Aquatic surface respiration improves survival during hypoxia in zebrafish ( Danio rerio ) lacking hypoxia-inducible factor 1-α

Hypoxia-inducible factor 1-α (Hif-1α), an important transcription factor regulating cellular responses to reductions in O 2 , previously was shown to improve hypoxia tolerance in zebrafish ( Danio rerio ). Here, we examined the contribution of Hif-1α to hypoxic survival, focusing on the benefit of aquatic surface respiration (ASR). Wild-type and Hif-1α knockout lines of adult zebrafish were exposed to two levels (moderate or severe) of intermittent hypoxia. Survival was significantly compromised in Hif-1α knockout zebrafish prevented from accessing the surface during severe (16 mmHg) but not moderate (23 mmHg) hypoxia. When allowed access to the surface in severe hypoxia, survival times did not differ between wild-type and Hif-1α knockouts. Performing ASR mitigated the negative effects of the loss of Hif-1α with the knockouts initiating ASR at a higher P O 2 threshold and performing ASR for longer than wild-types. The loss of Hif-1α had little impact on survival in fish between 1 and 5 days post-fertilization, but as the larvae aged, their reliance on Hif-1α increased. Similar to adult fish, ASR compensated for the loss of Hif-1α on survival. Together, these results demonstrate that age, hypoxia severity and, in particular, the ability to perform ASR significantly modulate the impact of Hif-1α on survival in hypoxic zebrafish.

Use of non-lethal endpoints to establish water quality requirements and optima of the Topeka Shiner (notropis topeka)

Water quality influences growth, development, and physiology of aquatic vertebrates. Current criteria on water quality assessments are primarily based on lethal level experiments (e.g. LC50 tests), which are poorly suited for assessing optimal water quality conditions or sub-lethal effects of common stressors. Measurements below threshold values may still impede organismal growth and development, especially considering the complex nature of compounding, low-level stressors. This is particularly important to consider for management of an endangered species that is actively cultured for reintroduction to extirpated locations. The endangered Topeka Shiner (Notropis topeka) is an ideal example for which this information is needed, as its remaining, stable populations display broad water quality optima and tolerance to naturally occurring stressors. We investigated the effects of dissolved oxygen, temperature (including acclimation), ammonia, nitrite, and chloride on Topeka Shiner using non-lethal endpoints by: (1) examining N. topeka's behavioral responses to a gradual reduction in oxygen, (2) determining thermal optima at different acclimation temperatures using swimming speed, and (3) determining the onset of effect of sub-lethal levels of nitrogenous compounds and chloride concentrations on swimming speed. We determined ASR50 and ASR90 (i.e. dissolved oxygen concentrations where 50 percent and 90 percent of fish use aquatic surface respiration) to occur at 1.65mg/L and 1.08 mg/L of dissolved oxygen, respectively. At 5.52 mg/L of dissolved oxygen, fish vertical position was significantly higher in the water column, presumably in preparation or aquatic surface respiration (ASR). With our thermal swimming tests, the optimum temperature range was etermined to be 17.7 to 28.0 degrees C, while the predicted incipient mortality to high temperature ranged from 33.7 to 40.3 degrees C, depending on acclimation temperature. Ammonia and sodium chloride significantly reduced swimming speed at concentrations below known LC50 values. Other than an initial drop from 0- concentration, nitrite did not reduce swimming speed, even at concentrations higher than known LC50 measurements. Although not all stressors were suitable to test with this methodology, emphasis on determining optimal conditions over tolerances, and sub-lethal effects over mortality, assists in selection of sites that have water quality suited for N. topeka to thrive after reintroduction.

Dermal lip protuberances associated with aquatic surface respiration in juveniles of the piscivorous characid Salminus brasiliensis (Actinopterygii: Characidae)

Some South American freshwater fishes can improve their capability of aquatic surface respiration (ASR) by developing dermal lip protuberances in the lower jaw. This adaptation was thought to be limited to omnivorous or herbivorous fishes. The present work provides the first evidence that juveniles of a piscivorous characid, Salminus brasiliensis, develop dermal lip protuberances during periods of hypoxia in floodplain ponds of the Salado River, in Argentina. The protuberance of S. brasiliensis involves dermal portions of both jaws exhibiting lateral lobes on the sides of the mouth arranged in the vertical plane. Water dissolved oxygen concentrations less than or equal to 1.05 mgl-1 were associated with a remarkable increase in lip protuberance. The lateral lobes of the protuberance in this species may limit the access of water to the anterior portion of the mouth which is positioned nearer to the oxygenated surface water during ASR. Finally, ASR, complemented with the development of dermal lip protuberances, can be considered a valuable strategy to survive in hypoxic environments, even for carnivorous fishes with elevated oxygen requirements.