The Mudskipper- A fish out of water
The Mudskipper is not like most fish. It spends 90% of its life out of water and is truly an air breathing fish. It belongs to one of the largest groups of fishes known as ‘gobies’ and is found in Indo-Pacific regions and the tropical western coast of Africa. Although this fish may seem out of place on land, it actually thrives in marshlands and mudflats. When the tide is low and the mudflats are uncovered, the mudskipper can emerge and perform activities such as, making burrows, defending territories, performing courtship displays and foraging. Considering that it is indeed a fish, it has had to adapt to the challenging intertidal environment in order to survive. Whilst the fish has not ‘escaped’ the water, it has evolved mechanisms to reduce competition with other fish, which has allowed it to thrive on land. Some of its adaptations include modifications of its morphology such as its skeleton, muscles, respiratory structures and eyes.
Living on land
Within the water, the body of the mudskipper is supported, however, ones out of the water, the mass of the fish is no longer supported by buoyancy. Modifications of the structure of the fins allows the mudskipper to be supported in the challenging terrestrial environment. In order to avoid predation pressures and desiccation stress the mudskipper has had to adapt to using its fins to not only support its body weight, but facilitate movement. Terrestrial movement of the mudskipper is primarily through the use of its pectoral and pelvic fins. To strengthen the fins of the fish, the rays are stiffer and vary in type from spinous and semi-spinous. In order to aid in the ‘skipping’ motion, the caudal tail axial muscles and the pelvic fins propel body whilst the hypaxial muscles produce up-thrust. During the movement of ‘crutching’, which is derived from the familiar term used when one uses crutches, the fish relies on the pelvic fins to move whilst the caudal tail acts as a third point of support. This motion is less continuous than skipping and is slower at a speed of 10cm/sec in an adult fish at approximately 140mm in total length. The modifications of the fins and musculature of the mudskipper enable efficient bodily support and movement on land.
A video clip showing how the mudskipper moves on land. Video credit: Videos of the wild
In order to perform terrestrial activities, the mudskipper has specialised respiratory adaptations which allow for aerial respiration. The three specialised areas are the buccal- branchial cavity, the epithelia in the gills, and the skin. The function that the buccal- branchial cavity plays is in the storage of a large volume of air in order that the mudskipper can respire. This holding place is also used for when the mudskipper is performing activities such as burrowing or territorial displays. As an example of how successful the mudskippers buccal- branchial cavity is, gobies that are not adapted to amphibious life have a cavity volume of 2-6%. Whereas the Malaysian mudskipper (Periophthalmodon schlosseri) has a cavity volume which makes up 16% of its entire body volume. The skin of the mudskipper is made up of three layers: the outer epidermis, the superficial epidermis and the stratum germinativum. These layers have hyper- vascularization which enable the skin to be ‘breathable’ in that respiration can occur between the epithelium layers allowing for gas exchange between the air and the circulating blood. In order for aerial respiration to occur through the skin, it must be kept moist which is achieved by the mudskipper rolling in the wet mud. A study also suggested that the most crucial respiratory adaptation is the skin.
In order for this amphibious fish to see in and out of water, its eyes have adapted to the two mediums. The behaviour of light is different in and out of water due to differences in refraction, therefore, when the fish comes out of the water, the refraction of both the lens of the eye and the cornea essentially combine and create ‘short-sightedness’. To combat this issue, the lens is flattened when the fish emerges from the water. As well as this issue, the fish also needs to drain the eye of excess water, and adapt to the stress of desiccation and increased light intensity to the eye. The problems of excess water on the eye of the mudskipper are similar to that of rain on the windscreen of a car. To solve this issue, the mudskipper has a drainage system in which water is removed quickly, due to the position of the eye, and stored in a groove at the edge of the corneal surface. As the mudskipper can’t produce tears due to the lack of lacrimal glands, it retracts its eye which allows it to be re-hydrated. To further avoid desiccation to the eye, the fish has a mucus membrane, conjunctiva, which covers the cornea. An additional feature of the mudskipper’s eye, which benefits is movements from water to land, is the protrusion of the eye which enhances the field of vision which increases vigilance to predation.
To conclude, the mudskipper has numerous morphological adaptations, enabling it to thrive in and exploit the water and land of the challenging intertidal environment.