Anatomical Investigation of The Swim Bladder in Grass Carp (Ctenopharyngodon idella) and Red Sea Halfbeak (Hyporhamphus gamberur) Fish Using Different Imaging Techniques Eman Ismail El behery, Shimaa A. Ezzeldein, Nehal N. Ibrahim, Eman A. A. Mahdy Abstract: The current study elucidated the gross anatomical description of the swim bladder in 10 fish from each species (grass carp and Red Sea halfbeak), with further X-ray, computed tomography, and ultrasonographic examinations. The study revealed that the swim bladder of the two examined species had significant differences. The swim bladder of the Red Sea halfbeak was vesicular, while the bladder of the grass carp was composed of two chambers (anterior and posterior). The characteristic arrangement of the vesicles on the surface of the Red Sea halfbeak swim bladder resembled a pearl necklace or soap bubbles. The grass carp had a connection between the swim bladder and the esophagus via a pneumatic duct, so it is considered to be of the physostomous type. In contrast, the Red Sea halfbeak fish’s bladder lacked this duct but had a large blood vessel in the cranioventral extremity and a smaller one in the caudodorsal extremity, making it of the physoclistous type. The plain radiography and computed tomography revealed the grass carp’s swim bladder as two radiolucent structures of different sizes, with a constriction (isthmus) in between. The study highlights the typical features of the swim bladder in two different species of fish for a better understanding of fish anatomy and possible swim bladder disorders.