Hi Gills! It's been a while but I've been working hard since my last post. One of my research projects involves looking at relationships between anatomy and the environment of swordfish and bigeye thresher sharks. While these fishes are unrelated, they are both highly active, predatory species inhabiting the open ocean or pelagic environment. Unlike other active, pelagic fishes, swordfish and bigeye thresher sharks routinely make long dives (10+) hours from warm surface waters (>60F) to cold depths (~40F). Despite
their similar ecologies, swordfish and bigeye thresher sharks exhibit different anatomies. Swordfish appear endothermic, while bigeye thresher sharks appear ectothermic. This week I had the privilege to examine the anatomy of a freshly landed swordfish. Endothermy (sometimes called warm-blooded) refers to elevation of body temperature above the environment. Humans are endothermic. We typically maintain a body temperature of 98.6F regardless of environmental temperatures. Some fishes are regionally endothermic. Tunas and lamnid sharks (e.g. great white, mako, porbeagle) elevate red muscle tissue, or that muscle used for sustained/marathon swimming, but not the temperature of the entire body, above water temperature. Swordfish are thought to be regionally endothermic because the red "marathon swimming" muscle is located close to the spine and the associated arteries and veins form a complex arrangement, called a counter-current heat exchanger, which keeps muscular produced heat at the muscle. The arrangement of the white muscle and the presence of the counter-current heat exchanger creates a "heated blanket" that warms the red muscle and travels with the fish. Most fishes are ectothermic ( sometimes called cold-blooded) and red muscle temperature is similar to water temperature. Bigeye thresher sharks are ectothermic because the red muscle is located directly beneath the skin and they do not possess the arteries and veins to form an exchanger. This means that while both species are active in cold waters, swordfish red muscle may not fully experience low temperatures because of the regional endothermy, while bigeye thresher shark red muscle must experience temperature changes during dives from the surface to cold depths.