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Top ⇑Toward Standard Methods

Both former techniques for the determination of clutch parameters have inherent problems because neither is based on a full understanding of the reproductive biology of these fishes. Moreover, the dichotomy of "immature" and "mature" eggs is not detailed enough to describe the entire process of ovum formation, hence, clutch production. As a result, structures in more than one stage of a more refined classification system may be used, resulting in significant errors in determinations of clutch parameters. The central question is: which ovarian cells does one count and measure to obtain accurate estimates of clutch parameters? The answer lies in an understanding of the ovum development process.

As the spawning season of a species nears, sexually mature, adult females begin the formation of eggs. In doing so, they progress from the latent (LA) stage through early maturing (EM) and late maturing (LM) stages of ovarian condition. Throughout this time only one group of oocytes is present in the ovaries, although the group often is bimodal in its size distribution in LM females. This group of oocytes is a recruitment stock from which successive clutches are formed.

As some, but not all, oocytes in LM females undergo vitellogenesis (yolk loading), they form a second, distinct group of larger oocytes, in the follicles. The entire group of oocytes progresses through three major stages of development to form the eggs (clutches) that will be spawned. Both the oocytes/eggs in the group of larger cells and the ovaries are classified using the same categorical terms. Thus, this discussion will focus on the changes in the group of larger oocytes, which represents the developing clutch.

In chronological sequences, mature oocytes are initially found in mature (MA) females. The mature oocytes later undergo final oocyte maturation as yolk loading ceases and are at that time referred to as ripening oocytes, which are found in ripening (MR) females. (MR stands for mature ripening.) The ripening process involves the hydration of the follicles, which eventually will burst, releasing ripe eggs into the ovarian lumina of the ripe (RE) females.

One should keep in mind that MA, MR, and RE females also have another group of gametic cells in their ovaries. These are the smaller oocytes, which we refer to as maturing oocytes, and which form the recruitment stock of oocytes from which groups of oocytes are drawn successively to produce clutches. Thus, after a group of vitellogenic oocytes has become separated from the recruitment stock of smaller, maturing oocytes, the oocytes themselves progress through three distinct stages of development during the formation of a clutch: mature oocyte, ripening oocyte, and ripe egg.

In RE females collected soon after ovulation, the ripe eggs will fill the centers of the ovaries and the maturing oocytes will form a layer around them. The ripe eggs will be concentrated at the posterior end of the ovary in females that have spawned a percentage of their eggs. Both maturing and mature or ripening oocytes, respectively, are present in the individual follicles of mature and ripening females. Thus, structures in different stages are interspersed among one another. Moreover, the individual oocytes must be extracted from the follicles and cleaned off to remove all follicular remnants. In ripe females, the ripe eggs are clustered like grapes and can easily be separated from one another after peeling the maturing oocytes away from the clump.

Data from field and laboratory studies have shown that females do not necessarily stop reproducing after producing just one clutch. Instead they are capable of producing multiple clutches as they cycle through the LM, MA, MR, and RE stages of ovarian condition. This cycle has been called the "clutch production cycle." Thus in a single collection taken during the spawning season, adult females may be found in one or more of the clutch production stages: LM, MA, MR and RE stages. All four stages will not necessarily be present in any one collection. LM females will have but one group of oocytes, although the group usually will show a bimodal size distribution. MA, MR, and RE females will have two easily separable size groups of gametic cells, although color, opacity, and transparency also may differ between the groups: maturing oocytes plus either mature oocytes, ripening oocytes, or ripe eggs.

As the close of the reproductive season draws near, females begin resorbing the oocytes in their follicles and become LA. This process usually seems to begin in LM and MA females.

     © Copyright 1995 by David C. Heins, All rights reserved.

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