Clutch size, egg size and clutch mass are important life history traits which are frequently measured for animals. The accurate determination of these clutch parameters is of critical importance in the study of life history evolution. Accurate and precise techniques, including standardized methods, allow researchers to benefit from the collective efforts of the scientific community in making broad comparisons based on studies of limited scope.
Historically, biologists studying minnows and especially darters have held two fundamentally different views about the processes by which ova are produced and spawned. The methods that have been adopted by individual researchers for the study of clutch parameters have been a reflection of these two views and as such differ greatly in the data that have been generated using them. Thus, two very different sets of techniques have been used to determine the clutch parameters of both darters and minnows in past studies. Because the two techniques are used to provide the same data and are based on completely different ideas of ovum production, both cannot be correct.
Presented here is a proposal for standard methods, which are based upon recent studies of ovum production in darters and minnows. These studies have important implications for different methods used for the study of life history traits. The techniques presented here are, therefore, based upon published results and appear to represent the approach, which will yield accurate and comparable data. Nonetheless, these methods may be modified as additional information becomes available.
During the spawning season, sexually mature females often, if not usually, have ovaries containing gametic cells, which comprise two distinct groups. These two groups of cells differ in size, color, opacity and transparency. The most important of these characteristics is size because it is the characteristic by which the two groups always can be separated.
The group of smaller cells appears much the same from one female to the next, being opaque and varying in color from white to cream to yellow or even dark yellow. The group of larger cells will vary greatly in appearance from female to female, depending on each female's reproductive condition. They may be opaque and yellow to dark yellow or sometimes even orange. In other cases, they may be translucent to transparent and yellow to amber or orange. Still other characteristics, which are discussed below, may be used to distinguish the stages in the formation of eggs.
Names applied to the gametic cells in the ovaries of fishes have varied among authors. One common approach has been to refer to all the cells as "ova" or "eggs" and to distinguish among them by using adjectives to describe the stage of formation, e.g. "immature ova" and "mature ova." To allow greater precision in the description and/or study of reproductive biology, modification of this nomenclature is needed, as shall be addressed later.
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