HEAT-PRODUCING ORGAN OF THE SWORDFISH (XIPHIAS-GLADIUS) - A MODIFIED EYE MUSCLE

In swordfish (Xiphias gladius), the dorsal rectus muscle of the eye (M . rectus dorsalis) is partly differentiated for heat production simila r to brown adipocytes. Other parts of the dorsal rectus show a typical muscular structure. This modified muscle and its anatomical relations hips to neighboring organs were investigated using macroscopic dissect ions, light and transmission electron microscopy, nuclear magnetic res onance tomography, and scanning electron microscopy of vascular corros ion casts. The ophthalmic artery off the efferent pseudobranchial arte ry supplies the dorsal rectus muscle. The ophthalmic artery branches i n a brush-like manner as it enters the origin of the muscle. In this r egion, numerous arterioles and venules are arranged in parallel, sugge sting a countercurrent heat exchange mechanism. Thermogenic cells, mor phologically similar to brown adipocytes, surround small vascular bran ches in the middle of the muscle. Muscular cells also extend into this area. The vessels then divide into a tree-like pattern, forming an ex tensively capillarized area near the surface of the muscle at the basi sphenoid bone. The basisphenoid is reduced to a thin connective tissue layer in this area, hence facilitating thermal transfer between the b lood in the muscle and the brain base, The Tela lymph-adiposa surround s the brain with large masses of fat. Additionally, voluminous masses of periocular fat provide thermal insulation lateral to the lateral re ctus muscle of the eye. It is suggested that this system for raising t he temperature of areas of the brain a few degrees above ambient tempe rature allows an optically oriented predator such as the swordfish to respond to optical input in cold water more efficiently. (C) 1997 Wile y-Liss, Inc.