Mudskippers are any of the 23 extant species of amphibious fish from the subfamily Oxudercinae of the goby family Oxudercidae.
Mudskippers can grow up to long, and most are a brownish green colour that ranges anywhere from dark to light. During , the males will also develop brightly coloured spots in order to attract females, which can be red, green or blue. Unlike other fish, the mudskipper's eyes protrude from the top of its flat head. Their most noticeable feature however is their side pectoral fins that are located more forward and under their elongated body. These fins are jointed and function similarly to limbs, which allow the mudskipper to crawl from place to place. Although having the typical body form of any other gobiid fish, these front fins allow the mudskipper to actively "skip" across muddy surfaces (hence the common name) and even climb low-hanging and Shrubland. Mudskippers have also been found to be able to leap distances of up to by laterally flexing and pushing with their tails.
Taxonomy
Oxudercinae is sometimes classified within the family Gobiidae (
Goby).
Recent molecular studies do not support this classification, as oxudercine gobies appear to be
paraphyletic relative to amblyopine gobies (Gobiidae:
Amblyopinae), thus being included in a distinct "
Periophthalmus lineage", together with amblyopines.
Mudskippers can be defined as oxudercine gobies that are "fully terrestrial for some portion of the daily cycle" (character 24 in Murdy, 1989
). This would define the species of the genera
Boleophthalmus,
Periophthalmodon,
Periophthalmus, and
Scartelaos as "mudskippers". However, field observations of
Zappa confluentus suggest that this monotypic genus should be included in the definition.
Behavior
Mudskippers typically live in burrows in
intertidal , and exhibit unique adaptations to this environment that are not found in most
, which typically survive the retreat of the
tide by hiding under wet
seaweed or in
.
These burrows are most often characterised by their smooth and vaulted ceilings. The way the males dig these burrows has been found to be directly linked to their ability to survive submerged in almost anoxic water. It has also been found to play a crucial role in the development of the eggs within the burrow. Mudskippers are quite active when out of water, feeding and interacting with one another, for example, to defend their territories and court potential partners. Once the male has completed digging his burrow he will resurface and will begin attempting to attract a female through assorted yet typical displays. These displays consist of body undulations, different postures and energetic movements. Once the female has made her choice she will then proceed to follow the male into the burrow where she will lay hundreds of eggs and allow them to be fertilized. After
Fertilisation occurs, the period of cohabitation between the male and female is rather short. Eventually, the female will leave and it is the male that ends up guarding the egg-filled burrow from predators.
Mudskippers are amphibious. When leaving the water and moving into a more dry environment on land, they are still able to breathe using water that is trapped inside their large gill chambers. They are also able to absorb oxygen from the lining of their mouth and throat, allowing them to stay out of water for long periods of time. It has been discovered that they spend up to three quarters of their life on land. They are found in tropical, subtropical, and temperate regions, including the Indian Ocean-Pacific Ocean and the Atlantic Ocean coast of Africa.
Adaptations
Compared with fully aquatic gobies, these specialised fish present a range of
anatomical and
ethological adaptations that allow them to move effectively on land as well as in the water.
Terrestrial movement
As their name implies, these fish use their fins to move around in a series of skips.
Breathing
Mudskippers have the ability to breathe through their skin and the lining of their mouth (the
mucosa) and throat (the
pharynx); this is only possible when the mudskippers are wet, limiting them to humid habitats and requiring that they keep themselves moist. The ability to breathe through their skin is associated with increased capillary density in their skin.
This mode of breathing, similar to that employed by
amphibians, is known as cutaneous respiration.
[ Another important adaptation that aids breathing while out of water is their enlarged gill chambers, where they retain a bubble of air. These chambers close tightly when the fish is above water, due to a ventromedial valve of the gill slit, keeping the gills moist, and allowing them to function while exposed to air. Gill filaments are stiff and do not coalesce when out of water.][
]
Diet
The different species have adapted to various diets on the mudflats. Boleophthalmus boddarti is detritivorous, while others will eat small crabs, insects, snails and even other mudskippers.[ Ecology of Kalimantan: Indonesian Borneo]
Burrowing
Digging deep burrows in soft sediments allows the fish to thermoregulate,
Ammonia regulation
To reduce toxic ammonia production, mudskippers can suppress amino acid breakdown when exposed to air.
Mudskippers can reduce the membrane permeability of their skin and acidify the water in their burrows to reduce levels of ammonia from the environment.
Blinking
Mudskippers evolved the ability to blink independently from terrestrial . Their eyes are located high on their head compared to other gobies, and they blink by lowering their eyes as a membrane called the dermal cup rises to cover them. Although other fully aquatic goby species do not have the ability to blink, mudskippers did not evolve different muscles or glands to blink with; their blinking is accomplished with the same muscles in a different configuration. Rather than having specialized glands to produce eye lubricant, the fluid film on their cornea is likely composed of mucus secreted by their skin and water from their environment, possibly stored in the infraorbital space behind the dermal cup membrane. Mudskippers likely evolved blinking in response to conditions of terrestrial life, such as to keep their eyes wet out of water (they blink more frequently in high evaporation conditions and only when colliding with things in water) and to clean and protect the eye from debris, which may adhere to the eye or approach at a faster, more dangerous speed when in air versus when in water. Their eyes are not elevated when they are still juveniles, which are fully aquatic.
Species
The genus Periophthalmus is by far the most diverse and widespread genus of mudskipper. Eighteen species have been described.[ It grows to a length of about 9.5 cm ][ and is a carnivorous opportunist feeder. It feeds on small prey such as small crabs and other arthropods.][
]
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