Bigmouth buffalo fish don’t appear to age, but new ones seldom survive their first year.
Increased age in the fish was not associated with telomere shortening (a biological sign of ageing), as would normally be expected. Instead, it seemed to be linked with better immune functioning, including a decreased neutrophil-to-lymphocyte ratio – a change which suggests that these fish get better at handling bodily stress and see increased immunity as they age.
Emphasis mine.