Do Social Interactions or Life on Land Serve as Stressors to the Bichir, Polypterus senegalus?

Abstract

Polypterus senegalus is a basal ray-finned fish that breathes both air and water, and can tolerate life on land for extended periods. Previous research reported that P. senegalus held on land for several months exhibited lower growth rates than counterparts fed equal amounts of food but held under aquatic conditions. Because chronic stress can impair growth, the present research aimed to determine whether P. senegalus experience stress when held under terrestrial conditions. P. senegalus were held for 1, 2, 4, or 8 weeks in aquatic or terrestrial conditions, following which tissues were sampled for analysis. Growth rates were significantly lower in fish held under terrestrial conditions, but neither plasma cortisol levels nor transcript abundances of genes of the stress axis differed between aquatic and terrestrial fish. Terrestrial fish exhibited lower transcript abundance in liver of the growth regulator insulin-like growth factor 1, and higher transcript abundances in muscle of myostatin, an inhibitor of muscle growth, and muscle RING-finger protein-1, an indicator of muscle breakdown. With knowledge from anecdotal observations that P. senegalus tend to be aggressive when housed with a conspecific, P. senegalus were held in a second experiment in pairs for 6 d to investigate stress axis function. Clear differences in territorial and aggressive behaviours between the individuals in a pair were consistent with one fish being dominant over the other, subordinate fish. Dominant fish tended to have higher cortisol levels than subordinate fish, although the difference was not significant, and cortisol levels were correlated with the frequency of aggressive and territorial behaviours, regardless of social status. However, transcript abundances of stress axis genes generally did not differ between dominant and subordinate fish. Collectively, these results suggest that P. senegalus is unusually tolerant of conditions that serve as stressors in other fish species, including emersion and social interactions.