Osmo-respiratory compromise in the mosshead sculpin (Clinocottus globiceps): effects of temperature, hypoxia, and re-oxygenation on rates of diffusive water flux and oxygen uptake

In nature, mosshead sculpins (Clinocottus globiceps) are challenged by fluctuations in temperature and oxygen levels in their environment. However, it is unclear how mosshead sculpins modulate the permeability of their branchial epithelia to water and O2 in response to temperature or hypoxia stress. Acute decrease in temperature from 13 to 6 oC reduced diffusive water flux rate by 22% and ṀO2 by 51%, whereas acute increase in temperature from 13 to 25 oC increased diffusive water flux rate by 217% and ṀO2 by 140%, yielding overall Q10 values of 2.08 and 2.47 respectively. Acute reductions in oxygen tension from >95% to 20% or 10% air saturation did not impact diffusive water flux rates, however, ṀO2 was reduced significantly by 36% and 65% respectively. During 1-h or 3-h recovery periods diffusive water flux rates were depressed while ṀO2 exhibited overshoots beyond the normoxic control level. Many responses differed from those seen in our parallel earlier study on the tidepool sculpin, a cottid with similar hypoxia tolerance but much smaller gill area that occupies a similar environment. Overall, our data suggest that during temperature stress, diffusive water flux rates and ṀO2 follow the traditional osmo-respiratory compromise pattern, but during hypoxia and re-oxygenation stress, diffusive water flux rates are decoupled from ṀO2.

Theme and variations: amphibious air-breathing intertidal fishes

Over 70 species of intertidal fishes from 12 families breathe air while emerging from water. Amphibious intertidal fishes generally have no specialized air-breathing organ but rely on vascularized mucosae and cutaneous surfaces in air to exchange both oxygen and carbon dioxide. They differ from air-breathing freshwater fishes in morphology, physiology, ecology and behaviour. Air breathing and terrestrial activity are present to varying degrees in intertidal fish species, correlated with the tidal height of their habitat. The gradient of amphibious lifestyle includes passive remainers that stay in the intertidal zone as tides ebb, active emergers that deliberately leave water in response to poor aquatic conditions and highly mobile amphibious skipper fishes that may spend more time out of water than in it. Normal terrestrial activity is usually aerobic and metabolic rates in air and water are similar. Anaerobic metabolism may be employed during forced exercise or when exposed to aquatic hypoxia. Adaptations for amphibious life include reductions in gill surface area, increased reliance on the skin for respiration and ion exchange, high affinity of haemoglobin for oxygen and adjustments to ventilation and metabolism while in air. Intertidal fishes remain close to water and do not travel far terrestrially, and are unlikely to migrate or colonize new habitats at present, although in the past this may have happened. Many fish species spawn in the intertidal zone, including some that do not breathe air, as eggs and embryos that develop in the intertidal zone benefit from tidal air emergence. With air breathing, amphibious intertidal fishes survive in a variable habitat with minimal adjustments to existing structures. Closely related species in different microhabitats provide unique opportunities for comparative studies.

Physiological, behavioral and biochemical adaptations of intertidal fishes to hypoxia

Hypoxia survival in fish requires a well-coordinated response to either secure more O2 from the hypoxic environment or to limit the metabolic consequences of an O2 restriction at the mitochondria. Although there is a considerable amount of information available on the physiological, behavioral, biochemical and molecular responses of fish to hypoxia, very little research has attempted to determine the adaptive value of these responses. This article will review current attempts to use the phylogenetically corrected comparative method to define physiological and behavioral adaptations to hypoxia in intertidal fish and further identify putatively adaptive biochemical traits that should be investigated in the future. In a group of marine fishes known as sculpins, from the family Cottidae, variation in hypoxia tolerance, measured as a critical O2 tension (Pcrit), is primarily explained by variation in mass-specific gill surface area, red blood cell hemoglobin–O2 binding affinity, and to a lesser extent variation in routine O2 consumption rate (). The most hypoxia-tolerant sculpins consistently show aquatic surface respiration (ASR) and aerial emergence behavior during hypoxia exposure, but no phylogenetically independent relationship has been found between the thresholds for initiating these behaviors and Pcrit. At O2 levels below Pcrit, hypoxia survival requires a rapid reorganization of cellular metabolism to suppress ATP consumption to match the limited capacity for O2-independent ATP production. Thus, it is reasonable to speculate that the degree of metabolic rate suppression and the quantity of stored fermentable fuel is strongly selected for in hypoxia-tolerant fishes; however, these assertions have not been tested in a phylogenetic comparative model.

Escaping to the surface: a phylogenetically independent analysis of hypoxia-induced respiratory behaviors in sculpins

Behavioral responses to progressive hypoxia exposure were assessed in several species of fish from the family Cottidae (sculpins), which are distributed along the near-shore marine environment and differ in their hypoxia tolerance. The use of aquatic surface respiration (ASR) and aerial emergence as a response to progressive decreases in environmental O(2) differed between intertidal and subtidal sculpins. Intertidal sculpins consistently displayed ASR followed by emergence behaviors, while the subtidal species performed these behaviors at low frequency or not at all. There was a significant negative correlation between the O(2) thresholds for the onset of ASR and critical O(2) tensions (P(crit), a measure of hypoxia tolerance), such that the hypoxia-tolerant species performed ASR at higher O(2) tensions than the more hypoxia-sensitive species. There was no relationship between the O(2) thresholds for emergence and P(crit). When restricted from accessing the water surface during progressive decreases in O(2), all species of sculpin displayed the same sequence of behaviors including agitation, attempts to escape, quiescence, and unresponsiveness. The only behavior from this suite that correlated with P(crit) was the first sign of agitation, which occurred at a lower O(2) tension in the most hypoxia-tolerant species. With the application of phylogenetically independent contrasts, all the relationships between behavior and P(crit) were nonsignificant, which limits our capacity to separate selection-driven changes in behavior from the phylogenetic signal.