I've noticed the same thing about shiners. Coastal shiners (Notropis
petersoni) from peninsular Florida are annuals, but I had one in captivity
for three years. Topeka shiners (N. topeka) usually live 2-3 years but
full-sized adults that we aquired 18 months ago are still going strong.
Maybe the absence of seasonal cues (changing water temperature, daylight,
etc) or interruption of reproductive cycyles prevent the fishes' internal
"senescence switches" from getting flipped on.
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