The Nurseryfish, Kurtus gulliveri, is known for “forehead brooding” parental care behavior in which the adult male carries the egg mass on a hook that overhangs the “forehead” region. This hook is derived from the supraoccipital bone, and it has been suggested to be formed through absorption rather than growth of the supraoccipital crest (SOC). Here we employed X-ray microtomography to investigate the neurocranium with focus on occurrence of the supraoccipital hook in a series of specimens from postflexion larva to adult male and female (10–200 mm SL). A slit-like cavity filled with soft tissue but not spongy bony tissue was revealed in the SOC in all examined specimens except the two smallest larvae (<15 mm SL), thus the cavity appears in the late postflexion-stage. The SOC can be divided into three parts from anterior to posterior in juveniles and adult females by reference to the cavity. 1. The crest rostrum is the most anterior end of the supraoccipital, and it increases in height with a sloped dorsal edge at about a 30° angle. 2. The intermediate segment in the middle of the crest contains a slit-like cavity filled with soft tissue, and its dorsal edge is horizontal and serrated. 3. The posterior process occupies the entire rear portion and is vertically expanded with two transverse ridges. The supraoccipital hook in adult males is derived from this basic configuration by the modifications: the crest rostrum is resorbed ontogenetically, and the dorsal anterior part of the intermediate segment protrudes forward to form the hook; the cavity within the intermediate segment extends into the hook termination. The function of cavity is not yet known; however, our results combined with the previous histological study imply that it may accommodate vessels nourishing the SOC in K. gulliveri. Apogonid fishes, putative relatives of the family Kurtidae, were included in this study for comparison. The SOC is a simple laminar structure without any cavity in Jaydia lineata and J. truncata. Based on the above findings, we suggest that: 1. the supraoccipital hook in adult males is formed by resorption of the crest rostrum combined with growth of the anterodorsal part of the intermediate segment of the SOC; 2. the SOC with a cavity is a unique osteological feature and may represent an anatomical pre-adaption allowing the development of the supraoccipital hook which made possible the evolution of the unique method of paternal forehead brooding in K. gulliveri.

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