Yellow Juvenile Color Pattern, Diet Switching and the Phylogeny of the Surgeonfish Genus Zebrasoma (Percomorpha, Acanthuridae)
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.
Document Type: Research Article
Publication date: September 1, 1998
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