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<br />Environmental Biology of Fishes 71: 233-245, 2004.
<br />© 2004 Kluwer Academic Publishers. Printed in the Netherlands.
<br />Fish humps in two Colorado River fishes: a morphological response to cyprinid
<br />predation?
<br />Donald E. Porte,' & Harold M. Tyusa
<br />aDepartment of Environmental, Population, and Organismic Biology, University of Colorado at Boulder,
<br />Boulder, CO 80309-0216, U.S.A.
<br />bCurrent address: Department of Wildlife, Fish, and Conservation Biology, One Shields Avenue, University of
<br />California, Davis, CA 95616-8751, U.S.A. (e-mail: deportz@ucdavis.edu)
<br />Received 10 August 2003 Accepted 22 January 2004
<br />Key words: humpback chub, Gila cypha, razorback sucker, Xyrauchen texanus, Colorado pikeminnow,
<br />Ptychocheilus lucius, nuchal hump, drag, lift, convergent evolution
<br />Synopsis
<br />Extant fishes endemic to the upper Colorado River of the American southwest include only cyprinids and
<br />catostomids. A curious attribute in species of both groups is the presence of a large nuchal hump. Largest
<br />cyprinid humps occur in humpback chub, Gila cypha, and largest catostomid humps occur in razorback
<br />sucker, Xyrauchen texanus. Several authors have suggested the humps confer a hydrodynamic advantage to
<br />life in fast flow, but this premise has not been confirmed with experimental work. To test the role of humps
<br />in Colorado River fishes, we subjected whole-body casts of preserved specimens with humps and with
<br />humps removed to controlled flows in an experimental tank. These tests confirmed that humps increased
<br />drag coefficients for X. texanus and G. cypha with no additional lift component. High energetic costs of
<br />locomotion and position-holding with a large hump, and the additional metabolic expense of forming large
<br />humps, suggest that the humps are not relict structures. Instead, we argue that these large humps represent
<br />convergent evolution prompted by predation from a cyprinid piscivore. Colorado pikeminnow, Ptychoc-
<br />hedus lucius, top piscivore in the Colorado River system, is the only native fish capable of consuming large
<br />X. texanus and G. cypha, and it also is sympatric with them. However, lack of jaw teeth and a relatively
<br />small jaw gape limit the maximum prey size that P. lucius can consume. Based on gape size, about 55% of
<br />X. texanus and 71% of G. cypha could be consumed by even the largest P. lucius. However, vulnerability
<br />would increase to 73 and 83% respectively if these species did not have humps. Coevolution tends to favor
<br />predator defense mechanisms in prey most vulnerable to such a voracious predator. Development of a large
<br />nuchal hump provides a deep body that is difficult or impossible for P. lucius to ingest.
<br />Introduction
<br />The Colorado River is home to an endemic fish
<br />fauna that has evolved over millions of years
<br />(Smith 1981, Minckley et al. 1986, Hoetker &
<br />Gobalet 1999). The upper mainstem Colorado
<br />River is unique in that its fishes are characterized
<br />by high endemism (i.e:, 87%), and historically has
<br />been represented by seven large river species con-
<br />sisting of only cypriniform fishes (Jordan &
<br />Evermann 1896, Miller 1958, Carlson & Muth
<br />1989). The top carnivore in this system is a large,
<br />voracious predator, the Colorado pikeminnow,
<br />Ptychocheilus lucius, largest: North American
<br />minnow, which may have once reached nearly
<br />1.8 m and 36 kg (Miller 1961). There are anecdotal
<br />reports of very large P. lucius from the historic
<br />Colorado River; however, fish larger than ca.
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