Gametogenesis in the Capelin, Mallotus villosus (Müller) from the Northwest Atlantic

Flynn, Shawn R. (1996) Gametogenesis in the Capelin, Mallotus villosus (Müller) from the Northwest Atlantic. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (10MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Capelin, Mallotus villosus (Muller), from the Northwest Atlantic were examined to describe the anatomical structure of the gonads and their positioning in the body cavity in relation to that of other fish. The testes of immature fish were thread-like, elongate, smooth and clear without obvious internal divisions in histological cross section. Those of maturing fish were elongate and smooth but much more robust, flattened and white. -- In maturing fish the final stages of spermatogenesis begin in the Fall of the year before spawning and are easily recognized as primary spermatogonia begin a process of division. This results in the division of the testes into lobules containing cysts. Within the cysts cells develop synchronously but the development of cells from different cysts is asynchronous. Sperm cells first appear in October and as development continues cysts filled with sperm break down and form larger pools of sperm starting in February. Fish examined immediately after spawning had gonads that were distended and contained only small amounts of residual sperm. -- The ovarian anatomy is unusual in that ovarian wall covers only the medial half of the organ and the oocytes develop in lamellae that extend freely into the body cavity. This type of ovarian structure was previously described by Brock, 1878 and Huxley, 1883. -- Four potential year classes of oocytes develop in the capelin ovary simultaneously and can be distinguished on the basis of size and cytological character. In immature fish all four are previtellogenic, but in maturing fish the fourth year class begins vitellogenic growth during the fall (September - October) of the year prior to spawning. This year class will mature over the winter passing through several developmental phases until final ovulation in June - August Mature eggs are ovulated directly into the body cavity and spawned through the gonopore located just posterior to the anus. The previtellogenic stages continue to develop during this time but at a slower rate. Their slight increase in size is accompanied by changes in the density of cytoplasm and the occurrence of such organelles as Balbiani bodies. -- Post-spawned capelin were captured at Middle Cove and maintained in aquaria at the Ocean Science Centre in Logy Bay. All the male fish died within several weeks but some female fish survived until the following year. Samples of these fish were taken in the winter and spring and showed clear evidence of development of gonads for the upcoming spawning season. The ovaries of recovering fish, soon after spawning, are distended and histological cross sections reveal thickened areas of folded tissue in the ovarian wall as well as loosely packed previtellogenic oocytes and residual eggs. These features have been used to develop a method to distinguish recovering females from first time spawners. -- The timing for the sequence of developmental steps of gametogenesis occurring in the last year before spawning, for both male and female capelin, was documented. It was shown that fish of different lengths have different rates of reproductive development.

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