Abstract
ABSTRACT Many South American characid fishes develop reversible dermal protuberances in the jaws to optimize aquatic surface respiration (ASR) during hypoxia. To date, basic aspects of this adaptation remain unknown, mainly due to the scarcity of experimental studies. In laboratory experiments, we determined time necessary for the complete formation and reversion of these structures in Piaractus mesopotamicus , and studied comparatively behavioral, morphological, and respiratory responses along gradients of dissolved oxygen (DO) concentration. Morphological changes during hypoxia consisted in dermal protuberances of lower lip, anterior border of maxillary and distal border of opercular valve, increasing the known number of structures modified. These structures developed completely in less than 6 hours and reversed in less than 3 hours. Most of observed traits showed a logistic response curve with threshold DO values between 0.90 and 2.70 mgL-1. Respiratory frequency and opercular valve development showed similar threshold values above the level of tolerance of DO, whereas ASR and dermal protuberances of the jaws showed threshold values below this level. This observation supports the functional link between these groups of behavioral and morphological traits. This study demonstrates that this species is able to modify reversibly portions of the respiratory system to optimize responses to hypoxia.
Highlights
Phenotipic plasticity is the ability of an organism to modify their phenotype in response to changes in the environment (Whitman & Agrawal, 2009)
Morphological traits showed rapid and significant increases in size in response to hypoxia followed by a rapid reversion when fish were passed to normoxic conditions (Figs. 1a-c, 2)
Our results showed that Piaractus mesopotamicus is capable to modify quickly morphological, behavioral and respiratory variables, displaying an integrated strategy to respond to the stress produced by an oxygen poor environment
Summary
Phenotipic plasticity is the ability of an organism to modify their phenotype in response to changes in the environment (Whitman & Agrawal, 2009) It has been proposed as a mechanism to exploit heterogeneous or changing environments (Lande, 2014). Given that DO exhibit wide and fine-grained variation along the time, and responses to hypoxia are generally costly in adaptive terms (Chapman et al, 2008), evolution should favor rapid trait reversal when oxygen conditions are reestablished. This aspect has been poorly explored for hypoxia-induced traits This aspect has been poorly explored for hypoxia-induced traits (e.g. Nilsson et al, 2012)
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