1 3 Mar Biol (2014) 161:575–584 DOI 10.1007/s00227-013-2360-z ORIGINAL PAPER Hypoxia-driven selective degradation of cellular proteins in jumbo squids during diel migration to the oxygen minimum zones Katja Trübenbach · Gonçalo da Costa · Cristina Ribeiro-Silva · Raquel Mesquita Ribeiro · Carlos Cordeiro · Rui Rosa Received: 25 April 2013 / Accepted: 20 November 2013 / Published online: 11 December 2013 © Springer-Verlag Berlin Heidelberg 2013 role in hypoxia tolerance, but further investigations are nec- essary to discover its full potential and pathways. Introduction The jumbo or Humboldt squid (Dosidicus gigas) is a jet- propelled oceanic top predator endemic to the Eastern Pacific, and one of its most pronounced features is the abil- ity to perform diel vertical migrations into oxygen mini- mum zones (OMZs). Consequently, it spends >70 % of the daytime below its critical oxygen partial pressure (P crit , ~1.6 kPa; Gilly et al. 2006; Seibel 2013). It is worth not- ing that active muscular squids were thought to be pre- cluded from hypoxic areas (Pörtner 2002), as consequence of their physiological and anatomical restraints (Pörtner 1994, 2002; Melzner et al. 2007). Unexpectedly, D. gigas thrives in mesopelagic OMZs via metabolic suppression (Rosa and Seibel 2008, 2010; Trübenbach et al. 2013). Marine organisms exposed to hypoxia commonly suppress their metabolism by 50–95 %, as energy reserve stores are limited and fermentative pathways less efficient, and sup- plement the remaining energy demand using a combination of aerobic (oxidative phosphorylation) and anaerobic meta- bolic pathways (Seibel 2011). The main biochemical fuel under hypoxia is glycogen (i.e., gastropods, bivalves, and sluggish fish; Shulman et al. 2002), but in squids, glyco- gen reserves are less than 0.4 % of body weight (Rosa et al. 2005a, b), and therefore, a few minutes of activity can sig- nificantly deplete them (Storey and Storey 1983). Moreo- ver, cephalopods, under well-oxygenated conditions, feed on proteins (O’Dor and Webber 1986; Lamarre et al. 2012), and available evidence does indicate that squids, at least, under extreme conditions like at the end of their migrations show high levels of muscle proteolysis due to fasting and Abstract The jumbo squid, Dosidicus gigas, is an oce- anic top predator in the eastern tropical Pacific that under- goes diel vertical migrations into mesopelagic oxygen minimum zones (OMZs). Besides glycogen breakdown, the pathways of the squid’s metabolic (suppression) strat- egy are poorly understood. Here, juvenile D. gigas were exposed to oxygen levels found in the OMZ off Gulf of California (1 % O 2 , 1 kPa at 10 °C) with the aim to identify, via proteomic tools, eventual anaerobic protein degradation as potential energy source at such depths. Under hypoxia, total protein concentration decreased nonsignificantly from 79.2 ± 12.4 mg g -1 wet mass to 74.7 ± 11.7 mg g -1 wet mass (p > 0.05). Yet, there was a significant decrease in heat-shock protein (Hsp) 90 and α-actinin contents (p < 0.05). The lower α-actinin concentration at late hypoxia was probably related to decreased protection of the Hsp90 chaperon machinery resulting in increased ubiqui- tination (p < 0.05) and subsequent degradation. Thus, the present findings indicate that D. gigas might degrade, at least under progressing hypoxia, specific mantle proteins anaerobically to increase/maintain anaerobic ATP pro- duction and extend hypoxia exposure time. Moreover, the ubiquitin–proteasome system seems to play an important Communicated by H.-O. Pörtner. K. Trübenbach (*) · R. Rosa Laboratório Marítimo da Guia, Centre of Oceanography, University of Science in Lisbon, Av. Nossa Senhora do Cabo 939, 2750-374 Cascais, Portugal e-mail: kjtrubenbach@fc.ul.pt G. da Costa · C. Ribeiro-Silva · R. M. Ribeiro · C. Cordeiro Department of Chemistry and Biochemistry, University of Science in Lisbon, Campo Grande, 1749-016 Lisbon, Portugal