|
242
<br />Animals have a wide array of morphological
<br />structures that function to reduce predation and
<br />there are many cases in which morphologies have
<br />evolved to provide defense against some biotic
<br />selective agent (Havel 1987, Adler & Harvell 1990,
<br />Harvell 1990). Predator-induced morphological
<br />defenses are well known in aquatic invertebrates.
<br />For example, cladocerans and rotifers produce
<br />progeny with neckteeth or enlarged crests on their
<br />head in response to biotic cues from consumers
<br />(Havel & Dodson 1987, Stemberger & Gilbert
<br />1987, Adler & Harvell 1990). Barnacles that come
<br />into contact with predatory gastropods early in
<br />their development are known to form asymmetri-
<br />cally, thus making it more difficult to open its
<br />opercular plates (Lively 1986, Adler & Harvell
<br />1990).
<br />A cyprinid fish, crucian carp, Carassius caras-
<br />sius, was the first vertebrate to be added to this
<br />long list of organisms with such adaptations
<br />(Bronmark & Miner 1992, Br6nmark & Pettersson
<br />1994, Nilsson et al. 1995). Bronmark & Miner
<br />(1992) discovered that C. carassius increased body
<br />depth in the presence of northern pike, Esox lucius,
<br />to reduce susceptibility to predation. Morpholog-
<br />ical change in C. carassius is so dramatic that ef-
<br />fected individuals have been mistakenly identified
<br />as a separate species (Bronmark & Miner 1992,
<br />Bronmark & Pettersson 1994). This morphologic
<br />change reduces the ability of gape-limited pisci-
<br />vores, such as pike, which consume only prey they
<br />can swallow whole (Hambright 1991). In addition,
<br />prey are generally swallowed head first, and their
<br />dorsoventral body depth determines whether they
<br />can be ingested (Hambright 1991). Prey body
<br />depth also increases handling time for gape-limited
<br />piscivores, which provides a greater opportunity
<br />for prey to escape.
<br />The major piscivore in the Colorado River
<br />System is P. lucius, which has exerted predation
<br />pressure over millions of years. If another cypri-
<br />nid, C. carassius can increase dorsoventral body
<br />depth in just 12 weeks (Bronmark & Miner 1992,
<br />Nilsson et al. 1995), why could not G. cypha and
<br />X. texanus evolve similar adaptations through
<br />natural selection over thousands of years? P. lucius
<br />is gape-limited and thus piscivory would be re-
<br />stricted by increasing prey size. Thus, the rela-
<br />tionship between body depth and piscivore mouth
<br />gape is important in selection of prey within the
<br />range of ingestible size (Gillen et al. 1981, Ham-
<br />bright 1991). Juvenile G. cypha and X. texanus are
<br />not associated with larger P. lucius until they leave
<br />their nursery grounds and then live sympatrically
<br />in pools and eddies with P. lucius (Tyus 1987,
<br />Karp & Tyus 1990, Tyus & Karp 1990). The tim-
<br />ing of this change in habitat preference is syn-
<br />chronous with the enlargement of the nuchal
<br />processes in X. texanus (Minckley et al. 1991,
<br />Converse et al. 1998, R. Valdez, unpublished).
<br />Because P. lucius lacks jaw teeth, it has no way of
<br />wounding or holding partially consumed prey;
<br />thus, it must engulf its prey whole. Inefficiency in
<br />successfully consuming large-bodied prey could
<br />have resulted in an adaptive response in prey
<br />species most affected, which would be other fishes
<br />that occupy preferred habitats of P. lucius. For
<br />sympatric species, such as G. cypha and X. texanus,
<br />enlargement of the nuchal region could provide a
<br />means of attaining a body shape that would be
<br />difficult or impossible for P. lucius to ingest.
<br />We offer an alternate hypothesis for the presence
<br />of large nuchal humps in G. cypha and X. texanus:
<br />the humps are in response to thousands of years of
<br />predation by a large, but gape-limited cyprinid.
<br />Humps would have developed in these two species
<br />and not others because they evolved in the same
<br />low velocity habitats foraged by adult P. Lucius.
<br />These humps ostensibly allow the two prey species
<br />to reduce predation pressure at a smaller length
<br />and age, thus increasing their fitness. This is
<br />especially valuable in such long-lived species which
<br />can attain ages of 15-20 years and more.
<br />Our findings show that based on gape size alone,
<br />P. lucius piscivory would be constrained by its
<br />relatively small gape and the presence of enlarged
<br />nuchal humps on its prey. Thus, gape limitations
<br />restricted predation to smaller sizes of the humped
<br />forms (Figures 5 and 6). Relating this to the
<br />present river system, the average size of adult P.
<br />lucius extant in four rivers was reported by Haw-
<br />kins (1992) as 536 mm TL (modal range 500-
<br />550 mm, n = 2 176). Based on hump growth,
<br />G. cypha (>210 mm) and X. texanus (>220 mm)
<br />would be immune to predation from an average
<br />size P. lucius. Furthermore, young of both prey
<br />species occupy shallow shoreline areas, moving
<br />offshore to deeper water at 100-300 mm for G.
<br />cypha (Converse et al. 1998, R. Valdez, unpub-
<br />lished observations), and about 127-196 mm for
|