Studying the Force-Length Relationship in the Adductor Mandibulae of Pink and King Salmon

Abstract

Specialized feeding behavior is generally reflected not only in skeletal anatomy (as has been the major focus of functional morphology literature) but also in muscular morphology and physiology. We show that this is the case for salmon feeding mechanics of king and pink salmon. King salmon (Oncorhynchus tshawytscha) eat small, fast fish; and pink salmon (Oncorhynchus gorbuscha) primarily filter feed on planktonic organisms by keeping their mouths open while swimming. Salmon close their jaws using the adductor mandibulae, which, like all skeletal muscles, is constrained by a strict relationship between muscle length and force. Muscles that are over-stretched or overshortened exert weaker forces than they do at optimal length, and muscle length corresponds to gape. We compared the force-length curves of king and pink salmon adductor mandibulae and demonstrated that the maximum bite force of king salmon is achieved closer to maximum gape (67% of max gape, n = 3). This may allow them to take advantage of optimal muscle length, and thus greater force production, when eating large or elusive prey. In pink salmon, the force-length curve is centered at a smaller relative gape, closer to mid-gape (43% of max gape, n = 6). This may facilitate filter feeding, allowing reasonably high forces at a range of medium gape sizes. The different feeding preferences of these species may have put different pressures on the evolution of jaw muscle physiology, resulting in distinct optimal solutions to the force-length constraint.