A walking fish, or ambulatory fish, is a fish that is able to travel over land for extended periods of time. Some other cases of nonstandard fish locomotion include fish "walking" along the sea floor, such as the handfish or frogfish.
Types of walking fish
Most commonly, walking fish are amphibious fish. Able to spend longer times out of water, these fish may use a number of means of locomotion, including springing, snake-like lateral undulation, and tripod-like walking. The mudskippers are probably the best land-adapted of contemporary fish and are able to spend days moving about out of water and can even climb mangroves, although to only modest heights. The Climbing gourami is often specifically referred to as a "walking fish", although it does not actually "walk", but rather moves in a jerky way by supporting itself on the extended edges of its gill plates and pushing itself by its fins and tail. Some reports indicate that it can also climb trees.
There are a number of fish that are less adept at actual walking, such as the walking catfish. Despite being known for "walking on land", this fish usually wriggles and may use its pectoral fins to aid in its movement. Walking Catfish have a respiratory system that allows them to live out of water for several days. Some are invasive species. A notorious case in the United States is the Northern snakehead. Polypterids have rudimentary lungs and can also move about on land, though rather clumsily. The Mangrove rivulus can survive for months out of water and can move to places like hollow logs.
There are some species of fish that can "walk" along the sea floor but not on land; one such animal is the flying gurnard (it does not actually fly, and should not be confused with flying fish). The batfishes of the Ogcocephalidae family (not to be confused with Batfish of Ephippidae) are also capable of walking along the sea floor. Bathypterois grallator, also known as a "tripodfish", stands on its three fins on the bottom of the ocean and hunts for food. The African lungfish (P. annectens) can use its fins to "walk" along the bottom of its tank in a manner similar to the way amphibians and land vertebrates use their limbs on land. 
In modern fish the "walking" ability differs from that of tetrapods (four-legged animals). The theory of evolution suggests that life originated in the oceans and later moved onto land, and paleontologists have long been looking for a missing evolutionary link between ocean-living and land-living animals. Of recent finds, reported in Nature (April 2006) is Tiktaalik roseae, which has many features of wrist, elbow, and neck that are akin to those of tetrapods. It belonged to a group of lobe-finned fish called Rhipidistia, which according to some recent theories were the ancestors of all tetrapods.
Comparison of fish with tetrapod-like features
A number of fish, both extant and prehistoric, have featured some characteristics related to locomotion that are typical of tetrapods.
|Species||venturing onto land||tetrapod-like spine||tetrapod-like appendages||digit-like bones|
|5 axial regions||interlocking vertebrae||fully ossified vertebrae||shoulder & skull separation||functional 'intra-fin' joints||fins adapted for walking rather than swimming||strong & muscled fins||humerus, radius & ulna bones||differentiated distal radial bones||jointed distal radial bones|
|Panderichthys rhombolepis †||?||No||No||No||No||?||No||Yes||Yes||Yes||No|
|Sauripterus taylori †||?||No||No||No||No||?||No||Yes||Yes||Yes||Yes|
|Tiktaalik roseae †||?||No||No||No||Yes||Yes||No||Yes||Yes||No||No|
|Tarrasius problematicus †||?||Yes||No||No||No||No||No||No||No||No||No|
|Leptolepis koonwarriensis †||?||No||No||Yes||No||No||No||No||No||No||No|
|Eastmanosteus pustulosus †||?||No||No||No||Yes||No||No||No||No||No||No|
|Polypterus bichir lapradei||Yes||No||No||No||No||No||No||Yes||No||No||No|
|Chelidonichthys cuculus||No||No||No||No||No||No||Yes (3 rays)||No||No||? (3 rays)||No|
- Climbing Fish
- "Maryland Suffers Setback in War on Invasive Walking Fish", National Geographic News July 12, 2002
- Shells, trees and bottoms: Strange places fish live
- "Tropical fish can live for months out of water". Reuters. 15 November 2007.
- Fish Lives in Logs, Breathing Air, for Months at a Time
- Fish Lives in Logs, Breathing Air, for Months at a Time
- Jones, AT; KJ Sulak (1990). "First Central Pacific Plate and Hawaiian Record of the Deep-sea Tripod Fish Bathypterois grallator (Pisces: Chlorophthalmidae)" (PDF). Pacific Science 44 (3): 254–7.
- Fish uses fins to walk and bound
- Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes
- A Small Step for Lungfish, a Big Step for the Evolution of Walking
- Edward B. Daeschler, Neil H. Shubin and Farish A. Jenkins, Jr (6 April 2006). "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan". Nature 440 (7085): 757–763. doi:10.1038/nature04639. PMID 16598249.
- C. A. Boisvert, E. Mark-Kurik, P. E. Ahlberg (4 December 2008). "The pectoral fin of Panderichthys and the origin of digits". Nature 456 (7222): 636–638. Bibcode:2008Natur.456..636B. doi:10.1038/nature07339. PMID 18806778.
- E. B. Daeschler, N. H. Shubin (8 January 1998). "Fish with Fingers?". Nature 391 (6663): 133–133. doi:10.1038/34317.
- Lauren Cole Sallan (23 May 2012). "Tetrapod-like axial regionalization in an early ray-finned fish" (PDF). Proceedings of the Royal Society B 279 (1741): 3264–3271. doi:10.1098/rspb.2012.0784.
- Shukla, J.P. (2007). Fish & Fisheries. Rastogi Publications. pp. 24–25. ISBN 81-7133-800-3.
- K. Trinajstic et al. (12 July 2013). "Fossil Musculature of the Most Primitive Jawed Vertebrates". Science 341 (6142): 160–164. doi:10.1126/science.1237275.
- Olsen, Stanley J. (1968). Fish, Amphibian and Reptile Remains from Archaeological Sites. Acme Bookbinding. p. 4. ISBN 0-87365-163-4.
- S. M. Kawano and R. W. Blob (August 2, 2013). "Propulsive Forces of Mudskipper Fins and Salamander Limbs during Terrestrial Locomotion: Implications for the Invasion of Land" (PDF). Integrative and Comparative Biology 53: 283–294. doi:10.1093/icb/ict051. PMID 23667046.
- C. M. Pace and A. C. Gibb (July 15, 2009). "Mudskipper pectoral fin kinematics in aquatic and terrestrial environments" (PDF). The Journal of Experimental Biology 212: 2279–2286. doi:10.1242/jeb.029041. PMID 19561218.
- H.M. King, N.H. Shubin, M.I. Coates, M.E. Hale (27 December 2011). "Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes". PNAS 108 (52): 21146–51. doi:10.1073/pnas.1118669109.
- Hanken, James; Hall, Brian K. (1993). The Skull, Volume 2: Patterns of Structural and Systematic Diversity. University of Chicago Press. p. 209. ISBN 0-226-31570-3.
- Goto, T., Nishida, K. and Nakaya, K. (Sep 1999). "Internal morphology and function of paired fins in the epaulette shark, Hemiscyllium ocellatum". Ichthyological Research 46 (3): 281–287. doi:10.1007/BF02678514.