Comparison of Larvae and Early Juveniles of Hybognathus Species (Cyprinidae) in Colorado and New Mexico

Authors: Darrel E. Snyder, W. Howard Brandenburg, John P. Sherrod, C. Lynn Bjork, Jennifer A. Charles, Sean C. Seal, Steven P. Platania, Kevin Bestgen, Darrel E. R.

Year: 2016 (xlviii)

Abstract

Hybognathus are small-bodied minnows with a small, slightly subterminal, crescent-shaped mouth, long coiled gut, dark peritoneum, typically 8 dorsal- and 8 anal-fin rays, dorsal fin origin before the pelvic fins, and about 35-40 lateral-line scales. Three of seven recognized species species are found in Colorado (CO) or New Mexico (NM). Rio Grande silvery minnow H. amarus (RGSM) is a federal and NM endangered species that was endemic to the Rio Grande and Pecos River basins, but is now restricted to the middle Rio Grande, NM. Plains minnow H. placitus (PM) is native to Great Plains drainages, including eastern CO, where it is state endangered, and NM, where, as a non-native species, it has displaced RGSM in the Pecos River. Brassy minnow H. hankinsoni (BM) is a more northern, very wide-spread species native to, and state threatened in, eastern-most CO; however, non-native populations have become established in limited reaches of the Colorado River in western CO (and Utah). RGSM and PM broadcast eggs in open, flowing water that quickly swell to large size, become semi-buoyant, and drift downstream, whereas BM broadcasts smaller, demersal, adhesive eggs in relatively still, vegetated shallows. Based mostly on reared series, larval development of the three species is very similar with small, oblique, terminal mouths forming shortly a fter hatching; oval eyes in earlier larvae (slightly oval in RGSM and some PM); typically 25-27 preanal and 36-38 total myomeres; and preanal lengths averaging 69-74% of standard length (SL). PM hatch, complete yolk absorption, and tend to become mesolarvae at larger sizes than the others (typically hatch at 5-6 vs 3-4 mm SL), whereas BM usually develop gut coils and transition to later developmental intervals at slightly larger sizes (typically juveniles by 15-16 mm SL). Just-hatched RGSM and PM have unpigmented eyes and bodies, whereas BM have no or little eye pigment but notable body pigment including a distinctive, wid ely scattered pattern of melanophores over the dorsum of head, eyes, and body’perhaps unique among North American cyprinids. Also unlike the others, later BM larvae develop a midventral line of melanophores from heart to vent and oblique branchial - preopercular lines under the head, and later juveniles develop stronger body pigmentation and a more complete lateral band. The mouths of juvenile RGSM and PM become more horizontal, ending near bottom-of-eye level, but in BM remain more oblique, ending at mid-eye level. Later juveniles of PM have longer snouts and smaller eyes than RGSM.