|
<br />,
<br />
<br />CATOSTOMID FISH LARVAE AND EARLY JUVENILES OF
<br />THE UPPER COLORADO RIVER BASIN -
<br />MORPHOLOGICAL DESCRIPTIONS, COMPARISONS,
<br />AND COMPUTER-INTERACTIVE KEY
<br />
<br />Darrel E. Snyder and Robert 1. Muth
<br />
<br />ABSTRACT
<br />
<br />Use of collections of fish larvae and young-of-the-year juveniles to help document fish
<br />spawning sites and seasons or assess larval production, transport, distribution, nursery habitat, survival,
<br />and other aspects of early life history, requires diagnostic criteria to accurately distinguish target species
<br />from all similar appearing taxa in the waters sampled. To facilitate identification oflarvae and early
<br />juveniles of the seven species of Catostomidae in the Upper Colorado River Basin (UCRB),
<br />developmental series of reared and collected specimens were studied for differences in morphology,
<br />meristics, pigmentation, size relative to developmental state, and skeletal features. The results are
<br />documented in detailed descriptive species accounts, a comparative summary, and a computer-
<br />interactive key, the first application of such to fish larvae.
<br />Early larvae of the endangered razorback sucker (Xyrauchen texanus) are most similar to
<br />bluehead sucker (Catostomus discobolus), whereas later larvae and early juveniles appear most like
<br />tlannelmouth sucker (c. latipinnis). Criteria for distinguishing razorback sucker from the early larvae
<br />of most species include early yolk absorption, few or no melanophores along the ventral midline
<br />between heart and vent, and generally sparse dorsal pigmentation. Criteria for diagnosis of later larvae
<br />and juveniles include up to 16 principal dorsal-fin rays, a correspondingly long dorsal fin base, a large,
<br />fan-shaped, first interneural bone, and a large, oval-shaped, frontoparietal fontanelle.
<br />Larvae of blue head sucker and mountain sucker (c. platyrhynchus), both subgenus Pantosteus,
<br />are best characterized by early scattering of dorsal pigmentation, early folding of the gut, early
<br />formation of dark peritoneal pigmentation, and relatively few dorsal-fin rays. The midventralline of
<br />pigmentfrom heart to vent is often complete in mountain sucker larvae but highly variable in bluehead
<br />sucker. Early juveniles of both species have a small, blocky, first interneural bone, a narrow fontanelle,
<br />moderate to small scales, lips well divided at the comers of the mouth, and a shallow incision
<br />separating lower lip lobes.
<br />The remaining four species represent subgenus Catostomus. Flannelmouth sucker larvae are
<br />distinguished from most other UCRB catostomids by their generally large size at hatching, yolk
<br />absorption, and onset of other developmental events; also by a relatively high count of dorsal-fin rays,
<br />delayed gut folding, moderate to few midventral melanophores anterior to the vent, and lines of dorsal
<br />pigment parallel to the midline that sometimes include obliquely oriented pairs of melanophores
<br />resulting in a distinctive herringbone pattern that is sometimes shared only by white sucker (c.
<br />commersoni); juveniles develop small scales. White sucker larvae have greater than 20 melanophores
<br />in a typically complete midventralline from before or over the heart to the vent; juveniles have large
<br />scales, usually well outlined with pigment, and typically develop a distinctive series of three eye-size
<br />lateral spots (behind head, above pelvic fins, and on caudal peduncle). Utah sucker (C. ardens) larvae,
<br />like tlannelmouth sucker, usually have much less midventral pigmentation than white sucker,
<br />sometimes none, like some razorback and bluehead sucker; dorsal pigmentation is often sparse like
<br />razorback sucker. Juvenile Utah sucker often have larger eyes relative to head length than the other
<br />catostomids and, like white sucker, have large scales, but they develop no distinctive eye-size lateral
<br />spots or rarely just the anterior two. Early larvae of longnose sucker (c. catostomus) are most similar
<br />to bluehead, mountain, and white suckers. All typically have a complete middorsal line of
<br />melanophores from head to tail, but longnose sucker larvae develop pelvic-fin buds earlier, and, unlike
<br />white sucker, they seldom have complete lines of melanophores lateral to the dorsal midline and
<br />sometimes have much less midventral pigmentation. Juveniles have smaller scales and develop no
<br />distinct eye-size lateral spots except sometimes one near the base of the caudal fin.
<br />
<br />1
<br />
|