Mudskippin’ Along

Guest Blogger: Luke Leuty

The following guest post was voted best in the class for an undergraduate assignment on fish adaptation from Humboldt State University’s Fall 2020 Fish Conservation and Management course taught by Dr. Andre Buchheister. The assignment required students to find a peer-reviewed article on a fascinating fish adaptation or species and to communicate the scientific information in a blog format to a general audience. 

Two male mudskippers performing an acrobatic scuffle to settle a territory dispute. The victor will have a chance at luring a mate to his burrow. (image credit: rangerrick.org)

What is that?

Imagine yourself walking along the beach, admiring the surf as it washes ashore. Suddenly, what looks like an unassuming piece of driftwood jumps! What is that weird creature hopping around? Believe it or not, that’s a fish! Now, you might be confused. What is a fish doing on the shore? Well, that driftwood look-a-like is a mudskipper, a name readily earned. Due to unique adaptations of their gills, skin, fins, and eyes, this frog-like fish is amphibious, spending a majority of its time on the mud, outside of the water!

The areas highlighted in red indicate where mudskippers naturally inhabit. These coastlines and estuaries consist of mudflats and mangrove swamps, the perfect living space for a mudskipper. (Image credit: repository.si.edu)

The name mudskipper is given to several genera members of the suborder Oxudercinae. These genera are Scartelaos, Boleophthalmus, Periophthalmus, and Periophthalmodon (Lee & Graham 2002). Mudskippers comprise 25 species inhabiting coasts and estuaries, ranging from West Africa all the way to New Guinea. Most species live on mudflats and in mangrove swamps, where they spend their days scouring for food and building their burrows. 

A breath of fresh air

The mudskipper is a rare exception among fishes in that it has the ability to “breathe” air outside of the water. This is due to unique adaptations of the fish’s skin and gills. Mudskipper gills have been designed to reduce their surface area, allowing them to retain more moisture (Sayer 2005). A fully aquatic fish would not be able to last as long on land, for their gills would soon collapse without moisture. The mudskipper gill design includes branched gill filaments which allows the amphibious fish to remain out of the water for over a full day (Sayer 2005; Gordon et al. 1968)!

A mudskippers burrow is constructed in a J shape. 
This provides them a small pocket of air at the end 
for breathing and protecting eggs until they hatch. 
(Image credit: Ishimatsu et al. 2007)

The gills are not the only place “where the magic happens.” The skin of the mudskippers is also designed to help with a more terrestrial lifestyle. In a classic study, the mudskipper’s amphibious abilities were examined on the island of Nosy Be, Madagascar to see how long they could survive without being submerged (Gordon et al. 1968). Turns out the design of their gills and skin enabled mudskippers to survive outside of an aquatic environment longer than an amphibian living in a similar environment! 

Even with these incredible designs, the mudskipper has yet another adaptation to breathe air. In order to survive the low oxygen and high ammonia environment of the mudflats, mudskippers create an air chamber inside their burrows. They do so by making a J shaped tunnel, where they then deposit gulps of air before high tide (Ishimatsu et. al. 2007). Once high tide hits, the mudskipper can then relax and take gulps of air from inside the safety of their burrow. For species in the subfamily Oxudercinae, these burrows have the added bonus of serving as a nesting site (Ishimatsu et. al. 2007). Eggs are laid on the sides of the air pockets, where they remain until fully developed. Then, the male mudskipper will submerge the eggs, causing them to hatch.

Movement on the flats

This illustration shows the motions of a mudskipper’s fins as it moves along the mudflats. (Image credit: Sayer 2005)

Another important adaptation the mudskipper utilizes in its semi-terrestrial life is its highly specialized fins. A mudskipper’s fins are unlike a fully aquatic fish. Instead of being designed for swimming, they are short and compact, giving the fish stability on land. Mudskipper pelvic fins rest just underneath the pectoral fins, allowing the fish to better balance out of water. This also allows the fish to move by what is called crutching. According to Sayer (2005), a mudskipper will start the motion by pushing off with its pectoral fins. Then, the pelvic fins extend, acting as a landing pad to keep the fish balanced. This video gives a perfect demonstration of just how the mudskipper moves. This process repeats itself rapidly, a full body coordination, as the mudskipper crutches along the land.  

Better to see you with!

Quite possibly the most comical feature of the mudskipper is the eyes. They sit on top of its head, giving it a frog-like appearance. Yet, these eyes are what keep the mudskipper safe from predators while it is out of the water. In fact, the mudskipper’s eyes give it better vision out of the water than submerged in it (Lee & Graham 2002)! Their superb vision is thanks to a highly modified lens in the eye. This lens is not as curved as the lenses in other fish, enabling them to see more clearly outside of the water (Sayer 2005). As if they did not have enough of a terrestrial advantage, mudskippers are also able to “blink.” They have folds of skin underneath their eyes that hold excess water, letting them moisten their eyes to prevent drying out (Lee & Graham 2002). With these specially designed eyes, mudskippers are able to both watch for terrestrial predators and search for food.

  Eyes on top of their head gives the mudskipper a 
  significant advantage in detecting both predators and 
  prey on land. (Image credit: H. Krisp)

The ultimate amphibious fish

The mudskipper is a unique fish with multiple designs that enable it to not only survive but also thrive on land. Living in a habitat where the water is low in oxygen and ammonia is high, an amphibious lifestyle is essential to the mudskipper’s survival. Gills that have a smaller surface area allow mudskippers to keep them moist with little water and enable mudskippers to breathe air even on land. The mudskipper’s skin is so efficient at retaining moisture that it outperforms a frog that lives in the same environment, and makes long terrestrial exposure times a piece of cake (Gordon 1968). Their fins are more leg-like, giving them the ability to scoot across mudflats. Bulbous eyes on top of their head, coupled with a set of uniquely curved lenses provide excellent vision on land. These special adaptations in their gills, skin, fins, and eyes, make that weird creature you mistook for driftwood on the beach the ultimate amphibious fish!

This fish is truly designed to not only survive but thrive 
  on land. Very few species of fish are able to do this in as 
  effective a manner as the mudskipper. (Image credit: Alpsdake)

About the Author:

Hello! My name is Luke Leuty. I am a Freshwater Fisheries major at Humboldt State University. I have always had a passion for outdoor recreation, even when I was young. I can’t remember a time when I did not have a fishing pole in my hands. One of the main highlights of my childhood was when my father and I would go on our annual deep sea fishing trips. Having parents willing to help me pursue my interests really enabled me to follow my passions. 

A major turning point in my life was when my cousin bought me my first 20-gallon aquarium. Fast forward a few years, and you will see my pride and joy, a 250-gallon North American show tank (see below)! As I grew older, I began to realize that I could turn my passion into a career. I knew that I wanted to be hands on with my work, so I have decided to become a game warden. Choosing to transfer to HSU to complete my bachelor’s degree has been one of the best decisions in my life and I continue to be amazed by the world of fisheries biology!

For other Humboldt State student posts, please see:

References:

Adhityo, W., Bambang, R., Parvez, A., and Saifullah, H. 2020. The water-hopping kinematics of the tree-climbing fish, Periophthalmus variabilis. Zoology. 169. 

Bian, C., Chen, J., Li, J., Shi, Q., Sun, M., Yi, Y., You, X., and Yu, H. 2018. Mudskippers and their Genetic Adaptations to an Amphibious Lifestyle. Animals. 8 (2): 24.

Boetius, I., Evans, D., Gordon, M., McCarthy, J., and Oglesby, L. 1969. Aspects of the Physiology of Terrestrial Life in Amphibious Fishes. I. The Mudskipper, Periophthalmus sobrinus. J. Exp. Biol. 50 (I): 41-49.

Ishimatsu, A., Yoshida, Y., Itoki, N., Takeda, T., Lee, H. J., and Graham, J. B. 2007. Mudskippers brood their eggs in air but submerge them for hatching. Journal of Experimental Biology. 210: 3946-3954. 

Graham, J. B., and Lee, H. J. 2002. Their game is mud. Natural History, 111: 42-47. 

Sayer, M. D. J. 2005. Adaptations of amphibious fish for surviving life out of water. Fish and Fisheries, 6: 186-211.

Graham, J. B. and Lee, H. J.  2003. Breathing in air: lessons from amphibious mudskippers on integrating aerial respiration with life on land. Comparative Biochemistry and Physiology. 134(Suppl. 1): 113.

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