Mormyrids: The Most Intelligent Fish You’ve Never Kept

By: Chelsea Steenfott

I love oddball fish. I live for the weird looking, strangely behaved and unusually intelligent. I’m not really driven by color, ornate fins, goliath monsters or teeny nanos. I just want something that challenges me and can in their own way “connect”. I value behavior. I would rather spend months or even years chasing a wildly uncommon fish than go to an LFS and pick up another fish that’s just a dime a dozen. These are the reasons that I love mormyrids.

Mormyrids are everything that is unusual. For starters they come from an area of the world that is very hard to access, immediately adding to their lore. West Africa possesses some of the most amazing fish on earth, fish that we don’t really see with any real regularity due to their being geographically challenging.

What is a Mormyrid? Mormyrids are commonly known as Elephantfish and are a diverse family, with a wide range of different sizes and shapes. The smallest are just 2 inches in adult length, while the largest can reach up to 5 feet. They do, however, possess several of the same unique features. They all have a greatly enlarged cerebellum which enables them to interpret complex bio-electrical signals, basically huge amounts of processing power. Theirs is one of the largest brain to body ratios of any animal.

Some species also possess modified mouthparts to facilitate electrolocating and to feed on small invertebrates buried in mud. The shape and structure of said mouthparts leads to their common name "elephant-fish". The “trunk” is  flexible, and equipped with touch, and possibly taste, sensors. There are also some species that lack these appendages, they’re often referred to as “Baby Whales” or “Baby Dolphins”. 

Mormyrids also possess electric organs that generate weak electric fields, and electroreceptors that detect small variations in these electric fields caused by the presence of prey or other objects of varying conductivities. This allows them to sense their environment in turbid waters where vision is impaired by suspended matter. Each species of Mormyrid emits its own unique electric pulse. At times the frequency may change, but the pulse waveform remains the same.

Electric fish can be classified into two types: pulse fish or wave fish. Pulse-type discharges are characterized by long intervals between electric discharges, whereas wave-type discharges occur when the interval between consecutive pulses is so brief that the discharges fuse together to form a wave. The electric discharge is produced from an electric organ that evolved from muscle, this can also be seen in gymnotiform electric fish, electric rays, and skates. Convergent evolution between the South American gymnotiforms and the African Mormyridae is remarkable, with the electric organ being produced by the substitution of the same amino acid in the same voltage-gated sodium channel despite the two groups of fish being from different continents and the evolution of the electric sense organ being separated in time by roughly 60 million years. Convergent changes to other key transcription factors and regulatory pathways in both Gymnotiforms and Mormyridae also contributed to the evolution of the electric sense organ.

Their retinas are called "grouped retina", which are eye structures seen in mormyrids and a few other fishes. Instead of being smooth, their retina is composed of tiny cups, acting like parabolic mirrors. Because of the murky waters they inhabit, the cones in their eyes have adapted to see only red light. The cups are made of four layers of light-reflecting proteins, funneling red light to areas of cones, intensifying its brightness 10-fold, while the rods are hit by light from other wavelengths. Only a single gonad is present, located on the left side of their body. The Mormyridae and the closely related genus Gymnarchus are also unique as being the only vertebrates where the male sperm cell does not have a flagellum.

The fact that Mormyrids are extremely intelligent makes it so you can actually interact with them. Providing enrichment can be beneficial and enjoyable for both you and your fish. You can do things like hide their food around the tank or in specific receptacles so they can go on the hunt for it, or give them a rubber die and watch them nose it around for fun. They’ll also swim up and take food right out of your hands, which is always a treat for us as keepers who rarely, if ever, get to touch their fish. 

They may not be the most colorful, but what they lack in showiness they make up for in every other aspect. In my experience they’re not super easy, but they’re not super difficult. Understanding their needs is paramount, unfortunately there isn’t a lot of personal experiences or literature surrounding them except a few scientific papers and forum posts. 

They need a fairly large tank as most species available in the aquarium trade such as Brienomyrus brachyistius or Brevimyrus niger, get around 6-9 inches with others getting even larger. A 75 gallon tank or larger would be best. Be mindful of tankmates as they are not aggressive fish and shouldn’t be kept with overly boisterous, territorial or aggressive tankmates. Schooling fish and small/medium well behaved cichlids, plecos, spiny eels, etc. will work, always keep an eye on the situation to make sure everyone is tolerating each other. Parameters are easily attainable; pH 6-7.5 and temperatures 72-82, as we always say, stability is best - don’t chase parameters. They’ll greatly appreciate plants and hardscape as they like spaces to retreat and hide and will often adopt one go-to spot. They’re nocturnal so they do prefer a dimly lit tank or lots of cover. 

I can promise you, out of all the brown fish, these are the ones you want. If you're a dedicated aquarist and want a semi-challenging and very rewarding oddball, give Mormyrids a try.