Meeting Abstract
79.6 Sunday, Jan. 6 The role of muscle force versus work in trophic specialization of cyprinid fishes GIDMARK, N*; TARRANT, J; KONOW, N; BRAINERD, E; Brown University njg@brown.edu
Vertebrates feed on a wide variety of foods that vary in material properties, so biomechanical traits of their feeding systems should vary with diet. Cyprinidae is a trophically diverse fish clade that offers an opportunity to compare how muscle function varies with feeding ecology in a simple, conserved mechanical apparatus. Food is broken down exclusively by the pharyngeal jaws (tooth-bearing modified gill arches), where a single muscle elevates the jaw, pushing the teeth against an occlusal surface. We measured in vivo lengths of this levator muscle using XROMM in two cyprinid specialists: a molluscivore and a herbivore. We then used in situ electrical stimulation experiments to measure the force-length relationship of the same muscle. While masticating small foods, black carp (Mylopharyngodon piceus, molluscivore) used small muscle strains (<10%, 3 animals, 78 chews), whereas grass carp (Ctenopharyngodon idella, herbivore) used larger muscle strains (>15%, 4 animals, 196 chews). In vivo muscle strain differences translated into different physiological operating lengths: both species initiated occlusion at near-optimal muscle lengths (ca. 0.95Lo), but the black carp levator shortened to a minimum length of 0.87Lo, compared with 0.81Lo for grass carp. The slight operating length difference led to a striking force difference in situ: at 0.87Lo, the black carp levator preserves much of its potential force (0.5-0.7Po), but at 0.81Lo, the grass carp levator only produces ca. 25% of its potential force (0.2-0.3Po). We hypothesize that for black carp, force alone is the functional optimality criterion, whereas for grass carp, work (force x distance) is important for shearing plant matter. Thus, our results underscore the role of in vivo muscle operating lengths in shaping the diverse vertebrate trophic strategies.
