Animal Kingdom. Volume 60, Issue 4, pp. 151-154
Publisher – New York Zoological Society, 1957

 

The Drifting Leaf that Is a Fish

From Eigenmann & Allen, Biological Bulletin, Vol. 41, No. 5, 1921. Drawing by W. S. Atkinson. 2 1/2 x life-sized.

 

By MYRON GORDON

 

When William R. Allen, an ichthyologist of the University of Kentucky, was collecting fishes in the Amazon in 1920 for a scientific survey of the South American fishes which he and Carl H. Eigenmann of Indiana University were making, he was told of a strange fish that lived a seemingly drifting life in the sluggish waters of Peruvian jungles. The savages, Allen said, called the fish Pira-caa, which means „a small fish that looks like a leaf,” or in short, a leaffish.

Allen was also told that, although the strange fish was a voracious predator and capable of catching and eating other fish its own size,  it appeared to be as innocuous as a waterlogged leaf.

Allen’s Indians poisoned a stream and the ichthyologist later wrote about it in „A Leaf-mimicking Fish” in the Biological Bulletin in 1921.

„In order to know if there was sufficient current to carry the poison to every part of the pool, I began tossing broken twigs on the water to observe their course with the current. One such twig had reached a standstill, when directly beneath it I saw what was apparently a dead leaf being wafted past the twig. I couldn’t understand why the twig was not moving too. At about that moment the leaf moved out into a path of sunlight, and toward the surface. There the resemblance to a fish became apparent, especially to one in search of the same.”

Allen was not the first to discover the leaffish in South America. Back in 1840 Ernst Heckel, the famous German zoologist, gave it its generic name Monocirrhus after studying some Amazonian specimens sent to him from the Rios Negro and Cupai. Sporadically some live leaffish were sent to German aquarists in the early part of 1900, probably from the Essequibo River of British Guiana. They certainly live there because George S. Myers of Stanford University said that he caught some there during the second World War.

After 1921, importations of the leaffish for aquarium use became more frequent but they did not cause any unusual excitement until Wm. T. Innes in 1933 publicized them by printing two remarkable pictures in his then new magazine, The Aquarium. These pictures firmly established the leaffish as one of the attractive novelties in the aquarium world, especially for those aquarists who had or could raise a superabundance of guppies to feed them.

In an aquarium the thin, flat Monocirrhus carries on its usual imitation of things that are plantlike; the fish’s coloring and black markings add to its success in the art of camouflage. If at any particular moment its body coloring does not match its  match its surroundings the leaffish, like a quick- change actor on the stage, can alter its appearance to suit an appropriate scene. If an aquarium plant is greenish-brown, the fish will become greenish-brown too. Several inquisitive aquarists have tested the limits of the fish’s ability to mimic its surroundings. When they placed in the aquarium a long-dead, water-logged leaf blotched by the decomposing action of fungi and bacteria, the leaffish soon matched every aspect of it, blotches and all.

The Leaffish at the upper right has its mouth almost closed, in the normal water-breathing position. But it can be opened enormously to form a membranous tunnel, as in the other fish. Photograph by William T, Innei Here a leaffish is guarding and aerating the eggs, which may be seen on the under side of the horizontal leaf. Vibration of the fish’s small pectoral fin creates a slow-moving current of water.

 The leaffish’s scientific name Monocirrhus refers to this single, fleshy, tendril-like goatee. When the time for mating comes along, it is the male leaffish that selects and prepares the spawning site. It may choose the underside of a large aquatic plant, such as a Cryptocoryne or a broad-leafed Echinodorus, a slab of slate or a flower pot. Lacking all of these conveniences one leaffish chose a vertical angle formed at the corner of the aquarium. By some subtle body movements the male courts the female and induces it to come to his side. Together they engage in a sort of prespawning dance in which their bodies shimmy rhythmically and their coordinated vibrations are punctuated by tail waggings and slappings. At the peak of their mutually stimulating contacts the pair turn upside down, the female oviposits one egg at a time — each egg, being adhesive, is firmly secured to the under surface of a plant or of a suitable artifact — and the ova are immediately fertilized by the male. About a hundred eggs are expelled, and each fertilized egg is clear, spherical and about one millimeter in diameter. When spawning is over in an aquarium the female, seemingly by prearrangement, retires to a neutral corner. If the pair had spawned in their natural water, she would probably wander off. In the early days some aquarists thought that the female rather than the male cared for the eggs, but the error has been attributed to the difficulty of sexing the fish accurately, at least until the time of their spawning. The male leaffish usually takes on the job of guarding the spawning areas and aerating the developing eggs; this he does by placing his flat body, tail up, head down, as close to the spawn as possible and then by rapidly vibrating his small pectoral fin. This creates an effective current of water that passes over the eggs. Intruders entering the spawning area are driven away by a threatening display which is unexpectedly vigorous in a fish whose movements are usually quite slow and rather deliberate. At a water temperature of 78° F. the eggs are likely to hatch in two or three days — but hatching does not immediately give the fry complete freedom, for they are tied to their nesting site by almost invisible threads. Perhaps this simple but effective device has evolved as an advantageous structure to prevent the helpless fry from sinking into the suffocating mud. It certainly facilitates their father’s job in guarding them at one central spot against their many enemies. The system is not fool-proof. Sometimes in the course of his duties the male may accidentally get his body so close to the spawn that he buffets his tethered young offspring free of their moorings. This  does no harm to the detached fry when they are in a clean-bottomed aquarium and in a few days all are capable of moving about under their own power. The young fish soon learn to swim about in that lazy drifting manner of their parents, stopping dead still from time to time, waiting,  watching, for the approach of a tiny crustacean such as a daphnia or newly hatched brine shrimp nauplii.

When they reach half an inch, the blunt-headed leaffish will stalk and capture baby guppies. Part of the young leaffish’s body is dark and part is translucent, giving the fish at this early age the great advantage of camouflage.

To the naked eye each fish seems ragged, like a bit of a torn leaf, but parts of their fins are really not torn at all but merely transparent. It is quite difficult to spot the leaffish when they are against a background of floating and submerged broken vegetation because in addition to their imitative, leaflike outlines, their bodies are finely dusted with white dots which add to their protective coloration.

 

These two Leaffish babies were photographed under a microscope soon after hatching.
Large dark spots on the head are developing eyes.

While the small leaffish eat all sorts of smaller aquatic game in the form of worms and crustaceans, the mature fish are generally piscivorous. In nature they hunt and capture live fish, but the aquarist Don L. Jacobs trained them to eat dead ones. First he held small live fish in front of the young „leafers” with his fingers; then he hand-fed them on freshly killed fish. Later, he substituted and taught them to take frozen fish (after, of course, these were well thawed out and reached the temperature of the aquarium water). This ingenious aquarist found frozen Dixie mosquito-fish a most convenient food for leafers and other fish like them. Jacobs could always get a good supply of these Gambusia in the quiet waterways in Georgia, and would deep freeze them until wanted.

Wm. T. Innes obtained his first leaffish in 1933 and he immediately set about photographing them. He was about to trigger the shutter of his camera when one of the leaffish opened its mouth and „yawned a mile long.” He snapped it and got a remarkable picture. Its mouth seemed to unfold deep from within itself and, when fully opened, it formed an enormous membranous oval tunnel.  It seemed unbelievable that all this large and intricate apparatus, consisting of multiple membranes and bony supports, could have been so beautifully collapsed and completely hidden from view. When at rest its mouth was as neatly folded as a closed umbrella. The purpose of the leaffish’s cavernous mouth seemed inexplicable to Innes. He guessed that the fish used it, when in a vertical position, to dig for living food in the bottom mud of the water by-ways of Brazil’s Rio Negro.

But later Jacobs watched the trigger action of its mouth when the leaffish was about to swallow an unsuspecting member of its community. The sudden opening of its enormous tubular mouth, he said, created a vacuum within its gullet, so strong that a fish an inch away and seemingly safe was instantly and violently sucked into its wide, gaping mouth. All this was done without noticeable body movement on the part of the leaffish.

C. W. Coates, Director of the New York Aquarium, often has had occasion to warn aquarists that the leaffish is not a community fish. It will swallow several large guppies without effort and eat a two-inch swordtail with possibly a trifle more zest. On one occasion Don Jacobs fed his Monocirrhus on more live top-minnows than the leafers could take at one helping. They gorged themselves within a few minutes, right up to the limit of their short gastrointestinal system. The small fish swam about with impunity, but the animated plant-like fish could not swallow another victim. Later, when the effects of gluttony wore off, the smaller fish became quite wary and were difficult to approach; some of the Gambusia swam close to the surface where the leaffish in their stilted and inverted position could not easily follow. This stalemate did not last long. One hungry leafer, Jacobs noticed, drifted to the surface, twisted its flat body to parallel that of the surface of the of the water and then, when it was floating half in and half out of the water, it moved imperceptibly toward its victim, keeping its mouth below the water. Suddenly its gaping mouth flew open and vacuumed a Gambusia into its gullet.

Separated by the wide Atlantic Ocean, the American leaffish has an old zoological family relative in Africa.. In a way, the American Monocirrhus and the African Polycentropis look somewhat alike and have the same general breeding habits. It is one of the mysteries of nature how these two leaffishes got separated by miles of sea. Zoologists, through their studv of comparative anatomy, have found additional members of the Family Nantidae to which these two leaffishes belong. And the mystery deepens because the other species, though few in number, live in India, Asia and some of the East Indian islands. The peculiar geographical distribution of the Nandidae has been used by some naturalists as one of their strongest arguments for the theory of the drifting apart of the continents during the geological history of the world.

Links:

http://archive.org/stream/annalendeswiener21840wien#page/438/mode/2up

http://www.jstor.org/stable/1536530?seq=1

http://archive.org/stream/archivosdomuseun17muse#page/n595/mode/2up