YELLOW JUVENILE COLOR PATTERN, DIET SWITCHING AND THE PHYLOGENY OF THE SURGEONFISH GENUS ZEBRASOMA (PERCOMORPHA, ACANTHURIDAE)

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1 BULLETIN OF MARINE SCIENCE, 63(2): , 1998 YELLOW JUVENILE COLOR PATTERN, DIET SWITCHING AND THE PHYLOGENY OF THE SURGEONFISH GENUS ZEBRASOMA (PERCOMORPHA, ACANTHURIDAE) Radu Cornel Guiasu and Richard Winterbottom ABSTRACT Optimization of yellow juvenile coloration on a previously published genus-level cladogram of acanthurid fishes predicts that such coloration is either plesiomorphic (given that the species with yellow juveniles are basal in their respective genera, 6 steps minimum), or that this coloration has developed independently (4 steps minimum). These hypotheses were tested by examining the phylogenetic relationships among the six currently recognized species of Zebrasoma, one of the genera with a species (Z. flavescens) possessing yellow juveniles. A linearly coded cladistic analysis of 14 osteological and external characters produced two equally parsimonious trees (22 steps, consistency index = 0.91). Both tree topologies indicated that: (1) Zebrasoma is a monophyletic group; (2) Z. veliferum is the sister group of the remaining five species; (3) Z. gemmatum is the sister group of the next four species; (4) Z. xanthurum + Z. rostratum + Z. flavescens + Z. scopas form a monophyletic group (the Z. scopas clade); and (5) Z. flavescens + Z. scopas form a monophyletic group. Running the analysis with the multistate characters unordered and employing a strict consensus tree collapses the Z. scopas clade into a polytomy. We argue that (6) Z. rostratum is the sister group to (5) above. The species with yellow juveniles, Z. flavescens, is one of the terminal two taxa in the genus, and not basal as one of the above optimizations predicted. Thus the plesiomorphic condition in Zebrasoma is most parsimoniously interpreted as non-yellow juveniles. Re-optimizing the juvenile coloration data on the genus-level cladogram predicts that a non-yellow juvenile color was the character state for all clades of acanthurids. Thus, yellow juveniles have evolved independently at least four times during the evolution of these fishes (in Acanthurus, Ctenochaetus, Prionurus and Zebrasoma). This suggests that it is an adaptation and that there may be (a) significant (but as yet unknown) selection pressure(s) at work. Possible forces driving these potential adaptations include lowered predation rates, increased access to food, or poster coloration. Preliminary information on diet in Zebrasoma confirmed that the basal diet in this genus consists of macroalgae, with a switch to filamentous algae in the ancestor of the Z. scopas clade (4 above), correlated with changes in the upper jaw teeth and the pharyngeal apparatus of these fishes. The power of microevolutionary explanations of character states in organisms depends in large part on whether those states are plesiomorphic or apomorphic. Plesiomorphic traits that are retained have a historical explanation, and may be so old that the microevolutionary processes responsible for them may no longer be discernable. Alternatively, apomorphic states may provide the opportunity to discover microevolutionary processes, especially when convergence (sensu Coddington, 1988) can be demonstrated. The vast majority of studies that utilize macroevolutionary predictions based on cladograms to explore scenarios of adaptation, speciation and biogeography do so by utilizing cladograms originally generated for other (often purely heuristic) purposes (Brooks and McLennan, 1991; Harvey and Pagel, 1991, and references therein). In contrast, the time seems ripe for the microevolutionary questions to begin driving the search for macroevolutionary patterns. This point is illustrated by examining the evolution of two char- 277

2 278 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 acters, yellow juvenile coloration, and diet (foraging preference), in a group of tropical marine reef fishes, the Acanthuridae. The members of the surgeonfish genus Zebrasoma are tropical, herbivorous, marine fishes of moderate size, found in the fairly shallow waters around coral reefs of the Indo- West Pacific Ocean (Randall, 1955a). The six currently recognized species of this genus are: Zebrasoma flavescens, Z. gemmatum, Z. rostratum, Z. scopas, Z. veliferum, and Z. xanthurum (Randall, 1955b). Some authors, such as Barlow (1974), considered the yellow Z. flavescens and the brown Z. scopas as two different color forms of the same species. However, Randall (1955b) concluded that Z. scopas and Z. flavescens were distinct species, based on consistently different color patterns and distributions and the modally different (but partially overlapping) anal and dorsal fin-ray counts. Mok (1977) found that Z. flavescens had a different gut pattern from that of Z. scopas and other species of Zebrasoma examined, and recommended, based on this and the previously cited evidence, that Z. scopas and Z. flavescens should be retained as separate species. The phylogenetic relationships of the six, currently recognized, acanthurid genera and the sister group relationships of Zebrasoma have been recently hypothesized (Guiasu, 1991; Guiasu and Winterbottom, 1993; Winterbottom, 1993). Those studies show Paracanthurus to be the sister group of Zebrasoma. The monophyly of Zebrasoma was supported by one osteological autapomorphy (Guiasu, 1991; Guiasu and Winterbottom, 1993) and one myological autapomorphy (Winterbottom, 1993). Intrageneric variation between species of Zebrasoma of such osteological characters as the shape of the fifth ceratobranchial and the urohyal was reported by Guiasu (1991), but these characters were not analyzed. Herein we present additional autapomorphies in support of the monophyly of Zebrasoma and elucidate the intrageneric phylogenetic relationships of the six, currently recognized, Zebrasoma species (Randall, 1955b). METHODS Data were analyzed using the cladistic methods initially described by Hennig (1950, 1966) and Wiley (1981) and the outgroup comparison method of Maddison et al. (1984). The data matrix (see Table 1) was analyzed with Phylogenetic Analysis Using Parsimony [(PAUP) Swofford, 1985; Version 2.4], using Farris optimization, rooted by outgroup and using the branch-and-bound option (to find all most parsimonious phylogenetic trees), and the apolist option (in order to obtain a list of the apomorphies of each node). The data were also analyzed using HENNIG86 (Farris, 1986) using the ie command. Paracanthurus was designated as the first outgroup. Ctenochaetus + Acanthurus and Prionurus were chosen as the second and third outgroups, respectively (Guiasu and Winterbottom, 1993; Winterbottom, 1993; Fig. 1). Dissections and drawings were made using a Wild M-5A dissecting microscope with a camera lucida attachment. Specimens for osteological study were doublestained for cartilage and bone, using the methods developed by Taylor and Van Dyke (1985). RESULTS The numbers of the characters described below correspond to those listed in Table 1. OSTEOLOGICAL CHARACTERS. (1) Upper jaw teeth. The teeth of all species of Zebrasoma, like those of all other acanthurids (except Ctenochaetus), are flattened and incisor-like (Guiasu and Winterbottom, 1993). In Paracanthurus, Z. gemmatum and Z. veliferum, the denticulations on the anterior edge of the upper jaw teeth are equal in size to those on the

3 GUIASU AND WINTERBOTTOM: ZEBRASOMA OSTEOLOGY, PHYLOGENY AND EVOLUTION 279 Table 1. Data matrix showing polarized characters for cladistic analysis. Character numbers (1 14) correspond to those listed in the Results section. The autapomorphies for Zebrasoma ( characters 15, 17 18) and the synapomorphy linking Paracanthurus and Zebrasoma (character 16) are not included. Character Taxon Prionurus A canthurus + Ctenochaetus Paracanthurus Zebrasoma veliferum Zebrasoma gemmatum Zebrasoma xanthurum Zebrasoma rostratum Zebrasoma flavescens Zebrasoma scopas posterior edge of these teeth (Fig. 2A,B), and this is therefore considered to be the primitive condition. Zebrasoma scopas, Z. flavescens, Z. rostratum and Z. xanthurum display the apomorphic condition in which the denticulations on the posterior edge of the upper jaw teeth are noticeably smaller than those of the anterior edge (Fig. 2C; see also fig. 1 of Randall, 1955b). Figure 1. Cladogram of acanthurid genera and the first three sequential outgroups with juvenile coloration optimized on it using ACCTRAN (if DELTRAN is used, all change occurs within the terminal nodes). Juvenile coloration: yellow in one or more (but never all) species YN; no species with completely yellow juveniles N.

4 280 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 Figure 2. Left lateral view of the upper jaw teeth of (A) Paracanthurus hepatus, USNM , 78 mm SL. (B) Zebrasoma veliferum, ROM 933CS, 50 mm SL. (C) Zebrasoma scopas, ROM 932CS, 59 mm SL. Bone indicated by stipple, cartilage, where present, indicated by circles. Scale bars = 1 mm. (2) Interopercle shaft width. The maximum width of the interopercle shaft represents 23 25% of its total length in Paracanthurus (Fig. 3A) and the species of Ctenochaetus + Acanthurus examined, 11 13% in Z. veliferum, Z. gemmatum and Z. xanthurum (Fig. 3B), and 5 9% in Z. rostratum, Z. flavescens and Z. scopas (Fig. 3C). The reduction in the width of the interopercular shaft does not seem to be correlated with the elongation of this shaft within Zebrasoma (see Characters informative at a higher level ), and these two features are treated separately. We have analyzed this multistate character as an ordered transformation series (23 25% 11 13% 5 9%). Analyzing this character as unordered makes no difference to the cladograms or the associated tree statistics. The opercular series, showing the position of the interopercle with respect to the opercle and subopercle in Z. rostratum is shown in Figure 3D. (3) Urohyal. The urohyal of all acanthurids is somewhat sickle-shaped and has a broadly concave posteroventral margin with a well developed groove on its anterior surface. This groove forms an articular surface for the attachment of the urohyal to the hypohyals. In Paracanthurus (Fig. 4A) and Z. veliferum (Fig. 4B), the length of the urohyal groove represents the dorsal 45 50% of the length of the anterior margin of the urohyal (plesiomorphic), whereas in the other five species of Zebrasoma the groove is deep and broad and, uniquely among acanthurids, virtually as long as the anterior edge of the urohyal

5 GUIASU AND WINTERBOTTOM: ZEBRASOMA OSTEOLOGY, PHYLOGENY AND EVOLUTION 281 Figure 3. Left lateral view of the interopercle of (A) Paracanthurus hepatus, USNM , 78 mm SL. (B) Zebrasoma xanthurum, ROM 64492, 114 mm SL. (C) Zebrasoma rostratum, BPBM , 70 mm SL. The arrows point to the interopercle shaft. (D). Opercular series showing position of the interopercle with respect to the opercle and subopercle in Zebrasoma rostratum, BPBM 1174, 70 mm SL. Figure abbreviations: IOP = interopercle; OP = opercle; SOP = subopercle. Scale bars = 5 mm. (apomorphic, Fig. 4C). At its broadest point, near the mid-point of its ventral half, the deep, long groove shared by all species of Zebrasoma except Z. veliferum is flanked by thick, broadly convex ridges on each side. Since we have no means to decide whether the long groove and the thick convex ridges are independent of one another, we treat these conditions as a single character. (4) Ventral hypohyal. The ventral edge of each of the ventral hypohyals of Z. flavescens, Z. rostratum, Z. scopas and Z. xanthurum bears two prominent, convex, ventral processes, one anterior and one posterior (Fig. 5C). These ventral processes are perpendicular to the ventral margin of the ventral hypohyal (apomorphic). In other acanthurids (including the remaining two species of Zebrasoma), the anterior process is absent and the posterior process, where present, projects posteriorly from the ventral margin of the ventral hypohyal (plesiomorphic, Fig. 5A,B). (5) Hypobranchial 2 process. Paracanthurus has a very small, barely noticeable, anterolateral process on hypobranchial 2, the length of which is 12 16% of the medial margin of the bone (Fig. 6A). Some species of all other acanthurid genera, except Naso, also have a very small anterolateral process on hypobranchial 2, which has a length of 5 16% of the length of the medial margin of the hypobranchial 2. The second hypobranchial of Z. veliferum and Z. gemmatum (Fig. 6B) has an anterolateral process, the length of which represents 10 30% of the length of the medial margin of the hypobranchial 2. These percentages, calculated for the other Zebrasoma species, are: 47 50% for Z. xanthurum, 43 58% for Z. rostratum and 75 86% for Z. flavescens and Z. scopas. We treat the states of the flange as an ordered transformation series (5 30% 43 58% 75 96%), providing evidence for the monophyly of the four latter species with further resolution of the

6 282 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 Figure 4. Left lateral view of the urohyal of (A) Paracanthurus hepatus, ROM 520CS, 67 mm SL. (B) Zebrasoma veliferum, ROM 933CS, 50 mm SL. (C) Zebrasoma scopas, ROM 1405CS, 100 mm SL. The arrows point to the groove on the anterior surface of the urohyal. Scale bars = 1 mm. sister group status of Z. flavescens and Z. scopas. Evaluating this transformation series unordered makes no difference to the topologies or to the tree statistics. (6) Cleft in hypobranchial 2 process. In Z. xanthurum, Z. rostratum, Z. flavescens and Z scopas, the second hypobranchial process is expanded into a long, pointed flange and a small but conspicuous cleft is present at the posteromedial tip of this flange (Fig. 6C,D). The elongated flange and its cleft are not found in any other acanthurids. The presence of the cleft thus provides evidence for the monophyly of these same four species. (7) Ceratobranchial 5 width. The maximum width of the ceratobranchial 5 is 28 30% of its length in Paracanthurus (Fig. 7A), 22 28% in Z. veliferum and Z. gemmatum (Fig. 7B) and 40 55% in the other four Zebrasoma species (Fig. 7C). We regard 22 30% as the plesiomorphic condition, and 40 55% as the apomorphic state. The interspecific differences in the width of the ceratobranchial 5 within Zebrasoma are correlated with differences in the shape of this bone (Guiasu, 1991). The fifth ceratobranchial of Paracanthurus, Z. veliferum and Z. gemmatum is narrow and fusiform (Fig. 7A,B). In the other four Figure 5. Left lateral view of the ventral hypohyal of (A) Paracanthurus hepatus, USNM , 78 mm SL. (B) Zebrasoma gemmatum, RUSI 17818, 35 mm SL. (C) Zebrasoma scopas, ROM 1405CS, 100 mm SL. The arrows point to the two convex processes perpendicular to the ventral margin of the ventral hypohyal. Scale bars = 1 mm.

7 GUIASU AND WINTERBOTTOM: ZEBRASOMA OSTEOLOGY, PHYLOGENY AND EVOLUTION 283 Figure 6. Left lateral view of the hypobranchial 2 of (A) Paracanthurus hepatus, USNM , 78 mm SL. (B) Zebrasoma gemmatum, RUSI 17818, 35 mm SL. (C) Zebrasoma xanthurum, BPBM 10514, 145 mm SL. (D) Zebrasoma flavescens, USNM , 67 mm SL. The arrows point to the cleft and flange, respectively, of hypobranchial 2. Scale bars = 1 mm. Zebrasoma species, the ceratobranchial 5 has the shape of a wide, ovoid plate (Fig. 7C). The presence of a wide, ovoid fifth ceratobranchial is a synapomorphy of Z. xanthurum + Z. rostratum + Z. flavescens + Z. scopas. (8 and 9) Ceratobranchial 5 teeth. Paracanthurus, Z. gemmatum and Z. veliferum have few (maximum 3 4 per row), fairly large, mostly dorsally-pointing teeth on the fifth ceratobranchial (Fig. 7A,B), whereas Z. flavescens, Z. rostratum, Z. scopas and Z. xanthurum have numerous (maximum 8 per row), small, mostly medially-pointing teeth (Fig. 7C). Differences between Z. flavescens and Z. veliferum in the shape of the fifth ceratobranchials, referred to as lower pharyngeals, and the size of the teeth, were first noted and illustrated by Jones (1968: fig. 19), but were not polarized or discussed further. Since there is no obvious reason why a broader based ceratobranchial 5 should necessarily carry smaller, more numerous teeth, we regard this as a separate character (#8) from the relative width of ceratobranchial 5, and thus evidence for the monophyly of Z. flavescens, Z. rostratum, Z. scopas and Z. xanthurum. Additionally, we regard the medially directed teeth (as opposed to dorsally directed) as a separate synapomorphy of these four species (#9). Figure 7. Dorsal view of the pair of ceratobranchials 5 of (A) Paracanthurus hepatus, USNM , 78 mm SL. (B) Zebrasoma gemmatum, RUSI 17818, 35 mm SL. (C) Zebrasoma scopas, ROM 1405CS, 100 mm SL. Scale bars = 1 mm.

8 284 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 Figure 8. Dorsomedial view of the left pharyngobranchials 2 4 of (A) Paracanthurus hepatus, USNM , 78 mm SL. (B) Zebrasoma gemmatum, RUSI 17818, 35 mm SL. (C) Zebrasoma flavescens, USNM , 67 mm SL. Each pharyngobranchial is numbered in the figure. Scale bars = 1 mm. (10) Pharyngobranchials 2 4. Zebrasoma flavescens, Z. rostratum, Z. scopas and Z. xanthurum have more and smaller teeth on pharyngobranchials 2 4 (Fig. 8C) compared to both Paracanthurus (Fig. 8A) and Z. veliferum and Z. gemmatum (Fig. 8B), indicating that the former four species form a monophyletic group. (11) First dorsal- and anal-fin pterygiophores. The posterolateral margins of the heads of the first dorsal- and anal-fin pterygiophores have fairly well developed posterolateral expansions in both specimens of Z. xanthurum examined. Tyler (1970) reported that Z. rostratum had well developed posterolateral expansions of the first dorsal and anal pterygiophores, but that the other two species of Zebrasoma (Z. flavescens and Z. veliferum) had very small, if any, such expansions (Tyler, 1970: fig. 13). However, we found that only one of three specimens of Z. rostratum we examined had pronounced posterolateral expansions of the first dorsal and anal pterygiophores. This specimen (illustrated by Tyler 1970: fig. 13B), is neither the largest nor the smallest of the three Z. rostratum specimens examined, and, therefore, the presence or absence of these well developed posterolateral expansions does not appear to be size or age related in this species (we were unable to check the specimens for sexual dimorphism). Another feature of Z. rostratum, the length of the snout, also shows great intraspecific variability which does not appear to be related to growth or sex (Randall, 1955b). All the specimens of Z. veliferum, Z. gemmatum, Z. scopas and Z. flavescens examined have very small if any posterolateral expansions of the first dorsal and anal pterygiophores. Paracanthurus shows little or no such expansions (Tyler, 1970: fig. 12). Thus, on the basis of first outgroup comparisons, the pres-

9 GUIASU AND WINTERBOTTOM: ZEBRASOMA OSTEOLOGY, PHYLOGENY AND EVOLUTION 285 ence of well developed posterolateral expansions of the heads of the dorsal and anal pterygiophores in Z. xanthurum and at least some Z. rostratum is considered to be a synapomorphy for these two species. (12 and 13) Dorsal fin. All species of Zebrasoma have an elevated dorsal fin, by comparison to other acanthurids. Zebrasoma veliferum (the sailfin tang) has an extremely elevated dorsal fin. We quantified the length of the dorsal fin by measuring the longest dorsal fin ray (#12) and the longest dorsal spine (#13), respectively, and converting these values to percentage standard length (SL). The same spine and fin ray were measured in all cases. The longest dorsal-fin ray is contained % SL for Z. veliferum and %SL in the other species of Zebrasoma (see also Randall 1955a, 1984). The ranges for the outgroups are: % for Paracanthurus, % in Ctenochaetus, % in Acanthurus and % for Prionurus. The longest dorsal spine is contained % SL in Z. veliferum and % in the other species of Zebrasoma. The measurements for the outgroups are: % for Paracanthurus, % for Ctenochaetus, % for Acanthurus and % for Prionurus. These measurements indicate that the longest dorsal-fin ray and spine are apomorphically elongated in all Zebrasoma species and, autapomorphically, extremely elongated in Z. veliferum. The two features have been treated as independent characters, under the assumption that elongation of the dorsal spines is independent from the elongation of the dorsal-fin rays. This assumption is supported by the fact that, in Zebrasoma, the anal-fin rays are apomorphically elongated, whereas the anal-fin spines are not. EXTERNAL CHARACTERS. (14) Setae on the posterior sides of the body. Zebrasoma xanthurum, Z. rostratum, Z. flavescens and Z. scopas have, uniquely among acanthurids, a conspicuous, bilateral oval patch of setae just anterior to the movable caudal spine. This patch of setae, which tends to be better developed in larger specimens, is much larger and made up of longer, more dense setae in Z. flavescens and, particularly, Z. scopas than in Z. xanthurum and Z. rostratum of similar size. In large specimens of Z. scopas, for instance, the patch of elongated, brush-like setae can extend anteriorly from the caudal spine half way to the base of the pectoral fin. In Z. rostratum, unlike the other Zebrasoma species which possess this character, the setae may be present only in males, and the presence of the setae may be a sex-related character in this species (Randall 1955b, 1984). Only two of three Z. rostratum specimens examined had setae on the sides of the body. The patch of setae, although present in both sexes, is larger and has longer setae in males of Z. scopas (Randall, 1986). The setae are absent in Z. veliferum and Z. gemmatum (Randall 1955b, 1984). In Paracanthurus, the cteni on a few scales anterior to the caudal peduncle are about three times longer than the scale cteni from any other region of the body (Randall, 1955b). These elongated cteni are here assumed to be homologous to the setae (which represent elongate scale cteni) of four of the six Zebrasoma species. The presence of long cteni or setae, anterior to the caudal spine, is thus a putative synapomorphy for Paracanthurus and some Zebrasoma. The presence of the oval patch of setae remains a synapomorphy for all Zebrasoma except Z. veliferum and Z. gemmatum, and the particularly well developed setae of Z. scopas and Z. flavescens is a synapomorphy for these two species. We have analyzed this character as an ordered transformation series. Analyzing this character as unordered results in only two trees being found (the same two as found when running the entire data set as ordered, but of 21 steps, 0.95 CI). This method (unordered states) can raise the CI in trees where homoplasy is present in the data set, since it

10 286 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 has the net effect of a posteriori optimizing the character on a tree generated by other data. CHARACTERS INFORMATIVE AT A HIGHER LEVEL. (15) Interopercle shaft length. The interopercular shaft, which extends anteriorly from the squarish head of the interopercle, attaches anteriorly to the retroarticular by a conspicuous ligament. The length of the interopercular shaft represents approximately 60 62% of the total length of the interopercle in Paracanthurus (Fig. 3A), and the Ctenochaetus and Acanthurus species examined. For all Zebrasoma species the length of the interopercular shaft is 69 81% of the total length of the interopercle (Fig. 3B,C). While this character is of no assistance in solving the intrarelationships of Zebrasoma, we include this apomorphy for Zebrasoma here since it has not been previously described. (16) Symplectic. The symplectic was reported to have a gentle, sigmoid flexure in Paracanthurus and Zebrasoma, an apomorphy uniting these two genera (Guiasu, 1991; Guiasu and Winterbottom, 1993). In these two previous studies, however, only four of the six species of Zebrasoma (Z. flavescens, Z. rostratum, Z. scopas and Z. veliferum) were examined. The current study confirms the presence of a symplectic with a gentle, sigmoid flexure in Paracanthurus (Guiasu and Winterbottom 1993: fig. 1), as well as in all of the six species of Zebrasoma. A sigmoid symplectic is homoplastically present in a few species of Acanthurus (Tyler, pers. comm.). (17) Number of dorsal-fin spines. All acanthurids, except Zebrasoma and several species of Naso, have six or more dorsal spines. The reduction of the number of dorsal spines is hypothesized to have occurred independently in Zebrasoma and the species of Naso with fewer than six dorsal spines, based on the overall parsimony analysis for acanthurid genera presented by Winterbottom (1993). Thus, the presence of only 4 5 dorsal spines is an apomorphy of Zebrasoma (Guiasu and Winterbottom, 1993). Within Zebrasoma, Z. veliferum and Z. gemmatum always have four dorsal spines, while Z. xanthurum and an overwhelming majority of the specimens of Z. rostratum, Z. flavescens and Z. scopas have five dorsal spines. A few specimens of the latter three species with four rather than five dorsal spines have also been recorded (Randall, 1955b). Since the character is dimorphic in three species, we are reluctant to polarize this information further. (18) Anal fin. The anal fin of Zebrasoma is more elevated than in other acanthurids. The longest anal ray is %SL in adult Zebrasoma. The ranges for the outgroups are: % for Paracanthurus, % for Ctenochaetus, % for Acanthurus and % in Prionurus. Given the cladogram of the evolution of Acanthuridae (Winterbottom, 1993), we hypothesize that the elongation of the longest anal ray has occurred independently in Zebrasoma and a few species of Ctenochaetus and Acanthurus. Interestingly, the longest anal spine of Zebrasoma is not, by and large, more elongated than the longest anal spine of Paracanthurus and some species of Acanthurus and Ctenochaetus. Therefore, the more elevated anal fin of Zebrasoma is due solely to the elongation of the anal-fin rays. This character has not previously been used as an apomorphy of Zebrasoma. CHARACTER NOT USED IN THE ANALYSIS. (i) Gut pattern. Mok (1977) reported that Z. flavescens had, uniquely among acanthurids, an unusually simplified state of the loop a of the gut. The other three Zebrasoma species examined by Mok (1977), Z. rostratum, Z. scopas and Z. veliferum all had a more complicated pattern of this loop. This character, which helps differentiate Z. flavescens from the similar Z. scopas, may be an autapomorphy for Z. flavescens. However, since the gut patterns of Z. gemmatum and Z. xanthurum have

11 GUIASU AND WINTERBOTTOM: ZEBRASOMA OSTEOLOGY, PHYLOGENY AND EVOLUTION 287 Figure 9. A. A strict consensus tree of the phylogeny of the species of Zebrasoma. B. Our preferred hypothesis of the relationships of Zebrasoma species (see text for discussion). Arabic numerals correspond to the numbered characters discussed in the Results section and listed in Table 1. Letters indicate the optimization of yellow juvenile coloration (Y = at least one species with yellow juveniles, N = no species with yellow juveniles) and predicted diet switch (M = macroalgae diet, F = filamentous algae diet). not been examined and since the gut pattern exhibits considerable intrafamilial variability within the Acanthuridae (Mok, 1977), we did not include this character in the phylogenetic analysis. DISCUSSION PHYLOGENETIC ANALYSIS. Two 22-step equally parsimonious trees with a Consistency Index (CI) of 0.91 and a Retention Index (RI) of 0.95 were found when PAUP and HENNIG 86 were run using additive coding (ordered transformation series) for multistate characters. In both these trees, Z. veliferum is the first species to diverge, followed by Z. gemmatum. The final four species form a monophyletic group (the Z. scopas clade), in which Z. rostratum is either the sister group of Z. xanthurum or is the sister species of Z. flavescens + Z. scopas. There are three trees when the data set is evaluated as non-additive (CI = 0.95, RI = 0.97, 21 steps). In addition to the above two trees, the third tree consisted of a trichotomy among Z. xanthurum + Z. rostratum, Z. flavescens, and Z. scopas. A strict consensus tree would thus consist of a polytomy involving each of these four species (Fig. 9A). Acanthuroids in general exhibit considerable plasticity of scale morphology. The enlarged scale cteni of Paracanthurus and of the terminal four species of Zebrasoma may be non-homologous; if so, there is no need to postulate a reversal linking Z. veliferum and Z. gemmatum, in which the scale cteni are not elongate (plesiomorphic). The single character allying Z. xanthurum with Z. rostratum (posterolaterally expanded tips of first dorsal-

12 288 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 fin pterygiophore in the former species and in one of three specimens of the latter species) we find to be less convincing (since the character is dimorphic in Z. rostratum) than the single character of a greatly reduced shaft of the interopercle linking Z. rostratum, Z. flavescens and Z. scopas. Our preferred phylogeny is given in Figure 9B. The monophyly of the Z. scopas clade is particularly well supported by nine synapomorphies (Fig. 9B). OPTIMIZATION OF JUVENILE COLORATION. Tracing the evolutionary origin(s) of yellow juvenile coloration in acanthurids is problematical. Numerous species of surgeonfishes, but not all, have at least some yellow in their color pattern, usually as juveniles and often as adults. Four of the six acanthurid genera contain both one or more species in which the juveniles are bright yellow with few, if any, other markings, and other species not exhibiting this color pattern. Genera lacking at least one species with yellow juveniles are Naso and Paracanthurus (monotypic). Syntopic juveniles of Prionurus punctatus may be either gray with numerous black spots or entirely yellow (Thomson et al., 1979; figure in Goodson, 1988: 164), whereas juvenile P. laticlavus are primarily yellow (Allen and Robertson, 1994). Zebrasoma flavescens is yellow as both a juvenile and an adult (Randall, 1955b), but Ctenochaetus strigosus is yellow only as a juvenile (Randall et al., 1990). Species of Acanthurus with yellow juveniles include the Caribbean A. coeruleus (Böhlke and Chaplin, 1968), and the Indo-Pacific A. olivaceus (Randall, 1956 misidentified as A. tennenti by Burgess and Axelrod, 1973:795 as pointed out by Randall and Anderson, 1993:33), as well as A. pyroferus (Randall and Randall, 1960). The evolutionary optimization of yellow juvenile coloration on the cladogram (Fig. 1) proposed by Winterbottom (1993, 12 taxa, 185 characters, length 200 steps, CI = 0.925) using ACCTRAN (see Wiley et al., 1991, for definitions) is equivocal, primarily because the states of some juveniles yellow and other juveniles non-yellow do not co-occur in any single species except Prionurus punctatus. This optimization required a minimum of six steps, and predicted that species with yellow juveniles were basal within their clade (because an additional step a reversal will be required for each node by which they are removed from the base). In a DELTRAN optimization a minimum of four steps (convergences) are required, assuming that the species of Acanthurus and Prionurus with yellow juveniles form a monophyletic unit within each genus. Interestingly, the Prionurus clade was the first to diverge among the lineages exhibiting at least one species with yellow juveniles. Thus, the possibility that the ancestral acanthurin species possessed the dimorphic state seen in P. punctatus remains. Parenthetically we note that there are several methods other than ACCTRAN and DELTRAN for analyzing such information (e.g., phylogenetic autocorrelation, independent comparisons see Harvey and Pagel, 1991, for a review). However, since most of these methods were designed to deal with continuous variables, we have retained the older methods here. There are thus two hypotheses for explaining the origin of yellow juvenile coloration. Should the species with yellow juveniles in each genus prove to be basal in the phylogenies of their respective genera, then we would conclude that the reversals reflect evolutionary history, and yellow is primitive for juvenile acanthurins. If so, then the conditions surrounding the origins of yellow juvenile coloration are probably so old as to be unidentifiable. [The analysis of character evolution assumes that the plesiomorphic character state is displayed by the basal member(s) of polytypic groups, because an extra step is needed for each node by which the species displaying the character state is removed from that base]. Alternatively, if yellow juvenile coloration has arisen independently in the four genera in which it occurs (convergence), it is possible that some powerful (but as yet

13 GUIASU AND WINTERBOTTOM: ZEBRASOMA OSTEOLOGY, PHYLOGENY AND EVOLUTION 289 unidentified) selective force is driving the system. This study of the phylogeny of Zebrasoma was designed to choose between these hypotheses by discovering whether the species with yellow juveniles, Z. flavescens, is basal in the clade or not. ORIGINS OF YELLOW JUVENILE COLORATION. We have presented two hypotheses to explain the distribution of yellow juvenile coloration. The phylogenetic patterns depicted (Fig. 9) allow us to repudiate the single origin hypothesis for yellow juvenile color (involving reversals) in acanthurins because the species of Zebrasoma with yellow juveniles, Z. flavescens, belongs to the most recently derived species pair (or to a derived clade of four species if one only accepts the consensus tree based on an unordered data set) in the genus. If yellow juveniles were the primitive state, we would have to postulate a minimum of four independent loses just to explain the distribution of states in Paracanthurus, Z. veliferum, Z. gemmatum, and a hypothetical ancestor for the three of the four members of the Z. scopas clade that lack yellow juveniles. More reversals would need to be invoked if the three members of the Z. scopas clade do not form a monophyletic group. The maximum number of such reversals (six) is required if our preferred hypothesis of Zebrasoma phylogeny (Fig. 9B) is correct. Further support for the multiple (convergent) origins of yellow juvenile coloration may be provided by examining the phylogenetic position of Ctenochaetus strigosus, the only species of its genus with yellow juveniles. Guiasu and Winterbottom (1993) presented some evidence to suggest that one species currently placed in Acanthurus, A. nigroris, was the sister group of Ctenochaetus. Thus, the yellow juveniles of C. strigosus cannot represent the ancestral state for that clade either because A. nigroris does not have yellow juveniles. It appears that yellow juvenile coloration has evolved independently in a minimum of two of the four lineages. Re-optimization of the distribution of yellow juvenile coloration on the cladogram of acanthurid genera results in the prediction of four independent acquisitions for this character state (i.e., the convergence optimization is most parsimonious). Identification of repeated convergent evolutionary events provides the starting point for an investigation of the adaptive function (if any) of that character (Coddington, 1988). Some possible adaptive hypotheses for the evolution of distinctive juvenile color patterns have been summarized by Thresher (1984). These include species camouflage, in which juveniles mask their species identity from territorial adults; intra-juvenile advertisement, associated primarily with the spacing out of territorial juveniles; and adult habituation, where conspicuously colored juveniles provoke increased attacks by territorial adults. The initial high rate of attacks leads to rapid habituation of the adults to the presence of the juveniles, which presumably avoid the attacks by hiding in crevices too small for the adults to enter (Thresher, 1984). Several adaptively based scenarios may be proposed to explain the evolution of yellow juvenile coloration in acanthurids. Randall and Randall (1960) hypothesized that A. pyroferus juveniles were mimicking the yellow-colored lemonpeel angelfish (Centropyge flavissimus). This hypothesis is strengthened by the observation that, like the angelfish but unlike almost all other acanthurids (juvenile or adult), juvenile A. pyroferus have a rounded caudal fin. Randall et al. (1990) suggested that the secretive habits of the angelfish cause predators not to bother hunting them (they did not mention other acanthurids with yellow juveniles but without rounded caudal fins, for which the same general explanation is available). Myers (1989: 247; pls. 127d,e) pointed out that at Belau (Palau) in Micronesia, where the C. flavissimus is absent, A. pyroferus juveniles have a somewhat

14 290 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 different coloration that is very similar to that of another secretive angelfish, Centropyge vrolicki. According to this hypothesis, then, yellow juvenile coloration has evolved as an anti-predator strategy. An alternative adaptive hypothesis may be erected from observations RW made in 1991 at Bora Bora, Society Islands. A (yellow) juvenile Ctenochaetus strigosus was seen feeding unmolested in the territory of a farmer damselfish (Stegastes nigricans); a pair of lemonpeels, also ignored by the damselfish, were in the territory as well. The damselfish vigorously chased away both the brown-colored adults of C. strigosus and the differentlycolored, schooling, herbivorous, juvenile siganids when they entered its territory. If Ctenochaetus and the damselfish feed on the same species of algae and the lemonpeel on a different resource not defended by the damselfish, then it is possible that the advantage lies in feeding strategies rather than predator avoidance. Both of these adaptive hypotheses suffer from the same problem. One of the species of Acanthurus with a yellow juvenile, A. coeruleus, occurs in the Caribbean, where there are no Recent monochromatic yellow angelfishes. However, the juveniles of Holacanthus tricolor are yellow, albeit with a large, blue-ringed ocellus below the soft dorsal fin (Böhlke and Chaplin, 1968), which is absent from the putative mimic. The relationship, if any, between the angelfish, territorial damselfish and juvenile A. coeruleus in the Caribbean has not been documented. Suffice it to say that A. coeruleus is considerably more abundant than yellow juvenile angelfish. Another explanation is offered by Thresher (1984). Juvenile A. coeruleus have bright yellow bodies and blue eye rings and irises. This color combination is ideal for hue discrimination and spectral sensitivity of reef fishes and is, therefore, highly visible to other fishes in the clear waters around coral reefs (Thresher, 1977). The juveniles are solitary and aggressively territorial, whereas adult A. coeruleus have dull blue bodies and are nonterritorial, schooling fishes. Thus, in this species at least, the bright yellow color of juveniles may represent a poster coloration serving as an intra-specific signal, advertising territorial ownership to conspecific juveniles (Thresher, 1984). OPTIMIZATION OF DIET SWITCHING. Winterbottom and McLennan (1993) optimized dietary information on the cladogram of acanthurid genera, and suggested that feeding on macroalgae was primitive for the clade of Paracanthurus, Zebrasoma, Acanthurus and Ctenochaetus. The phylogenetic analysis presented in this paper can be used in interpreting some of the preliminary information available on the diets of certain Zebrasoma species. Differences between the diets and feeding strategies of Z. veliferum, on one hand, and Z. scopas and Z. flavescens, on the other hand, have been observed by Barlow (1974) and well documented for the former two species by Robertson et al. (1979). Such differences are of particular interest in analyzing the evolution of feeding strategies within Zebrasoma, because Z. veliferum is the sister group of all other Zebrasoma, while Z. scopas and Z. flavescens are both members of a well corroborated monophyletic group that also includes Z. xanthurum and Z. rostratum. In a detailed study of the behavioral ecology of some Indian Ocean surgeonfishes conducted at Aldabra Atoll, Robertson et al. (1979) reported that Z. scopas defended feeding territories and fed exclusively on turfs of microalgae, whereas Z. veliferum patrolled vast areas around the reef and fed on soft macroalgae. These findings support some of the earlier observations of Barlow (1974). Jones (1968) did not mention any differences between the diets of Z. flavescens and Z. veliferum, studied at Hawaii and Johnston Islands, and classified both species as herbivores feeding on filamentous and small fleshy algae. He did not mention the exact diet of

15 GUIASU AND WINTERBOTTOM: ZEBRASOMA OSTEOLOGY, PHYLOGENY AND EVOLUTION 291 either of these two species of Zebrasoma separately. Therefore, it is possible that Hawaii and Johnston Islands Z. veliferum feed primarily on macroalgae, whereas the Z. flavescens feed mostly on fine, filamentous algae. However, pronounced intraspecific differences between the diets of the Aldabra and Hawaii populations of Z. veliferum cannot be ruled out. ORIGIN OF DIET SWITCHING. Robertson et al. (1979) suggested two alternative hypotheses to account for the divergence in diets between Z. scopas and Z. veliferum. Firstly, leafy macroalgae may represent the optimal diet for Z. veliferum, or, secondly, Z. scopas, due to its smaller size, may outcompete Z. veliferum in the use of microalgae, thus forcing the latter species to feed on the larger algae. Our data on the differences between the pharyngeal teeth of Z. veliferum and those of Z. scopas and Z. flavescens tend to support the first hypothesis. Pharyngeal teeth, which include the teeth present on the pharyngobranchials 2 4 and those found on the fifth ceratobranchials, can play a major role in the mastication process in teleosts (Bond, 1979). The pharyngeal teeth of cichlids, for example, are involved not only in food transport, but also in food manipulation, preparation and mastication (Liem, 1971). These teeth, together with gizzards, are particularly important for herbivorous fishes, many of which must grind and tear plant cells extensively to allow the digestive acids and enzymes access to the cell contents (Bond, 1979; Moyle and Cech, 1982). Again, Z. veliferum has fewer and larger pharyngeal teeth than the four members of the Z. scopas clade. Our analysis of the morphological and ecological data within a phylogenetic context indicates that Z. veliferum eats macroalgae because it has inherited that trait from an ancestor, and not because it has been forced to shift diet. The numerous, small pharyngeal teeth of the members of the Z. scopas clade are apomorphic; these teeth may have evolved in response to a shift in the diet from macroalgae to filamentous algae. Thus, a dietary shift from macroalgae to filamentous algae appears to have taken place in some Zebrasoma species, matching a similar dietary shift that has taken place independently in some other herbivorous acanthurids. While there are no specific studies on the role of pharyngeal teeth in the feeding behaviour of Zebrasoma species, it is reasonable to assume that consistent interspecific differences in jaw and pharyngeal teeth within this genus may be linked to interspecific differences in diets and feeding strategies. Unfortunately, to date there is no empirical information available on the diets and feeding strategies of Z. gemmatum, Z. rostratum and Z. xanthurum, the three rare Zebrasoma species (Randall, 1955b; Smith, 1966). However, since Z. rostratum and Z. xanthurum also have small and numerous pharyngeal teeth, we predict, based on the information presented above, that the diets of these two species also consist mainly of filamentous algae. Elucidation of the diet of Z. gemmatum then becomes critical to answering whether diet switching in the Z. scopas clade is correlated with these morphological changes in jaw and pharyngeal teeth (if Z. gemmatum eats macroalgae) or whether the diet change preceded the morphological changes (if Z. gemmatum eats filamentous algae). Many more detailed studies of the behaviour and ecology of all species of Zebrasoma, at various locations throughout their respective ranges, are needed in order to show whether or not there are consistent interspecific behavioral and ecological differences within Zebrasoma. If such differences are uncovered, they could then be interpreted in the light of the currently hypothesized phylogeny of Zebrasoma.

16 292 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 MATERIAL EXAMINED. Institutional abbreviations for material examined follow Leviton et al. (1985). The standard lengths of the specimens are given in millimiters, in parentheses, following each catalog number. The notation CS following catalog numbers indicates cleared and stained specimens. Zebrasoma: Zebrasoma flavescens, USNM CS (67). Zebrasoma gemmatum, RUSI 1243CS (155); RUSI (2, 35 and 44; only the smaller of the two specimens was CS). Zebrasoma rostratum, ANSP CS (97); ROM 64491CS (98); BPBM 11774CS (70). Zebrasoma scopas, ROM 932CS (4, 39 59); ROM 1405CS (100). Zebrasoma veliferum, ROM 933CS (5, 42 50); ROM (84); ROM (107). Zebrasoma xanthurum, ROM 64492CS (114); BPBM (145). Paracanthurus: Paracanthurus hepatus, ROM 520CS (2, 56 and 67); USNM (78). Many other acanthurid specimens, belonging to the remaining four acanthurid genera, were examined during a previous study and are listed in the Material Examined section of another paper (Guiasu and Winterbottom, 1993). Some of the information on acanthurid osteology obtained from that study has been used in polarizing the characters described in this paper. ACKNOWLEDGMENTS We thank J. E. Randall (Bernice P. Bishop Museum, Honolulu, Hawaii), P. C. Heemstra (J.L.B. Smith Institute of Ichthyology, Grahamstown, South Africa), G. D. Johnson (National Museum of Natural History, Washington, D. C.) and W. F. Smith-Vaniz (Academy of Natural Sciences of Philadelphia, Pennsylvania) for providing us with several valuable specimens and for allowing us to clear and stain, as well as dissect some of this material. We are especially grateful for the input of D. A. McLennan on earlier versions of this manuscript. Valuable comments were also received from E. J. Crossman, K. D. Doyle, A. S. Harold, R. L. Mayden and R. W. Murphy. K. D. Doyle performed the PAUP and HENNIG86 analyses, and S. Guiasu (York University, Toronto, Canada) also offered valuable assistance with the computer analysis. Thanks to J. C. Tyler for unpublished information on Acanthurus. Financial support for this study was provided by University of Toronto Open Doctoral Fellowships to RCG and by a Natural Sciences and Engineering Research Council of Canada research grant (OGP ) to RW. This is Contribution No. 12 of the Centre for Biodiversity and Conservation Biology of the Royal Ontario Museum. LITERATURE CITED Allen, G. R. and D. R. Robertson Fishes of the tropical eastern Pacific. Univ. Hawaii Press, Honolulu. Barlow, G. W Contrasts in social behaviour between Central American cichlid fishes and coral-reef surgeon fishes. Amer. Zool. 14: Böhlke, J. E. and C. C. G. Chaplin Fishes of the Bahamas and adjacent tropical waters. Livingstone Publ. Co., Wynnewood, Pennsylvania. Bond, C. E Biology of fishes. Saunders College Publishing, Philadelphia, Pennsylvania. Brooks, D. R. and D. A. McLennan Phylogeny, ecology and behaviour. A research program in comparative biology. Univ. Chicago Press, Chicago, Illinois. Burgess, W. E. and H. R. Axelrod Pacific Marine Fishes. Book 3. Fishes of Sri Lanka, Maldive Islands and Mombasa. T.F.H. Publications, New Jersey. Coddington, J. A Cladistic tests of adaptational hypotheses. Cladistics 4: Farris, J. S Hennig86, Version 1.5. Port Jefferson Station, New York. Goodson, G Fishes of the Pacific Coast. Stanford Univ. Press, California Guiasu, R. C Phylogeny and osteology of the genera of the family Acanthuridae (Perciformes, Pisces). Master s thesis, Univ. Toronto, Toronto, Ontario, Canada. and R. Winterbottom Osteological evidence for the phylogeny of recent genera of surgeonfishes (Percomorpha, Acanthuridae). Copeia 1993:

17 GUIASU AND WINTERBOTTOM: ZEBRASOMA OSTEOLOGY, PHYLOGENY AND EVOLUTION 293 Harvey, P. H. and M. D. Pagel The comparative method in evolutionary biology. Oxford Univ. Press, Oxford. Hennig, W Grundzüge einer Theorie der phylogenetischen Systematik. Deutsche Zentralverlag, Berlin Phylogenetic systematics. Univ. Illinois Press, Urbana, Illinois. Jones, R. S Ecological relationships in Hawaiian and Johnston Island Acanthuridae (surgeonfishes). Micronesica 4: Leviton, A. E., R. H. Gibbs, Jr., E. Heal, and C. E. Dawson Standards in herpetology and ichthyology. Part I. Standard symbolic codes for institutional resource collections in herpetology and ichthyology. Copeia 1985: Liem, K. F Evolutionary strategies and morphological innovations: cichlid pharyngeal jaws. Syst. Zool. 20: Maddison, W. P., M. J. Donoghue, and D. R. Maddison Outgroup analysis and parsimony. Syst. Zool. 33: Mok, H.-K Gut patterns of the Acanthuridae and Zanclidae. Jp. J. Ich. 23: Moyle, P. B. and J. J. Cech, Jr Fishes: an introduction to ichthyology. Prentice-Hall Inc., Englewood Cliffs, New Jersey. Myers, R. F Micronesian reef fishes. Coral Graphics, Guam. Randall, J. E. 1955a. An analysis of the genera of surgeon fishes. Pac. Sci. 9: b. A revision of the surgeon fish genera Zebrasoma and Paracanthurus. Pac. Sci. 9: A revision of the surgeon fish genus Acanthurus. Pac. Sci. 10: Red Sea reef fishes. IMMEL Publishing, London Acanthuridae. Surgeonfishes, tangs, unicornfishes. Unpaginated in W. Fischer and G. Bianchi, eds. FAO species identification sheets for fishery purposes. Western Indian Ocean; (Fishing Area 51). FAO, Rome. vol Family No. 243: Acanthuridae. Pages in M. M. Smith and P. C. Heemstra, eds. Smith s sea fishes. Macmillan South Africa (Publishers) (Pty), Ltd., Johannesburg.,G. R. Allen and R. C. Steene Fishes of the Great Barrier Reef and Coral Sea. Univ. Hawaii Press, Honolulu, Hawaii. and R. C. Anderson Annotated checklist of the epipelagic and shore fishes of the Maldive Islands. Ich. Bull. Smith Inst. 59: and H. A. Randall Examples of mimicry and protective resemblance in tropical marine fishes. Bull. Mar. Sci. Gulf Carib. 10: Robertson, R. D., N. V. C. Polunin, and K. Leighton The behavioral ecology of three Indian Ocean surgeonfishes (Acanthurus lineatus, A. leucosternon and Zebrasoma scopas): their feeding strategies, and social and mating systems. Env. Biol. Fish. 4: Smith, J. L. B A rare acanthurid fish from Mauritius in South Africa. Ann. Mag. Nat. Hist. 9: 5 8. Swofford, D. L PAUP phylogenetic analysis using parsimony. Version 2.4. Illinois Nat. Hist. Surv., Champaign, Illinois. Taylor, W. R. and G. C. van Dyke Revised procedure for staining and clearing small fishes and other vertebrates for bone and cartilage study. Cybium 9: Thompson D. A., L. T. Findley and A. N. Kerstitch Reef fishes of the Sea of Cortez. John Wiley and Sons, New York. Thresher, R. E Eye ornamentation of Caribbean reef fishes. Z. Tierpsychol. 43: Reproduction in reef fishes. T.F.H. Publications, New Jersey. Tyler, J. C Osteological aspects of interrelationships of surgeon fish genera (Acanthuridae). Proc. Acad. Nat. Sci. Philadelphia 122: Wiley, E. O Phylogenetics: the theory and practice of phylogenetic systematics. John Wiley and Sons, New York.