Claw Transformation and Regeneration in Adult Snapping Shrimp: Test of the Inhibition Hypothesis for Maintaining Bilateral Asymmetry

9 Pages·1997·3.8 MB·English

by  A T Read

Reference:Bio/. Bull. 193:401-409. (December. 1997)
Claw Transformation and Regeneration in Adult
Snapping Shrimp: Test of the Inhibition Hypothesis
for Maintaining Bilateral Asymmetry
A. T. READ AND C. K. GOVIND*
LifeSciences Division, University ofToronto atScarborough, 1265 Military Trail.
Scarborough, Ontario, Canada MIC 1A4
Abstract. Inthepairedasymmetricclawsofadultsnap- shrimpisgovernedbyastrongintrinsiclateralizingmech-
ping shrimp, Alpheus heterochelis, the minor, or pincer, anismin whichthe snapperclaw inhibitsthepincerfrom
claw may transform intoamajor, orsnapper, claw ifthe advancing to another snapper.
existing snapper claw is damaged or lost, implying that
anintact snapperclawnormally inhibitsthecontralateral Introduction
pincerclawfromadvancingtoasnapper.Wefindthatthe
pincer-to-snapper advancement in external form occurs Among crustaceans, bilateral asymmetry of the first
almost immediately afterthe snapper is lost even as late pairofchelipeds is common; one ofthe paired claws is
asthepremoltstage.Thetransformingclawinturn inhib- more enlarged and elaborate (majorclaw) than the other
its the newly regenerating pincerclaw from becoming a (minorclaw). In snapping shrimps ofthe family Alphei-
snapper, but if the dactyl of the transforming claw is dae,the major, orsnapper,claw isalmostaslargeasthe
cut, then snapper-based inhibition is removed and the abdomen and has ahammeron the moveabledactyl that
contralateral claw mayregenerate as a snapper, resulting fits into areciprocal socketon the fixedpollex (Fig. 1A)
inshrimpwithpairedsnapperclaws.However,damaging (Przibram, 1901). Theclosingactionofthehammerinto
anestablished snapperclaw will notallow anothersnap- the socket is made with such tremendous force that it is
per claw to regenerate at the pincer site, implying that accompaniedbyaloudpoppingsoundandajet-expulsion
lessinhibitionisrequiredtorestrictanewlyregenerating ofwater, bothofwhich are used in agonistic encounters
claw to a pincer than to arrest an existing pincer claw. (HazlettandWinn, 1962;Ritzmann, 1974)orincrushing
Inhibitionmaybemanifestedlargely intermsofquantity bivalve shells(McLaughlin, 1982).Theminor,orpincer,
ofinnervation. Hencethegreaterinnervationofthesnap- claw is much smaller and is used in burrowing and
persideoverthepincersidewouldinhibitthepincerside, feeding.
accounting for the regeneration of paired claws in their An unusual feature of claw bilateral asymmetry in
previousconfigurationfollowinglossofbothclaws.Loss snappingshrimpistheabilitytoreverseitsconfiguration.
ofthepairedclawsintwoconsecutivemoltsretardstheir Loss ofthe snapper early in an intermolt results in the
development so that both claws often appearas pincers, transformationofthepincertoasnapperandtheregenera-
but in succeeding molts one usually differentiates into a tion ofa new pincer at the snapper site at the next molt
snapper and bilateral asymmetry is restored. In contrast, (Przibram, 1901;Wilson, 1903). Inadditiontolossofthe
shrimpwithpairedsnapperclawsretainthisconfiguration snapper claw, less drastic measures such as its denerva-
over several molts unless one or both of the claws are tion(MellonandStephens, 1978),dactylotomy(Readand
lost; inthatcase,regenerationrestoresbilateral asymme- Govind, 1997),orclosermuscletenotomy(Govindetal,
try.Thus,bilateralasymmetryofthepairedclawsofadult 1988) are also sufficient to triggertransformation ofthe
pincerintoasnapper.Becausetheexistingsnapperclaws
R*eAcuetihvoerdto13whMoamyc1o9r9r7e;spaocncdeepntceeds1h9ouAludgbuestadd1r9e9s7s.ed.E-mail:govind rpeopsasierssthpeamisreeldvessn,apspuecrhclparwosc.edSuirnecseptrhoedsuecmeansihpruilmaptitohnast
@scar.utoronto.ca that induce pincer-to-snapper transformation involve
401
402 A T. READ AND C.K. GOVIND
Figure 1. Adult snappingshrimpshowingdifferentconfigurationsoftheirpairedclaws. (AlPristine
asymmetricconfigurationinwhichthesnapper(rightclaw)isextremelyhypertrophied,withapronounced
hammerand socket, and the pincer (leftclaw) is small, slender, and lacks the snapping apparatus. (B)
Newlyregeneratedpairedpincerclawswhicharesmallerthantheirpristinecounterparts.(C)Pairedsnapper
clawswithnewlyregeneratedsnapper(rightclaw)anddactyl-lesstransformedpincer(leftclaw).(D)Newly
regeneratedpairedsnapperclawswhicharemuchsmallerandnotashighlydifferentiatedastheirpristine
counterparts.(E)Pristinepairedsnapperclawsinwhichthepairedclawsaresimilarinsizeanddifferentia-
tion.Scalebar 10mm. X2.5
damage to the nervous system ofthe snapperclaw, it is asanarrestedsnapper.Wehaveadoptedthisschemewith
likelythatthetransformationresultsfromlossoftheneu- the assumption that the inhibitory signal has a neural
ral inhibition by which the snapper claw prevents the basis.Thatassumptionlargelyrestsonthefactthatcutting
pincerclawfromcompletingitsdevelopmenttoasnapper thenerveinthesnapperclawisenoughtotriggertransfor-
(Wilson, 1903). Because regenerating claws in adult mation ofthe pincer to a snapper. The inhibitory signal
shrimp pass through a distinct pincerlike stage before may therefore be easily manipulated with minor surgery
differentiatingintoasnapperclaw(Wilson, 1903;Darby. of the claws. Here we describe experimental manipula-
1934:ReadandGovind, 1997),wehavedevisedascheme tionsdesignedtoexploresomeramificationsoftheinhibi-
in which loss ofthe snapperclaw removes its inhibition tion hypothesis for claw bilateral asymmetry in adult
on the pincer, which then advances to a snapper and in snapping shrimp. Ourfindings support theexistenceofa
turn inhibits the newly regenerating claw to a pincer. lateralizing mechanism that is based on inhibition from
Snapper-based inhibition of the pincer claw can also thesnapperclaworfromitssiteandcanswitchfromone
explainwhylossofthepincerclawinadultshrimpresults side to the other.
in the regeneration ofanotherpincer(Wilson, 1903). To
explain the fact that simultaneous loss ofboth claws re- Materials and Methods
sults in claw regeneration in the same configuration, we
would have to assume that snapper-based inhibition pre- Adult snapping shrimp, Alpheus heterochelis, ofboth
vailsevenintheabsenceofclaws.Thus,nomatterwhich sexeswerecollectedatlowtidesoffthecoastofBeaufort,
claw is lost, inhibition by the snapper claw (or its site) NorthCarolina,andtransportedtoScarborough.Ontario,
on the pincer claw (or its site) ensures regeneration of where they were held inthe laboratoryat roomtempera-
bilateral asymmetry. A simple and economical scheme ture, 23C. The shrimp were housed in 25-liter glass
forexplaining how bilateral asymmetry is maintained in aquaria partitioned into 12 compartments with plastic
the face ofclaw loss and regeneration in adult snapping screening(Youngetal., 1994) oneshrimppercompart-
shrimpisthatofWilson (1903).whoregardedthepincer ment.Theywerefedat2-3dayintervalswithaspecially
CLAW BILATERAL ASYMMETRY 403
prepared diet consisting ofa mixture offish, beefheart, was 10.theequivalentofapristinesnapper;theminimum
carrots, and commercial trout chow. Under these condi- was 0, equivalent to a pristine pincer.
tionstheshrimpshadanintermoltperiodbetween 19and
26days,foranaverageof23days.Adetailedmolthistory Results
wmaansipkuelpattfioornseawcehraeniemaarln,edanodutin1moorst2cdaasyessaefxtpeerreicmdeynstiasl. Hcltaww hluetereinmotvheedi?ntermoltcan inhibition to thepincer
All animals were allowed to molt twice before being
selected forstudy in orderto ensure that the claws were In most previous studies the snapper was removed a
fully differentiated (Read and Govind, 1991). Several dayortwoaftertheshrimpmolted,thusprovidingenough
typesofexperimentalmanipulationsweremade.Thesim- time for transformation and regeneration of claws that,
plestonewastoinducetheanimaltoautotomizeitsclaw by the next molt, both claws appeared at an advanced
after ecdysis by gently pinching the limb with forceps state. Can inhibition be removed later in the intermolt,
justdistal totheautotomy plane. Formore complex ma- evenperhapsaslateasthepremoltstage?Snapperswere
nipulations,theshrimpwerefirstanesthetizedbycooling; autotomized in 37 shrimps at various stages ofthe molt
then, either the dactyl ofthe snapperclaw was cut close cycle, as determined by measuring epidermal retraction
to its attachment to the claw so that most of it was re- and setal development in the pleopods (Aiken, 1973);
moved, or the nerve in the snapper claw was sectioned. pleopod stages may range from (intermolt) to 5.5 (late
The latter was accomplished by cutting a small flap of premolt). Followingecdysis,thecontralateralclawswere
cuticle in the ventral sideofthe merus andpullingnerve examinedandatransformationindex,representingdegree
2 (the largeroftwo) so that it broke more proximally at of transformation from pincer to snapper (Fig. 2). was
the autotomy plane. Aftera substantial length ofnerve 2 calculated on a scale of (the equivalent of a pristine
had been removed, the cuticular flap was replaced and pincer) to 10 (a pristine snapper). The results indicated
the shrimp were treated with a wide-spectrum antibiotic thattheeffectofa snapperautotomyon thecontralateral
(Paragon) for the next two days. Limb immobilization pincer was not restricted to the intermolt stage or even
was achievedby anesthetizing the shrimp, then applying theearlypremoltstage.Whendonemidwayorevenfairly
cyanoacrylate glue to the thoroughly dried converging late into the molt cycle i.e., as late as pleopod stage
edges ofthe propus and dactyl. Experimental manipula- 4.0 the pincer in some cases showed clear signs ot
tions were usually performed 1-2days aftera molt, and transformation (Fig. 3). There was a gradation in the ef-
theexperimentalshrimpwereobservedoverthenexttwo fect that was related to the time in premolt when the
to three intermolts. snapper was autotomized and the interval between the
Fibercompositionoftheclawclosermusclewasdeter- snapperautotomy and the ensuing ecdysis.
minedbyobtainingfrozencross-sectionsoftheclawsand Inaddition, for9 animals the transformingpincerwas
stainingthesehistochemicallysothatstandardtechniques selected for histochemical analysis, to characterize the
for detecting myofibrillar ATPase activity (Ogonowski fibertypeoftheclosermuscle.Intheseshrimp,thesnap-
and Lang. 1979) could be applied. perhadbeenautotomizedatpleopodstagesrangingfrom
To evaluate the degree of transformation ofpincer to to 3.5, and the transforming pincers had a transforma-
snapperclaw,atransformationindex,basedonfivemor- tion index ranging from 2.5 to 6.0. All 9 animals still
phological features, each representing a graded measure retained the band of fast muscle that is unique to the
ofaunique snappercharacteristic, wasdevised. The first pincer(Govindetal, 1986), although inmostanimals it
feature,baseduponthepresenceanddevelopmentofthe had begun to degenerate (Fig. 4). Muscle degeneration
plunger and socket, was given a weighting of 4 points signifies that transformation had begun even in late pre-
anddeterminedbycomparingthetransformingclawwith molt shrimp, suggesting that snapper-based inhibition is
astandard developmental series similarto that in Figure removedalmostimmediatelyandisunrelatedtothestage
2. Forexample, a transforming claw with no plunger or ofthe moltcycle.
socket (similar to a pristine pincer) would be assigned
points, while one withaplungerand socket (similarto As an intact transforming claw necessary to limit claw
apristinesnapper)wouldbeallotted4points.Thesecond regeneration to apinceratthe contralateral
feature, stoutness, was weighed at 3points and based snappersite?
upontheratioofpropuslengthtowidth.Thesmallerthe
ratio, the stouter and more snapper-like the claw, and Wehave shownpreviously that denervating the trans-
hence the more points allocated. The last three features forming pincer claw will allow regeneration ofa pincer
were weighted at 1 point each and based upon the pres- or a snapper claw at the contralateral snapper site; the
ence and development or abundance of the transverse appearanceofasnapperclawimpliesthelossofsnapper-
groove, tubercles, and plumose setae (Read and Govind, basedinhibition(Youngetal., 1994).Topursuethisidea
1991).Themaximumpossibletransformationindexscore further, we wondered whetherdactylotomy alone would
404 A. T READ AND C.K. GOVIND
8'
O
to
E
C
ra
B
<aD.
41
09
<D
2345
01
.V
J>,
Pleopod stage in premolt shrimps
Figure 3. Relationship between pleopod stage in premolt shrimp
subjectedtosnapperautotomyandpincer-to-snappertransformationin-
dex defined by as pristine pincer and 10 as pristine snapper. The
degree to which the pincer transforms is linearly related (regression
line)tothestageatwhichthesnapperisremovedinpremoltshrimps.
JV= 37.r = 0.577(P< 0.005).slopeb= -1.162.
into a snapper; i.e., it hypertrophied and developed a
D dactyl socket, despite lacking a dactyl (Table la). In 7 ofthose
10shrimp,asnapperregeneratedonthecontralateralside,
transverse groove resulting in a symmetrical animal, albeit one snapper
lacked a dactyl (Fig. 1C) and the regenerated snapper
propus often did not grow to pristineproportions. In the other3
shrimp,apincerregeneratedatthe snappersite,resulting
in a reversal of asymmetry. Thus pincer dactylotomy.
mimicking a mild form of denervation, appeared to be
sufficient to remove snapper-based inhibition of the re-
generating claw.
Can snapper-basedinhibition ofa regeneratingpincer
claw he removed?
pollex
tbwyoFaisgehularemectme2ed.rst(Aaagreprsroiw(sB)t.inaCen)dpiisnntococekraetpcrli(asdwtoiun(beAl)senpaarrporpgoerwre)sc,sliaavwetlr(yaDnldsevcveheraslreoapcgitrneorgoivzveei,da sannoaLtpohpseesrroprfiesnttcrheierctpcsilrnaecwgeerpnrecerlsaautwimoarnbelsauytlttbsheicsiansuitstheeettorheeagceponinentrcraeatrlia(otWneirlao-lf
and hypertrophy ofthe entire claw. The slender pristine pincerclaw son, 1903). Is it possible to remove this snapper-based
(A) acquires all the snapper features after the first molt (B). and in inhibitionbydamaging thesnapperwhen regenerationis
subsequentmolts(C.D)becomeshypertrophiedwithfurtheraccentua- taking place at the pincersite?Wetested thispossibility
tionofthesnapperfeatures.Scalebar3mm. x6 byremovingthepincerclawandatthesametimecutting
the snapper dactyl. These procedures were successfully
be equally effective. Ten animals were successfully ma- accomplishedinnineshrimp(TableIb).Atthenextmolt,
nipulated so that the snapper was autotomized and the anormal-appearingpincerregeneratedattheoriginalpin-
pincerdactylotomized.Inallcases,thepincertransformed cer site in these shrimp. On the contralateral side the
CLAW BILATERAL ASYMMETRY 405
sidebeweakenedtoallowregenerationofasecondsnap-
peronthecontralateralside?Wethereforetriedremoving
both snapper and pincer claws at a time when snapper-
basedinhibitionmightnotbewellestablished.Thiswould
bethecaseimmediatelyafterreversalofasymmetrywhen
the newly transformed snapper claw is not as elaborate
nor as hypertrophied as a pristine snapper, nor is the
newly regenerated pincer claw as well differentiated as
thepristinepincer(Przibram, 1901:Wilson, 1903).Forty-
sevenanimalsweresuccessfullysnapper-autotomizedand
atthenextmolt,followingareversalofasymmetry, sub-
jected to a paired autotomy. Relative to the most recent
configuration, thelocationofthe pincerand snapperwas
maintained in 33 and reversed in 14 animals (Table Id).
Despite reversal of bilateral asymmetry, snapper-based
Figure4. (A)Cross-sectionofapristinepincerclawinwhichthe inhibition remained intact.
closermuscle,stainedhistochemicallyformyofibrillarATPase.shows We next tried snapper dactylotomy as a means of in-
acharacteristiccentralbandoffastfibers(dark-staining)flankedbyslow ducing pincer transformation. Out of 21 animals. 11
afibsenrasp(pleirghti-nstwahiinicnhg)f.as(tB)fiCbreorsss-(sdeacrtkiosntaoifniangp)inicnerthcelacwenttrraalnsbfaonrdminhgavteo showedtransformationofthepincerintoasnapperwhile
degenerated, while the flanking slow fibers(light staining)are intact the damaged snapper repaired itself, resulting in shrimp
Scalebar 1 mm. Xl5 withpairedsnapperclaws. Followingpairedautotomyin
these 11 animals.8regeneratedlimbstomimictheorigi-
nal asymmetric configuration, but 3 regenerated double
intact snappershowedlittle regeneration ofits cut dactyl snappers (Table le). Both the regenerated snapperclaws
for at least two subsequent molts. Snapper dactylotomy
didnotinduceregenerationofasnapperclawatthepincer
site. Tahlt 1
In an earlier experiment, transecting nerve 2 in the
tsrnaanpspfeorrcmliangwaptitnhceeroldclsanwapppeerrmsiittet,eidmprleygienngertahteiornemoovfala irCneognnefiniregiruoarittasitoimnoannnitopfuuplnaacitrieo/ndbscoltahwssitienssiinmpr/etsinpto;nssehrimpfollowing
ofsnapper-based inhibition (Young etal., 1994). There-
fore, we transected nerve 2 from the autotomy plane to Claw
mid-merus in the pristine snapperclaw and autotomized configuration
the pincer claw at the same time. The procedure was Snapper Pincer
successfulin 15shrimp(TableIc).Noneoftheseanimals Manipulation site site # %
regenerated a snapper: 7 hadregenerated apincerby the
endofthe firstmoltcycle, 12 hadapincerby theendof a. Pincerdactylotomywithsnapper
the second molt cycle, and all had regenerated a pincer autotomy psinnacpeprer ssnnaappppeerr 37 3700
bytheendofthethirdmoltcycle. In 11 oftheseanimals, h. Pincerautotomywithsnapper
snapper function was restored, but to varying degrees: 4 dactylotomy snapper pincer 9 100
could open and close the dactyl but not snap, 5 could c. Pincerautotomywithsnapper
snap weakly, and 3 could snap with moderate force. In denervation snapper pincer 15 100
mofostthecafsiersst,smnoalptpecyrcfleu.nctTihounsbesgnaapnpreertudrennienrgvanteiaronthfeaielnedd d. pSaniarpepderauatuottootmoymyatatsefcirosntdmomlotl;t spinnacpeprer psinnacpeprer 3134 7300
to bring about regeneration of another snapper claw at e. Snapperdactylotomyatfirstmolt;
the pincersite. pairedautotomyatsecondmolt snapper pincer 8 73
snapper snapper 3 21
Can snapper-side inhibition be weakenedtopermit f. Paauitroetdomayutaottsoemcyonatdfmiroslttmolt;paired snapper pincer 46 96
regeneration ofa snapperatthepincersite? pincer snapper 1 2
When both snapperand pincer claws are removed si- g. Pairedautotomyatfirstmolt;paired snapper snapper 1 2
multaneously, regeneration at these sites is ofa similar autotomyatsecondmoltand
type claw (Przibram, 1901), suggesting that snapper- snapper-sidelimbbudimmobilized
bsansaepdperinchliabiwt.ioCnanisthpirsesiennhtibietvieonn pirnestehnetaobnsetnhceesnoafpptehre atthirdmolt ssnnaappppeerr psinnacpeprer 226 7219
406 A T READ AND C.K. GOVIND
were relatively small in comparison to the animal (Fig. Can snapper-s\innietr\ be maintainedfollowing
ID),althoughthelimbontheoriginalsnappersidetended claw loss?
ptfioirnstcbeedresmslioitngehsttwlriyatthliaoarngnereoxftthaaanntsnsiatnpsappceporuenrctlecarlpwaawrrte.ogneTnhteihresatoiwpnapgsosaitttheea ctiloaTnwhsseabbgoyeuntreertgahetenieosrtnaatboiilfointaydoufolftathssihesrciuomnnupdsuwsainltahpcpoepnradiirrteaiidosne.ssnTaqhpuepesese-r
side.
snapper-symmetric shrimp retained their symmetry fol-
lowingasubsequentmolt,atwhichtimethesecondsnap-
Can snapper-side inhibition sun-ire successive perassumedmorepristineproportions(Fig. IE).Symme-
cUi\v loss? trywasretainedinfiveshrimpthatunderwentthreesubse-
quent molts, showing that once snapper-symmetry is
A newly regenerated limb is usually smaller than a established,itisretainedthroughlatermolts.Ontheother
pristine limb, suggesting that the regenerating limb is in hand, if the second snapper is removed, as was done
an immature state, whichcould probably be exaggerated in three shrimp after they had molted once, the shrimp
with asecond round ofregeneration. Underthesecondi- regeneratedapincerinitsplace.Evenifthesecondsnap-
tions, does snapper-side inhibition still prevail? A group per was removed after three molts, as was done in two
of48shrimpweresuccessfullysubjectedtotwoconsecu- shrimp, a pincerregenerated in its place. Finally, in five
tive paired autotomies of their claws and the returning shrimp with similar-sized paired snapperclaws, removal
asymmetry observed (Fig. 5; Table If). After the paired ofbothclawsresultedintheregenerationofpairedasym-
clawshadregeneratedforthefirsttime, 75%wereasym- metric claws in the original configuration. Clearly, the
metric, although they wereconsiderably smallerthanthe snapper-symmetric condition is relatively stable but not
pristineclaw:theremainderwerenomoreadvancedthan permanent, aslossofoneorbothclawsallows regenera-
stage 5 limb buds or stage 6 pincers (Govind and Read, tion ofasymmetric claws.
1994). After they had regenerated for the second time,
they were even smaller. Only 34% showed slight asym- Discussion
metry, and 66% resembled stage 5 limb buds or stage 6 Ourexperimentsproducedpairedregeneratedclawsin
pincers (Fig. IB). Over the next two molts, however, the usual asymmetric configuration ofpincer/snapper as
these pincer-symmetric shrimp reverted to the original wellas,inafewcases,intheunusualsymmetricconfigu-
snapper/pincerconfiguration. Of48 animals,46regener- rations ofpincer/pincer or snapper/snapper. The pincer-
ated limbs to the original configuration, one experienced symmetricconditionappearstobeephemeral,becausein
a reversal, and one regenerated paired snappers. These succeeding molts, given adequate time fordevelopment,
results show that snapper-based inhibition usually per- oneoftheclawsbecomesasnapper.Incontrast,thesnap-
sisted through at leasttwo successive paired autotomies, per-symmetric condition can be maintained overseveral
althoughoccasionallyitcouldbereversedorevenabsent. molts, assuming arelatively permanentstate. Indeed, we
Asshownintheaboveexperiment,aftertwosuccessive have maintained snapper-symmetric shrimp for five
pairedautotomiesthenewlyregeneratedclawsweresmall molts, providing there is no loss of or damage to the
and the snapperwas not highly differentiated. Occasion- claws (Pearce and Govind, 1987). Loss of one or both
ally only the faintest trace of a snapper for example, clawsinthesesnapper-symmetricshrimpimmediatelyre-
a poorly developed hammer was seen, and in overall stores the asymmetric configuration of the paired claws
dimension the claw was more like a pincer. To test the uponregeneration.Theobservationsthatthe pincer-sym-
possibility that manipulation of the snapper-side limb. metricconditionisephemeralandthesnapper-symmetric
even in this relatively immature condition, could reverse conditionismore stable tendtosupporttheviewthatthe
clawasymmetry,weperformedthefollowingexperiment. pincerrepresentsastage in the developmentofthesnap-
Agroupof28 shrimp were subjected totwoconsecutive per, with the final condition ofclaw regeneration being
paired autotomies, then the snapper-side limb bud was that ofa snapper (Wilson, 1903; Darby. 1934).
gluedshut,effectivelyrestrictingmovementofthedactyl With this developmental sequence in mind, lateraliza-
ofthisregenerating limb (Fig. 5;Table Ig). Most (22)of tion ofthe paired claws would be easily achieved ifthe
these animals regenerated claws resembling the original snapperclaworitsputativesitearrestedthedevelopment
configuration. However, six animals regenerated paired ofthe contralateral claw to a pincer state. Although the
snappers,showingthatthelimbbudatthepincersitewas natureoftheinhibitory mechanismisnotknown,consid-
capable of developing into a snapper. When compared erable evidence suggests that it has a neural basis and
to the previous experiment in which regeneration was that inhibition is removed most readily with loss ofthe
assessedaftertwosuccessivepairedautotomies,thenum- entire claw but also with nerve transection (Mellon and
berofsnapper-symmetric shrimps in the present experi- Stephens. 1978), closermuscle tenotomy (Govind etai.
ment proved to be significant (X2 -= 5.612, P < 0.02). 1988),ordactylotomy(ReadandGovind. 1997);inother
CLAW BILATERAL ASYMMETRY 407
molt molt molt molt
|
i
Pristine Pairedautotomy Pairedautotomy Regenerate
snapper
pmcer
Pristine Pairedautotomy Pairedautotomy Regenerate,
immobilizesnapper
sidelimb
snapper
pmcer
Y
Figure5. Pictorial representation ofthe paired asymmetric snapper(large circle) and pincer(small
circle)clawsofadultshrimpintwoexperiments.Intheupperseriestheshrimpsunderwenttwosuccessive
pairedautotomiesandregeneratedpairedpincerlikeclawsafterthethirdmolt:afterthefourthmolt,these
claws differentiated into snapper/pincer claws in the pristine configuration. The same experiment was
repeated inthelowerseries,butafterthethird molt thesnapper-sidelimbwasimmobilized(shaded); at
thefourthmolt,pairedclawsappearedin thepristinesnapper/pmcerconfigurationbutalsoinasnapper/
snapperconfiguration.
words, with some damage to the nervous system. Once Elimination of the inhibitory influence should allow the
theinhibitionisremovedthepincercontinuesitsdevelop- regenerating claw to develop into a snapper, and this was
ment to a snapper, even as late in the intermolt as the thecaseinshrimpinwhichremovingtheclosermusclein
premolt stage, when the new exoskeleton is being laid the transforming claw or transecting its nerve 2 allowed
down. These changesinvolvetheexoskeletonandcloser regenerationofasnapperclawontheoppositeside(Young
musclecomposition,whereasthemotorinnervation(Ste- etai, 1994).Wenowreportthatsimplycuttingthedactylof
phensandMellon, 1979;QuigleyandMellon, 1984;Mel- thetransformingclawissufficienttoeliminateitsinhibitory
lonetai, 1981),sensoryinnervation(GovindandPearce, influence and permit regeneration of a snapper claw, re-
1988), and vascularization (Guchardi andGovind, 1990) sultinginshrimpwithpairedsnapperclaws.Thefirstsnap-
occur later. per arises because the pincer, released from its inhibition
The next step in the lateralizing mechanism is for the bysnapperautotomy,continues itsdevelopmenttoasnap-
claw advancing from the pincer to the snapper stage to per. The second snapperarises because,owing todactylo-
exertan inhibitory influence onthe regeneratingcontralat- tomy ofthe transforming snapper, the newly regenerating
eral claw and hold its development to the pincer stage. claw is not restrictedto apincerstage.
408 A. T. READ AND C.K. GOVIND
Underthisschemethepincerwouldalsoadvancetothe the form ofthe removal ofthe snapper-based inhibition
snapperstageifthesnapper-basedinhibitionwasremoved ofthecontralateralpincerclaw. A useful analogy forthe
withoutlossofthe snapperclaw. Weknowthatthishap- lateralizing mechanism is that ofa seesaw in which the
pens readily when neural input is reduced in the snapper beam, balanced on its fulcrum, can assume one of two
clawandtheexistingpincercontinuesitsdevelopmentto inclined positions but rarely a horizontal position.
a snapper, resulting in shrimp with paired snapperclaws Our assumption that the snapper-based inhibition of
(MellonandStephens, 1978:Govindettil., 1988). Along the pincer claw is neural in origin gained some support
similarlines isourearlier finding thatcutting nerve 2 of fromourfindingthatrestrictingdactyl movementsofthe
thetransformingclaw(Youngetai, 1994)orourpresent regenerating snapper claw permitted the regenerating
findingthatcuttingoffthedactylofthetransformingclaw claw on the opposite side to develop as a snapper. In
allows the regeneration of another snapper claw on the these few cases, interfering with reflex activity of the
opposite side. But when these same surgeries were per- regenerating snapperclaw was sufficient toremove inhi-
formed on a pristine snapper claw and the pincer claw bition of the pincer claw. Differences in reflex activity
wasautotomizedatthesametime,anotherpincerregener- betweenthetwosideswerealsoresponsiblefordetermin-
ated in its place. In this case the presence ofa pristine, ingclawbilateralasymmetryindevelopinglobsters(Gov-
although damaged, snapper was sufficient to inhibit re- indandPearce, 1986).Insnapping shrimp,differencesin
generation to a pincer stage, but the presence ofa dam- reflex activity are subserved by differences in numbers
agedtransforming snapperwas not. Bearingin mindthat ofaxons to the paired claws (Govind and Pearce, 1988).
the snapperclaw has almost twice as many axons as its
pincercounterpart in the pristine condition (Govind and Acknowledgments
Pfeoarrmcien,gc19l8a8w),resitultissilnikaelmyutchhatgrdeaactteyrlroetdoumcytioonfoafatrxaonnss- We thank W.D. Kirby-Smith for generous supplies
compared to the opposite side than does dactylotomy of ofsnapping shrimps and Rosalind Coulthard and Joanne
thepristinesnapperclaw.Moreover,becausedactylotomy Pearce forcritical comments. Financial support was pro-
affects similar structures in the transforming or pristine vided by Natural Sciences and Engineering Research
snapperclaw,theresultscastdoubtonthepossibilitythat Council ofCanada.
qualitative aspects ofthe innervation are responsible for
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