Pufferfish inflation: Functional morphology of postcranial structures in Diodon holocanthus (Tetraodontiformes)

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Abstract
For the purpose of defending itself against predators, the balloonfish (Diodon holocanthus) can triple its body volume by pumping water into its stomach. Diodon holocanthus exhibits striking structural and functional specializations for inflation. The stomach of the balloonfish is highly extensible and has lost its digestive function, it is repeatedly folded upon itself and is lined with transitional epithelium. The peritoneal cavity of the balloonfish is also large and extensible. During inflation, the stomach expands into an existing peritoneal space surrounding the axial musculature and presses the folded peritoneum out into potential peritoneal spaces ventral to the head and surrounding the dorsal fin, anal fin, and caudal peduncle; only the dorsal and lateral surfaces of the head are unaffected. Balloonfish skin is also specialized for inflation. Because the collagen fibers in the dermis of unstretched skin are wavy, little force is required to extend the skin up to 40% of its rest length. At high strains, the collagen fibers are stressed directly and the skin suddenly becomes stiff. Stiff skin surrounding a ball of incompressible water provides a rigid framework for the support of the erectile spines, and the balloonfish becomes a formidable, spiny sphere. However, not all of the structural and functional features that contribute to the ability of balloonfish to inflate are evolutionary adaptations specifically for inflation. Absence of pleural ribs, absence of a pelvis, and the orthogonal arrangement of dermal collagen sheets in the skin were all present in the ancestor of pufferfishes, and thus, must be considered exaptations. Sagittal plane flexibility of the vertebral column may be an epiphenomenon of the lengthening of the body cavity for more spherical inflation. Together these structural and functional features—whether adaptations, exaptations, or epiphenomena—combine to produce a highly effective mechanical defense.