Gross and Histological Morphology of the Swimbladder and Lack of Accessory Respiratory Structures in Lepidogalaxias salamandroides, an Aestivating Fish from Western Australia

Abstract

The swimbladder morphology of Lepidogalaxias salamandroides was investigated. This small (maximum size is 67 mm SL) freshwater fish is endemic to southwestern Australia. It apparently survives droughts by aestivating in the bottom mud. The physostomous swimbladder is poorly developed, consisting of a thin serous membrane that extends from the dorsal body wall to the gut. It does not have the tissue normally associated with a teleost swimbladder. Instead, it is composed of a double-layered membrane that consists of squamous epithelium with a thin basement layer of collagen fibers. It appears more like dorsal mesentery, and almost certainly cannot be used in aerial respiration. Comparisons were made with Australian galaxiids and an American umbrid fish all of which possess a well-developed physostomous swimbladder. The skin of L. salamandroides contains numerous goblet (mucous) cells, and the opercular chamber lacks adaptations for air breathing. We suggest four hypotheses whereby L. salamandroides can survive the desiccation of its habitat without air breathing.