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r Larva: Period of fish development between hatching or birth and (1) acqmsltlon of adult complement of fin spines and rays (principal and rudimentary) in all fins, and (2) loss beyond recognition of all finfold not retained by the adult. Protolarva: Phase oflarval development characterized by absence of dorsal-, anal-, and caudal- fin spines and rays. (Standard length measured to end of notochord.) Mesolarva: Phase of larval development characterized by presence of at least one dorsal, anal, or caudal-fin spine or ray but either lacking the adult complement of principal soft rays in at least one median (dorsal, anal, or caudal) fin or lacking pelvic-fin buds or pelvic fins (if present in adult). (Standard length measured to end of notochord or, when sufficiently developed, axial skeleton.) Preflexion Mesolarva: Among fishes with homocercal tails, subphase of meso larval development characterized by absence of caudal-fin rays. (Posterior portion of notochord remains essentially straight and standard length measured to end of notochord. When first median-fin ray is a caudal ray, as in most fishes, larva progresses directly from protolarva to flexion mesolarva.) Flexion Mesolarva: Among fishes with homocercal tails, subphase of meso larval development characterized by an incomplete adult complement of principal caudal-fin rays. (Posterior portion of notochord flexes upward and standard length measured to end of notochord.) Postflexion Mesolarva: Among fishes with homocercal tails, subphase of meso larval development characterized by adult complement of principal caudal-fin rays. (Notochord flexion essentially complete and standard length measured to posterior-most margin of hypural elements or plates.) Metalarva: Phase of larval development characterized by presence of (1) adult complement of principal soft rays in all median fins and (2) pelvic-fin buds or pelvic fins (if present in adult). (Standard length measured to posterior end of axial skeleton, hypural elements or plates in fishes with homocercal tails.) Yolk-sac, Yolk-bearing, With Yolk, Without Yolk: Examples of modifiers used with any of the above period or phase designations to indicate presence or absence of yolk material, including oil globules. Characteristics Useful in Identification of Cypriniform Fish Larvae The following discussion of taxonomically useful characters is reprinted with minor modifi- cation from Snyder (1981) and Snyder and Muth (1988). Fishes of the families Cyprinidae (min- nows and carps) and Catostomidae (suckers) are closely related and morphologically similar. Together the two families account for nearly half of over 50 species in the Upper Colorado River System. Generalizations with respect to the order Cypriniformes refer specifically to North American species of these families. Fig- ures 2 and 3 identify the more obvious morpho- logical features and structures of catostomid (and cyprinid) eggs and larvae. Identification of fish larvae is in part a pro- cess of elimination. Even before examination of a single specimen, the number of candidate species can be substantially reduced by a list of known or likely species based on adult captures in the study area or connected waters. However, there are cases in which the presence of certain species was first documented by collection and identification of larvae. Incidental transport of eggs or larvae from far upstream or distant tributaries also must be considered. Knowledge of spawning seasons, temperatures, habitats, and behavior coupled with information on egg deposition, larval nursery grounds, and larval behavior are also useful in limiting possibilities. 7