Extent of gill pathology in the toadfish Tetractenos hamiltoni caused by Naobranchia variabilis (Copepoda: Naobranchiidae)

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1 Vol. 35: DISEASES OF AQUATIC ORGANISMS Dis Aqut Org Published Februry 26 Extent of gill pthology in the todfish Tetrctenos hmiltoni cused by Nobrnchi vribilis (Copepod: Nobrnchiide) Frnk R. Roubl* Deprtment of Prsitology, The University of Queenslnd, Brisbne Queenslnd, Austrli ABSTRACT: Snguinivorous Nobrnchi vribilis prefer the first gill rch, externl hemibrnch nd nterior end of the gill rch. The smllest N. vribilis observed ttched to fish by thin filment which connects fused tips of second mxille to 'plug' inserted into the gill tissue. Second mxille enlrge to encircle nd incresingly compress the gill filment, which results in thin lyer of epithelium nd connective tissue overlying the crtilginous supporting br. Erly juveniles cuse little tissue prolifertion, but the extent of proliferted epitheli] nd connective tissue (PR) djcent to the mxille increses from lte juveniles to subdult nd dult copepods. Most vntion in length of gill filment dmge (PL, proliferted nd compressed tissue) mong ge clsses is explined by mxill length (ML, length of compressed gill filment); dult trunk width (TRW) explins n extr, smll mount of vrition, but not trunk length (TRL) or totl fish length (TL). Most vrition in ML is explined by TRW of dults, subdults nd lte juveniles, nd TRL of edrly juveniles. PR is explined by TRW for dults, but by ML for other ges. These ptterns re due to elongtion of the juvenile trunk during growth nd lterl expnsion of dult egg pouches during mturtion. Up to 38 N. vribilis, verge (vg.) = 9.3, infected individul Tetrctenos hmiltoni nd dmged up to 3.4 % (vg. 0.72%) of totl filment length nd 8.6% (vg. 2.1 %) of gill filments per fish. KEY WORDS: Nobrnchi vribilis. Copepod. Prsite. Ecology - Pthology - Todfish. Tetrctenos hrniltoni INTRODUCTION Nobrnchi vribilis infects wide rnge of mrine fish in the Atlntic nd in Indi, s well s the bnded todfish Spheroides pleurosticus in Moreton By, Austrli (Kbt 1968). The prsite lso occurs on the common todfish Tetrctenos hmiltoni in Moreton By. The prsite ttches by using highly modified second mxille tht encircle the gill filment, but little is known bout its biology, including the life cycle of this widespred prsite, nd in prticulr bout the dmge tht it cuses to fish gills. There re few quntittive dt for the extent of pthology cused by ectoprsites in wild fish popultions. Mesurements of the dmge cused by prsitic copepods to the gill 'E-mil: tylo jc3usq.edu.~ surfce re of fish hve been mde in few studies, e.g. Lopmnn (1940), Kbt & Cousens (1977), Sutherlnd & Wittrock (1985). Roubl (1987, b). The present study ws done to investigte the pthology cused by Nobrnchi vribilis on the gills of Tetrctenos hmiltoni. The sptil distribution of the prsite ws mpped, nd the extent of gill dmge ws quntified nd ssessed ginst prsite size, fish size nd intensity of infection. MATERIALS AND METHODS Tetrctenos hn~iltoni were collected by bem trwl from shllow wter in western Moreton By, Queenslnd, Austrli, nd plced in plstic bgs on ice for up to 2 h, then preserved in 4 % formldehyde. Fish were wshed in fresh wter, the totl length (TL, cm) O Inter-Reserch 1999 Resle of full rticle not permitted

2 204 Dis Aqut Org 35: , 1999 mesured nd the gills excised. The number, size nd distribution of Nobrnchi vribilis on the gills were recorded. The length (TRL) nd width (TRW) of the trunk, nd length of the second mxill (ML; mesured prllel to the long xls of the primry gill filment) were mesured. The copepods were identified s either erly juveniles (trunk bsent or much smller thn neck; second mxill smll), lte juveniles (trunk formed but smll, with no evidence of egg pouches), subdults (egg pouches evident with internl supports but no eggs) or dults (eggs present in t lest 1 pouch or pouches swollen but eggs shed, or trunk lrge but no eggs present). Representtive copepods from ech ge clss were clered in 80% lctic cid nd drwn with the id of clibrted cmer lucid. Some prsites were detched with shrp probe from the primry gill filment nd the under!ying dmge exmined. Ci!l filments with ttched prsites from freshly preserved mteril were embedded in wx, sectioned nd stined with hemtoxylin nd eosin (H&E). To determine if gut pigment in N. vnbilis ws derived from blood diet, the rte of bleching of the pigment in hydrogen peroxide ws exmined (Llewellyn 1954). N. vribilis were cut in hlf nd plced in 30 mg ml-' (10 volume) hydrogen peroxide for up to 12 h. Control tissues lso plced in hydrogen peroxide t the sme time were the gut contents of the snguinivorous copepod Lernnthropus trox, nd pieces of fin nd kidney with melnin deposits. All of these tissues hd been fixed in formlin nd stored in 70% ethnol, then wshed severl times in fresh wter. The number of gill filments from 17 fish (TL 4.3 to 10.8 cm) were counted. The length of every fifth filment from the externl nd internl hemibrnchs of ech of the 3 gill rches on 1 side of the hed ws mesured with clibrted microscope. The verge gill filment length ws clculted nd multiplied by the totl number of filments to estimte totl gill filment length (modified from Roubl 1987). Lest squres regression nlyses were done to derive the reltionship of best fit between totl filment length (TFL, mm) nd fish size (TL, cm). Stepwise regression ws done to investigte the reltionships between length of dmged gill filment (PL), proliferted tissue (PR), ML, TRL nd TRW of individul prsites within ech ge clss, s well s TL. The extent of dmge cused by the ssemblge of Nobrnchi vribilis on fish ws the sum of dmge cused by individul prsites. In some cses prsites nd their ttchment were dmged or dislodged during dissection, so ML nd PL were estimted from the derived regression equtions. These estimtes were used only to clculte totl extent of gill dmge. RESULTS Sptil distribution Individuls of Nobrnchi vribilis were situted on the externl surfce of hemibrnchs nd orientted with the mouth prts nd trunk pointed towrds the gill rch (Fig. 1). Most dult, subdult, lte juvenile nd erly juvenile N. vnbilis ttched to the first gill rch nd the fewest to the third gill rch (Tble 1). Most ttched ner the bse of the gill filment, in the nterior zone nd on the externl hemibrnch; few N. vribilis ttched to the middle zone of ny gill rch. Ten fish with only 1 N. vribilis were exmined; these prsites preferred either the first or second gill rch, nd the nterior zone (Tble 1). Pthology A recently settled prsite ttched itself by mens of thin filment rising from the fused distl tips of the second mxille (Fig. 2) nd inserted into the gill tissue. The end of the filment in the gill tissue ws swollen into bell-shped plug (Fig. 2). This smll lrv (<OS mm) eroded the gill tissue in its vicinity. Further metmorphosis of Nobrnchi vribilis ws ssocited with n increse in size. The second mxille enlrged into strp-like structures, with centrl bnd of muscle, tht encircled the primry gill filment in opposite directions to join beneth the prsite so tht the seprte strps ly side-by-side (Fig. 3). The juvenile mxille cused smll depression in the gill Fig 1. Gills of Tetrctenos hmiltoni infected with Nobrnchi vribilis. (A) Hevy infection on externl hemibrnch of first gill rch. (B) Reltive sizes of 2 lrge dults, 1 smll dult nd subdult. Scle br (A] = 6 mm, (B) = 3 mm

3 Roubl. Nobrnchi vribilis pthology 205 Tble 1. Sptil distribution of 4 ge clsses of Nobrnchj vribilis on the gills of Tetrctenos hmiltoni. The distribution of N. vribilis in 10 fish with 1 prsite only is shown in prentheses Adult Subdult Lte Erly Sum 1 uven~le juvenlle Gill (1) 68 (2) 25 (1) 26 (2) 321 (6) 2 99 (1) 27 (2) 24 (0) 13 (1) 163 (4) (0) Hernibrnch Externl 215 (2) 66 (4) 38 (1) 24 (1) 343 (8) Inteirnl 128 (0) 42 (0) 26 (0) 22 (2) 218 (2) Zone Anterior 316 (2) 99 (4) 61 (1) 41 (3) 517 (10) Middle (0) Posterior (0) filment (Fig. 3). Further growth of the prsite ws ssocited with enlrgement of the neck, trunk nd mxille (Tble 2). The mxille incresed in length, width nd thickness, which incresed the length of gill filment ffected nd incresingly conlpressed the underlying gill tissue (Figs. 4 & 5). The encircling mxille were wider t the mrgin of the gill filment wy from the prsite (Figs. 3 & 4). Bnds of muscle within the mxille (13 to 18 in ech dult mxill) were oriented in the sme direction round the gill filment s the mxille, with the thickest bnds t the centre of ech strp. The cuticle between the muscle bnds becme thicker, nd ws thlckest long the middle of ech strp nd on the mrgin between the djcent strps. Fig. 3. Juveniles of Nobrnchi vrib&s ttched to gill filment. (A) Erly juvenile with thin mxille encircling filment (rrowhed), with httle proliferted tissue; neck (n) distinct but trunk (t) smll. (B) Lte juvenile with ttched mle, more tissue dmge (rrow), nd lrger encircling mxille (m), neck nd trunk. Scle br = 0.5 mm There ws little ~ L tissue U prolifertion beside the encircling mxille of erly juveniles; the mount of prolifertion incresed s the prsite mtured. There ws complete loss of respirtory lmelle beneth the mxille (Figs. 4 & 5). The dimeter of the gill filment encircled by the mxille ws much reduced, nd Tble 2. Size (pm) of Nobrnchl vribhs. Trunk length (TRL), trunk width (TRW), mxill length (ML) nd length of gill filment dmged (PL) re shown for dults, subdults nd lte nd erly luveniles. Proliferted tissue (PR) = PL - ML. Vlues given re smple size (N), men, minlrnum (Min), mximum (Mx) nd stndrd dev~tion (SD) Age clss N Vrible Men Min Mx SD Fig. 2. Erly stge of Nobrnch vribilis ttched to gill fdment. (A) Filment (rrow) inserted in fl tissue extends from second mxille (m). Smll trunk (t) nd neck (n) with mouth prts lso indcted. Stippled re: eroded gill tissue. (B) Enlrged view of distl end of fused second mxde with filment. Scle br (A) = 0.25 mm, (B) = 0.1 mm Adult 423 TRL TRW ML PL PR Subdult 145 TRL TRW ML PL PR Lte juvenile 48 TRL TRW ML PL PR Erly juvenile 37 TRL TRW ML PL PR

4 206 Dis Aqut Org 35: , 1999 Fig. 4. Subdult Nobrnchi vribilis. Egg pouches (e) evident. Proliferted tissue stippled. Scle br = 0.5 mm thin epithelium covered the crtilginous br nd constricted fferent nd efferent blood vessels within the primry gill filment. In some cses where the hed of the prsite ws close to the gill rch there ws loclised prolifertion of epithelilim on the side of the gill rch djcent to the prsite. Histologicl sections reveled tht the mjority of proliferted tissue djcent to the compressed region consisted of hyperplstic epithelil Mlpighin cells with the bsl lyer distinctly columnr. There ws proliferted connective tissue within the dermis, but few infiltrting cells. The fferent nd efferent blood vessels within the proliferted region djcent to the compressed regions, prticulrly with young prsites, were engorged with red blood cells (Fig. 6A). Compressed epithelil cells surrounded the compressed gill filment. There ws little connective tlssue, nd the blood vessels were difficult to locte. The epithelil nd derml tissues were compressed onto the crtilginous supporting br within the gill filment (Fig. 6B). The crtilge ws little ffected, but hd rounded sur- Fig. 6. Histologicl sections of ttched Nobrnchi vribilis. (A) Juvenile second mxille with muscle bnds (crosshtched) encircle filment nd compress epithelium (stippled). Blood vessel (b) compressed beneth mxille but engorged on either side. Crtilge (c) nd djcent connective tissue (ct). (B) Second mxille of dult with severl muscle bnds (cross-htched) encircle filment nd compress epithelium (stippled) onto thin lyer of connective tissue (ct) nd crtilge (c). No blood vessels evident in this or djcent sections. Spce between prsite nd tissue is rtifct. (C) Attched erly juvenile with rounded second mxille nd feeding site (rrow). Scle br (A & B) = 0.05 mm. (C) = 0.2 mm Fig. 5. Adult Nobrnchi vribilis. Egg pouches with eggs extend trunk lterlly. ProMerted tissue stippled; compressed ~ LLI filment encircled by second mxde (m). Scle br = 0.5 mm fce projections ssocited with the res of thickened cuticle in the mxille. In histologicl sections, the gill filment in the vicinity of the mouth prts of the erly juvenile ws eroded nd hd proliferted epithelium; this feeding site ws seprte from where the prsite ttched (Fig. 6C). A drk pigment observed in the gut of the prsite ws evident in ll ge clsses. The pigment ws bleched by hydrogen peroxide t rte similr to blood residue in the gut of the snguinivorous Lernnthropus trox, but the melnin control in the fin nd kidney bleched only l~ttle in the sme period. The gut pigment is most likely residue from blood diet.

5 Roubl: Nobrnchi vnbilis pthology 2 07 Extent of pthology The extent of dmge to the gills cused by individul prsites (PL) ws restricted lrgely to the gill filment where the prsite ttched, nd consisted of con~pressed gill filment beneth the second mxill (= ML) s well s proliferted gill tissue (PR = PL - ML) on either side of the mxill. The sizes of prsites in different ge clsses, s well s ML, PL nd PR, re shown in Tble 2. There ws distinct increse in men vlues of ll vribles s the prsite mtured, but the rnge of vlues overlpped between djcent ge clsses. In one cse there ws no pprent prolifertion cused by n dult; this prsite ws smll (TRL = 420 pm, TRW = 420 pm) with few eggs. Stepwise regression of PL on ML, TRW, TRL nd TL over ll ge clsses showed tht ML explined most vrition wheres TRW ws significnt only for dults; TRL nd TL were insignificnt fctors in ll ge clsses (Tble 3). ML explined lest PL vrition in the dult ge clss nd most in erly juveniles. Stepwise regression ws done to determine which of TRL, TRW nd TL ccounted for significnt vrition in ML. Over ll ge clsses, TRW ws more importnt vrible thn TRL, nd TL hd significnt, negtive effect lthough it explined little of the totl vrition (Tble 3). Among ge clsses, TRW explined most vrition but ws not significnt fc- tor for erly juveniles, wherein TRL ws the only significnt vrible. TRL explined significnt, but smll mount of vrition for dults nd subdults. In ddition, TL hd smll, significnt, but negtive effect on subdult ML. There were no significnt correltions (p > 0.5) between ML nd TL for subdult, lte nd erly juveniles, but TL nd dult ML were correlted positively (r = 0.29, p < 0.001). Prtil correltions between ML nd TL djusted for either TRW or TRL were not significnt for erly nd lte juveniles, but TL nd subdult ML were correlted negtively when djusted for either TRW (r = -0.21, p = 0.14) or TRL (r = -0.19, p = 0.02). Stepwise regression of PR on ML, TRW, TRL nd TL showed tht over ll ge clsses TRW explined most vrition with smll mount explined by ML (Tble 3). This result ws similr in the dult ge clss, but ML explined most vrition in PR for subdults, nd no reltionships were found for juvenile prsites. When PR ws regressed on ML, significnt reltionships were found for dult nd subdult ge clsses only (Tble 3). However, subdults hd greter slope thn dults. Thus, compression cused more dmge thn prolifertion (Tble 2), but reltive to length of mxill subdults cused more prolifertion thn dults. Adult Nobrnchi vribilis cused the most dmge (Tble 2). Tble 3 Nobrnchi vribilis size nd extent of pthology. Reltionships between rnxlll length (ML, pm), trunk width (TRW, pm), trunk length (TRL, pm), fish length (TL, cm), dmge to gill filment (PL, pm) due to proliferted tissue (PR, pm) nd tissue compressed beneth mxill (= ML). Equtions were derived by stepwise regression; significnce level nd percent of vrition explined by ech vrible re shown. Age, prsite ge clss (Ad: dult, S: subdult, U: lte juvenile, EJ: erly juvenile); DV. dependent vnble. n: not pplicble; nf: not fitted; Sig.: significnce level (ns p t 0.05, 'p < 0.05, "p < 0.01, "'p < 0.001) Age DV Eqution Independent vribles Intercept ML TRW TRL TL Sig. Sig. Oh Sig. % Sig. % Sig. % All PL ML TRW TRL ns "' 79 "' ns Ad PL ML+0.28TRW ns "' 52 "'? ns ns SA PL ML ns "' 54 ns ns ns LJ PL ML ns "' 62 ns ns ns EJ PL ML ns "' 80 ns ns ns All ML TRW TRL TL "' n 73 "' 1.9 " 0.4 Ad ML TRW TRL... n "' 1.7 ns SA ML TRW TRL TL "' n 48 " 2.6 " 2.3 U ML TRW... n.. 35 ns ns EJ ML TRL ns n ns.. 39 ns All PR TRW ML... 2 "' 55 ns ns Ad PR TRW+0.22ML ns "' 1 "' 26 ns ns SA PR ML ns "' 18 ns ns ns LJ, EJ PR No eqution ns ns ns ns ns All PR ML ns "' 49 nf nf nf Ad PR ML nf nf n f SA PR ML ns "' 18 nf nf nf LJ, EJ PR No eqution ns ns nf nf nf....

6 208 Dis Aqut Org 35: , 1999 Morphometrics of todfish gills Todfish in the TL rnge 4.3 to 10.8 cm (n = 17) hd between 330 nd 460 gill filments, nd the nonliner reltionship between number of filments nd TL ws described by the cubic eqution: number of filments = TL TL TL (r2 = 0.93). The corresponding rnge of TFL in these fish ws 443 to 2471 mm. The reltionship between TFL (mm) nd TL (cm) ws described dequtely by liner regression eqution: TFL = TL (r2 = 0.96). These equtions were used to estimte the number of gill filments nd totl filment length in order to determine the percentge of these vlues dmged by ll Nobrnchi vribilis on individul fish. Totl gill dmge per fish Infected todfish hd n verge of 9.3 (1 to 38) prsites per fish (Tble 4, Fig.?A). Of the 88 infected fish, 71 fish hd t lest 1 dult (vg. = 7.6 per fish), 61 fish hd t lest 1 subdult (vg. = 2.98), 22 fish hd t lest 1 lte juvenile (vg. = 2.4), nd 16 todfish hd t lest 1 erly juvenile prsite (vg. = 2.6). The gretest intensities occurred in the 9 to 10 cm TL rnge. The gretest dmge, expressed s totl dmged filment length (Fig.?B), occurred between 9 nd 10 cm TL. Reltive to the intensity dt in Fig. 7A, there ws generl decline in vlue of points reltive to the mximum vlue, especilly in those fish infected with smll prsites. When dmge ws expressed s percentge of TFL (Fig. 7C), vlues for fish <8.5 cm incresed wheres vlues for fish >l0 cm decresed reltive to Fig. 7B becuse smller fish hve less totl gill filment length. Totl gill dmge did not exceed 3.4 % of TFL, most of which ws due to the dult prsites (Tble 4). Since it ws rre for more thn 1 prsite to ttch to primry gill filment, the percentge of gill filments ffected (Fig. 7D) ws similr to the intensity of infection (Fig. 7A), nd, unlike TFL, there ws little difference in number of filments between lrge nd smll fish. Between 0.22 nd 8.6% (vg. = 2.14 O/O) of gill filments were dmged (Tble 4). DISCUSSION Like mny monogenen nd copepod ectoprsites, Nobrnchi vribilis shows definite site preference on its host. This preference is for the ends of the gill rch, prticulrly the first gill rch. Only in hevy infections re less prcferred res occupied. The lerneopodid Alell mcrotrchelus ttches to the gill tips of Acnthopgrus ustrlis, but unlike N. vribilis, individuls of A. mcrotrchelus tend not to occur in close proximity to ech other (Roubl 1989). This my be explined by differences in resource utilistion. The histophgous A. mcrotrchelus browse on the tips of the gill filments nd tend to exclude conspecifics from their vicinity, wheres the snguinivorous N. vribilis hve usully unlimited food supply, so there is little intrspecific competition for food nd the prsites cn ttch close to ech other. Reproductive success is not ffected, whether or not the femle prsites re in close proximity (N. vribilis) or scttered over the gills (A. mcrotrchelus), despite the fct tht in both groups the mle is smll nd short-lived. The life cycles of the Nobrnchiide hve not been elucidted. Kbt (1981) suggested close evolutionry reltionship between the Nobrnchiide nd Tble 4. Extent of gill dmge per fish cused by different ge clsses of Nobrnchi vribilis. Intensity shows number of fish infected, men number of prsites per fish nd rnge for number of prsites per fish (N, men, rnge). Dmge cused per fish is expressed s totl length of g111 filments ffected (dmge/fish, mm), percentge of totl filment length (%TFL) ffected nd percentge of totl number of g111 filments (%fils) ffected. Percentge of TFL ffected by individul prsites (% TFL ind.) lso shown Age clss Intensity Totl gill pthology % TFL ind. Dmge/fish % TFL % fils Adult N, men 71, Rnge Subdult N, men 61, rnge Lte juvenile N, men 22, Rnge Erly juvenile N, men Rnge Pooled N, men Rnge

7 Roubl: Nobrnchi vribllls pthology C l p m m C 2.5 m E 50-! m 1- A l l :, 8 l B l. l : + l0. l c g * l l l : E 3, l : *, -.- m 2. *. S 1 l 8 l.# * : l Of prticulr interest is the filment used for ttchment by the young, l l post-copepodid stge of Nobrnchi vribilis observed here. The terminl plug inserted into the gill tissue nd * connected by filment to the # mxille resembles the frontl fill * ment secreted by the frontl glnd. 8 l l nd used by copepodid nd chlimus I stges of lerneopodids for ttch ment. The bsl plug of the filment in these stges is grsped by clws of the : second mxille prior to the develop ment of the bull; the bull is secreted l by the frontl glnd nd held by fused l tips of modified second mxille (Pi secki 1989). Like the bull of lerneopodids, the filment of N. vribilis is 8 held by the fused tips of the second mxille, but the filment clerly does not hve mnubril ducts nd other ; fetures of bull (Kbt & Cousens 1972). Post-copepodid stges of N. vnbilis my retin the filment used by the copepodid for ttchment, or - 8 l t * l the filment my be nother structure * l such s vestigil bull tht hs.; t l re r, become redundnt during evolution, given the possible lerneopodid-like ncestry of Nobrnchiide (Kbt 1981). In either cse, this filment provides ttchment whilst the second mxille grow to encircle the gill filment. Detiled observtions on the l, ontogeny of the ttchment pprtus of the Nobrnchiide re needed.. Selection hs fvoured the loss of the verstile lerneopodid bull in fvour of strp-like outgrowths of the nobrnchid mxille tht restrict them to gill filments (Kbt 1981). Concomitnt with this restriction to the gills is switch from the lrgely histophgous diet in lerneopodids to snguinivorous diet in nobrnchids. The dmge cused by individul Nobrnchi vnbilis is usully restricted to 1 gill filment nd consists of compressed tissue nd blood vessels beneth the mxille with proliferted djcent epitheli1 nd connective tissue. This cuses loss of surfce re nd interrupts blood flow, but there ws no evidence of gill necrosis in Tetrctenos hrniltoni due to restricted blood supply. Ergsilid copepods such s Ergsilus lbrcis nd Ergsilus mplectens pinch off the gill filment blood supply, cusing necrosis dstl to the l 12 Fish length (cm) Fig. 7. Dmge to gills of Tetrctenos hmiltoni of different sizes infected with Nobrnchi vribilis. (A) intensity, (B) totl gill dmge, (C) percentge of totl glll filment length dmged, (D) percentge of gill filments dmged with t most 1 prsite per filment Clvell- brnch of the Lerneopodide. The ClveUbrnch hs n bbrevited life cycle (Heegrd 1947, Shotter 1971), but Pisecki (1989) concluded tht there ws no need to discern 2 types of life cycle within the Lerneopodide nd considered the pup of the 'Clvell-brnch' to be delyed copepodid with the chlimus I stge inside. It is not known how mny moults occur in the Nobrnchiide once the eggs re htched nd once the infective copepodid ttches to the gills.

8 210 Dis Aqut Org 35: , 1999 ttchment site, but nobrnchids cuse only prtil blockge of the blood supply (Kbt 1984). Perhps the muscle bnds within the nobrnchid second mxille permit some occsionl lessening of the prsite's grip nd llow blood flow to the distl end of the gill filment. No more thn 3.4 % of totl gill filment length nd 8.6% of gill filments were ffected t the levels of infection by Nobrnchi vribilis observed in this study. Individuls of the lerneopodid Alell mcrotrchelus ffected up to 2% of gill filment length of Acnthopgrus ustrlis, but the pthology ws distributed over the tips of severl gill filments; in smll fish the filments were frequently swollen nd fused (Roubl 1987b). Hevy infections by A. mcrotrchelt~s in quculture re lethl (Ueki & Sugiym 1979). Lopmnn (1940) found tht no more thn 50% of gill surfce re of Tinc vulgris could be dmged by the copepod Ergsilus sieboldi; more severe infection cused deth. Kbt & Cousens (1977) reported 16 to 25% of gill surfce of Oncorhynchus nerk dmged by 24 to 56 dults of the lerneopodid copepod Slmincol cliforniensis, wheres 15 dult S. cliforniensis dmged up to 7.7% of gill surfce re of lrger Oncorhynchus mykiss. Compred to these dt, it is unlikely tht the dmge cused by N. vribilis t the intensities observed in this study would be hrmful to Tetrctenos hmiltoni. Lrge (dult) prsites cused more dmge (PL = compression nd prolifertion) thn smll (young) prsites. ML ws n exct mesure of compression dmge, nd ws better predictor thn either TRW, TRL or TL of PL mong ge clsses. Growth of Nobrnchi vribilis ws ssocited with n increse in size of the trunk, neck nd second mxille. Once the prsite mtured nd ws fertilised, eggs were deposited in lterl egg pouches nd TRW incresed more thn TRL (Roubl unpubl. dt). TRW hd smll positive effect on PL in ddition to ML for the dult ge clss. This is explined by the lrge contribution of TRW to PR for dult prsites, wheres in other ge clsses, especilly juveniles in which the trunk is little developed, ML explined most vrition in PL nd PR. The influence of dult TRW on PL nd PR mens tht, for dults with the sme ML, prolifertion increses more for those dults with wider trunk. A wider trunk signifies greter reproductive ctivity nd n older ge (Roubl unpubl. dt). ML ws best explined by TRL in erly juveniles, wherein growth ws ssocited with incresing trunk length, but in older clsses TRW hd more influence due to development nd mturtion of egg pouches. Although subdults cuse less dmge thn dults, the slope of PR on ML is greter for subdults thn dults, which implies tht unit increses in dult ML hd lessened effect on prolifertion; increses In TRW were more significnt s shown bove. Juveniles cuse little tissue prolifertion compred to compression nd neither ML, TRW nor TRL explin vrition. It is not esy to explin why fish TL should hve negtive effect on subdult ML once TRL nd TRW hve been fitted. A possible explntion is tht lrger fish hve thicker gill filments, nd for prsites with the sme trunk size, the mxille tht encircle the thicker gill filment re thinner. As fish increse in length, gill filments increse in length, but fewer gill filments re dded (Frrell 1980, Hughes 1984). Thus, the number of filments incresed 39 % for Tetrcfenos hmiltoni between 4.3 nd 10.8 cm TL whilst the corresponding filment length incresed 558%. Thus, % gill filments infected ws similr to intensity of hrzohrnchi vribilis mong fish becduse ech infected filment usully hd only 1 prsite ttched. nd there ws not much difference in number of filments mong fish. However, reltive to intensity of infection the % totl filment length dmged ws less in lrger fish nd greter in smller fish becuse lrger fish hve greter totl filment length. Irrespective of fish size only smll mount of dmge ws done to the gills by N. vribilis. Acknowledgements. I m grteful to Dvid Grhm nd students in the Zoology Deprtment, University of Queenslnd, for supplying the fish. A reviewer mde useful comments on the mnuscript. LITERATURE CITED Frrell AP (1980) Gill morphometrics, vessel dimensions, nd vsculr resistnce in ling cod, Ophiodon elongtus. Cn J Zoo1 58: Heegrd P (1947) Contribution to the phylogeny of the rthropods: Copepod. Spoli Zoologic Musei Hun 8:l-236 Hughes GM (1984) Generl ntomy of the gills. In: Hor WS. Rndll DJ (eds) Fish physiology, Vol X. Gills. Prt A. Antomy. gs trnsfer nd cid-bse regultion. Acdemic Press. New York, v 1-73 Kbt Z (1968) copepod prsitic on Austrlin fishes VIII. Fmilies Lerneo~odide nd Nobrnchiide. J Nt Hist 2: Kbt Z (1981) Copepod (Crustce) prsitic on fishes: problems nd perspectives. Adv Prsitol 19:l-71 Kbt Z (1984) Diseses cused by metzons: crustcens In: Kinne 0 (ed) Diseses of mrine nimls, Vol IV, Prt 1. 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