UC San Diego

Signals are evolved to help animals communicate and thereby maximize their inclusive fitness. To increase the efficiently of communication, selection favors animals that are able to increase the effectiveness or amplify their signals over evolutionary time. Relatively few studies of signal amplification have been conducted on fishes although fishes include many species whose behavior is well-known. The Sarcastic Fringehead (Neoclinus blanchardi, Teleostei) exhibits an extreme version of a common aggressive display, the “gaping display” in which an open mouth is presented toward an opponent. This species has extremely long maxillae that are flared laterally during the display. In this study, display behaviors of three live specimens and 14 videos of N. blanchardi were observed, described, and analyzed. The comparative anatomy of N. blanchardi and the related species N. uninotatus and N. stephensae, were examined to document the structures related to the unusual gaping display. Fluorescence of the buccopalatal membrane was investigated. Finally, geometry morphometry, including the truss network system and thin-plate spline, and PCA were used to study the evolution of the maxilla under the heterochrony framework. The results show that the unusual gaping display is used for intraspecific territorial defense. Three main morphological modifications related to amplification of this display are enlargement of the buccopalatal membrane, enlargement of the adductor mandibulae muscle complex, and lengthening of the maxilla. The elongate maxilla of N. blanchardi evolved via acceleration (faster growth compared to outgroups) and hypermorphosis (continued growth to a larger body size), both forms of peramorphosis. All of these characters play a role in the amplification of the gaping display in N. blanchardi.