Wet Beast Wednesday: elephant fish
It's the last Wet Beast Wednesday of fresh-uary, the month where I only cover freshwater species. I started the month with a fish and am ending it with a fish as well. The elephant fish are a group of very bizarre-looking fish known for their long trunks. More than just looking weird, these little fish are potentially quite intelligent animals, much like their namesakes. Let's nose our way in and learn.
(Image: Peter's elephant fish, Gnathonemus petersii, in a tank. It is a long fish with mostly brown scales except for white stripes on the back half. It's dorsal and anal fins are large and near the rear of the body, giving it al almost arrow-shaped profile. The head is long, with small eyes and a very small mouth. Under the mouth sprouts a fleshy, trunk-like structure. End ID)
Elephant fish are also called elephant-nose fish and trunkfish, among a number of other names. They are members of the superfamily Mormyridae, though not every Mormyrid has the famous trunk. There are around 200 species, which can range in length from 5 cm to 1.5 m in the largest species, Mormyrops anguilloides (which does not have a trunk). The typical elephant fish is a smaller fish with a forked tail, large dorsal and anal fins located near the tail, small eyes, mouth, and gill openings, and a long, trunk-like protrusion emerging from below the mouth. In most elephant fish, the trunk is a modified lower lip and is muscular and somewhat flexible, but has no skeletal support. This type of trunk has the amazing scientific name of schnauzenorgan. Some mormyrids have alternatively developed extended jaw muscles and mouthparts that form a rigid trunk with a the mouth at the tip, but these trunks are usually shorter than the schnauzenorgans. The skin of Mormyrids, but especially the head and trunk, are covered with sensory pits known as ampullary organs or ampullae of Lorenzini. These kind of organs have evolved multiple times independently in several fish lineages and the variety that Mormyrids have are called mormyromasts. The purpose of these organs is to detect electric fields created by other living things.
(Image: Campylomormyrus elephas, an elephant fish. It has a very similar body structure to the first picture, but is black all over. Its trunk is actually an extension of the jaw and looks like a tube with a tiny mouth at the end. A small fleshy schnauzenorgan hangs from under the mouth. End ID)
In addition to sensing electric fields, elephant fish can create their own. They are considered weakly electric fish, because they actively generate an electric field around themselves, but the electricity they produce is not strong enough to be used as a weapon like in electric eels and torpedo rays. The ability to do this has also evolved independently several times. The electric field is generated by stacks of electricity-generating cells called electrocytes that are located in the tail. The use of the electrocytes creates an electric field that surrounds the fish like a bubble. when electric fields interact, they alter each other, and the elephant fish detects these alterations with its mormyromasts, altering it to the location of other organisms. Because all animals generate a weak electric field as a consequence of muscle movement and neuron activity, the elephant fish can essentially sense the location of animal whose electric field intersects with its. The trunk being covered in mormyromasts makes it extra sensitive to electrical fields.
(Image: Campylomormyrus elephas seen from the front, better showing the mouth at the tip of the trunk and the small schnauzenorgan hanging from beneath it. End ID)
To interpret all the signals it receives, the elephant fish needs a large brain. Their brain to body mass ratios are among the highest of any vertebrates, slightly higher than that of humans and their brain to body oxygen consumption ratio is the highest of any vertebrate, three times that of humans. Though their brains are simple in structure, they are capable of advanced cognition. In one test, elephant fish were shown to be able to recognize objects with one sense that they had only examined with another sense. For an example of this, humans can examine an object with their hands while blindfolded, then recognize it later when able to see. This is thought to be a rare ability in the animal kingdom, only found in intelligent mammals like dolphins, apes, and rats, and was formerly believed to require a cerebral cortex, something elephant fish lack. However, the fish were able to visually recognize objects they had previously only observed with their electric sense. It is currently not clear how they do this. There is also some evidence that elephant fish near each other can share information by linking their electric fields. If one fish senses something, it can transmit that information to all other nearby fish by altering its electric field. This could be highly useful in predator avoidance, where if one fish detects a predator, they all do. Elephant fish in captivity have also been reported engaging in play behavior (a sign of intelligence) and, like many fish, can recognize individual humans. I wasn't able to find any information on their memories, but fish with much smaller brains that use less oxygen have been shown to have at least several months worth of memory, so I would guess elephant fish can do the same.
(Image: a group of 4 blunt-jawed elephant fish, Campylomormyrus tamandua, in an advertisement for an aquarium company. They have the same body plan as Campylomormyrus elephas, but with a shorted trunk. Their bodies are striped with yellow and black. End ID)
Mormyrids are a highly successful group of fish, making up about 10% of all species of freshwater fish in Africa, where they are endemic. Mormyrids can be found in virtually every body of water in the continent, though elephant fish in particular tend to favor slower-moving, murky waters with lots of vegetation. While capable of using vision to examine their surroundings, electroreception is an even more valuable skill in murky water. Their electric sense allow elephant fish to navigate and hunt in water other fish would be blinded in. Elephant fish are carnivores, but their small mouth restrict them to very small prey, usually zooplankton, insect larvae, and other small invertebrates. Some trunkless species have larger mouths. Trunked species use their trunks to poke around in vegetation and sediment, looking for buried or hiding prey. The schnauzenorgans are reported to be flexible and can aid in carrying food to the mouth in a similar manner to an elephant's trunk. Hunting elephant fish have been seen doing a 'dancing' motion, which is believed to be used for reorienting themselves to better detect prey. They are active mostly during dawn and dusk and spend day and night hiding in vegetation to avoid predators. Mormyrid reproduction is understudied, but they have mating seasons that coincide with the rainy season. It appears that water acidity and soil content help trigger reproduction. Some species will leave the main waterways to mate in temporary pools created by the rain. Thy to not appear to provide parental care to their eggs or larvae. Elephant fish are known to be difficult to breed in captivity, limiting our understanding of how they reproduce.
(Cyphomyrus psittacus, a species of Mormyrid without a trunk. It has the same body plan as the above pictures, but its head is round and looks much more like a normal fish, except for a small, fleshy bulb below the mouth. It has silver scales. End ID)
Mormyrids are known to be vulnerable to pollution and are threatened by habitat loss, pollution, and climate change. As a group, they are doing well, though individual species may be threatened. Elephant fish have entered the pet trade due to their unique looks, with Gnathonemus petersii, Peter's elephant fish, being the most commonly found in personal aquariums. The same species is also highly used in laboratory experiments studying electrogenesis, fish intelligence, and bioelectricity. The difficulty of breeding them in captivity has led to a marked for wild caught fish. Some of the largest Mormyrid species are used for food.
(Image: a group of Peter's elephant fish in an aquarium. There are two of a similar size swimming next to each other. Two smaller specimens and fish of a different species can also be seen. End ID)
