Positive feedback promotes terrestrial emergence behaviour in an amphibious fish

Major ecological transitions such as the invasion of land by aquatic vertebrates have been hypothesised to be facilitated by positive feedback between habitat choice and phenotypic plasticity. We tested whether aquatic hypoxia, emergence behaviour, and plastic changes in gill surface area could create such a positive feedback loop and drive an amphibious fish to spend increasing amounts of time out of water. We found that terrestrially acclimated amphibious mangrove rivulus Kryptolebias marmoratus were more sensitive to, and less tolerant of, aquatic hypoxia relative to water-acclimated fish, which are necessary trade-offs for positive feedback to occur. Next, we acclimated fish to normoxic or hypoxic water with the opportunity to emerge for 7d to test the predictions that fish in hypoxic conditions should regularly leave water, reduce gill surface area, and become less hypoxia tolerant. Consistent with these predictions, fish in severe hypoxia spent almost 50% of the time out of water and coverage of the gill lamellae by an inter-lamellar cell mass almost doubled. Hypoxia acclimated fish were also more sensitive to acute aquatic hypoxia (emergence at higher oxygen levels), and lost equilibrium faster in hypoxic water compared to control fish. Thus, we show that a positive feedback loop develops in amphibious fish where emergence behaviour begets further emergence behaviour, driven by gill remodelling which reduces aquatic respiratory function. Such a scenario may explain how amphibious behaviour has repeatedly evolved in fishes that occupy hypoxic aquatic habitats despite the associated challenges of life on land.

Interspecific Differences in the Metabolic Rate, Gill Dimension and Hematology of Fish in an Amazonian Floodplain Lake

It has been hypothesized that respiratory physiology in fish is closely associated with ecological traits. Therefore, data on gill morphometrics (lamellae frequency, gill surface area and barrier thickness), metabolic rate (oxygen consumption) and blood oxygen capacity (hematology) were analyzed in several fish, including benthic, bentho-pelagic and pelagic species collected in an Amazon floodplain lake. Similar to other teleostean species, the 2nd and 3rd gill arches have numerous large filaments in both pelagic and benthic species, as these characteristics tend to increase the gill surface area. A large gill area (4 to 7 cm2 g-1, mass-specific) is associated with a high (100 to 300 mg O2 h-1 kg-1) routine oxygen consumption rate and has been observed in active pelagic swimmers, such as Cichla monoculus and Pygocentrus nattereri. Benthic dwelling fish (e.g., Pterygoplichthys pardalis and Sorubim lima) have low metabolic rates (20 to 50 mg O2 h-1 kg-1), small gill dimensions (2 to 3 cm2 g-1, mass-specific), low hemoglobin levels (3 to 5 g dL-1), reduced numbers of circulating red blood cells (1 to 2 Í106 mm-3) and low hematocrit values (25 to 35%) compared to pelagic species. These results demonstrated that pelagic fish have high routine oxygen consumption rates compatible with their large gill surface area and high blood oxygen capacity, whereas benthic species have low metabolic rates, small gill dimensions and reduced blood oxygen capacity.

Calcified gill filaments increase respiratory function in fishes

The morphology of fish gills is closely linked to aerobic capacity and tolerance of environmental stressors such as hypoxia. The importance of gill surface area is well studied, but little is known about how the mechanical properties of gill tissues determine function. In some fishes, the bases of the gill filaments are surrounded by a calcified ‘sheath' of unknown function. We tested two non-exclusive hypotheses: (i) calcified gill filaments enhance water flow through the gill basket, improving aquatic respiratory function, and (ii) in amphibious fishes, calcification provides support for gills out of water. In a survey of more than 100 species of killifishes and related orders, we found filament calcification was widespread and thus probably arose before the evolution of amphibious lifestyles in killifishes. Calcification also did not differ between amphibious and fully aquatic species, but terrestrial acclimation caused calcium deposition on the filaments of the killifish Kryptolebias marmoratus , suggesting a possible structural role when out of water. We found strong evidence supporting a role for filament calcification in enhancing aquatic respiratory function. First, acclimation to increased respiratory demands (hypoxia, elevated temperatures) induced calcium deposition on the filaments of K. marmoratus . Next, gentle removal of filament calcification decreased branchial resistance to water flow, indicating disruption of gill basket positioning. Thus, the mechanical properties of the gill filaments appear to play an important and previously unappreciated role in determining fish respiratory function.

Gill slits provide a window into the respiratory physiology of sharks

Metabolically important traits, such as gill surface area and metabolic rate, underpin life histories, population dynamics and extinction risk, as they govern the availability of energy for growth, survival and reproduction. Estimating both gill surface area and metabolic rate can be challenging, especially when working with large-bodied, threatened species. Ideally, these traits, and respiratory physiology in general, could be inferred from external morphology using a faster, non-lethal method. Gill slit height is quick to measure on live organisms and is anatomically connected to the gill arch. Here, we relate gill slit height and gill surface area for five Carcharhiniform sharks. We compared both total and parabranchial gill surface area to mean and individual gill slit height in physical specimens. We also compared empirical measurements of relative gill slit height (i.e. in proportion to total length) to those estimated from field guide illustrations to examine the potential of using anatomical drawings to measure gill slit height. We find strong positive relationships between gill slit height and gill surface area at two scales: (i) for total gill surface area and mean gill slit height across species and (ii) for parabranchial gill surface area and individual gill slit height within and across species. We also find that gill slit height is a consistent proportion of the fork length of physical specimens. Consequently, relative gill slit height measured from field guide illustrations proved to be surprisingly comparable to those measured from physical specimens. While the generality of our findings needs to be evaluated across a wider range of taxonomy and ecological lifestyles, they offer the opportunity that we might only need to go to the library and measure field guide illustrations to yield a non-lethal, first-order approximation of the respiratory physiology of sharks.

The Gills Area of Two Species of Marine Water Crabs: The Blue Swimming Crab, Portunus pelagicus (Linnaeus, 1758), and the Chinese Mitten Crab, Eriocheir sinensis H. Milne Edwards, 1853 from the North- West Arabian Gulf and Shatt Al-Arab River , Iraq.

The present study was conducted on the gills structure and the gill surface area of 40 individuals of the Blue swimming crabs, Portunus pelagicus, and 40 Chinese mitten crabs, Eriocheir sinensis, specimens of the species were collected from the different regions of Shatt Al- Arab and North-west Arabian gulf. Both species were having eight phyllobranchiatesas of the same structure but different in length and surface area. The second gill in P. pelagicus shows major difference in shape from the other gills with missing platelets from one side of the filament. The results showed there were positive relationships between dry body weight with gill surface area, total number of platelets and number of platelets surface area in each gill. The gill surface area had no significant difference (p>0.05) with dry weight in the two species.

Gill dimensions in near-term embryos of Amazonian freshwater stingrays (Elasmobranchii: Potamotrygonidae) and their relationship to the lifestyle and habitat of neonatal pups

This comparative study of gill morphometrics in near-term embryos of freshwater stingray potamotrygonids examines gill dimensions in relation to neonatal lifestyle and habitat. In embryos of the potamotrygonids Paratrygon aiereba, Plesiotrygon iwamae, Potamotrygon motoro, Potamotrygon orbignyi, and cururu ray Potamotrygon sp. the number and length of filaments, total gill surface area, mass-specific surface area, water-blood diffusion distance, and anatomical diffusion factor were analysed. In all potamotrygonids, the 3rd branchial arch possessed a larger respiratory surface than the other gill arches. Larger embryos had more gill surface area and large spiracles, which are necessary to maintain the high oxygen uptake needed due to their larger body size. However, the higher mass-specific gill surface area observed in near-term embryos may be advantageous because neonates can use hypoxic environments as refuges against predators, as well as catch small prey that inhabit the same environment. As expected from their benthic mode of life, freshwater stingrays are sluggish animals compared to pelagic fishes. However, based on gill respiratory morphometry (such as gill area, mass-specific gill area, the water-blood diffusion barrier, anatomical diffusion factor, and relative opening of the spiracle), subtypes of lifestyles can be observed corresponding to: active, intermediate, and sluggish species according to Gray's scale.