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. 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. 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. ![]() 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. 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. 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. Eels have sex-specific life-history strategies. ![]() ![]() Females develop ovaries directly from the ambiguous primordial gonad whereas males pass through a transitional intersexual stage before developing testes. Catadromous eels enter fresh water as sexually undifferentiated glass eels and develop into males and females before migrating back to sea as silver eels.
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