Which parts of tetraodontiform bodies show reductive evolutionary trends?

The evolution of tetraodontiforms is characterized by significant reductive trends in several anatomical structures. These changes, which involve the loss, consolidation, or failure of certain elements to develop, include the following areas:

• Skeletal elements: There is a notable reduction in the total number of bones compared to generalized perciforms.1
• Fin structures: Anal fin spines are absent, and dorsal fin spines are either entirely absent or present in reduced numbers (never exceeding six). Similarly, the pelvic fin is either completely absent or significantly reduced to no more than one spine and two small soft rays.12
• External openings: Both the mouth and the gill openings are reduced in size, with the latter restricted to a relatively short slit on the side of the head.12
• Dentition: There is a reduction in the number of teeth, often accompanied by their fusion with jawbones.12

These reductive tendencies have occurred alongside elaborative trends in other systems, such as the development of specialized scales and skin, the inflation apparatus, and the evolution of poisonous flesh.1