Mudskippers – blurring the line between amphibian and fish


Please welcome back Frank Indiviglio to That Fish Blog.
Those with an interest in unique aquarium fishes need look no further than the mudskipper. These odd little creatures seem to straddle the line between fishes and amphibians, leaving the water for long periods of time to chase insects across mudflats and even climbing up onto tree trunks.

Mudskippers, the largest species of which reach a length of 12 inches, inhabit tidal flats, river mouths and mangrove swamps in East Africa, Southeast Asia, Australia, and along the Red Sea.

The mudskippers are unusual in having highly modified pectoral, pelvic and anal fins that enable them to move about quite well on land – they can even leap (“skip”) about very rapidly. In addition, the fused pectoral fins form a suction disc that allows these little acrobats to climb up onto mangrove roots and tree trunks. The eyes are situated at the top of the head and are, for a fish, quite movable.

Gill covers tightly seal the gill chambers, and water stored there keeps the gills moist and provides oxygen to the fish as it scuttles about on land. Mudskippers also absorb moisture from the damp mud upon which they usually travel when out of water. Although it is tempting to think of mudskippers as representing an early stage in the development of amphibians, the creature that gave rise to frogs and salamanders was more like the Australian lungfish, Neoceratodus fosteri, in appearance and in its method of breathing (utilizing primitive lungs).

The most commonly available mudskipper in the pet trade is Periopthalmus barbarus, a fairly hardy species that reaches a length of 6 inches. Like all mudskippers, it hails from brackish water areas.

Mudskippers are fairly tolerant in their salinity requirements, and will do well under typical brackish water aquarium conditions (salinity of 1.005-1.015) and temperatures of 75 – 80F. They require a “beach” area, which can be a separate, drainable plastic container within the main aquarium or designed as small islands fashioned from non-toxic tree roots, coral heads and rocks. The popular “aqua-terrariums” now on the market make excellent mudskipper homes as well. Remember to keep the water shallow, or to provide easy access to land, as they are poor swimmers (not something you usually worry about when keeping fish!).

Most mudskippers do well in captivity if provided with a suitable habitat. Males, however – distinguished by their large dorsal fins and bright colors – are very territorial, and dominant specimens will make life miserable for others, so plan your group and space accordingly.

Although they prey upon live invertebrates such as crabs and insects in the wild, mudskippers adjust well to frozen foods such as prawn and clams. I also provide a vegetable-based frozen food from time to time, and find they accept this readily as well. Their food should be placed on land, as most species will not feed while submerged. Mudskippers are especially fond of live crickets, small shrimp and other such foods, and these should form a large portion of their diet. Their acrobatics when chasing live food – they often flip over in their excitement – never fail to delight me.

Brackish water community tanks containing mudskippers and fiddler crabs make fascinating exhibits. The interactions between the crabs and mudskippers (assuming they are properly matched in size!) go on all day long. If you establish a deep water area (mudskippers will do okay as long as they can exit the water easily) you can add such fascinating fishes as four-eyed fish, Anableps spp., scats, Scatophagus argus and rubrifus, monos, Monodactylus argenteus, and, of course, the amazing archer fish, Toxotes chatareus. In fact, archer fish are at their best in an aquarium containing a land area because in such they can show off their incredible ability to knock crickets from land into water. Somehow compensating for the refraction of light through water, archerfish eject streams of water at insects (best observed by placing crickets on branches positioned over the water’s surface), hitting them unerringly and thus securing a meal. They will also aim water at your eye movements, so be careful!
I’ll cover the creation of such aquariums in future articles. Until then, please share your observations and write in with your questions. Thanks, Frank.

For more information on establishing aquariums for brackish water fish, please see the article Brackish Water Basics, posted on on February 26, 2008:

Blind Cave Fish: Their Discovery, Initial Collection and Care – Part 2

Blind Cave Fish

Click here to read the first part of Blind Cave Fish
Blind cave fish navigate entirely through the use of the lateral line – a system of sensory organs possessed by all fish but, it seems, very highly developed in this species. The movement of water (caused by currents or the fishes’ own swimming) bouncing off objects is sensed and used to guide the fish in their travels. I am tempted to compare the process to echo-location in bats, or the use of electricity by the elephant nose fish, Gnathonemus petersi, but it is, of course, quite distinct. If you have an opportunity, observe how well cave fish can move about – in an exhibit at the NY Aquarium they speed through a series of glass barriers unerringly. Individuals introduced to new exhibits may “crash” on occasion, so there may be some learning involved as well.

They are also amazingly adept at locating food – a school I kept at the Prospect Park Zoo in NYC hit food dropped on the water’s surface as quickly as do most sighted fish. In fact, blind cave fish do quite well in aquariums housing other fish species.

Recently (January, 2008) it was discovered that young blind cave fish can detect light via unique compounds in the brain’s pineal gland (the embryos begin to develop eyes, but these degenerate rapidly). This ability declines with age.

In contrast to most cave-adapted fish – which require cold, hard water if they are to thrive – blind cave fish are quite undemanding pets. In fact, they do best at 78-82 F, as their native waters are quite warm. If kept alone, slightly hard water should be provided, but they adapt easily to conditions suited to most community-type tropical fish. Despite a very specific natural diet (see above), blind cave fish remain healthy on almost any commercial fish food – I have successfully used a mix of omnivore flakes and pellets, along with frozen foods. They ravenously devour black worms, brine shrimp and such, and are especially fond of crushed crickets and other insects. Their reaction to insect food brings, at least to my mind, an image of feeding behavior in their native cave. At feeding time, they compete quite well with other fish and rarely require special attention.

Blind cave fish are placed within the order Charachiformes, an extremely diverse group of fishes containing well over 1,500 species, including tetras and the infamous piranha. I will write about piranhas in a future article, and will include photos of some that became “attached” to me while I was seining for knife fish in northern South America. Until then, please forward your comments and questions. Thank you. Until next time, Frank.

A fascinating account of the first expedition to collect blind cave fishes, including original drawings and photos, is given in Zoo Expeditions, by William Bridges (William Morrow & Co., 1945). Long out of print, this book is well worth searching for.

An interesting article on the evolution of eye regression in this fish is posted at:

Thanks Frank,

Until Next Time,

Amazing Coral Story

With this weeks passing of Earth Day 2008, I thought I would write a blog about this great article that I had read recently on Some of the darkest days in U.S history involve the nuclear weapons use and testing during and after WWII. Most people learned about the bombs dropped on Japan during WWII in history class, or from family members who lived in that era.

Much less well known nuclear testing was done in the years following WWII as the cold war escalated, and the demand for bigger and bigger bombs grew. From 1946 to 1958 the U.S. Government conducted nuclear bomb tests on the remote Pacific Island Group of Bikini Atoll, which is part of the Marshall Islands. In 1954 the U.S. detonated, what was at the time, the largest hydrogen bomb ever tested. The bomb was code named Castle Bravo, and was 15 megatons (1,000 times more powerful that the bomb dropped on Hiroshima). The blast vaporized 3 islands, raised the water temperature to 55,000 degrees, and left a crater that was over a mile wide and more that 200 feet deep. Needless to say, there was nothing left of what was a thriving tropical island group and surrounding reefs.

Enough of the bad news, this story has a happy ending. Recently a group of international scientists returned to Bikini Atoll to see what was there, almost 50 years later. Plant life on the surface had returned, but is still contaminated with radiation (don’t eat the coconuts). What the group found underwater was truly amazing. As they planned their dive into the Bravo Crater, expectations were running wild. The last time the area was surveyed it looked like part of the moon, and was irradiated.

What they found was a thriving coral reef ecosystem that had completely self seeded itself in the once barren wasteland. Porites corals that reached 25ft in the water, huge formations that looked like trees reaching for the surface. The belief is that water currents from untouched neighboring areas brought larval corals to Bikini, where they settled and matured. The corals had recolonized as much as 80% of the habitat in some of the areas studied.

Compared to studies performed at Bikini prior to the testing, the results show that there has been a serious impact on the diversity of corals to the area. The new study showed that 40 species that were documented to have been there prior to testing, where no longer there, and appear to be locally extinct. I found it amazing that what was there had reclaimed space that had felt the worst of what human kind can offer.

This news shows that, given the chance, reefs can recover from even the most severe destruction. Maybe by looking at the species that are thriving in the Bravo Crater, we can use them to as a guide to recolonizing reefs that have been destroyed by shipping, fishing, and pollution around the world. The main thing that I took from the story is that if we as a society can get our act together as far as protecting our natural resources, that Mother Nature can fight back pretty hard if we let her.

Until Next Blog,


Blind Cave Fish: Their Discovery, Initial Collection and Care – Part 1

Blind Cave Fish

Please welcome back Frank Indiviglio for another fascinating species profile.
Perhaps because of its lack of color, the blind cave fish, Astyanax fasciatus mexicanus, does not receive nearly the attention it warrants. This 5 inch-long fish was first discovered by Salvador Coronado, a young employee of Mexico’s Department of Fisheries, and it was he who later led a New York Aquarium sponsored expedition to collect the bizarre creatures. The species’ entry into the aquarium world’s consciousness is recounted by expedition member William Bridges, of the Bronx Zoo, in his initial report – “…at three o’clock on a March afternoon, a thousand feet inside the cave, two of us dragged a net out of the black water and revealed a dozen little flopping white fish…”. Amazingly, fish from that first collection appeared in three distinct forms – with normal eyes, with eyes of reduced size and without eyes. Fortunately for we fish enthusiasts, captive-born blind cave fish eventually found their way into the pet trade.

The blind cave fish is found in only one cave, La Cueva Chica (the “Little Cave” – the entrance is only 15 feet wide) near Pujal, in San Lois Potosi, Mexico (related species have since been found in other caves). This fact alone should grant it special status among fish keepers, but there is much more to distinguish it – including a rare glimpse at fish adaptation in progress. Apparently, fish with normally developed eyes live in a nearby river, the Rio Tampaon, and are regularly swept into the cave by the current. There, deprived of light and the need for sight, the eyes degenerate. As time goes on, their descendents are born with gradually more reduced eyes, until the eyeless form emerges. Although I have not read evidence of it, I imagine that inter-breeding with eyeless fish already in the cave may hasten the process.

Fish in pools nearest the cave’s entrance rely upon flies, spiders and other invertebrates for food, while those deep inside subsist upon bat guano, dead bats and, I would venture a guess, fleas and other invertebrates that fall from the bats’ fur.

Check back on Friday to read the rest of this article.
Until Than,

Motherhood in Crayfish, A personal observation

Please welcome back Frank Indiviglio to That Fish Blog. Frank gives his unique perspective on another interesting, sometimes aquarium inhabitant, the crayfish.

Freshwater crayfish, found on all continents except Africa and Antarctica (the southeastern United States, home to 80% of the world’s species, is a hotspot of crayfish diversity), are often purchased as an “oddity” or scavenger to add to the aquarium. However, these active Crustaceans make fascinating pets in their own right and are well worth more attention. I will write more about the specifics of crayfish care in future articles, but would now like to recount my experience with the maternal instincts of one species, the red swamp crayfish, Procambarus clarkii.

It is difficult to house crayfish in groups, as they tend to consume tank-mates that have recently molted (newly-molted crayfish are soft and defenseless). I was, therefore, fortunate in having the opportunity to observe a female with her young in an aquarium. I came across her while she was traveling overland (they do this on occasion) between ponds at the Prospect Park Zoo in NYC. In typical crayfish fashion, several dozen young clung to the swimmerets (feathery organs) on her underside. (Note: the red swamp crayfish is native to the southeastern USA but widely introduced elsewhere. Non-native crayfish cause serious problems in many parts of the world – please do not release unwanted pet crayfish).

Established in a 5 gallon aquarium, the female soon became quite bold and allowed me a peek at her version of maternal care. Any disturbance caused her to rear up, claws extended towards the threat – she definitely seemed more aggressive than crayfish I had kept in the past. The young remained on the swimmerets for over two weeks and then began making short feeding forays on their own but, to my surprise, returned unerringly to their mother after eating. At this point they also began to scamper about the rest of her body, sometimes covering most of her head from view. Knowing of this creature’s pugnacious disposition, I wondered when her “patience” would reach its limit. That limit came after about three weeks, when she promptly began devouring the prodigy she had so carefully nurtured until then. The survivors took refuge in the hiding spots (cracked clay flower pots) that I had provided for them, after which I moved the group to a larger aquarium.

A number of crayfish species are readily available and do well in aquariums. Particularly interesting are stream-dwelling forms, such as the red-tip crayfish, Orconectes erichsonianus, which seem determined to re-arrange every stone in their tank in an effort to establish the perfect home. Others you might consider are the P. alleni, a blue strain of which has been developed for the pet trade, dwarf species such as O. compressus, and the bright blue Australian yabbie, Cherax quadricarinatus.

I’ll write again soon and highlight other species. Until then, I’d appreciate hearing about your own experiences.

A good deal of interesting information, including a key to help you identify the crayfish you may come across, is sponsored by the Carnegie Museum of Natural History:

Thank you, Frank.