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The maximum frequency attainable decreases with increasing size of the fish. Wavelength relative to body length is generally assumed to remain constant within a species, but there is some reason to suspect that longer fish may use slightly shorter specific wavelengths. The body tends to be heavier and more rounded anteriorly compared to anguilliform swimmers. The caudal peduncle is fairly deep; the caudal fin which it bears has a rather low aspect-ratio, with its posterior margin almost straight or only moderately indented (“scooped out”) in the center (Fig.

Hence three “ terms, subcarangiform,” “carangiform,” and “thunniform,” will be used, the latter as a more convenient substitute for “carangiform with large lunate tail” (Lighthill, 1969) or “carangiform mode with semilunate taiI” (Webb, 1975). Some characteristics of each of these propulsive modes are given in Table 11, and examples of taxonomic groups in which some (but not necessarily all) members use that mode. 2. ANGUILLIFORMMODE Anguilliform is a purely undulatory mode of swimming, in which most or all of the length of the body participates.

The body tends to be heavier and more rounded anteriorly compared to anguilliform swimmers. The caudal peduncle is fairly deep; the caudal fin which it bears has a rather low aspect-ratio, with its posterior margin almost straight or only moderately indented (“scooped out”) in the center (Fig. l C , D). The caudal fin in a subcarangiform swimmer tends to be flexible, and is provided with intrinsic muscles (Fig. 7D) which can slightly open or shut the “fan” of caudal fin rays so as to alter the fin area by as much as 10% at different phases of one beat (Bainbridge, 1963).