Sex Determination in Freshwater Eels and Management Options for Manipulation of Sex |
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Authors: | Andrew J H Davey Donald J Jellyman |
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Institution: | (1) National Institute of Water and Atmospheric Research, PO Box 8602, Christchurch, New Zealand |
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Abstract: | Catadromous eels enter fresh water as sexually undifferentiated glass eels and develop into males and females before migrating
back to sea as silver eels. Females develop ovaries directly from the ambiguous primordial gonad whereas males pass through
a transitional intersexual stage before developing testes. Eels have sex-specific life-history strategies. Males may grow
faster than females initially, but this difference is soon reversed and females attain a greater age- and size-at-metamorphosis
than males. Male fitness is maximized by maturing at the smallest size that allows a successful spawning migration (a time-minimizing
strategy) whereas females adopt a more flexible size-maximizing strategy that trades off pre-reproductive mortality against
fecundity. Although heteromorphic sex chromosomes have been identified in some species, the sex of developing gonads is labile
and gender is determined principally by environmental factors. Individuals experiencing rapid growth prior to gonad differentiation
tend to develop as males, whereas eels that grow slowly initially are more likely to develop as females. Paradoxically, males
tend to predominate under conditions of high density, which may be because a male “grow quickly, mature early” strategy increases
an individual’s chances of survival during periods of intraspecific competition. High temperatures and saline conditions have
also been proposed to favor development as males but experimental studies have failed to demonstrate a clear effect of either
on sex determination. High proportions of female silver eels migrating from some upstream areas, lakes and large rivers may
be due to low population density or poor conditions for growth in these habitats. Manipulating sex ratios in favor of females
has the potential to increase eel production in aquaculture and to buffer natural populations against fishing pressure. Sex
steroids (oestrogens and phytoestrogens) have a strong feminizing effect on undifferentiated individuals and are most effective
when targeted at younger eels and administered at high doses for prolonged periods. Modifying local environmental conditions,
in particular reducing eel density and limiting interference and social stress, may also promote the development of females.
Further research into the timing and mechanisms of sex determination in eels is required to effectively and efficiently manipulate
sex for conservation and/or economic benefit. |
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Keywords: | Anguillidae life history strategy metagamic sex sex differentiation |
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