Gross anatomy and histology of the hook and skin of forehead brooding male nurseryfish, Kurtus gulliveri, from northern Australia

Mature males of nurseryfish have a hook on their head to which the eggs become attached and are carried like a bunch of grapes. This paper examines the anatomy and histology of the hook. The osteological basis of the hook is shown to be a modi cation of the supraoccipital crest of the skull covered by typical teleost skin. The integument in the cleft of the hook, where the eggs are attached, is considerably different from ordinary fish skin. The stratified epidermis is devoid of secretory mucus and neurosensory cells and is folded into crypts that extend deeply into the dermis. This may be a specialization that facilitates adhesion of the sticky egg mass. Field observations show that this cleft area of the hook is edematous, and histology confirms that the area is highly vascularized. We speculate that this may facilitate gas exchange and/or nutrition between the male and the egg mass, but this can only be confirmed by physiological experiments with pregnant males in captivity. Engorgement with blood in the highly vascularised dermis of the hook may help hold the egg mass in place.