7723 - Laserfiche WebLink

Home Browse Search

A COMBINED DEVELOPMENTAL INTERVAL TERMINOLOGY It is often convenient and desirable to divide the ontogeny of fish into specifically defmed intervals. If the intervals selected are used by many biologists as a frame of refer- ence, such division can facilitate communica- tion and comparison of independent results. The largest intervals, periods (e.g., embry- onic, larval, juvenile, and adult), are often sub-divided into phases and sometimes into steps (Balon 1975b and 1984); the word "stage," although commonly used as a syno- nym for period or phase (e.g., Kendall et al. 1984), should be reserved for instantaneous states of development. The larval phase terminologies most commonly used in recent years, particularly for descriptive purposes, are those defined by Hardy et al. (1978 -- yolk-sac larva, larva, prejuvenile; modified from Mansueti and Hardy 1967), Ahlstrom et al. (1976 -- preflex- ion, flexion, postflexion; expanded upon by Kendall et al. 1984), and Snyder (1976b and 1981 -- protolarva, mesolarva, metalarva). Definitions for all three terminologies were presented by Snyder (1983b). During a work- shop on standardization of such terminolo- gies, held as part of the Seventh Annual Larval Fish Conference (Colorado State Uni- versity, January 16, 1983), it became obvious that these are not competing terminologies, as they often are treated, but rather comple- mentary options with subdivisions or phases defined for different purposes. As such, it is possible to utilize all three terminologies simultaneously to: (1) facilitate comparative descriptions and preparation of keys based on fish in similar states of development with res- pect to morphogenesis of finfold and fins; (2) segregate, for fishes with homocereal tails, morphometric data based on standard lengths measured to the end of the notochord prior to and during notochord flexion from those measured to posterior margin of the hypural plates following notochord flexion; and (3) approximate transition from at least partially endogenous nutrition (utilization of yolk material) to fully exogenous nutrition (depen- dence on ingested food) based on presence or absence of yolk material. The combined terminology presented below and utilized herein effectively inte- I grates principal subdivisions and functions of the three component terminologies. In doing so, Ahlstrom's "preflexion- flexion-postflexion" terminology is treated, for fishes with homo- cereal tails, as a subset of Snyder's mesolarva phase. Since notochord flexion in the caudal region usually begins when the first caudal fm rays appear and is essentially complete when all principal caudal fm rays are well defmed, and since presence of fm rays can be more precisely observed than the beginning or end of actual notochord flexion, fm rays are used as transition criteria. As a result, all proto- larvae are preflexion larvae, and all meta- larvae are postflexion larvae. Although most fish pass sequentially through all phase sub- divisions designated, some pass pertinent points of transition prior to hatching or birth and begin the larval period in a later phase or possibly skip the period entirely. The definition for the end of the larval period is necessarily a compromise deleting all requirements (some taxon-specific, others difficult to determine precisely) except acqui- sition of the full complement of fm spines and rays in all fms and loss of all finfold (last remnants are usually part of the preanal fm- fold). Provision for taxon-specific prejuvenile (or transitional) phases are also deleted. In some cases, finfold persists through the end- point for such special intervals, and the inter- vals are effectively included in the larval period. Timing of complete yolk absorption varies from well before notochord flexion and initial fin ray formation, as in most fishes with pelagic larvae, to postflexion stages after all or most of the fin rays are formed, as in many salmonids. Accordingly, the interval during which fish larvae bear yolk should not be represented generally as a separate phase preceding phases based on fm formation as it has been treated by Kendall et al. (1984). The Hardy et al. terminology effectively dis- tinguishes between larvae with and without yolk by modifying the period name with the adjective "yolk-sac" when yolk material is present. Any period or phase name of the combined terminology can be similarly modi- fied to indicate presence or absence of yolk material (e.g., yolk-bearing larva, yolk-sac 5