Sexual segregation of the European eel


Sexual segregation of the European eel

The objective of this study is to evaluate the bathymetric distribution, sex ratio, stages of gonad maturation and reproductive strategy of the European eel (Conger conger) in coastal inshore waters of the Costa Brava (north-west Mediterranean) and to answer many questions about its biology. The observations confirm the absence of both males and mature or spent females in the area. The only males obtained were caught at greater depths (between 600 and 800 m) than females (100 to 400 m). Ten morphometric characters were used to calculate morphometric indices and compare anatomical differences between males and females. To describe the different stages of maturity, gonads were sectioned, stained and observed under a microscope. Gonadosomatic and hepatosomatic indices were also calculated. Results confirm that males show various morphometric differences from females of the same size, which may be related to the different depths at which they live. A histological examination of ovaries demonstrated that C. conger has synchronous ovaries. Although some females were in the early vitellogenic stages of gonad development, the great majority were still in the pre-vitellogenic stages, characterized by a large amount of adipose tissue between the germ cells, which decreases progressively with maturation. When females are fully mature, their ovaries have grown enormously; the germinal tissue has grown and consumed the adipose tissue, a process that is believed to take place during pre-reproductive migration to deeper spawning grounds. Males’ mature gonads show an increase in weight, but never of the magnitude of females. Some males in our sample were completely mature and full of sperm. In conclusion, our observations are clearly consistent with a semelparous strategy for females. However, males seem to be capable of recovering after spawning and no signs of degeneration have been detected. The difficulty in obtaining a larger sample of males means our results should be taken as approaches, pending further studies to corroborate them.

In conclusion, in the northwest Mediterranean area individuals of C. conger are segregated by sex. Males inhabit deeper waters than immature individuals and females in pre-vitellogenic or early vitellogenic stages. Morphometric differences observed in the two sexes, for equal sizes, may be related to the different habitat needs. Females get bigger than males and become gravid at very big sizes; when vitellogenesis starts, they migrate to deeper spawning grounds to find males. At this time, the weight of their ovaries increases heavily, above one third of their total weight, and the gonad adipose tissue disappears making it very difficult for them to swim, hunt or feed. Consequently, it is assumed that these females consume more and more of their own reserves and tissues. Once spawning finishes, they have difficulty recovering and probably die, like other semelparous species.

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