Morphological plasticity associated with environmental hypoxia in characiform fishes from neotropical floodplain lakes Pablo A. Scarabotti & Javier A. López & Romina Ghirardi & María Julieta Parma Received: 26 April 2010 / Accepted: 8 May 2011 / Published online: 13 July 2011 # Springer Science+Business Media B.V. 2011 Abstract The ability to develop reversible dermal extensions on the lower jaw in some South American characiform fishes has been proposed as a way to optimize the performance of aquatic surface respiration (ASR) during hypoxic conditions. These structures are formed by edema of the hypodermal tissues and can develop in a large proportion of individuals inhabiting a lake within a few hours following daily changes in dissolved oxygen. Our study report the development of dermal lip protuberances in eleven species of characiform fishes associated with periods of strong environmental hypoxia in floodplain lakes of Salado River, Argentina. Protuberances occurred in different basic forms in fishes with divergent head morphology (genera Astyanax, Ctenobrycon, Aphyocharax, Brycon, Mylossoma, Tri- portheus, Oligosarcus and Acestrorhynchus). The dis- covery of dermal projections on the anterior border of maxillary bone extends the known range of structures affected by lip protuberances. Dermal structures began to develop simultaneously in both jaws below dissolved oxygen (DO) concentrations of 1.20–1.75 mgl -1 and rose in a steep manner as oxygen level decreased. The degree of morphological plasticity differed among traits and species. The shape of response of morphology to DO was similar to that previously reported on ASR, providing additional evidence about the functional link between these traits. Our results suggests that dermal lip protuberances are widely spread among characiform fishes, affecting several mouth structures. The different types of protuberances would make up for the limi- tations imposed by body size and mouth shape and position on the performance of ASR in fishes with divergent morphology. Keywords Aquatic surface respiration . Dissolved oxygen . Dermal lip protuberances . Paraná River Basin Introduction Floodplain lakes of large tropical and subtropical rivers experience wide environmental fluctuations driven by the recurrent seasonal increase in water level, known as flood pulse (Junk et al. 1989; Junk 1997; Welcomme 2001). In these environments, dissolved oxygen (DO) shows strong spatial and temporal variation, from supersaturation to complete Environ Biol Fish (2011) 92:391–402 DOI 10.1007/s10641-011-9850-y P. A. Scarabotti (*) : J. A. López : R. Ghirardi : M. J. Parma Instituto Nacional de Limnología (CONICET-UNL), Ciudad Universitaria, Paraje el Pozo s/n, 3000 Santa Fe, Provincia de Santa Fe, Argentina e-mail: pascarabotti@gmail.com R. Ghirardi Instituto de Botánica ‘Carlos Spegazzini’, Avenida 53 Nº 477, 1900 La Plata, Provincia de Buenos Aires, Argentina P. A. Scarabotti : J. A. López : M. J. Parma Universidad Nacional del Litoral, Facultad de Humanidades y Ciencias, Ciudad Universitaria, Paraje el Pozo s/n, 3000 Santa Fe, Provincia de Santa Fe, Argentina