A skin breathing fish and its other unique adaptations

Killifish (families Aplocheilidae, Cyprinodontidae, Fundulidae, Profundulidae and Valenciidae) are egg-laying fish that are mostly adapted to living in ephemeral waters, many with eggs able to survive out of water and for periods of partial dehydration. They are found in fresh and brackish water on the Americas, southern Europe and much of Africa, middle-east, but they are not naturally occurring in Australia or northern Europe.

We know the skin of fish performs many functions, including gas and ion exchange, and nitrogen excretion when fish are emersed. The Mangrove Killifish (Kryptolebias marmoratus) lives in brackish water along the eastern coast of the Americas. It has a wide tolerance to salinity (0-68‰) and temperature (7-38ºC) and can survive out of water (Taylor 2012, Heffell et al. 2018). And, if that is not amazing enough, it can alter the thickness of its skin in response to salinity and exposure to air. The ventral epidermis gets thicker in response to increasing salinity, does not change with exposure to air (e.g. emersion) but the dermis gets thinner in response to emersion (Heffell et al. 2018). This fish spends about 70% of its time on its ventral surface when out of water. So, it looks as if this extremely adaptable killifish is managing its water balance by changing the thickness of its skin, as emersion increases skin water flux, i.e. it is able to take up water in hypersaline conditions and when emersed due to the thinner ventral dermis.

Although it is commonly thought or described as a mangrove species, Taylor (2012) lists its micro-habitats as intermittently dry shallow, stagnant pools; crab burrows (specifically, those of Cardisoma guanhumi, great land crab, and Ucides cordatus, mangrove land crab); temporally flooded swales; ditches (including anti-mosquito ditches); inside/under logs and mangrove leaf litter; solution holes; empty beer cans and coconuts; and even several meters inside anchialine cave systems. This is probably one of the reasons its geographical range includes most of the tropics and subtropics of North, Central and South America – about 52º of latitude and 91º of longitude. It has also been collected in very poor water quality and is able to survive in hypoxic conditions (<1.00ppm; Dunson and Dunson 1999).

It gets better! After terrestrial acclimation, Type X collagen, which is associated with endrochondral bone growth, increases ninefold to provide extra strength and stiffening that enables a fish that would have normally gained support from the water to move in terrestrial environments (Turko et al. 2017).

This same fish exists primarily as hermaphrodites with preferential self-fertilization (Harrington 1961), but this species also exhibits environmental sex determination, with males developing from embryos incubated at low temperatures (i.e. 18-22ºC; Harrington 1967, Harrington and Kallman 1968) and then remain sexually labile throughout its life.

The Mangrove Killifish has these amazing and quite bizarre adaptations which enable it to survive in a variety of conditions. Australian fauna are also truly unique but their special adaptations will be the focus of another post. Have you got a favourite Australian species with unique adaptations that you want explained?

References

Dunson, W. A., and D. B. Dunson. 1999. Factors influencing growth and survival of the killifish, Rivulus marmoratus, held inside enclosures in mangrove swamps. Copeia 1999:661-668.

Harrington, R. W. 1961. Oviparous hermaphroditic fish with internal self-fertilization. Science 134.

Harrington, R. W. 1967. Environmentally controlled induction of primary male gonochorists from eggs of the self-fertilizing hermaphroditic fish, Rivulus marmoratus. Biology Bulletin 132.

Harrington, R. W., and K. D. Kallman. 1968. The homozygosity of clones of the self-fertilizing hermaphroditic fish Rivulus marmoratus Poey (Cyprinodontidae, Atheriniformes). American Naturalist 102.

Heffell, Q., A. J. Turko, and P. A. Wright. 2018. Plasticity of skin water permeability and skin thickness in the amphibious mangrove rivulus Kryptolebias marmoratus. Journal of Comparative Physiology B 188:305-314.

Taylor, D. S. 2012. Twenty-four years in the mud: what have we learned about the natural history and ecology of the mangrove rivulus, Kryptolebias marmoratus? Integr Comp Biol 52:724-736.

Turko, A. J., D. Kultz, D. Fudge, R. P. Croll, F. M. Smith, M. R. Stoyek, and P. A. Wright. 2017. Skeletal stiffening in an amphibious fish out of water is a response to increased body weight. Journal of Experimental Biology 220:3621-3631.

Photo – Mangrove Killifish (Kryptolebias marmoratus) taken from https://adlayasanimals.wordpress.com/2013/04/24/mangrove-killifish-kryptolebias-marmoratus/

Addendum – We had some factual errors in last weeks post. The post has now been corrected but it should have indicated Brushtail Possums and Bilbies were released outside the fenced area at Matuwa, and Golden Bandicoots and Boodies were released inside the fenced area.