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<br />8 DEVELOPMENTAL INTERVALS
<br />
<br />ANATOMICAL FEATURES OF EGGS AND LARVAE
<br />
<br />Figures 2 and 3 identify most of the more
<br />obvious morphological structures of cypriniform fish
<br />eggs and larvae.
<br />
<br />TERMINOLOGY OF DEVELOPMENTAL INTERVALS
<br />
<br />Classification of Developmental Intervals
<br />
<br />The development of a fish is a continuous and
<br />somewhat gradual process, but there are differential
<br />and frequently varying rates of development for
<br />specific structures or physiological processes, and
<br />certain events occur rather suddenly. In the stud~.
<br />of larval fish ecology and development, as in the
<br />preparation of formal descriptions and keys, it is
<br />most convenient and useful to divide the developmental
<br />continuum into specific recognizable intervals.
<br />Balon (1971 and 1975) suggested a four-tier hierarchy
<br />of developmental intervals, the period, phase, step
<br />or threshold, and stage. In fish, we typically
<br />recognize the largest intervals as the embryonic,
<br />larval. juvenile and adult periods. Balon also
<br />suggested a senescent period--an interval usually
<br />considered part of the adult period. Each period can
<br />be divided into two or more phases, each of which can
<br />be further subdivided into steps. Balon (1979)
<br />characterized steps as "natural intervals of
<br />ontogeny during which changes in form and function
<br />represent no significant alteration in the animal's
<br />environmental relationships. Only certain combina-
<br />tions of synchronous qualitative change will result
<br />in the attainment of a threshold, which is an abrupt
<br />functional change in ontogeny that produces a new
<br />environmental (external or internal) relationship,
<br />and therefore, a new step. In this manner, develop-
<br />ment occurs by a process of saltation." Stage is
<br />defined as a specific point in the developmental
<br />continuum and in this sense is misused by
<br />many fisheries biologists. This guide will use
<br />specifically-defined intervals of the period and
<br />phase type as a framework for presentation of
<br />developmental information.
<br />
<br />Need for a Standard Terminology
<br />
<br />In past literature, over 60 different but often
<br />synonomous terms have been applied to periods and
<br />phases of development between hatching or parturition
<br />and attainment of sexual maturity (Snyder 1976b).
<br />Snyder (1976b) called for standardization of the terms
<br />and definitions used with respect to the larval period
<br />and its phases of development to better facilitate
<br />comparability of descriptions, keys, and reports of
<br />studies on fish larvae. Though the need had been
<br />expressed many times in the past. the call had
<br />remained largely unheeded. To promote such standard-
<br />ization, he reviewed, compared, and pointed out the
<br />difficulties in application of 15 candidate terminolo-
<br />gies including those proposed and used by Hubbs
<br />(1943), Faber (1963). Mansueti and Hardy (1967),
<br />
<br />Balon (1975), and Ahlstrom (Pers. Comm., 1968, and
<br />et a1. 1976), and one that he and Maryann Mulhall
<br />Snyder had developed after several years of experimen-
<br />tation with various modifications of existing termi-
<br />nologies. The following criteria were suggested for
<br />selecting a standard: l)"a standard terminology
<br />should be practical, precise, and easy to use without
<br />requiring intricate or time consuming techniques." 2)
<br />"the terminology should provide intervals indicative
<br />of relative age and state of development. It should
<br />therefore be based on a sequence of developmental char-
<br />acters which follow nearly the same course in all
<br />fishes." And 3) "the intervals must facilitate the
<br />production of comparable formal descriptions of fish
<br />larvae, keys to their identity, and reports of perti-
<br />nent field and laboratory studies..."
<br />
<br />The terminology proposed by the Snyders has
<br />proven to be useful to many larval fish biologists
<br />and is gaining acceptance for standard usage in
<br />descriptions and keys. Its use has been promoted by
<br />general, though not exclusive, usage in the
<br />Transactions of the American FisheY'ies Society and by
<br />agreement among many of the participants in the
<br />Second Symposium on Larval Fishes which was sponsored
<br />by TVA in Knoxville, Tennessee, 21-22 February 1978.
<br />Accordingly, it will be used exclusively in this
<br />guide.
<br />
<br />Terms and Definitions
<br />
<br />The following discussion and definitions are
<br />paraphrased from Snyder (1976b).
<br />
<br />The larval period is defined arbitrarily to
<br />consist of three distinct sequential phases: proto-
<br />larva, mesolarva and metalarva. These phases, and
<br />therefore the period, are based on one of the most
<br />consistent and obvious sequences of development in
<br />all. or nearly all, bony fishes--the morphogenesis
<br />of the median finfold and fin elements (spines and
<br />rays). In addition, paired fins (pectoral and pelvic)
<br />are included in defining the last or metalarval
<br />phase. The definitions specify structures or features
<br />readily observed under low range magnification (less
<br />than 30x) and do not require dissection, clearing or
<br />staining. Not all fish pass through all three phases;
<br />salmon (Salmonidae), catfish (Ictaluridae) and certain
<br />killifish (Byprinodontidae), for example, hatch as
<br />mesolarvae. It is likely that some fish may hatch or
<br />be born as juveniles, lacking a larval period entirely.
<br />However, no examples of such are known among North
<br />America's freshwater or anadromous fishes (Snyder
<br />1976b erroneously gave the mosquitofish, Gambusia
<br />affinis, as an example; since that fish lacks pelvic
<br />fins at birth, it by definition has both a mesolarval
<br />and a metalarval phase).
<br />
<br />The specific definitions of the larval period
<br />and its phases are as follows:
<br />
<br />LaY'vaZ PeY'iod - The period of bony fish development
<br />characterized by obvious fin morphogenesis following
<br />hatching or parturition. Transition to the juvenile
<br />period is based on the following three criteria, each
<br />of which must be met: 1) finfold and atrophying
<br />fins, if any (very rare), must be absorbed beyond
<br />recognition; 2) the full adult complement of fin
<br />spines (actinotricha) and rays (lepidotrichia),
<br />including secondary rays, must be distinctly formed
<br />(visually well defined) in all fins; and 3) segmenta-
<br />tion must be evident in at least a few of the rays of
<br />each fin that is characterized by segmented rays in
<br />the adult.
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