines,for providing Chanos chanos eggs for this study,and Mrs.Marietta Duray and Miss Doris Bargarinao of the same institute for technical as-sistance.Our thanks also go to Mr.Kiyoshi Fujita,Tokyo University of Fisheries,for his valuable suggestions on caudal osteology.

This is contribution No.162 of the SEAFDEC

Aquaculture Department.

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(YT: Tokyo University of Fisheries, 4-5-7, Konan, Minato-ku, Tokyo 108, Japan, formerly concurrently at SEAFDEC Aquaculture Department; HK and SH: Aquaculture Department, Southeast Asian Fisheries Development Center. (SEAFDEC), P.O. Box 256, Iloilo City, Philippines)

サバヒーの鰭支持骨と鰭条の初期発達

多紀保彦・河野 博・原 士郎

サ バ ヒー 仔 稚魚 の 鰭支 持 骨 と鰭 条 の 発 達 過 程 を記 載 し

た.尾 骨 の形 成 は体 長4.70mmで 始 ま り,30mm前 後 で

骨 化 が ほ ぼ 完 了 した.尾 鰭 椎 前 脊椎1と 尾 鰭 椎1の あい

だ で,小 軟 骨 片 を介 在 して軟 骨 癒 合 が観 察 され た。 主 尾

鰭 条は 体 長6.60mmで 形 成 が始 ま り,7.35mmで 定 数

に達 した.背 ・轡 鰭 の担 鰭 骨 は そ れ ぞ れ体 長6.70mm,

6.65mmか ら形成 され,近 担 鰭 骨 の全 教 は と も に8.15

mmで 出現 し た.鰭 条 は両 鰭 と も に体 長8.60mmか ら

形 成 を 開始 し,そ れ ぞれ10.00mm,10.70mmで 定 教 に

達 した.胸 鰭 で は,体 長4.65mmで 擬 鎖骨 ・烏 口=肩

甲軟 骨 ・射 出軟 骨 板 がす で に形 成 を始 め て い た 。 中 烏 口

骨 は 烏 口=肩 甲軟 骨 よ り派 生 し,骨 化 の 過 程 で 分 離 独 立

す る こ とが観 察 され た.鰭 条 は体 長13.80mmか ら出 現

し,20.00mmで 定 数 に達 した.腹 鰭 の 基担 鰭 骨 は体 長

13.00mmで 初 め て認 め られ た.鰭 条 は体 長13.80mm

よ り出 現 し,17.15mmで 定 数 に達 した.

(多紀:108東 京都港区港南4-5-7東 京水産 大学;河

野 ・原:フ ィリピン イロイ ロ市 東南 アジア漁業開発

セ ンター養殖部局)

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