Early development of preleptocephalus larvae of the Japanese eel in captivity with special reference to the organs for larval feeding

Early development of preleptocephalus larvae of the Japanese eel in captivity with special reference to the organs for larval feeding

著者 Yoshimatsu Takao journal or

publication title

三重大学大学院生物資源学研究科紀要

volume 37

page range 11‑18

year 2011‑02‑01

その他のタイトル ウナギプレレプトケファルス幼生の形態変化と摂餌

器官の発達

URL http://hdl.handle.net/10076/11533

Introduction

TheJapaneseeel

Anguillajaponicai

snatural l ydi stri butedi ntheFarEastAsi aandacommerci al l y i mportantspeci es.Thei ntensi vecommerci alcul tureofeelwasi ni ti atedi nJapanmorethanonehundred yearsago,andnowadaystheeelcul turei sani mportanti ndustrynotonl yi nJapanbutal soi nTai wanand Chi na

^1,2,3）

.Inspi teofi tsl argeaquacul tureproducti on,thecul tureofJapaneseeelsti l lcompl etel yrel yon thef i el d- col l ectedwi l dgl asseel sasseedl i ngs,buttheannualcatchofgl asseel si sreportedtobef l uctuati ng wi th aconti nuousl ydecreasi ngtrend caused byvari ousreasons

^2）

. Theref oretheestabl i shmentof producti ontechni quesofgl asseeli shi ghl ydesi redamongf i sh- f armers.

Thef i rstcapti ve- bredgl asseelwasproducedi ntheNati onalResearchInsti tuteofAquacul ture

（NRIA）

i nJapanon2002usi ngaki ndofpastyl i qui d- f orm di etmadef rom eggsofspi nydogf i sh

（Squalusacanthias）

asani ni ti alf eed

^4,5）

.However,thereari ngtechni quesf ormassproducti onhavenotbeenestabl i shedsof ar i nspi teoftheef f ortsbymanyresearchers.Thesurvi valofeell arvaeduri ngi tsearl ydevel opmentalstage i ncapti vi tyi sveryl owtodate.Themai nprobl emassoci atedwi ththedevel opmentofseedl i ngproducti on oftheeeli sthei ni ti alf eedusedf orthel arvi cul tureduri ngtheearl ydevel opmentalstagesoftheeel .The

February，2011

EarlyDevelopmentofPreleptocephalus LarvaeoftheJapaneseEelinCaptivitywith SpecialReferencetotheOrgansforLarvalFeeding

TakaoY

OSHIMATSU^＊

GraduateSchoolofBioresources,MieUniversity,Tsu,Mie514-8507

Abstract

ToinvestigatetheunavailabilityofBrachionusrotifersasaninitialfoodforrearingpreleptocephalus larvaeoftheJapaneseeel,theontogeneticmorphologicalchangesduringtheearlydevelopmentalstagesof eelwithspecialreferencetotheorgansforlarvalfeedingwerestudiedincaptivity.

Fertilizedeggswereobtainedfrom maturedadultsinducedbyhormonaltreatments.Theeggswere incubatedat21℃and25℃in33-34psuseawater,andthenewlyhatchedlarvaewerereareduntilthe completionofyolk-absorption.Hatchingoflarvaebegan45.0hafterthefertilizationat21℃and31.5h at25℃,andterminatedwithin3.5to4.0haftertheonsetofhatching.Theabsorptionofyolkcompleted on10dphat21℃（211.7day-degrees）and8dphat25℃（201.5day-degrees）,andsimultaneouslyfour pairsofcharacteristicsharp-pointedlarvalteethfullydevelopedonbothofupperandlowerjaws.Aseries ofhistologicalsectionsofoesophagealpartfrom 6dphto10dphspecimenshowedthateellarvaehada characteristicthickertissuelayerandwell-developedcircularmusclessurroundinglarvalgulletwithout mucouscells.

KeyWords:Japaneseeel,larvalrearing,earlydevelopment,oesophagus,mucouscell

2010年7月14日受理

＊ Forcorrespondence（e-mail:[email protected]）

TakaoYOSHIMATSU

mostcruci alconstrai ntwoul dbethatprel eptocephal usl arvaeoftheJapaneseeelcoul dnotberai sedby f eedi ng

Brachionus

roti f ers,unl i keothermanymari netel eostsl arvaerearedi nhatcheri es

^4-8）

.

Inthi sresearchtheearl ydevel opmentofprel eptocephal usl arvaeoftheJapaneseeelwi thspeci al ref erencetothel arvalf eedi ngorgansandthei rf uncti onswerestudi edtocl ari f ythereasonwhyroti f ers werenotavai l abl ef orthel arvalreari ngoftheJapaneseeel .

MaterialsandMethods

Thel arvalreari ngoftheJapaneseeelwasconductedusi ngthef aci l i ti esofNRIA i n2004- 2007.

Ferti l i zedeggswereobtai nedbyhandstri ppi ngmaturedadul tsi nducedbyhormonaltreatmentsusi nga so- cal l edNRIAprotocol

^9,10）

.Af terseparati on,thef l oati ng

（f

erti l i zed

）

eggswerestockedtopol ycarbonate cyl i ndri caltanksandi ncubatedunti lhatchi ngattwotemperaturesof21

℃（21.

0- 21. 5

℃,2004-

2006

）

and 25

℃（25.

0- 25. 1

℃,2006-

2007

）

i n32- 34psusand- f i l teredseawater.Thedayl arvaehatchedwasdef i ned as0dph

（daypost-

hatchi ng

）

. Thenewl y- hatchedl arvaewererearedi nadarkcondi ti onunti lthe compl eti onofyol k- absorpti onunderthetwodi f f erenttemperatures.

Theobservati onofembryoni candl arvaldevel opmentunti lthecompl eti onofyol k- absorpti onwas conductedunderami croscopeusi ngl i vespeci mensf ol l owi ngthebodysi zemeasurements

（n=10dai

l y, mean

±SD）

af teranestheti zedwi thasmal lamountoftri cai nemethanesul f onate

（MS-

222

）

.Larvaewere preservedi n5% f ormal i nsol uti onandhaematoxyl i n- eosi nstai nedsecti ons

（thi

ckness:6

μ

m

）were

suppl i edf ortheobservati onofl arvalmorphol ogyandhi stol ogi calstudi esofthedi gesti vesystemsofeel l arvae. Inaddi ti ontothat,ascanni ngel ectronmi croscopi c

（SEM;JSM-

6320F,JEOLLtd. ,Tokyo

）

observati onwasmadeonl arvalteethdevel opmentaf terf i xati onwi thgl utaral dehyde- tanni caci dand post- f i xati oni nosmi um tetroxi deusi ng25dphsampl esof2006wi thf ul l ydevel opedl arvalteethrearedon pastyl i qui d- f orm di et.

Results

Thedi ameterofhand- stri ppedeggsf rom maturedf emal eswas0. 92

±0.

05mm at21

℃

and0. 98

±0.

03 mm at25

℃.Af

terthef ormati onoftheperi vi tel l i nespace,theeggdi ameterreachedi tsmaxi mum si zeof about1. 6- 1. 7mm i n3. 5hat21

℃

and2. 5hat25

℃.Thenormal

l y- devel opi ngeggsoftheJapaneseeel weretransparent,non- adhesi ve,f ree,pel agi candspheri cali nshapewi thacharacteri sti cwi deperi vi tel l i ne space.Ini ti aldi spersedsmal loi lgl obul esgradual l ycl usteredandeventual l yf ormedasi ngl el argeoi l gl obul eduri ngthel ateembryoni cstage. Theeggsexhi bi ted atypi calmerobl asti ccl eavageaf ter i nsemi nati on.Thesequenceofeventsthatoccurredduri ngembryoni cdevel opmentwasal mosti denti cal tothatoftheprevi ousreportsontheJapaneseeel

^6,7）

andthoseofmari netel eostswi thpel agi ceggs

^11,12）

.

Morphol ogi calchangesoftheearl ydevel opmentalstageoftheprel eptocephal usl arvaeof

Anguilla japonica

wereshowni nFi g.1.Hatchi ngofeggsbegan45. 0haf terthef erti l i zati onat21

℃

and31. 5hat 25

℃,andtermi

natedwi thi n3. 5to4. 0haf tertheonsetofhatchi ng.Themeantotall ength

（TL）of

newl y- hatchedeell arvaewas2. 99

±0.

07mm at21

℃

and2. 75

±0.

08mm at25

℃.Theemergentl

arvae hadanel ongatedpear- shapedyol ksacandasi ngl eoi l - gl obul esi tuatedattheanteri orpartofyol ksac.

Ini ti alrowsofbranchedmel anophoresontheti pofcaudalparti ncreasedi tsnumberandeventual l y

f ormedaf an- shapedstrongbatchofmel anophoreswi thl arvalgrowth.On6to7dphatabout7mm TL,

thecharacteri sti coval - shapedeyenearl ycompl etedi tspi gmentati on.On10dphat21

℃（6.

82

±0.

16mm

TL,211. 7day- degrees

）

and8dphat25

℃（7.

13

±0.

14mmTL,201. 5day- degrees

）

,mostl arvaecompl eted

12

Fig.1 Anguillajaponicapreleptocephalirearedinthelaboratoryat25℃:A,0dph,2.75

±0.08mm TL;B,1dph,4.96±0.14mm TL;C,2dph,5.96±0.22mm TL;D, 3dph,6.58±0.14mm TL;E,4dph,6.63±0.11mm TL;F,5dph,6.90±0.19 mm TL;G,6dph,7.18±0.13mm TL;H,7dph,7.26±0.08mm TL;I,8dph, 7.13±0.14mm TL.

TakaoYOSHIMATSU

theabsorpti onofyol k- sac.Atthi sstageeell arvaeshowedastrongnegati vephototaxi sandhadatendency togatherandswi m aroundatthebottom ofthereari ngtankwhenl i ghtwason.Larvaeal sohadwel l devel opedl argeaudi toryvesi cl esasshowni nFi g.1G- I.Af terthecompl eti onofyol kabsorpti on,eel l arvaestartedtodi e,andby11dphacompl eteanni hi l ati onofl arvaeoccurredat25

℃

wi thoutf eedi ng.

Oneofthebi ggestmorphol ogi calcharacteri sti csofprel eptocephal usl arvaeofeeli ssharpandpoi nted l arvalteeth,i .e.f ourpai rsonbothupperandl owerj awsthatappearedat7to8dph

（Fi

g.1H,I

）

. Rudi mentall arvaltoothanl agenf i rstl yappearedontheventralsi deofl arvalj awsandtheygradual l y shi f tedtotheti pofsnoutwi thgrowth.Whenl arvaecompl etedyol kabsorpti on,thei rl arvalteethwere f ul l ydevel opedwi thsharp- poi ntedti p,andthej awmovementbecamemoreacti vethani tsprevi ousstage.

UnderaSEM observati onf ortheti pofl arvalteeth

（Fi

g.2

）

,i twascl ari f i edthatthesel arvalteethhad pl aneandsmoothsurf acewi thoutanyspeci alstructuresonthem.Inthecapti vecondi ti oneell arvae someti mesexhi bi tedS- shapedbodyposi ti oni nwatercol umnbutnevershowedacti veandspontaneous attacki ngbehavi ourtowardpreyusi ngthel ongpoi ntedl arvalteeth.Aseri esofl ongi tudi nalhi stol ogi cal secti onsoftheoesophagealpartf rom6dphto10dphspeci menat25

℃（Fi

g.3

）

showedthateell arvaehad thi ckerti ssuel ayerati nsi deoftheepi thel i um andwel l - devel opedci rcul armuscl essurroundi ngthepart ofl arvalgul l et.Anotherhi stol ogi calcharacteri sti cduri ngthi sstageofeell arvaewasthatmucouscel l sdi d notdevel opyetonthei nsi deofthel arvalgul l etf rom 6dph

（twodaysbef

orethecompl eti onofyol k

）

ti l l 10dph

（twodaysaf

terthecompl eti onofyol kabsorpti on

）

undercapti vecondi ti onat25

℃.

Discussion

Thei nf ormati ononthebi ol ogyoftheearl yl arvalstagesofthepresentspeci esi ntheoceani svery l i mi ted

^13,14）

.Recentl ythespawni nggroundoftheJapaneseeelwassuccessf ul l ypi n- poi ntedtobeneara seamount

（theSuruga Seamount）

,westoftheMari ana Isl andsi n theWestern Paci f i cby f i el d researchers

^15）

.Neverthel essnaturalpreysthatprel eptocephal usstageeell arvaei ngesti nthei rnurseryarea oftheopenoceanaresti l lnotcl ari f i edyet.Theref ore,atthemoment,wehavetodevel opl arvalf eedsf or reari ngeell arvaebytri alanderrorwi thoutusef uli nf ormati onf rom thei rnaturalf eedi nghabi tand behavi our.

Mari ne

Brachionus

roti f ersarethemosti mportantandi ni ti alf oodi tem f orproduci ngmari nef i shl arvae usedcommonl yal lovertheworl d

^16,17）

.Theuseofcul turedroti f ersasani ni ti alf oodsubstanti al l yreal i zed themassproducti onofmari nef i shl arvaei nhatchery

^18）

.Tanaka

etal

.

^19）

f i rstreportedthatsomeofthe eell arvaei ngestedroti f ersundercapti vecondi ti onsandKurokawa

etal

.

^20）

showedthateell arvaeabsorbed protei nsderi ved f rom roti f ersi nto thei rl arvali ntesti ne,by usi ng i mmuno- hi stol ogi calmethods.

Neverthel esssuccessf ulreari ngofeell arvaebyf eedi ngroti f erasani ni ti alf oodhasnotbeenachi evedyet,

asmosteell arvaedi dnei theracceptnori ngestroti f erseventhoughasuf f i ci entnumberofroti f erswas

gi veni nvari ousways

^8）

.Insomeoccasi ons,roti f erscoul dbef oundi ntheoralcavi tyofeell arvaeaf ter

bei ngf edroti f ers.Al thougheell arvaenevershowedacti vef eedi ngbehavi ourtof eedi temsbyusi ngthei r

l arvalteeth,theysometi mestakei nroti f ersnon- spontaneousl ythroughhal f - cl osedmouthatthebottom

l ayerofthetankwhenf edwi throti f ers.Eventhel arvaewerei nveryheal thyandacti vecondi ti on,they

j ustkeptonhol di ngroti f ersi nthei roralcavi tywi thoutswal l owi ngandsendi ngroti f erstol arvali ntesti ne

wi thperi stal si s

^8）

.Fewyearsl ateraf terthef i rstreportonthei ngesti onofroti f erbyeell arvae,Tanaka

et al

.

^4,5）

successf ul l yproducedthef i rstcapti ve- bredgl asseelnotbyf eedi ngroti f ersbutapastyl i qui d- di etf or

morethan250days.Vari ouski ndsofsol i df oodi temsweretestedwi thoutanysuccess,

i.e.zoopl

ankton,

phytopl ankton,mi cro- f ormul ateddi etsf ormari nef i sh,f i sheggs,etc.

^4,5）

bef oretheyf oundtheavai l abi l i ty

14

ofthi spastyl i qui d- di etf oreell arvae.Atthepresentmoment,onl ythesof tandpastyl i qui d- f orm di et madef rom spi nydogf i sheggi sappl i cabl ef orl arvalreari ngofeeli ncapti vi ty.Ontheotherhand,there havebeennoreportsonthesuccessf ulreari ngofeell arvaebyf eedi ngroti f erssof ar

^8）

.

Tanaka

^21）

studi edthedi f f erencei nstructureandf uncti onofdi gesti vesystemofthemanytel eostsl arvae.

Inthatstudyheshowedthati nmostspeci esmucouscel l sappearedi ntheepi thel i um bytheti meof f eedi ngcommencementandthosemucouscel l sstartedtosecretePAS- posi ti vemateri al s

（i.e

.muco- pol ysacchari des

）

i ntothel umeni mmedi atel yaf terthei rappearance.Accordi ngtotheresul tsobtai nedi n thepresentstudy,notl i kemanytel eostsl arvae,nomucouscel l sappearedonthei nsi deofthegul l etwhen eell arvaecompl etedyol kabsorpti onandwereexposedtwodaysstarvati on.Muco- pol ysacchari def rom mucouscel l sprobabl yf aci l i tatestheswal l owi ngandtransf erri ngofthef oodparti cl estof ore- gutaf terthe i ngesti on.Inaddi ti ontothat,i twasshowni nthepresentstudythatl arvaehadathi ckerti ssuel ayerat thei nsi deofepi thel i um andwel l - devel opedci rcul armuscl esi nthei roesophagusasshowni nFi g.3.Thi s wel l - devel opedci rcul armuscl essurroundi ngl arvalgul l etmi ghthavearol etocontrolthepassageoff ood i temsbychangi ngi tsopeni ng- si zeofthegul l et.From thesenewf i ndi ngs,i tmi ghtbepossi bl etomakea rati onalexpl anati onwhywecoul dnotrai seeell arvaebyf eedi ngroti f ersasf ol l ows.Ifeell arvaeweref ed onsol i d- f ormf oodsi ncl udi ngroti f ers,theycoul dnotswal l owthemeventheparti cl esi zewasenoughsmal l topassgul l etduetothecharacteri sti cstructurethateell arvaepossessed;namel y,verythi ckti ssuel ayer i nl arvalgul l etwi thoutmucouscel l sandwel l - devel opedci rcul armuscl esurroundi ngi t.

Thei nf ormati onontheearl yl arvalstageofthi sf i shi ssti l lscanty.Wehavetoaccumul atemore i nf ormati onontheearl ydevel opmentalstagesofeell arvaef rom bothofwi l dandcapti ve- bredsampl esto establ i shtheproducti ontechnol ogyofgrasseel s.Buttheresul tsobtai nedi nthepresentresearchwoul d becomeani mportantcl uetodevel opnovelreari ngf eedsf orl arvaeoftheJapaneseeeli nf uture.

Acknowledgements

Theauthori si ndebtedtoDrs.H.Tanaka,K.Nomura,andS.K.Ki m ofNRIA f orthei rval uabl e advi cesandprovi di ngf erti l i zedeggstoconductthereari ngexperi ment.Thanksareal soextendedtoDr.

M.N.D.Khanf orhi scri ti calreadi ngandusef ulsuggesti ons.Thi sstudywasconductedunderaresearch proj ectsponsoredbytheAgri cul ture,ForestryandFi sheri esResearchCounci lofJapan

（Proj

ectresearch

Fig.2 ScanningElectronMicroscope（SEM）photomicrographofthefirstpairof larvalteeth（upperjaw）ofAnguillajaponicapreleptocephalusat25dph.Bar indicates10μm.x500.

TakaoYOSHIMATSU

16

Fig.3 Photomicrographsofhistologicalsections（longitudinal）foroesophagealpartof Anguillajaponicapreleptocephalifrom 6dphto10dph.Barsindicate10μm:

oe,oesophagus;in,intestine;no,notochord;og,oilglobule;yo,yolk;cc,cardiac cavity;he,heart.

onthedevelopmentofseedlingproductiontechnologiesoftheJapaneseeelandlobster）.

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TakaoYOSHIMATSU

18

ウナギプレレプトケファルス幼生の形態変化と摂餌器官の発達

吉松 隆夫

三重大学大学院生物資源学研究科

要 約

ホルモン処理によって人工催熟した親魚から搾出した卵及び精子を人工授精し，その受精卵から孵化 仔魚を得た。孵化仔魚は卵黄吸収が終了するまで，21℃と25℃，塩分32-34，0-数lxの暗条件下で飼育 管理し，日々の連続標本からその外部形態及び摂餌に関わる器官の発達を観察した。孵化仔魚は，水温 21℃では日齢10（211.7℃・日），25℃では日齢8（201.5℃・日）で卵黄吸収を終了し，口には上下顎 それぞれ4対の針状幼歯が特徴的となった。この時期，HE染色された組織切片を見ると，咽頭部に食 道壁の肥大により狭窄した部位が観察され，またその内壁には粘液細胞もほとんど発達しないため，ワ ムシのような固形分の嚥下の困難が想像された。