There are over 200 species of amphibious fish that exploit the land by venturing out of the water. The benefits and extent of emersion depends on the species. Some only emerge for a few hours a day at low tide to avoid predators or to migrate to different tidal pools. While others have lost the ability to swim almost completely and so live out their lives in the splash zone. Emersion on land allows a fish to avoid low water oxygen levels, (hypoxia) and potentially access a food resource or breeding space with very low competition from other species.

Whatever the case may be there are three major environmental challenges they all have to deal with.  Amphibious fish either have to respond to hypoxic environment or adapt their breathing method or both. Depending on the degree they exploit marine and terrestrial habitats. Regardless of their niche role they must all be able to deal with gravity effects which they wouldn’t feel in the water as much and they must all be able to find water to keep damp. (Ord et al. 2017; Polgar et al. 2017; Turko et al. 2017)

File:Kriptolebias marmoratus.jpg
Kriptolebias marmoratus kilifish Source; wikimedia commons Arthor; Cardet co6cs

Breathing.

At best, water only has about 3% of the amount of oxygen that is available in the air. When emerged, the surface normally used for gas exchange in the water, the lamellae in the gills, collapses and sticks together. This impairs the fishes breathing ability through the gills. If carbon dioxide concentrations were to build up in the blood, the pH would drop. The pigment responsible for oxygen transport (Heamogloblin) would have a lower affinity for Oxygen, compromising the fish’s ability to breath.

Most amphibious fish have adapted cutaneous exchange where they breathe through their skin as well as gills. Extensive capillary networks are under the skin in the mouth and throat allow for gas exchange during emersion on land. Some have lung like organs like the lung fish or the labyrinth fish that develops its pseudo-lung as it grows. Labyrinth fish do not leave the water but can access oxygen if the pools become hypoxic (Ultsch, 1987;  Dejours 1988; Turko et al. 2014).

Not only do amphibious fish have to be able to breath but they have to be able to respond to hypoxic environments to prevent suffocation.

In general, amphibious fish are hypoxia tolerant and have adapted chemoreceptors, neuroepithelial cells (NECs) to detect oxygen. In aquatic fish these receptors are in the gills. For amphibious fish, receptor location is not limited to them and seems to be species dependent. NECs occur internally and externally to detect hypoxia in the blood and environment.  For instance, air-breathing bichirs (Polypterus delhezi and P. ornatipinnis) have chemoreceptors on the surface of the swim bladder (Zaccone et al. 2008; Regan et al 2011)

Gravity.

Amphibious fish have to be able to support their weight on land in a way that they wouldn’t have to in water.

Adaptions depend a lot on the life style of the fish. Killifish can spend months on land and so will acclimatise over the course of a week by adapting reinforcing their skeleton for support. by: both the modification of existing tissues- production of collagen to stiffen bone and the formation of new bone. (Turko et al. 2017)

(Video from Noah Bressman)

Staying hydrated.

Different fish emerge for different reasons. Some species need to find new pools as quickly as possible while others are more at home on land. They all need to keep their skin wet to breath though. To do this fish need to be able to find water and be mobile enough to navigate the terrain to get there.

Eyes have been adapted for terrestrial life and in some species. The mudskippers’ vision is better on land than it is water (Polgar et al. 2017).

Mudskippers spend most of their daily cycle on land and are quiet accomplished diggers and climbers- for a fish that is on land. Its muscular body allows it to travel by hopping (Gilman, 2016; Polgar et al. 2017)

Mummichog spend much less time out of water and only really leaves it to find better rock pools. As a result, its’ morphology is not as well suited for land living. To travel, they bend their head to tail and spring launch themselves in hopefully the right direction (Gilman 2016).

Fish example:

Name: Clarias batrachus Walking Catfish                                                                                                                          Found: South east Asia, Africa. Invasive to Florida                                                                                                          Reason to be on land: Search for food                                                                                                                                    Can travel impressive distances over land with muscular body and adapted pectoral fins. Protrusions from gills help it breath. (Morelle, 2006)

Pacific leaping blenny. Alticus arnoldorum Source; Flickr Arthor; pattfwl

Name: Kryptolebias marmoratus Killifish                                                                                                                                   Found: South and Central America mangrove forests                                                                                                   Reason to be on land: hypoxia in water.                                                                                                                Killifish relatively tolerant of extreme water conditions but they are often found out of water on: mangrove roots, in hypoxic blue crab burrows, puddles, leaf litter or in insect dug crevices in wood. Can survive out of water for months (Taylor et al. 2008; Regan et al 2014; Turko et al. 2014)

Name: Alticus arnoldorum Pacific leaping Blenny                                                                                                 Found: Micronesia coast.                                                                                                                                                     Reason to be on land: predator avoidance and access to breeding grounds                                                                    Four to eight centimetres long. Very territorial, rarely ever returns to water. Many species in the Blenny family are to some degree amphibious (Ord et al. 2011; Ord et al. 2017)

In summary

Amphibious fish have evolved for different reasons as shown by the examples. this i an example of convergence evolution. Not all adaptations are shared between species, although some, like cutaneous breathing, are fairly common. Adaptations depend largely on life style, chiefly, how much time spent emerged and how variable their environment maybe.

References

 Dejours, P. (1988). Respiration in Water and Air. Adaptations-Regulation-Evolution. Amsterdam: Elsevier.

Gilman C, (2016). Amphibious fish prop up when seeking water. Journal of Experimental Biology. 219: 1586-1587; doi: 10.1242/jeb.130146

Morelle R. (2006) African fish leaps for land bugs. BBC news channel.{online} http://news.bbc.co.uk/1/hi/sci/tech/4902784.stm

Ord, T. J., Hsieh, S. T., (2011). A highly social, land-dwelling fish defends territories in a constantly fluctuating environment. Ethology 117:918–927.

Ord T.J., Summers, T.C., Noble, M.M., Fulton, C.J.,(2017) Ecological Release from Aquatic Predation Is Associated with the Emergence of Marine Blenny Fishes onto Land, The American Naturalist 189, no. 5: 570-579

Polgar, G., Ghanbarifardi, M., Milli, S., Agorreta A., Aliabadian M., Esmaeili H.R., Khang T.F (2017) Ecomorphological adaptation in three mudskippers (Teleostei: Gobioidei: Gobiidae) from the Persian Gulf and the Gulf of Oman 795: 91. https://doi-org.ezproxy.bangor.ac.uk/10.1007/s10750-017-3120-8 .

Regan, K.S., Jonz, M.G., Wright, P.A., (2011) Neuroepithelial cells and the hypoxia emersion response in the amphibious fish Kryptolebias marmoratus. Journal of Experimental Biology. 214: 2560-2568; doi: 10.1242/jeb.056333

Taylor, D.S., Turner, B.J., Davis, W.P., Chapman, B.B., (2008). A novel terrestrial fish habitat inside emergent. logs.Am. Nat. 171, 263-266.

Turko, A.J., Kültz, D., Fudge, D., Croll, R.P., Smith, F.M., Stoyek, M.R., Wright, P.A., (2017) Skeletal stiffening in an amphibious fish out of water is a response to increased body weight. Journal of Experimental Biology 220: 3621-3631; doi: 10.1242/jeb.161638

Turko, A.J., Robertson, C.E., Bianchini, K., Freeman, M., Wright, P.A., (2014) The amphibious fish Kryptolebias marmoratus uses different strategies to maintain oxygen delivery during aquatic hypoxia and air exposure. Journal of Experimental Biology. 217: 3988-3995; doi: 10.1242/jeb.110601

Ultsch, G.R. (1987). The potential role of hypercarbia in the transition from water-breathing to air-breathing in vertebrates. Evolution 41, 442-445

Zaccone, G., Mauceri, A., Maisano, M., Giannetto, A., Parrino, V.,     Fasulo, S. (2008). Neurotransmitter localization in the neuroepithelial cells and unipolar neurons of the respiratory tract in the bichir, Polypterus bichir bichir. Acta Histochem. 110, 143-150.

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