Structure of an air-breathing organ and the swim bladder in the Alaska blackfish, Dallia pectoralis Bean

Abstract

Specimens of the Alaska blackfish, Dallia pectoralis Bean, were examined for an air-breathing organ. The swim bladder is modified for gas secretion, with rete mirabile and gas gland. However, the swim bladder lacks epithelial capillaries, as found in the mudminnows (Umbra). Further examination of the digestive tract has shown that the oesophagus is modified as an accessory respiratory organ. There is a sphincter between the oesophagus and stomach. Blood supply is from a branch of the coeliac artery, and venous blood from the oesophagus enters the anterior sections of the postcardinal veins. The blood vessels extend to the oesophageal epithelium, with an extensive arrangement of epithelial capillaries in the respiratory section of the oesophagus. The muscularis externa of the oesophagus is well developed, consisting of an outer transverse layer of striated muscle and inner longitudinal muscle bundles.