eduKate Learning Manual
Science | Animal World
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Mudskippers

How a Fish Walks, Blinks and Breathes on Land Without Becoming an Amphibian

Did You Know Some Fish Spend Much of Their Time Out of Water?

A fish is supposed to stay in water.

Then a mudskipper climbs onto a mangrove mudflat, props itself up on pectoral fins, looks around with raised eyes, gulps air, blinks, fights, feeds and skips across the ground.

It is still a fish.

Mudskippers are gobies that have evolved an amphibious way of life. They keep respiratory surfaces moist, exchange gases through specialised skin and mouth tissues, retain water in enlarged gill chambers, use fins and the body for terrestrial locomotion, and solve the waste problem created when ammonia can no longer simply diffuse away into surrounding water.

fish body + mangrove physics + physiological redesign → life across water and land.

The surprising claim becomes more precise as we go deeper: mudskippers did not become amphibians, and their ancestors were not the direct ancestors of frogs or tetrapods. They are a separate fish lineage that independently evolved solutions to many of the same problems faced by vertebrates moving onto land.

That makes them a natural experiment in convergence.

Read how mudskipper blinking illuminates adaptation to life on land →

Someone Used Singapore Mudskippers to Solve a Waste Problem: Yuen K. Ip and Colleagues

Professor Yuen K. Ip and collaborators in Singapore have spent decades studying how tropical air-breathing fishes survive chemically difficult environments.

The giant mudskipper Periophthalmodon schlosseri lives in mangrove burrows that can become low in oxygen, high in carbon dioxide and rich in ammonia. That creates a serious physiological question: fish normally excrete nitrogenous waste mainly as ammonia into surrounding water, but what happens when the animal is on land or the burrow water already contains large amounts of ammonia?

Ip’s group showed that giant mudskippers use multiple strategies, including active ammonium excretion, acidification of the local environment, reduced ammonia permeability and changes in nitrogen metabolism.

do not ask only how an animal breathes on land; ask how every ordinary water-dependent physiological process must be renegotiated.

Read the Singapore-led study of active ammonium excretion in giant mudskippers →

Big Question: What must change when a fish moves from water into air but still keeps fish gills, fins, skin, kidneys and nitrogen metabolism?

This manual begins with a Primary-level habitat and adaptation puzzle, opens into Secondary respiration and excretion, then reaches JC-level diffusion, acid–base physiology, osmoregulation, biomechanics, sensory evolution and convergent evolution.

Quick Answer

• Skin respiration contributes to gas exchange when skin remains moist.
• Buccopharyngeal surfaces inside the mouth and throat are highly vascularised in many species.
• Enlarged gill chambers help retain water around gills.
• Pectoral fins and body motion support terrestrial locomotion.
• Elevated eyes improve vision above the surface.
• Blinking wets, cleans and protects exposed eyes in studied species.
• Ammonia-management strategies allow survival when water is limited or chemically hostile.
• Burrows provide refuge, moisture, reproduction sites and chemically challenging microhabitats.

Part 1 — Why Gills Are Difficult on Land

Fish gills work beautifully in water because thin lamellae are supported and continuously bathed by flowing water. In air, delicate gill surfaces can collapse or dry, reducing effective area.

Mudskippers partly avoid this by keeping water inside enlarged branchial chambers and by relying on other respiratory surfaces.

Part 2 — Skin Can Become a Respiratory Surface

Gas exchange through skin requires a thin, moist, well-perfused barrier. Oxygen dissolves in the wet surface layer, diffuses across tissues and enters blood.

Dry skin would reduce that pathway sharply, which is why mudskippers repeatedly wet themselves or remain in humid mudflat environments.

air breathing through skin still requires water at the microscopic boundary.

Part 3 — The Mouth and Throat Help Breathe

In species such as the giant mudskipper, the buccal and pharyngeal epithelia contain dense capillary networks and short diffusion distances.

Air held in the mouth cavity can therefore exchange gases across tissues normally thought of mainly as feeding structures.

Part 4 — Why Stay Moist?

Moisture protects respiratory surfaces, limits dehydration and supports eye function. Mudskippers roll in wet mud, return to pools and use burrows to control exposure.

Amphibious life is therefore not escape from water. It is careful management of where and how water remains around the body.

Part 5 — How Does a Fish Walk With Fins?

Pectoral fins can act as supportive appendages. Mudskippers rotate and plant them against the substrate while the body flexes and the tail contributes thrust or stabilisation.

The movement is not identical to tetrapod walking because the skeleton and joint architecture remain fish-like.

Part 6 — Skipping Uses the Whole Body

Rapid escapes and territorial movements can involve strong body bending followed by extension. The fish uses stored elastic and muscular energy to launch across mud or shallow water.

Different mudskipper species use different combinations of fin-supported crutching, body undulation and jumping.

Part 7 — The Eyes Moved Up

Mudskipper eyes project above the top of the head. This allows the fish to survey the air while most of its body remains low against the mud or water surface.

The visual problem changes on land because refraction, glare, distance cues and evaporation differ from underwater conditions.

Part 8 — Mudskippers Blink

Most fully aquatic fishes do not blink like terrestrial vertebrates. Mudskippers independently evolved a blink-like movement.

Research published in 2023 showed that studied mudskippers retract the eye into a cup, allowing a passive membrane to cover it. The blink wets, cleans and protects the cornea.

same terrestrial problem → different anatomical route → similar useful function.

Part 9 — Why Blinking Matters More in Air

Water continuously bathes an aquatic eye. Air does not. Exposed corneal surfaces lose water by evaporation and collect particles differently.

In experiments, mudskippers blinked more under higher evaporation conditions, and blinking could remove debris from the eye surface.

Part 10 — Nitrogen Waste Becomes Harder on Land

Protein metabolism generates nitrogenous waste, much of it as ammonia in fishes. Ammonia is toxic at high concentrations but dissolves readily in water.

An aquatic fish can often diffuse or actively excrete ammonia across gills into a large surrounding water volume. On land, that sink disappears.

Part 11 — Giant Mudskippers Can Actively Excrete Ammonium

Singapore-led experiments on Periophthalmodon schlosseri showed active ammonium excretion across branchial surfaces even during emersion.

The fish also acidifies fluid around the gills. Lower pH shifts the ammonia equilibrium toward charged ammonium, NH4+, which crosses membranes less freely than uncharged NH3.

acidify boundary water → trap nitrogen as NH4+ → reduce back-diffusion of NH3 → improve excretion.

Part 12 — Skin Chemistry Helps Prevent Ammonia From Coming Back In

The giant mudskipper’s skin shows low permeability to gaseous ammonia under challenging conditions. Lipid composition and physiological regulation help reduce passive NH3 entry.

The animal therefore solves both sides of transport: push nitrogen out and reduce unwanted return.

Part 13 — Why Not Convert Everything to Urea?

Mammals convert ammonia to urea because urea is less toxic and can be concentrated for excretion with limited water.

Mudskippers use more varied strategies. Research on P. schlosseri shows some capacity for urea synthesis, but air-exposure survival cannot be explained simply as “the fish switches to mammal-like urea excretion.” Reduced amino-acid catabolism, active ammonium transport and acid trapping can be more important.

Part 14 — Burrows Are Physiological Infrastructure

Mudskipper burrows remain humid, provide refuge from predators and heat, and can be used for reproduction. But burrow water may become low in oxygen, high in carbon dioxide and high in ammonia.

The burrow is therefore both shelter and chemical challenge.

Part 15 — Why Build Air Pockets?

Some mudskippers maintain air in burrows, which can improve oxygen availability for adults or developing eggs where surrounding water is hypoxic.

This behaviour demonstrates that an animal can engineer its own respiratory microenvironment rather than merely tolerate whatever chemistry the habitat provides.

Part 16 — Land Changes Feeding Too

Feeding in air requires different mechanics from suction feeding in water. Some mudskippers capture prey directly from mud or exposed surfaces using rapid jaw and head movements.

Again, the move onto land changes multiple systems simultaneously: respiration, locomotion, vision, feeding and excretion.

Part 17 — Mudskippers Are Not “Fish Becoming Frogs”

Mudskippers belong to a modern teleost lineage. Tetrapods originated from ancient lobe-finned fishes hundreds of millions of years earlier.

The comparison is valuable because it is convergent, not ancestral. Mudskippers show how similar environmental demands can repeatedly favour similar functional solutions.

Follow One Oxygen Molecule on Land

1. Oxygen is present in air above the mudflat.
2. It dissolves in moisture covering skin or buccopharyngeal tissue.
3. It diffuses across a thin vascularised epithelium.
4. Blood carries oxygen toward tissues.
5. Cells use oxygen in aerobic respiration.
6. Carbon dioxide diffuses back toward respiratory surfaces.

Follow One Nitrogen Atom

1. An amino acid is broken down during metabolism.
2. Nitrogen enters an ammonia-related metabolic pool.
3. The fish limits accumulation through metabolic regulation and transport.
4. NH4+ is actively moved toward a branchial surface.
5. Local acidification favours charged ammonium.
6. The nitrogen leaves into retained water or the external environment.

Think Like a Scientist: How Do We Know a Mudskipper Breathes Through Skin?

• Measure oxygen uptake while different respiratory surfaces are exposed or isolated.
• Examine capillary density and diffusion distance histologically.
• Measure blood-gas changes during air exposure.
• Compare wet and dry skin conditions.
• Compare species with different degrees of terrestriality.

Observation vs Inference

• Observation: mudskippers remain active on exposed mud for long periods.
• Observation: skin and mouth tissues are richly vascularised.
• Observation: measured gas exchange occurs outside water.
• Inference: these tissues contribute significantly to aerial respiration.

Common Misconceptions and Better Models

Misconception	Better model
Mudskippers are amphibians.	They are fishes with amphibious adaptations.
They breathe air because they have lungs.	They use moist skin, buccopharyngeal tissues and retained-water gill systems rather than tetrapod lungs.
They can stay dry indefinitely.	Moisture is essential for respiratory surfaces and water balance.
Fins became legs.	Pectoral fins remain fish fins but are used for terrestrial support and movement.
Mudskippers are ancestors of frogs.	They are a separate modern lineage showing convergent solutions to terrestrial life.
All waste becomes urea on land.	Mudskippers use multiple nitrogen-management strategies, including active ammonium excretion.
Blinking proves eyelids evolved exactly as in tetrapods.	Mudskippers evolved a different anatomical mechanism with similar functions.

Checkpoint Questions

1. Why do ordinary fish gills perform poorly in air?
2. What conditions make skin useful for respiration?
3. How can the mouth lining help gas exchange?
4. Why must mudskippers stay moist?
5. How do fins help terrestrial locomotion?
6. What does blinking do for a mudskipper eye?
7. Why does ammonia excretion become difficult on land?
8. How can acidification aid ammonium excretion?
9. Why are mudskipper burrows both useful and difficult?
10. Why is the comparison with early tetrapods convergent rather than ancestral?

Answer Key

Open after attempting the questions

1. Gill lamellae can collapse or dry without water support.
2. Thin, moist, vascularised surfaces with strong gas gradients.
3. Rich capillaries beneath moist epithelium allow diffusion between air and blood.
4. Water is needed for diffusion surfaces, eye health and osmotic balance.
5. They act as supporting and propulsive appendages with body movement.
6. It wets, cleans and protects the exposed cornea in studied species.
7. There is less surrounding water to dilute and carry ammonia away.
8. Lower pH shifts NH3 toward NH4+, limiting back-diffusion.
9. They provide shelter and moisture but can become hypoxic, hypercapnic and ammonia-rich.
10. Mudskippers evolved terrestrial traits independently in a modern teleost lineage.

Can You Explain WHY?

• Why does breathing air still depend on wet surfaces?
• Why can the same fish need both gills and skin?
• Why would raised eyes become useful on a mudflat?
• Why does leaving water create an excretion problem?
• Why can a burrow need active environmental management?
• Why is convergent evolution useful evidence about environmental constraints?

Singapore Field Connection

Mudskippers are among the most visible animals on Singapore’s mangrove mudflats. NParks highlights the giant mudskipper Periophthalmodon schlosseri at Sungei Buloh Wetland Reserve and notes its ability to remain out of water, keep gills moist and exchange gases through moist skin.

Singapore scientists have also contributed substantially to understanding mudskipper nitrogen metabolism, turning a familiar mangrove animal into a globally useful model for amphibious physiology.

Open NParks’ Sungei Buloh guide →

Observe From the Boardwalk

1. Observe at low tide from a permitted boardwalk.
2. Watch whether a mudskipper is in water, mud or air.
3. Record pectoral-fin use during movement.
4. Look for eye retraction or wetting behaviour without approaching closely.
5. Note how often the fish returns to water or wet mud.
6. Separate observed behaviour from inferred physiological function.

Primary Science / PSLE Bridge

• Fish are animals with characteristic body structures.
• Animals need oxygen.
• Body structures can be adapted to habitats.
• Different organs can contribute to the same function.
• Environmental conditions affect survival.
• Classification should use biological evidence, not habitat alone.

Go Beyond Primary Science

Simple idea	Higher-resolution route
Fish breathes on land	Cutaneous respiration, buccopharyngeal gas exchange, diffusion distance
Fish walks	Fin biomechanics, axial locomotion, substrate forces
Fish blinks	Corneal hydration, convergent evolution, sensory biomechanics
Fish removes waste	Ammonia transport, acid trapping, membrane permeability
Burrow holds air	Hypoxia, hypercapnia, behavioural environmental engineering
Fish resembles early land vertebrate problem	Convergence, functional morphology, evolutionary constraint

Deep Science Window — Land Is a Chemistry Change

Moving from water to air changes much more than locomotion. Gas diffusion becomes easier, but surfaces dry. Nitrogen waste loses a huge solvent reservoir. Eyes need wetting. Body weight is no longer buoyantly supported.

Terrestriality is therefore a whole-body transition.

Deep Science Window — Acid Trapping Is Applied Equilibrium Chemistry

NH3 and NH4+ exist in a pH-dependent equilibrium. By acidifying the external boundary, mudskippers shift more nitrogen into charged NH4+, reducing its ability to diffuse back across lipid membranes.

An animal can therefore alter environmental pH to change the direction and form of molecular transport.

Deep Science Window — Mudskippers Let Evolution Run the Experiment Twice

Tetrapods and mudskippers did not inherit blinking from one recent amphibious ancestor. Similar terrestrial demands independently favoured eye-wetting and protection mechanisms in separate vertebrate lineages.

Convergence helps identify which environmental problems are powerful enough to repeatedly shape anatomy.

Evidence Boundaries

• Mudskipper ≠ amphibian. It is a teleost fish.
• Air breathing ≠ lung breathing. Multiple moist surfaces contribute.
• Skin respiration ≠ dry-skin respiration. Moisture is essential.
• Walking fish ≠ fish with tetrapod legs. Fins and body mechanics remain fish-derived.
• Blinking convergence ≠ direct ancestry. Similar function evolved independently.
• Ammonia strategy ≠ identical across all mudskippers. Species differ in terrestriality and nitrogen metabolism.
• One laboratory condition ≠ entire mangrove life. Tide, temperature, salinity and behaviour interact.

Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK

KNOW

Know amphibious fish, cutaneous respiration, buccopharyngeal respiration, branchial chamber, ammonia, NH4+, convergence and blinking.

CONNECT

Connect air exposure to drying, moist tissue to gas exchange, land to ammonia difficulty, and terrestrial vision to blinking.

EXPLAIN

Explain how a fish can remain physiologically fish-like while operating across air and water.

APPLY

Compare mudskippers with lungfish, amphibians, intertidal crabs and early tetrapod constraints.

CHECK

Ask which adaptation solves respiration, locomotion, vision, water balance or excretion.

Where to Go Next

• eduKate Learning Manual: Archerfish
• eduKate Learning Manual: Horseshoe Crab
• Animal World | Bodies, Behaviour, Evolution and Living Systems

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Teaching Guide for Parents, Tutors and Teachers

For the people who teach because somebody depends on them.

Why Begin With “A Fish on Land”?

The learner already owns the category “fish = water.” The mudskipper forces a useful repair: classification and habitat are related but not identical. The animal remains a fish because ancestry and anatomy matter more than where it happens to stand.

The Central Reasoning Model

leave water → gills risk collapse/drying + skin loses water + eyes dry + ammonia sink disappears + body weight shifts → multiple physiological and mechanical adaptations become necessary.

Why Yuen K. Ip and Colleagues Are Here

Their work prevents the article from becoming a simple “fish that walks” story. A true transition across environments changes hidden chemistry too. The child should learn to ask what happens to waste, acid–base balance and membrane transport when a habitat changes.

Teach in This Order

1. Classify the animal as a fish.
2. Break the gill-on-land problem apart.
3. Add skin and mouth respiration.
4. Add moisture dependence.
5. Move the fish with fins.
6. Raise the eyes and add blinking.
7. Then reveal the ammonia problem.
8. Finish with burrow chemistry and convergence.

Questions That Reveal Understanding

• Why does air contain plenty of oxygen yet still create a respiratory problem for fish gills?
• Why is moist skin more important than simply thin skin?
• Why does land make ammonia harder to remove?
• Why is blinking useful evidence for convergence?
• What physiological problem would you investigate next after respiration?

If the Learner Is Stuck

Make two columns: “water does this for a fish” and “air removes that service.” Start with gill support, dilution of waste, eye wetting and buoyancy. The adaptations then become solutions rather than facts to memorise.

If the Learner Is Ready for More

Open into acid–base equilibria, nitrogen transporters, epithelial permeability, biomechanics, convergent evolution, visual optics and environmental physiology.

Evidence Discipline

Do not turn mudskippers into living early tetrapods. Do not say all species use identical air-breathing or nitrogen strategies. Keep experimental claims attached to the studied species and preserve the distinction between analogy, convergence and ancestry.

Research Sources and Further Reading

• PNAS — The origin of blinking in mudskippers and tetrapods is linked to life on land
• Journal of Experimental Biology — Active NH4+ excretion in giant mudskippers
• Air breathing and ammonia excretion in Periophthalmodon schlosseri
• Active ammonia excretion during emersion
• Post-feeding nitrogen excretion and urea synthesis in giant mudskippers
• NParks — Sungei Buloh Wetland Reserve DIY trail guide

eduKate Learning Manuals are written so that a learner can begin simply, a parent can teach confidently, and both can keep going until the school model opens into real Science.