Yellow juvenile color pattern, diet switching and the phylogeny of the surgeonfish genus Zebrasoma (Percomorpha, Acanthuridae)

Optimization of yellow juvenile coloration on a previously published genus- level cladogram of acanthurid fishes predicts that such coloration is eithe r plesiomorphic (given that the species with yellow juveniles are basal in their respective genera, 6 steps minimum), or that this coloration has deve loped independently (4 steps minimum). These hypotheses were tested by exam ining the phylogenetic relationships among the six currently recognized spe cies of Zebrasoma, one of the genera with a species (Z. flavescens) possess ing yellow juveniles. A linearly coded cladistic analysis of 14 osteologica l and external characters produced two equally parsimonious trees (22 steps , consistency index = 0.91). Both tree topologies indicated that: (1) Zebra soma is a monophyletic group; (2) Z. veliferum is the sister group of the r emaining 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. scop as form a monophyletic group. Running the analysis with the multistate char acters unordered and employing a strict consensus tree collapses the Z. sco pas clade into a polytomy. We argue that (6) Z. rostratum is the sister gro up to (5) above. The species with yellow juveniles, Z. flavescens, is one o f the terminal two taxa in the genus, and not basal as one of the above opt imizations predicted. Thus the plesiomorphic condition in Zebrasoma is most parsimoniously interpreted as non-yellow juveniles. Re-optimizing the juve nile coloration data on the genus-level cladogram predicts that a non-yello w juvenile color was the character state for all clades of acanthurids. Thu s, yellow juveniles have evolved independently at least four times during t he evolution of these fishes (in Acanthurus, Ctenochaetus, Prionurus and Ze brasoma). This suggests that it is an adaptation and that there may be (a) significant (but as yet unknown) selection pressure(s) at work. Possible fo rces driving these potential adaptations include lowered predation rates, i ncreased access to food, or poster coloration. Preliminary information on d iet in Zebrasoma confirmed that the basal diet in this genus consists of ma croalgae, with a switch to filamentous algae in the ancestor of the Z, scop as clade (4 above), correlated with changes in the upper jaw teeth and the pharyngeal apparatus of these fishes.