Gill morphology of the mangrove killifish (Kryptolebias marmoratus) is plastic and changes in response to terrestrial air exposure

Abstract

Amphibious mangrove killifish, Kryptolebias marmoratus (formerly Rivulus marmoratus), are frequently exposed to aerial conditions in their natural environment. We tested the hypothesis that gill structure is plastic and that metabolic rate is maintained in response to air exposure. During air exposure, when gills are no longer functional, we predicted that gill surface area would decrease. In the first experiment, K. marmoratus were exposed to either water (control) or air for 1 h, 1 day,1 week, or 1 week followed by a return to water for 1 week (recovery). Scanning electron micrographs (SEM) and light micrographs of gill sections were taken, and morphometric analyses of lamellar width, lamellar length and interlamellar cell mass (ILCM) height were performed. Following 1 week of air exposure, SEM indicated that there was a decrease in lamellar surface area. Morphometric analysis of light micrographs revealed that there were significant changes in the height of the ILCM, but there were no significant differences in lamellae width and length between any of the treatments. Following 1 week of recovery in water, the ILCM regressed and gill lamellae were similar to control fish, indicating that the morphological changes were reversible. In the second experiment, V̇CO2 was measured in fish continuously over a 5-day period in air and compared with previous measurements of oxygen uptake(V̇O2) in water. V̇CO2 varied between 6 and 10 μmol g–1 h–1 and was significantly higher on days 3, 4 and 5 relative to days 1 and 2. In contrast to V̇O2 in water, V̇CO2 in air showed no diurnal rhythm over a 24 h period. These findings indicate that K. marmoratus remodel their gill structures in response to air exposure and that these changes are completely reversible. Furthermore, over a similar time frame, changes in V̇CO2 indicate that metabolic rate is maintained at a rate comparable to that of fish in water, underlying the remarkable ability of K. marmoratus to thrive in both aquatic and terrestrial habitats.

The amphibious fishKryptolebias marmoratususes alternate strategies to maintain oxygen delivery during aquatic hypoxia and air exposure

“…Observing bucco-opercular respiration was highly unexpected, considering gill surface area is reduced during air exposure and the branchial region was presumed to be non-functional (Ong et al, 2007;Cooper et al, 2012). Instead, aerial O 2 uptake in this species was thought to be achieved entirely across the cutaneous epithelium (Wright, 2012).…”

“…Augmented cutaneous blood flow increases O 2 uptake and probably allowed mangrove rivulus to reduce evaporative water loss by decreasing aerial ventilation frequency (Burggren and Mwalukoma, 1983; Burggren and Moalli, 1984). Air exposure also induces enlargement of the inter-lamellar cell mass, reducing the effective gill surface area, which may further limit water loss during bucco-opercular ventilation (Ong et al, 2007).…”

“…Control fish were maintained under standard rearing conditions (~21kPa O 2 ). Air-acclimated fish were maintained on moist filter paper (15‰ brackish water, ~21kPa O 2 ) in plastic rearing containers (Ong et al, 2007). Hypoxia in both water and air was achieved by introducing a finely regulated flow of nitrogen into closed chambers containing experimental fish (Regan et al, 2011).…”

“…Augmented cutaneous blood flow increases O 2 uptake and probably allowed mangrove rivulus to reduce evaporative water loss by decreasing aerial ventilation frequency (Burggren and Mwalukoma, 1983; Burggren and Moalli, 1984). Air exposure also induces enlargement of the inter-lamellar cell mass, reducing the effective gill surface area, which may further limit water loss during bucco-opercular ventilation (Ong et al, 2007).Observing bucco-opercular respiration was highly unexpected, considering gill surface area is reduced during air exposure and the branchial region was presumed to be non-functional (Ong et al, 2007;Cooper et al, 2012). Instead, aerial O 2 uptake in this species was thought to be achieved entirely across the cutaneous epithelium (Wright, 2012).…”

Rhesus glycoprotein gene expression in the mangrove killifishKryptolebias marmoratusexposed to elevated environmental ammonia levels and air

“…Fish exposed to terrestrial conditions were placed in individual plastic containers containing a moist paper of 15‰ seawater at 25°C, as described previously (Ong et al, 2007). After 24·h, fish were decapitated and skins were dissected, frozen immediately in liquid nitrogen and kept at -80°C until analysis within 1 week.…”