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AMER. ZOOL., 37:470-481 (1997)

InteractionofC ortisoi ndThyroid Hormone in theLarval Developmentof Pacific Threadfin1B O N G G. KIM AND CHRISTOPHER L. B R O W N2

Hawaii Institute ofMarine Biology, School ofOcean and Earth Science and Technology,University of Hawaii, Box 1346Coconut Island, Kaneohe, Hawaii 96744 USASYNOPSIS. We have conducted three hatchery-scale experiments designed to ex-aminetheactionsandinteractionsofcortisoiandthyroid hormonesin thedevel-opment of a larval marine fish. Survival among controls varied significantlybe-tween the3replicate experim ents. The threadfin Polydactylus sexfilis)consistentlyresponds to 1 hr posthatch immersionin a combination oftriiodothyronineandcortisoi (T3+ F)with accelerated gutdevelopment and increased survival com-pared with untreated controls (C).Survival among larvae treated withT, or Fseparately was significantly improved over controls in one of the three experiments.The frequency ofspinal deformities was reduced bycortisoi treatment, aloneorin combination with T3. Growthdidnot vary with treatm ent, except that variancein larval length was reducedin (T, + F)vs.C in all 3experiments.Ahormone-induced increaseinuniformity could lead to reduced cannibalism, which isaprob-lemin thecultureofthreadnns. These results suggest interactive hormonal regu-lationof developmental processes, working withinthecontextofother biologicalvariables.

INTRODUCTIONA variety of bioactive or potentiallybioactive substances are transferred fromthe maternal circulation into the yolk of

vertebrate eggs. These include hormones,neurohormones , neuro t ransmit te rs , andRNAs which encode growth factors andother compounds (reviewed byBrownandNunez, 1994). The biological importance ofmaternally-derived regulatory compoundsin developmentisstill very muchanincom-plete picture.In teleost fishes, the thyroid hormoneshave received the most attention,inpart be-cause of a perceived potential for applica-tions in fish culture (Higgs et al, 1982;Lam, 1990). Thyroid hormones have beenknown for decades to have stimulatory ef-fectsondeveloping larval and juvenile fish-es (reviewed by Lam, 1985;Brown andBern, 1989). For example, thyroxine (T4)reduced the time required for prehatchingdevelopment in the chum salmon, Onco-

1From the Symposium Developmental Endocrinol-S y f Non-Mammalian Vertebrates presented at theAnnual Meeting of the Society for Integrative andComparative Biology, 26-30 December 1996, at Al-buquerque, NewMexico.2E-m ail: cbrowniniki.soest.hawaii.edu.

rhynchus keta (DalesandHoar, 1954).Cor-tisoi accelerated hatching and thyroxineboth acceleratedandsynchronized hatchingin the steelhead trout, O. mykiss, (Yeoh,1993).Since thepresence of thyroid hormoneswas first reported in fish eggs (Kobukeetal, 1987;Tagawa and Hirano, 1987),T4and triiodothyronine (T3) have been detect-edin theeggsofmore than30species (Ta-gawa et al, 1990; Brown and Nunez,1994). Maternal deposition of thyroidhor-monesinfish eggs occurs againstaconcen-tration gradient, possiblybywayofbindingto vitellogenin (Babin, 1992), althoughnotall data support this hypothesis (reviewedby Specker and Sullivan, 1994).Notonlymay thyroid hormones adheretovitellogen-in, they also have been shown to stimulatevitellogenesis in the guppy, Poecilia reti-culata(Lam and Loy, 1985) and to promotethe uptakeof vitellogenin by trout oocytesin vitro O.mykiss; Shibata et al, 1993).Thus a theoretical casecan bemadefor amechanism by which circulating thyroidhormones could promote their owntrans-port andsequestration into eggs.

Varied patterns of circulating thyroidhormones characterize thetime of ovarian47

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CORTISOL AND T H Y R O I D H O R M O N E IN L A R V A L T H R E A D F IN 471

developmentinfemale fishes, which appearconsistent with ovarian hormone uptake(see Norberg et al., 1989). In general,theT3/T4 ratio is higher in marine fish eggsthan in those of freshwater fishes (Tagawaet al., 1990). Marine fish eggsarealsousu-ally smaller in diameter, produced inlargernumbers, they are more widely dispersed,and embryonic development is more rapid(for details, see Blaxter, 1988). Thus it ispossible that variationsin theconcentrationof T3in fish eggs may help to set an ap-propriate rate of development in r-selectedmarine larvaeand in AT-selected freshwaterspecies.Acyclic increasein theT3/T 4ratioduring oogenesis appears to favor the up-takeofT3by theeggsof Atlantic halibuta marine species (Bjornsson et al, 1997).Peripheral 5'monodeiodinase activityis in-hibited by estradiol in freshwater rainbowtrout, presumably resultingin areductioninthe T3/T4 ratio during oogene sisinthisspe-cies(Cyret al, 1988). These reports invitethe associationof differential hormone pro-duction in marine and freshwater environ-ments, which could account for the rela-tively higher concentrationsofT3inmarinefish eggs. However, not allreports fit thisgeneralization. Clarias batrachus, a fresh-water catfish, exhibitsashiftinthe maternalbalance of circulating thyroid hormonessuggesting increased peripheral 5'monod-eiodination during the time of yolk depo-sition (Sinha and Singh, 1990). To ourknowledge,thehormone contentofClariasspp. eggshas not been determined.Our recent studies have focused on thePacific threadfin Polydactylus sexfilis)as amodel for examination of the actions ofhormones in early development. Thisspe-cies,knownas moi inHawaii, spawnsingroupsofon etothree females together withseveral males.The small marine larvaeofthis species are responsivetoeither yolk en-richment with thyroid hormones,viamater-nal injection, or brief posthatch hormoneimmersions in a dose-dependent fashion(Brown and Nunez, 1994). We have alsofound that an apparent interaction of corti-soland T3has beneficial effects on devel-oping larvae which is consistent with inter-actions of these hormones in a numberofother vertebrate development models

(Brownand Kim,1995).In that study,theeffects of these two hormones were testedindividuallyand in combination, duringthedevelopment of larval threadfins in an ex-perimental marine hatchery. Apositive ef-fect onsurvival wasseen inlarvae receiv-ing the combination treatment, possiblyasa consequence of a hormone-induced ad-vancement of the initiation of digestivefunction.Thepresent study was designedtoexamine further the interactive effects ofcortisol and T3on development of Pacificthreadfins. In the three large-scale experi-ments reported herein, we have monitoredthe growth, development, and survival ofthreadfin larvae as affected by immersionin cortisol,T3, or acom bination ofthetwohormones.

MATERIALS AND METHODSPolydactylus sexfilis eggs were obtainedby natural spawning at the Oceanic Insti-tute, Waimanalo, Hawaii, and transferredwithin 10hours to the Hawaii InstituteofMarine Biology, during the early embry-onic stage. The experiment was repeatedthree times to minimize or eliminate the

batch effect. Experiments 1and 2were con-ducted using eggs spawned on 6 Octoberand 13November 1994,respectively,andExperiment 3 was conducted with eggsspawnedon 12 May 1996. All batchesofeggs were from the same broodfish keptinan earthen pondfed on am ixtureof frozensquid, krill,andartificial pelle ts. Emb ryonicdevelopment indicated that eggs were frommultiple females, and spawns occurredwithin 2-4 hoursin alloccasions.Similar handlingofeggsandrearing con-ditions were provided in all experiments.Eggs were incubated indoors in a 30 literdownwelling chamber suspended in a 250liter tank. Filtered sea water was suppliedtothechamber from aboveat arateof 120liter/hranddrained throughabottom screen(150 u,m mesh) into the tank which wasoutfitted with an overflow drainage. Morethan70% of larvae hatched within 30 minaftertheonsetofhatching,andthe timewasconsidered time 0 (DO).Because the ga-metes used for these experiments camefrom an unknown number of parents, wewere carefulto mix them thoroughly before

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47 B. G. KIM AND C. L. BROWN

distributing them into treatment groups.Newly hatched larvae were transferred to12 liter buckets for the one-hour hormoneimmersion treatments, one bucket per treat-ment. The four treatment groups were: 2.6ppm of triiodothyronine (T3, T), 0.1 ppm ofcortisol (F), combination of both hormonesat same concentrations (TF), and untreatedseawater as a control (C).Following hormone treatment, larvaewere stocked into rearing tanks (250 literfiberglass tank with a conical black bot-tom), three tanks per treatment. The initialstocking density was determined as 36-49larvae/liter for Experiment 1, 19-26 larvae/liter for Experiment 2, and 25-35 larvae/liter for Experiment 3. Tanks were main-tained as a flow-through system with a turn-over rate of 700% tank volume per day (1.2liter/min). The water level was maintainedusing an inside standpipe screened with amesh; 120 urn during a rotifer period, 220and 350 u.m during yolksac Artemia (nau-plii) and one-day-oldArtemia (metanauplii)periods, respectively. Illumination was pro-vided with fluorescent lights at a surface in-tensity of 950 lux. ( lig ht: dark 14:10).Rearing temperature (25.8-28.0 C), salin-ity (32 ppt), and dissolved oxygen concen-tration (6.2 0.3 ppm) were ambient, andwere similar between experiments. The ex-periments were terminated at the onset ofmetam orphosis, between D24 and D2 8. Ex-periment 2 was terminated on D24, prior tometamorphosis, due to high mortality. Earlyin this experiment, water exchange was in-terrupted for two days by a main water pipefailure, and the Artemia feeding scheduledescribed below was not maintained cor-rectly o n days 16 and 17.Rotifers {Brachionus plicatilis) were in-troduced as first food upon initial mouthopening, three days after hatching (D3). Atthis time the rotifer density was maintainedat 20-25 rotifers/ml, and the green algae,Nannochloropsis sp. was added at a con-centration of approximately 5,000 cells/mlto support the nutritional value of rotifers.When larvae were capable of improvedsearching and feeding success, starting onD 5, the rotifer density was reduced to 15-20 rotifers/ml. Yolksac Artemia sp. nauplii(0.35 nauplii/liter) were introduced starting

on D9 for 3 days along with rotifers. Yolk-sac Artemia nauplii were the sole feed onbetween D l l and D13, at concentrations of1-2.1 nauplii/liter. Selco enriched one-day-old Artemia metanauplii were fed tolarvae beginning on D14 at a density of23.5 metanauplii/liter. Feeding was conduct-ed at a frequency of once every 2-3 hoursthroughout the light phase.Approximately 30-50 larvae from eachtank were sampled at 4- to5-dayintervals,and larval growth was determined by mea-suring standard lengths using a Nikon vi-deomicroscopy system. Spontaneous growthrate was calculated from a linear regressionafter logarithmic transformation of standardlengths, and the treatment effect was testedusing a one-way ANOVA comparison. Pos-sible treatment-dependent effects on growthwere tested by comparing sizes on eachsampling day using a one-way AN OVA fol-lowed by Fisher's PLSD comparison whenappropriate to identify differences P

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CORTISOL AND THYROID HORMONE IN LARVAL THREADFIN

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TFFIG. 1. Survival of larval threadfin from hatching to metamorphosis after hornomal treatment at hatching; onehour immersion with T3(T), cortisol (F), combination of T3and cortisol (TF), and no exogenous hormone (C).A, B, and C represent the experiment 1, 2, and 3, respectively. Each bar shows the mean survival for a treatment(n = 3 per treatment) with a standard error of the mean as a vertical line. Results of Fisher's PLSD comparisonwere indicated at 5 confidence level for the experiments 1 and 2, and at 10 level for the experiment 3.

vival was calculated from the difference be-tween the initial number of larvae stockedand the final number remaining at the onsetof metamorphosis. Results were analyzedusing a one-way ANOVA followed byFisher's PLSD comparison. Two tanks (oneof each F and TF groups) from Experiment1 were excluded in survival analysis due tooverflow from clogged screens.

RESULTSSurvival

Differences in survival were seen bothbetween experiments and between treat-

ments, with the extremes ranging from 11to 61 survival from hatching to meta-morphosis (Fig. 1). The best survival (36-6 1%to 28 days) occurred in Experiment 1.The relatively poor survival (11-28 to 24days,pre-metamorphosis) in Experiment 2may have been a consequence of the stressfrom the static water during the early rear-ing period. Despite the differences in sur-vival between experiments, fairly consistenttreatment-dependent differences were ap-parent. All treatments promoted survivalsignificantly during Experiment 1 P

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pendent differences (F vs. C, T vs .C) werenot significant in Experiments 1 P =0.09,MS = 155) and 3 P = 0.26, MS = 53).The Fisher's PLSD test indicated, however,that the difference between C and TFgroups was significant at 10% confidencelevel P = 0.02 for Experiment 2 and P =0.08 for Experiment 3).We found it impractical to quantify dailysurvival because of the patch distribution oflarvae and their extremely delicate condi-tion. Nevertheless, an episodic increase inmortality was observed in three stages inall tanks; hatching through the first feeding(DOD5), immediately prior to notochordflexion (D12D13), and post flexion priorto m etamor phosis (D20D22). Evidence ofa treatment-dependent difference in surviv-al was first noticed during the late stage ofnotochord flexion (D18). Larvae subjectedto the combination (TF) treatment had lesspronounced mortality, which was confirmedat the end of the experiments. Less tankvariability in mortality patterns was alsoobserved among TF treated larvae. In Ex-periment 3, higher mortality was evidentprior to notochord flexion (D12-D14).Growth

The onset of metamorphosis was ob-served when larvae reached 21 mm in stan-dard length, or between D24 and D28 de-pending on rearing temperature. Despite theuniformity in size at hatching (1.59 mm 0.07, mean standard deviation), larvaegrew at different rates resulting in large sizevariation at the onset of metamorphosis(9.02 mm 1.93). A bimodal distributionwas observed in all groups of larvae. TFtreated larvae grew more uniformly with re-gard to variance in total length (Fig. 2). Thepronounced variation of D5 larvae in the Tgroup in Experiment 1 may have been aresult of a high rate of skeletal deformities,and consequently reduced body lengthamong some of the sampled larvae.No difference in spontaneous growth inlength was observed among all groups oflarvae in any of the experiments P =0.68,MS = 3.28 X 10 5 for Experiment 1;P =0.11, MS = 7.09 X 105 for Exp 2; P =0.63, MS = 6.13 X 105 for Exp 3). Thismay be because the tank variability within

treatments overshadowed any possibletreatment effects. In addition, density-de-pendent effects due to differential survivalrates probably also impacted growth rates.While the treatment-dependent growth insize was evident starting D12 P

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CORTISOL AND THYROID HORMONE IN LARVAL T H R E A D F I N 475

B

9 4 8days after hatching

22 25

FIG. 2. Size variation of larval threadfin from hatching to metamorphosis after hormonal treatment at hatching;one hour immersion with T3 (T), cortisol (F), combination of T3 and cortisol (TF), and no exogenous hormone(C). A, B, and C represent the experiment 1, 2, and 3 , respectively. Each bar shows the mean variation for atreatment (n = 3 per treatment) with a standard error of the mean as a vertical line.

TABLE 1. Hormonal treatment effect on DASPEI fluorescence of larval threadfin from 56 to 86 hr after hatchingat 26.2 0.8C.

TreatmentcFTTF

O.O0.80.2a0.7

56 hr 0.00 0.13 0.11 0.14

62 hr0.7 0.141.8 0.130.8 0 .171.8 0.11

68 hr1.5 0.231.8 0.111.8 0.132.0 0.00

Ag e74 hr

1.6* 0.191.9a 0.081.8' 0.132.6 0 .15

80 hr1.8a 0.182.7 0.142.5 b 0.192.7 0 .14

2.62.92.73.0

86 hr0.190.08 0.14 0.00

One hour immersion with 2.6 ppm of T3 (T), 0.1 ppm of cortisol (F), combination of T3 and cortisol (TF),and no exogenous hormone (C). Indexes indicate: 0, no fluorescence; 1, faint fluorescence in hindgut; 2, forstrong fluorescence in mid- and hindgut; 3, strong fluorescence in gut lining and other organs. Results of ANOVAtest for each sampling day indicated: 56 hr, P = 0.000 (MS = 1.632);62 hr,P = 0.000 (MS = 4.410); 68 hr,P = 0.114 (MS = 0.521); 74 hr,P = 0.000 (MS = 2.306); 80 hr,P = 0.001 (MS = 2.299); 86 hr,P = 0.077(MS = 0.472). Results of Fisher's PLSD comparison were indicated at 5% confidence level. Superscript letters(a b) indicate statistical d ifferences.

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4 7 6 B. G. KIM AND C. L. BROW N

FIG. 3. Hormonal treatment effect on DASPEI fluorescence of larval threadfin at 62h after hatching at 26.2 0.8 degree Celsius; one hour immersion with a combination of triiodothyronine and cortisol (TF) compared tono exogenous hormone (C).

found P > 0.10 in all cases), although theaverage length of larvae at the time of flex-ion suggested that C group larvae reachedthe flexion stage at a larger larval size thanother treated groups. High mortality oc-curred during Experiment 2, especiallyamong the Control group tanks, which wasrepresented in an increase in flexion ratesin four days.The most common deformity evidentamong larval threadfins was fused vertebraefollowed by scoliosis and lower jaw defor-mities. The development of these skeletaldeformities was rather life stage specific;much deformity was observed during pre-flexion stage, though the survival of theselarvae to later stages is doubtful (Fig. 5).The overall rate of deformities was highestin Experim ent 1. A treatment-dependent ef-fect on the rate of deformity was also evi-dent. Control and T 3-treated larvae consis-tently had the highest rates of deformities,

and significantly fewer occurred among lar-vae treated with F and TF at D5, near thecompletion of yolk and oil absorption.DISCUSSION

A one-hour immersion in a combinationof triiodothyronine and cortisol significant-ly improved survival, as we have reportedearlier (Brown and Kim, 1995). We find itespecially interesting that this effect per-sisted despite major differences in survivaland rates of deformities among the three ex-periments. Based on daily monitoring ofmortality, it appears that the net survivaleffect may have been a result, at least inpart, of the amelioration of episodic mor-tality events around D0-5 and D12-13.These are both stages involving dietarytransitions, which is consistent with the ideathat the advancement of development as in-dicated by DASPEI fluorescence may haveconferred some survival benefits.

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CORTISOL AND THYROID HORMONE IN LARVAL THREADFIN 47 710 0

80 -

60 1T3Ii 40 -20 -

standard lengt im m)

(mm)

5 48 J S 4 b 5 b.8,04 ,1 ,SGSb 8.61 C 3? li8,28c

D15D19

TFFIG.4. Percent larvae that reached notochord flexion after hormonal treatment at hatching; one hour immersionwith T3 (T), cortisol (F), combination of T3 and cortisol (TF), and no exogenous hormone (C). A, B, and Crepresent the experiment 1, 2, and 3, respectively. Each bar shows the mean percent larvae that underwentnotochord flexion for a treatment (n = 3 per treatment) with a standard error of the mean as a vertical line.

Differences in survival among controls inthe three experiments indicate that partiallycontrolled variables such as parental genet-ics and nutrition may have an impact on egg quality as manifested in survival.Although the same broodstock animalswere used throughout the study, it was notpossible to identify the specific parentage ofeach spawn. The increasing age of thebroodstock population may also have af-

fected egg quality; eggs used in experiment3 were from females 1.5 years older thanthey were in experim ents 1 and 2. The timeof year of each of the three replicate ex-periments differed, relative to the beginningof the spawning season, which in this spe-cies normally extends from May throughNovember in Hawaiian waters. Experi-ments 1 and 2 used eggs produced late inthe season while experiment 3 used eggs

GANSER LIBRARY

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478 B. G . K IM AND C. L. BROW N

10 15 20post hatching (day) 30

FIG. 5. Cum ulative deformity of larval threadfin from hatching to metamorp hosis after hormonal treatment athatching; one hour immersion with T 3 (T), cortisol (F), combination of T3 and cortisol (TF), and no exogenoushormone (C). A, B, and C represent the experiment 1, 2, and 3, respectively.

from near the beginning of the season. Eggsize and free fatty acid composition declinewith successive spawns in the cod Gadusmorhua;Marst0l et al., 1993). Egg size didnot change throughout our experiments(0.86 0.01 mm) but we have not under-taken chemical analyses.Although efforts were made to maintainconsistency in environmental and nutrition-al variables, subtle changes within the 19months that elapsed between experiments 1and 3 may have had some bearing on larvalsurvival. Different batches of broodstockfeed materials and slight weather variationcan not be discounted as having potentialeffects on the viability of eggs produced in

captivity. Clearly the second experimentwas compromised by a break in a watermain which interrupted the flow of filteredseawater to larval tanks, and by a brieflapse in the feeding schedule. Despite thephysical and human variables unintention-ally imposed on the experimental design inthe course of Experiment 2, we includethese data because they illustrate a pointabout the consistency of the actions of acombination of cortisol and triiodothyro-nine, which had a substantial and beneficialeffect on survival which persisted underthese suboptimal circumstances. The dis-tress endured by larvae in Experiment 2may account for reduced overall survival

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CORTISOL AND THYRO ID HORMO NE IN LARVA L THREADFIN 4 7 9

among controls relative to the other exper-iments (Fig. 1). In addition, Experiment 2alone had more spinal deformities in con-trols than in T3-treated larvae. If in fact theeffectiveness of T 3was altered by compro-mised water quality in Experiment 2, thismay help explain paradoxic larval re-sponses to exogenous thyroid hormones(reviewed by Brown and Nunez, 1994).The effects of the individual hormoneswere less consistent than those of cortisoland T3 administered together. In two of thethree experiments, the survival among T orF groups did not differ from the controls,although survival among these groups in-creased by about 50% in Experiment 1. Wetherefore conclude that these hormones in-teract to promote one or more vital devel-opmental processes, which convey survivaladvantages even under compromised con-ditions. When applied alone, the effects ofthe individual hormones may be masked oroverridden by other variables.Our DASPEI results (Table 1) indicatethat the development of the digestive andurinary tracts was advanced to some extentby cortisol treatment, and slightly more soby a combination of cortisol and T 3. A fun-damental preadaptive role of thyroid hor-mones in the preparation of larval intestinaltissues for exploitation of new food sourceshas been proposed (Specker, 1988). Thetwelve-hour advancement of absorptivefunction by the gastrointestinal tract implic-it in our results (Table 1) coincided with theintitial introduction of rotifers, and there-fore may have had an impact on larval sur-vival. We believe the importance of thistemporal advance is that it would reduce thelapse between exhaustion of endogenousenergy, in the form of yolk and oil, andinitial utilization of exogenous energy fromfirst-feeding. We do not have any evidencewhich would enable us to discriminate be-tween possible direct or indirect actions ofthese hormones on gut development, al-though these hormones have been shown tohave direct peripheral interactions on de-veloping target tissues in the Japaneseflounder, Paralychthys olivaceus (de Jesuset al., 1990).A clear difference in the effects of theindividual cortisol and T3 treatments be-

came apparent in the quantification of skel-etal deformities (Fig. 4). Cortisol-treatedgroups were always among those larvaewith the lowest frequency of skeletal de-formities, whether considering F or TFgroups. The T group displayed variable in-cidence of skeletal deformities, but in bothExperiment 1 and Experiment 3, exhibitedthe highest rate of deformities of all fourtreatments. Exogenous thyroid hormonesreportedly often induce skeletal deformitiesin fish (Higgs et al., 1982), however thistrend was reversed in both experiments 1and 3 among the larvae exposed to bothcortisol and T3. The influence of cortisol ev-idently precluded the increased rate of de-formities seen in the T groups. At this pointwe do not have sufficient data to explainthe mechanism of interaction of these hor-mones. Our working hypothesis is that thepromotion of absorptive function by corti-sol may have aided in the provision of nu-trients needed for skeletal differentiation;shortages of any number of substrate ma-terials in T3-stimulated developing tissuesmay result in scoliosis, opercular malfor-mations, and other skeletal deformities.This does not preclude the possibility of di-rect peripheral interactions of the two hor-mones in the developing vertebral and gilltissues.No consistent relationship of hormonetreatments with the timing of flexionemerged from these experiments (Fig. 4).Control larvae were larger than the TF lar-vae at the time of flexion in Experiment 1,and on D19 in Experiment 2. The inconsis-tency of these data suggests that the hor-mone treatments may have been secondaryto other factors in determining the timingor size of larvae at notocord flexion. Therelatively large size and high frequency oflarvae at flexion in Experiments 1 and 2could reflect a transient increase in growthrate among the control larvae, resultingfrom mortality-induced reduction of larvaldensity. A definitive explanation will re-quire further study.Survival effects were not accompaniedby any correlative effect on growth. Thetreatment groups exhibiting improvementsin survival rate grew at essentially the samerate as controls, although this is a notewor-

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thy observation since mortality in controltanks reduced larval densities, whichshould favor more rapid growth. Those lar-vae subjected to a combination of cortisoland T3 grew at somewhat more uniformrates than controls (Fig. 2). The amount ofvariance in length within a cohort is a crit-ical determinant of cannibalism inChannastriatus(Qin and Fast, in press). This rela-tionship probably holds among most spe-cies prone to cannibalism, including thethreadfins, which typically show cannibal-istic behavior beginning at the time ofmetamorphosis (~D25).The use of thyroid hormones in potentialfisheries applications has been limited byseveral complicating factors. Thyroid hor-mone treatments are viewed as inconsistentin their effectiveness, and replete with un-toward side effects such as the promotionof deformities and cannibalism. For exam-ple, some beneficial effects of exposure ofwalleye JStizostedion vitreum) to thyroidhormones may be overshadowed by ob-served hormone-induced 8- to 10-fold in-creases in the incidence of cannibalism(Heyet al., 1996). It is possible that a fur-ther examination of the interactive effectsof development-promoting hormones maylead to combination treatments which pro-mote survival without such side-effects.

ACKNOWLEDGMENTSThe authors wish to thank the organizersof the symposium for the invitation to par-ticipate. We also express thanks to Mr. W.Douglas Crompton and to Mr. Scott Clem-ent for assistance with larval care and sam-pling.This research was sponsored in part

by a grant from the State of Hawaii De-partment of Land and Natural ResourcesAquaculture Development Program. Fund-ing for this work was also provided by pro-ject R/AQ-57, which is sponsored by theUniversity of Hawaii Sea Grant CollegeProgram, under Institutional Grant No.NA36RGO5O7 from the National Oceanicand Atmospheric Administration Office ofSea Grant, Department of Commerce. Thisis Sea Grant Publication number UNIHI-SEAGRANT-JC-97-09.

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