Macroalgal grazing selectivity among herbivorous coral reef fishes

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1 Vol. 352: , 27 doi: /meps755 MARINE ECOLOGY PROGRESS SERIES Mr Ecol Prog Ser Published December 2 Mcrolgl grzing selectivity mong herbivorous corl reef fishes Chrystl S. Mntyk, Dvid R. Bellwood* School of Mrine nd Tropicl Biology, nd Austrlin Reserch Council Centre of Excellence for Corl Reef Studies, Jmes Cook University, Townsville, Queenslnd, 4811, Austrli ABSTRACT: Despite growing interest in exmining functionl groups mong corl reef species, few studies hve exmined the reltive functionl impcts of individul herbivorous fish species in corl reef ecosystem processes. We investigted potentil functionl roles within n herbivorous reef fish ssemblge by ssessing the feeding selectivity of fishes on mcrolge. Trnsplnted multiplechoice lgl ssys nd remote sttionry underwter digitl video cmers were used to quntify the feeding selectivity of 6 herbivorous reef fish species: Signus dolitus nd S. cnlicultus (Signide); Chlorurus microrhinos, Hipposcrus longiceps, nd Scrus rivultus (Lride); nd Pomcnthus sexstritus (Pomcnthide). The signids strongly selected Srgssum sp. (Pheophyt) nd, to lesser extent, Hypne sp. (Rhodophyt), but strongly voided clcified mcrolge. However, S. dolitus displyed strong selection for Hypne sp., wheres S. cnlicultus exhibited stronger selection for Srgssum sp. In contrst, the prrotfishes exhibited similr selectivities, with ll 3 species selecting hevily clcified Hlimed spp. (Chlorophyt), but strongly voiding Glxur sp. (Rhodophyt) nd other red nd brown mcrolgl species. The pomcnthid ws the lest selective of the 6 fish species, exhibiting no cler selection or voidnce. These results emphsize the potentil vrition mong species within presumed functionl groups, nd the need for cre when defining functionl groups t the fmily or the genus level. The extent of speciliztion, with strong selection nd voidnce, suggests tht functionl redundncy mong herbivores my be less thn previously ssumed. KEY WORDS: Selectivity Herbivorous corl reef fishes Functionl roles Redundncy Remote underwter video Resle or republiction not permitted without written consent of the publisher INTRODUCTION Functionl groups re unified by shred ecologicl role irrespective of their txonomic ffinities (Steneck 21). A number of functionl groups hve been described within the mrine herbivore ctegory on corl reefs (e.g. grzers, mcrolgl browsers, nd detritivores; see e.g. Wilson et l. 23, Chot et l. 24, Mumby et l. 26). Such functionl groups hve provided useful bsis for considering the role nd importnce of herbivorous reef fishes in mintining corl reef resilience nd voiding corl-lgl phseshifts (Hughes et l. 23, 27, Bellwood et l. 24, 26, Mumby et l. 26). However, little is known of the functionl groups tht regulte this trnsition from corl to mcrolgl dominnce, prticulrly in the Indo-Pcific. For exmple, which species et mcrolge on inshore reefs, where mcrolge re most undnt? A criticl component in understnding this corl lgl trnsition is knowledge of selectivity mong herbivorous reef fishes for mcrolgl species. For exmple, herbivorous reef fish species within functionl group my be considered functionlly redundnt if the species in tht group exhibit the sme feeding selectivity. However, it is not known to wht extent reef fish species exhibit selection nd/or voidnce. If similr herbivorous reef fish species exhibit mrkedly different feeding selectivity, they my no longer be functionlly redundnt. The loss of one species my therefore *Corresponding uthor. Emil: dvid.bellwood@jcu.edu.u Inter-Reserch 27

2 178 Mr Ecol Prog Ser 352: , 27 hve disproportiontely lrge effect on the diversity nd community composition of lge on reefs. We exmined the potentil of corl reefs to cope with chnge by evluting the extent to which resident herbivores will prey on mcrolge. Severl fctors hve been exmined s potentil determinnts of food selectivity nd preferentil feeding in herbivorous fishes. These fctors include: lgl nutritionl chrcteristics (e.g. Pillns et l. 24, Rubenheimer et l. 25), sptil nd temporl vrition in lgl vilility (e.g. Pillns et l. 24), lgl defenses in the form of chemicl nd/or morphologicl trits, such s secondry metolites, toughness, nd clcifiction (e.g. Pul & Hy 1986, Trgett & Trgett 199, Duffy & Pul 1992, Meyer et l. 1994), digestive cpilities of fish (e.g. Clements & Chot 1997), nd orl jw biomechnics (e.g. Bellwood & Chot 199, Purcell & Bellwood 1993). In studies of food selectivity of herbivorous fishes, the most frequently used technique for determining the degree of food selectivity is gut content nlysis (e.g. Rndll 1967, Ojed & Muñoz 1999, Rubenheimer et l. 25). Gut content nlyses re dvntgeous in tht one cn identify selection from wide rnge of nturl prey. However, the presence nd undnce of n lgl species in the gut my be relted to lgl vilility in the field, rther thn selection. In this, it reflects the relized niche rther thn the potentil niche. Alterntive techniques used in exmining food selectivity in herbivorous fishes include direct observtions (Lewis 1985, Bruggemnn et l. 1994), inference from cging experiments (Ceccrelli et l. 26), qurium-bsed feeding experiments (Trgett & Trgett 199, Ojed & Muñoz 1999), nd video observtions (Bellwood et l. 26). Ech technique ddresses different spects of selectivity. Direct observtions provide vlule informtion on the nturl diet of fishes in the field, but the fishes re restricted to ville resources, nd the presence of the observer my hve significnt effects on the fish s nturl feeding behvior (Bellwood et l. 26). Cging experiments my reflect selectivity t the fmily or system level, but cnnot identify the feeding selectivity of individul consumers. Aqurium-bsed feeding experiments llow for isoltion of specific individul herbivorous fish species in the lortory, but the extent to which they reflect nturl ptterns my be questionle (e.g. Ojed & Muñoz 1999). Video observtions, where remote sttionry underwter digitl video cmers re used to film feeding selectivity, provide compromise in tht they offer mens to record the feeding selectivity of herbivorous fishes in the field, on nturl or modified substrt, without the presence of n observer. They provide direct evlution of potentil functionl roles. In the pst, similr designs without the use of video cmers hve been used to offer specific food items on nturl or modified substrt to detect lgl deterrence (e.g. Pul & Hy 1986). However, the use of cmers enles the fish species exhibiting this selectivity to be identified. The centrl gol in the present study ws to evlute the potentil niche of herbivorous fishes. This reflects their cpcity to cope with chnge. One of the most widely reported chnges on corl reefs is mrked increse in mcrolgl cover (e.g. Mumby et l. 26, Hughes et l. 27). If mcrolge become widespred, which fish species would be le nd motivted to feed on vrious mcrolgl species? The specific im of this study, therefore, ws to ssess the feeding selectivity of 6 reef fish species: Signus cnlicultus nd S. dolitus (Signide); Chlorurus microrhinos, Hipposcrus longiceps, nd Scrus rivultus (prrotfishes, Lride; Westnet et l. 25); nd Pomcnthus sexstritus (Pomcnthide), using video observtions nd trnsplnted multiple-choice lgl ssys. These 6 species re functionlly the most importnt herbivorous reef fish species t the study site nd re mong the most undnt herbivorous fishes on Gret Brrier Reef (GBR) inshore reefs (Fox & Bellwood 27, Mntyk & Bellwood 27). MATERIALS AND METHODS Study site nd mcrolge. The study ws conducted between Februry nd Mrch 26 in Pioneer By on the leewrd side of Orpheus Islnd (18 35 S, E) on the inner shelf of the GBR, Austrli. Two fringing reef crest sites locted t the northern end of the by were selected. Both study sites re protected from commercil nd recretionl fishing nd hve reltively low levels of mcrolgl cover (Fox & Bellwood 27). Twelve species of mcrolge were exmined, of which 8 were collected long the inner nd mid-intertidl reef flt of Pioneer By (Chlorophyt: Hlimed cylindrce, H. discoide, nd H. opunti; Rhodophyt: Amphiro sp. nd Lurenci sp. 2; nd Pheophyt: Pdin sp., Srgssum sp., nd Turbinri ornt). Glxur sp. (Rhodophyt) ws collected from the outer reef flt nd crest, Chlorodesmis fstigit (Chlorophyt) from the reef crest, nd Lurenci sp. 1 nd Hypne sp. (Rhodophyt) from buoyed mooring line, pproximtely 25 m from the reef crest. The mcrolgl species selected for this study were chosen to represent ll 3 divisions of mcrolge (Chlorophyt, Rhodophyt, nd Pheophyt) nd disply wide rnge of morphologies, nd they were loclly undnt. Pdin sp., Srgssum sp., nd Glxur sp. were prticulrly undnt on the djcent reef flt. Mcrolgl species

3 Mntyk & Bellwood: Grzing selectivity in reef fishes 179 were removed, ensuring tht the holdfst ws intct where possible, nd trnsferred to outdoor recirculting sewter tnks until they were used in the feeding trils (within 24 h of collection). Mcrolge were identified to species where possible, but in most cses were only identified to genus becuse of the difficulty in identifying mcrolge to species level in the field, especilly rhodophytes. Algl txonomy in the Indo- Pcific is not well resolved, nd unfortuntely, significnt txonomic chllenges remin. However, Glxur sp. closely resembled G. rugos, nd Lurenci sp. 1 resembled L. filiformis. Mcrolgl selectivity trils. Selection differs from preference in tht mcrolgl selection is the process by which n individul chooses n lg, wheres mcrolgl preference is the likelihood tht n lg will be selected if offered on n equl bsis with others (Mnly et l. 22). In the present study, the mcrolgl selectivity of the 6 herbivorous reef fishes ws mesured through cfeteri-style selectivity trils using multiple-choice lgl ssys. These ssys differed from feeding preference experiments in tht ech ssy contined 1 proportionl-sized specimen of ech of the 12 mcrolgl species to reflect their usul growth form (rther thn equl-sized specimens s required for mesures of preference). The verge msses of the mcrolgl species used in the ssys were 1.1 g (Chlorodesmis fstigit), 6.8 g (Turbinri ornt), 8. g (Hlimed discoide), 1.3 g (Lurenci sp. 2), 13.8 g (Hypne sp.), 14. g (Pdin sp.), 14.1 g (H. cylindrce), 19.5 g (Lurenci sp. 1), 22.9 g (Amphiro sp.), 29.3 g (H. opunti), 45.7 g (Srgssum sp.), nd 62.6 g (Glxur sp.). Selectivity herein differed from ecologicl selectivity in tht it did not directly exmine selection versus nturl vilility. Ech specimen ws tied in rndom order onto 1 m long piece of fishing line t pproximtely 8 cm intervls. Dmged or discolored lge were not used in the ssys. The lge were trnsported in plstic selfseled bgs nd deployed on the crest of ech site using SCUBA. New sites were selected for ech deployment. Alge were secured by tying the end of the fishing line to the reef. A sttionry underwter digitl video cmer ws plced on tripod pproximtely 1 to 2 m from the multiple-choice lgl ssys to record the feeding ctivity of the herbivorous reef fishes. Mcrolgl selectivity trils commenced t 8: nd t 13: h, over 7 d. Feeding ctivity ws recorded for 3 consecutive hours with obligtory tpe nd bttery chnges t 9:3 nd 14:3 h. In totl, 28 filmed selectivity trils (84 h) were recorded, 14 t ech site (7 morning, 7 fternoon). Quntifiction of selectivity. Food selectivity of the 6 herbivorous reef fish species ws quntified by recording the totl number of bites tken by ech fish species on ech mcrolgl species, for the entire 3 h feeding tril (totl 84 h). Observtions were divided into 5 min intervls to permit further subdivision if necessry. All observtions were restricted to dult fishes: Signus dolitus >15 cm, S. cnlicultus >25 cm, Chlorurus microrhinos >3 cm, Hipposcrus longiceps >25 cm, Scrus rivultus >25 cm, nd Pomcnthus sexstritus >3 cm, nd only included those trils in which ll 6 study fish species were present. During the selectivity trils, no gonistic interctions were observed between ny of the 6 study species (or ny other species in the vicinity), nor did the fishes rech such high undnces tht ccess to the full rnge of lge ws reduced. It ppered tht ech fish species ws le to express selectivity without impct from the other species. For ech of the 6 fish species, the number of times ech lg ws selected first in selectivity trils ( first bites ) ws lso recorded to test for selection bsed on vision nd/or olfction (i.e. before consumption). A note ws mde of the time tken for mcrolgl species to be physiclly reduced by herbivory to size tht could not be visully identified/detected on the video footge. Sttisticl nlyses. The mcrolgl selection could not be nlyzed with n ANOVA becuse ech lgl specimen within ech selectivity tril ws not independent (i.e. the selection of one lgl species my be dependent on the presence of other lgl species in tril). Selection by ech of the 6 herbivorous reef fish species ws therefore nlyzed using 2 seprte techniques: Friedmn s test nd Struss Liner Selection Index. In the first method, the men number of bites per mcrolgl species ws compred nd nlyzed with non-prmetric Friedmn test, followed by Friedmn posteriori multiple comprison tests (Conover 1999). In the second method, selection ws mesured bsed on selectivity index, Struss Liner Selection Index (L): L = r i p i, where r i is the number of bites from mcrolg i, s percentge of the totl number of bites from ll mcrolge during ech feeding tril, nd p i is the percentge of the totl lgl mss presented t the beginning of every feeding tril belonging to mcrolg i. Selectivity indices for ech mcrolgl species were verged over ll feeding trils for ech of the 6 herbivorous reef fish species, nd 95% confidence intervls (CI) were clculted. CI vlues ove. indicte selection, vlues less thn. indicte voidnce, nd CI vlues tht encompss. indicte tht selection of the mcrolg did not differ significntly from rndom. These 2 methods were chosen to represent different pproches used throughout the literture. Thus, if both techniques produce similr results, one cn be more confident tht the results re likely to provide n ccurte representtion of lgl selectivity by the 6 fish species.

4 18 Mr Ecol Prog Ser 352: , 27 RESULTS Lurenci sp. 2 Lurenci sp. 1 Hypne sp. Glxur sp. Amphiro sp. Turbinri ornt Srgssum sp. Pdin sp. Hlimed opunti Hlimed discoide Hlimed cylindrce Chlorodesmis fstigit f cdef def cdef de d b Time (min) Fig. 1. Time required for mcrolge to be reduced to size tht could not be visully identified/detected on video footge. Brs represent mens ± SE. Anlysis ws by Friedmn s test followed by Friedmn s multiple comprisons test. Letters indicte homogenous subgroups, n = 23 to 25 for ech species. Blck brs: Rhodophyt; open brs: Pheophyt; grey brs: Chlorophyt The totl number of bites tken from mcrolge by the 6 herbivorous reef fish species during the first hour of feeding (85%) ws more thn 5 times greter thn the second (13%) nd third hours (2%) combined. Although mcrolgl species vried significntly in the time tken to rech the point where they were too smll to be detected on the video footge (Friedmn test, χ 2 = , p <.1; Fig. 1), ll mcrolgl species were present throughout the first hour of feeding trils, with the sole exception of Lurenci sp. 2 (lost on verge fter 35 min, Fig. 1). Given tht lmost ll mcrolge were present during most of the first hour of feeding trils, nd tht the vst mjority of bites occurred during this period, ll subsequent nlyses were restricted to the first hour of dt when the full rnge of mcrolge were ville to best evlute selectivity. Signus dolitus nd S. cnlicultus displyed strong feeding selectivity. In S. dolitus, the number of bites tken from Srgssum sp. nd Hypne sp. ws significntly greter (6% of ll bites) thn from ny other lgl species (Friedmn test, χ 2 = , p <.1; Fig. 2). Struss Liner Selection Index indicted tht S. dolitus selected Srgssum sp., Hypne sp., Lurenci sp. 2, nd Turbinri ornt (Fig. 2b), nd voided 6 of the remining 8 lgl species. In comprison, S. cnlicultus took significntly lrger number of bites from Srgssum sp. (6% of ll bites) thn from ny other mcrolgl species (Friedmn test, χ 2 = , p <.1; Fig. 2). Struss Liner Selection Index indicted tht S. cnlicultus selected Srgssum sp. nd Pdin sp., nd voided the green mcrolge (Hlimed opunti, H. cylindrce, H. discoide, nd Chlorodesmis fstigit), Glxur sp. nd Amphiro sp. (Fig. 2b). Thus, bsed on both techniques, S. dolitus consistently selected Srgssum sp., Hypne sp., Lurenci sp. 2 nd, to lesser extent, T. ornt. In contrst, S. cnlicultus consistently selected Srgssum sp. nd, to lesser extent, Pdin sp. over ll other mcrolgl species. Both fish species voided the green mcrolgl species nd the red Glxur sp. nd Amphiro sp. In contrst to the signids, the prrotfishes Hipposcrus longiceps, Chlorurus microrhinos, nd Scrus rivultus ll significntly voided Srgssum sp., but exhibited selection for Hlimed opunti (Friedmn tests: Hipposcrus longiceps, χ 2 = , p <.1; C. microrhinos, χ 2 = 121.5, p <.1; S. rivultus, χ 2 = , p <.1; Fig. 2c,d). The most interesting result from Struss Liner Selection Index ws tht ll 3 prrotfish species consistently selected 1 or more species of Hlimed (H. opunti, H. cylindrce, or H. discoide; Fig. 2d). Furthermore, ll 3 species consistently voided Glxur sp., nd, to lesser extent Hypne sp., Lurenci sp. 1, Lurenci sp. 2, Srgssum sp., Pdin sp., nd Turbinri ornt. Pomcnthus sexstritus exhibited the lest selectivity. The Friedmn test indicted significntly higher bite rte on the red mcrolg Hypne sp. compred to tht of ny of the brown or green mcrolge, but it ws not significntly different from either of the Lurenci species (χ 2 = , p <.1; Fig. 2). Struss Liner Selection Index lso showed selection for Hypne sp., but voidnce for the green nd brown mcrolge (Fig. 2b). However, these reltionships were not s distinct s those in either the signid or prrotfish species. When first bites were compred to p <.1 the totl number of bites per lg, there n = ws generl reltionship between the 2 vriles for ech of the 6 dominnt herbivorous reef fish species (Fig. 3). As first bites incresed for prticulr mcrolgl species, the totl number of bites for tht mcrolgl species lso incresed. Overll, the 6 herbivorous reef fish species did not pper to be selecting mcrolge t rndom, but rther they ppered to trget specific mcrolgl species from the beginning of every selectivity tril nd then continued to feed on those lge until they were no longer ville.

5 Mntyk & Bellwood: Grzing selectivity in reef fishes 181 Signus dolitus Signus cnlicultus Pomcnthus sexstritus Men number of bites ± SE () Friedmn n = 28 c n = 23 d d c c b A n = 17 c c Men selectivity index ± 95% CI (b) L (Liner) Hipposcrus longiceps Chlorurus microrhinos Scrus rivultus Men number of bites ± SE Men selectivity index ± 95% CI (c) Friedmn c (d) L (Liner) n = 12 n = 23 b c n = 19 C. fstigit H. cylindrce H. discoide H. opunti Pdin sp. Srgssum sp. T. ornt Amphiro sp. Glxur sp. Hypne sp. Lurenci sp. 1 Lurenci sp. 2 C. fstigit H. cylindrce H. discoide H. opunti Pdin sp. Srgssum sp. T. ornt Amphiro sp. Glxur sp. Hypne sp. Lurenci sp. 1 Lurenci sp. 2 C. fstigit H. cylindrce H. discoide H. opunti Pdin sp. Srgssum sp. T. ornt Amphiro sp. Glxur sp. Hypne sp. Lurenci sp. 1 Lurenci sp. 2 Fig. 2. Feeding selectivity by 6 herbivorous reef fish species. (,b) Signus dolitus, S. cnlicultus, nd Pomcnthus sexstritus; (c,d) Hipposcrus longiceps, Chlorurus microrhinos, nd Scrus rivultus. (,c) Men number of bites per lgl species, nlyzed using Friedmn s test followed by Friedmn s multiple comprisons tests; letters indicte homogenous subgroups. (b,d) Men selectivity estimted using Struss Liner Selection Index (L). Full lgl species nmes given in Fig. 1

6 182 Mr Ecol Prog Ser 352: , 27 6 () Signus dolitus n = 28 6 (b) Scrus rivultus n = (c) Signus cnlicultus n = 23 6 (d) Hipposcrus longiceps n = 12 Totl number of bites (e) Pomcnthus sexstritus n = 17 6 (f) Chlorurus microrhinos n = Totl number of first bites Fig. 3. Reltionship between the totl number of first bites on ech lg versus the totl number of bites tken from tht lg by 6 herbivorous reef fish species, over ll selectivity trils. Ech dt point represents seprte mcrolgl species. Lines represent trend lines DISCUSSION The herbivorous reef fishes in Pioneer By exhibited considerle selectivity in their feeding, with the extent nd nture of selectivity vrying within nd mong fmilies. Signus dolitus selected Hypne sp., Lurenci sp. 2, nd Srgssum sp., while S. cnlicultus selected Srgssum sp. nd Pdin sp. Both signids voided the green mcrolge nd the red Glxur sp. nd Amphiro sp. No previous food selection studies hve exmined either of these 2 signid species on the GBR. However, previous studies of selectivity in signids hve reported strong selectivity. For exmple, using multiple-choice ssys in the lortory, Pillns et l. (24) observed tht S. fuscescens ( close reltive of S. cnlicultus) from Southern Queenslnd (Austrli) preferred red lge (Acnthophor spicifer nd Grcilri edulis) over brown lge (Dictyot dichotom nd Lobophoro vriegt). Using combintion of direct observtions in the l nd gut content nlyses in the field in the Philippines, Von Westernhgen (1973, 1974) likewise

7 Mntyk & Bellwood: Grzing selectivity in reef fishes 183 found tht S. conctent (synonym of S. gutttus), S. ormin (synonym of S. cnlicultus), S. striolt (synonym of S. spinus), nd S. virgt (synonym of S. virgtus) selected vrious fleshy green nd red mcrolge of the gener Enteromorph, Grcilri, Hypne, nd Lurenci, nd ignored brown nd clcreous lge. Furthermore, Tsud & Bryn (1973) used feeding preference experiments in outdoor tnks on Gum nd reported tht Chlorodesmis fstigit ws voided by juvenile S. spinus but ws redily consumed by juvenile S. rgenteus (see lso Pul et l. 199). Given this level of selectivity, the extent of functionl redundncy nd the potentil for the replcement of Signus dolitus nd S. cnlicultus by other functionl groups in Pioneer By my be limited. Of the 33 roving herbivores recorded from the by, only 2 others hve been reported to feed on Srgssum to ny significnt extent: Scrus rivultus nd Pltx pinntus (Bellwood et l. 26). The ltter ws the dominnt browser of lrge mcrolgl strnds, but ws not observed feeding on mcrolge in the selectivity trils in the present study. Thus, it could be rgued tht this system currently displys limited functionl redundncy in terms of feeding on Srgssum spp. nd tht S. dolitus nd S. cnlicultus represent importnt species to conserve loclly for the mintennce of low Srgssum cover on inshore reefs. Mcrolgl selectivity by prrotfishes differed gretly from the signids in tht the prrotfishes consistently selected 1 or more of the green hevily clcified Hlimed species, while voiding the brown mcrolge, including Srgssum sp. nd the mjority of the red mcrolge, especilly Glxur sp. The reltively uniform selectivity of these 3 prrotfish species suggests tht this reef system my exhibit some functionl redundncy t the species level, in terms of feeding on Hlimed spp., but tht ll 3 species should be considered importnt for mintining low biomss of clcified lge on inshore GBR reefs. However, the prrotfishes would not likely be le to replce the signids nd vice vers. All 3 prrotfish species voided Srgssum sp., nd both species of signids voided the clcified Hlimed species nd Amphiro sp. Thus, the prrotfishes nd signids disply distinct functionl seprtion nd exhibit no reciprocl redundncy. Previous reserch on mcrolgl selection mong prrotfishes hs concentrted on the genus Sprisom in the Cribben, nd contrsts mrkedly with the present study. For exmple, the segrss-dwelling species S. rdins demonstrted voidnce of Hlimed incrsst in field observtions (Lobel & Ogden 1981, Trgett et l. 1986). Tnk-bsed preference experiments likewise found the genus Hlimed to be rejected by S. viride nd S. urofrentum; Srgssum, Turbinri, nd Pdin were preferred (Lewis 1985). However, such Cribben observtions must be interpreted with cution when exmining Indo-Pcific ptterns. The dominnt Cribben prrotfish genus Sprisom belongs to distinct linege (formerly Sprisomtine) tht feeds in n excvting mnner on epilithic lge (S. viride) (Bruggemnn et l. 1994) or, more often, in browsing mnner on mcrolge nd segrsses (Rndll 1967, Bellwood 1994, Streelmn et l. 22, Westnet et l. 25). Species in this linege re reltively rre on Indo-Pcific corl reefs (6 of c. 71 species; Bellwood 1994). The 3 prrotfishes investigted in the present study re in seprte linege (formerly the Scrine). In terms of feeding modes, Chlorurus microrhinos is n excvtor, nd Hipposcrus longiceps nd Scrus rivultus re both scrpers of the reef substrtum (Bellwood & Chot 199). Although ll 3 species re primrily regrded s grzers of the epilithic lgl mtrix (Bellwood & Chot 199, Bellwood 1994, Wilson et l. 23, Chot et l. 24, Fox & Bellwood 27), ll hve previously been reported to feed on mcrolge under experimentl conditions (Bellwood et l. 26, Mntyk & Bellwood 27). Thus, the min similrity between Cribben species nd the Indo-Pcific prrotfishes exmined in the present study, t this loction, is tht in both systems, prrotfishes exhibit significnt selection, lthough they my vry considerly in their individul feeding modes. Most pomcnthids re benthic omnivores tht feed on vriety of sponges, tunictes, scidins, soft corls, nd foliose clcreous or turf lge (Allen et l. 1998), nd hve therefore not been considered to be significnt herbivores. In keeping with this omnivorous diet, the pomcnthid Pomcnthus sexstritus displyed limited selectivity. The observed differences in feeding selectivity in the present study my reflect functionl cpilities in different fish fmilies. Herbivorous prrotfishes including Chlorurus microrhinos, Scrus rivultus, nd Hipposcrus longiceps possess severl ttributes tht fcilitte the consumption of hevily clcified food. The specilized teeth, fused dentl pltes, phryngel mill, powerful jw structure, nd lck of n cidic stomch enle species in this fmily to ingest diet high in crbontes (Bellwood & Chot 199, Bellwood et l. 23, Chot et l. 24). Thus, the selectivity of Hlimed by these species of prrotfishes my simply be reflection of their ility to exploit redily ville food source tht is highly defended both morphologiclly nd chemiclly (Littler et l. 1983, Lewis 1985, Hy & Fenicl 1988). The clcifiction in Hlimed species my be successful in deterring herbivory by signids, which hve smller, weker jw structures

8 184 Mr Ecol Prog Ser 352: , 27 nd cidic stomchs. Similr results hve been reported by Lewis (1985), who found tht lightly clcified lge nd those with tough, lethery thlli were susceptible to grzing by sprisomtine prrotfishes, but were voided by surgeonfishes (which hve more delicte jws). In contrst, it is interesting to note tht the prrotfishes in our study voided the red mcrolg Glxur sp. Although prrotfishes pper to be physiclly cple of consuming this robust nd lightly clcified lg, no consumption ws observed. Glxur sp. is not known to possess potent nti-herbivore chemicls, but it still ppers to be le to deter prrotfish feeding (possibly due to tougher thlli or limited nutritionl content rther thn clcified tissues). The differences in selectivity observed here re not solely due to fish morphology. For exmple, the signids (Signus dolitus nd S. cnlicultus), the smllest of the 6 fish species exmined, selectively te tough, non-clcified mcrolgl species, Srgssum sp. Furthermore, S. dolitus nd S. cnlicultus pper to shre typicl signid jw morphology nd possess thin-wlled, highly cidic stomchs, but S. cnlicultus selected different mcrolgl species (Srgssum sp. nd Pdin sp.) compred to S. dolitus (Hypne sp., Lurenci sp. 2, nd Srgssum sp.). Selectivity is not simple process, nd it is possible tht ecologiclly similr species of herbivorous reef fishes hve different physiologicl responses to structurl components nd lgl secondry metolites (Pul et l. 199, Meyer et l. 1994). Digestive physiology of herbivorous fishes cn vry mong species in terms of stomch ph, resident intestinl protozons (nd other microbes), nd gut length (Meyer et l. 1994). These chrcteristics cn lso vry in n individul species over reltively short periods of time (Meyer et l. 1994). While gut content nlyses hve provided vlule informtion on the reltive importnce of dietry components nd feeding ptterns in herbivorous reef fishes (Rndll 1967, Wilson et l. 23, Chot et l. 24), food selection studies provide different perspective. Gut content nlysis reflects selection in the context of vilility. For exmple, in our study, Srgssum sp. ws highly selected by the signids. However, Srgssum sp. would probly not be undnt in signid gut contents on regulr bsis, s it is not redily ville on the reef crest. Thus, gut content nlyses my reflect processes tht mintin lgl distributions in helthy reef ecosystem (relized niche) rther thn processes tht cn estlish ptterns (potentil niche) or the cpcity of system to cope with chnge (resilience) such s incresing mcrolge. Our findings lso emphsize the need to consider the criteri for describing functionl groups. Although prrotfishes nd signids re ll nominlly grzers of the epilithic lgl mtrix, when conditions chnge nd mcrolge re ville (s simulted in this experiment), new potentil functionl groups re reveled. Under these conditions, prrotfishes re clcified lgl browsers nd the signids re highly selective mcrolgl browsers. Thus, the functionl role of species is highly context dependent; the role of species under norml nd disturbed conditions my be profoundly different. These disturbed conditions my be experimentlly induced, s in this study, but incresingly, humn impcts, nturl disturbnce, nd globl climte chnge re modifying the benthic community composition of corl reefs (Hughes et l. 23, Bellwood et l. 24), with incresed mcrolge being common chrcteristic of disturbed reef ecosystems (Mumby et l. 26, Hughes et l. 27). In some respects, most of the species in our study represent sleeping functionl groups sensu Bellwood et l. (26) in tht their ilities my not be fully exposed until conditions chnge. 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