We describe the complex shapes of myomeres and myosepta in the mackerels and tunas (Scombridae: Teleostei), and we reveal the orientation of two major systems of collagen fibers in myosepta and horizontal septa with respect to points of attachment to skeleton and skin. Our goal is to identify the likely pathways of the transmission of muscle forces during locomotion. Our primary conclusions are (1) that the collagen fibers of myosepta, horizontal septa, and skin are the organs that transfer locomotor forces from the contraction of myomeres to the backbone and caudal fin during locomotion, and (2) that locomotor muscle pulls against a three-dimensional structure of tendons, septa, and skin that is kept in tension by the radial expansion of the contracting muscle. The main horizontal septum is formed by the convergence of myosepta and is likely to be the major transmitter of muscle force to the axial skeleton. The geometry of the myomeres, the position of red muscle, and particularly the geometric conformation of crossed-fiber arrays of collagen in the main horizontal septum suggest specific mechanisms for the transfer of muscle force to the backbone among scombrid fishes. Morphometrics and the construction of physical models help us to identify musculoskeletal mechanisms of locomotion, and we present two quantitative models of locomotor mechanics in fishes. © 1993 Wiley-Liss, Inc.

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Background: Evolutionary trees illustrate relationships among taxa. Interpreting these relationships requires developing a set of "tree-thinking" skills that are typically included in introductory college biology courses. One of these skills is determining relationships among taxa using the most recent common ancestor, yet many students instead use one or more alternate strategies to determine relationships. Several alternate strategies have been well documented and these include using superficial similarity, proximity at the tips of a tree, or the fewest intervening nodes in the tree to group taxa. Results: We administered interviews (n = 16) and pencil-and-paper questionnaires (n = 205), and constructed a valid and reliable assessment that measured how well students determined relationships among taxa on an evolutionary tree. Our questions asked students to consider a focal taxon and identify which of two additional taxa is most closely related to it. We paired the use of most recent common ancestor with one of three alternative strategies (i.e., similarity, proximity, or node-counting) to explicitly test students' understanding of the relationships among the taxa on each tree. Conclusions: Our assessment enables us to identify students who are effectively distracted by an alternative strategy, those who use the most recent common ancestor inconsistently, or who are guessing in order to determine relationships among taxa. Our 18-question tool (see Additional file 1) can be used for formative assessment of student understanding of how to interpret relationships on evolutionary trees. Because our assessment tests for the same skill throughout, students who answer incorrectly, even once, likely have an incomplete understanding of how to determine relationships on evolutionary trees and should receive follow-up instruction.