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The Black Marble Hoplo: Megalechis thoracata

Breeding the black marble hoplo.

By Mike Hellweg |

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black marble hoplo

Black marble hoplo
(Megalechis thoracata) by Gary Lange.

Aquarists have always had a fascination with catfish. Some of the earliest fish kept in the aquarium hobby were catfish, and to this day they are still sought-after by beginners and experts alike. There is even a bi-annual convention held in the Washington, D.C., area that caters solely to catfish enthusiasts. It’s hard to call a catfish beautiful, but if there were a catfish beauty contest, the black marble hoplo would be right there in the running.

The name hoplo comes from the old genus name of Hoplosternum. Hobbyists always knew there were several species, though in the literature of the time, only two or three were recognized. Then in 1997, Dr. Roberto Reis looked at the group more closely. He broke the old species group in the genus “Hoplosternum” into several species in three different genera. One of those, Hoplosternum thoracatum, became Megalechis thoracata. There are many common names, too, such as bubblenest catfish, black-spotted catfish and just plain old hoplo.

The most often encountered trade name, and the most appropriate, is black marble hoplo. As you can see from the photo, this fish is a cream to tan color with large black blotches or patches on the body and fins. This color pattern is evident in young fish and remains with the fish as they reach maturity. The only difference is that in adult fish, the cream color often darkens to a nut brown. At spawning time, adult males take on a bluish sheen to the belly, while at other times their bellies remain a creamy white. Adult females have a white belly at all times.

The easiest way to sex hoplos is to look at their pectoral fins. The adult male’s fin is unmistakable. The first ray is thickened, and the whole fin is enlarged and triangular. At spawning time, this ray takes on a deep orange color. In adult females and young males, the pectoral fin is oval and the first ray, while thicker than the rest of the rays in the fin, is not prominent. In addition, if you look at them from underneath, the bony plates that cover the chest, called coracoid bones, are different. In females, these bones are small, rounded and open, forming a wide “v.” In males, they are much larger, pointed and come close together, forming a tight “v.” This feature is so striking that Dr. Reis even refers to it in their generic name; Megalechis is from the Greek word meaning “large plate.”

Megalechis thoracata is found all over the northern half of South America, from the Amazon northward. They are found on the island of Trinidad, and escapees from fish farms have even established themselves in Florida. As you might surmise from this distribution, hoplos prefer warmer water, with a temperature in the upper 70s Fahrenheit being ideal. You also might guess from this wide area of distribution that they are not picky about water parameters. They are found in the wild in both hard and soft water (ranging from less than 50 ppm total hardness to well over 450 ppm total hardness), with a pH range from below 6.0 to above 8.0. Salinity also fluctuates, and they are tolerant of fairly brackish water. They will tolerate less-than-ideal water pollution conditions where other fish might perish. They can even absorb oxygen by swallowing air into their modified gut. This doesn’t mean that they can tolerate a lack of care on the part of the aquarist but rather that they are tough fish.

Black marble hoplos grow into fairly substantial fish. They can reach an adult size of nearly 5 inches in the aquarium, and specimens reaching nearly 6 inches have been reported in literature. They are a schooling fish that can be found in groups numbering into the thousands in the wild. In the aquarium, groups of five or six fish are ideal. It is best to house only one male per tank, as multiple males will get along just fine until spawning time, when the dominant male may kill other males. A tank with a large floor space is ideal, with a 30-gallon tank being the absolute minimum to successfully house a school of a half dozen black marble hoplos.

As large-bodied fish, they require a correspondingly large amount of food to keep them going. Pelleted foods designed for catfish are ideal, with supplemental feedings of frozen meaty foods, such as bloodworms or brine shrimp. To condition them for spawning, increased feedings and the addition of meaty live foods like black worms, white worms and earthworms to their diet will help get them ready.

Remember that their larger size and heavy diet will also mean a corresponding amount of waste. Regular water changes are in order, with 50 percent or more per week being ideal. A good filter is also in order, but if you want them to spawn, a filter that does not disturb the surface would be better than one that does, with a canister filter being ideal.

Megalechis thoracata has an unusual method of spawning for a catfish. The males build a bubblenest at the surface. They will spend days constructing the nest, incorporating pieces of plant matter such as stems, leaves and twigs. A really large nest can cover a third of the surface of the tank to a height of an inch or more. Many breeders give them a chunk of Styrofoam or a yellow coffee can lid floating on the surface. For some reason, yellow is a preferred color. This becomes the base for the nest. The male hoplo expels the bubbles from his gills. They are covered with sticky mucus that helps stick them together and keeps them from dissolving for several days.

When his nest is ready, the male will chase and court the females. A willing female will follow him up to the nest, and they will assume a “T” position. The female will be on her back forming the top of the “T” as she lays a clump of a dozen or so sticky eggs into a pouch that she forms with her ventral fins. She will then place the clump of eggs into the nest and move off. The male immediately moves under the nest upside down, fertilizes the eggs and blows more bubbles to help fix the eggs into the nest. This is repeated until upwards of 500 eggs are laid. Then the female is driven off. If there are more ripe females in the tank, the male may also spawn with them, though he is just as likely to drive them off as mate with them. At this point, it is best to remove the female. The male will guard the nest fiercely and will attack anything that enters the tank, including nets and hands.

When guarding the eggs, the male normally does not eat, so there is no need to feed him. He will repair the nest as needed, adding more bubbles and retrieving any eggs that fall from the nest. If any eggs wind up on the bottom, they will hatch there, so there is no need for concern.

After about four days at a temperature of 80 degrees Fahrenheit, the eggs begin hatching. At this point, it is best to remove the male, as he might begin considering the fry as food. The newly hatched young may hang around the nest for two or three days but are usually free-swimming within a day or so after hatching. As they become free-swimming, they head to the bottom of the tank. The young will have completely absorbed their yolk sac within the egg, so they will need to be fed about 24 hours after hatching. If there is a layer of mature gravel on the bottom, they will find all kinds of supplemental food in the mulm that naturally accumulates. They should also be fed several times a day with microworms, newly hatched brine shrimp and fine-powdered fry foods.

Early growth is rapid, and having the fry reach a size of one-and-a-half inches in just eight weeks is very common. At this point, they will eat the same foods as their parents and the rapid growth will mean correspondingly heavy feeding, good filtration and a lot of water changes. Raising 300 or more juveniles to this point is not uncommon, so it would be wise to have more tanks available to thin out the growing school. At this point, it’s a good idea to find new homes for the many juveniles. Fortunately, with their unique coloration, they are always in demand. Locally owned fish stores are always happy to take healthy young fish in trade from their regular customers, so make sure you support your local fish store.

If you reach this point, congratulations! You’ve had another successful Adventure in Fish Breeding.

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Reader Comments

Robert    Floyds Knobs, IN

8/28/2010 9:05:01 PM

I bought 3 from Mike at the 2009 ALA convention in Indianapolis. Mike is such a nice guy. He has spoken at our club, and even got a couple of my articles published in the Darter. The Hoplos are beautiful and fun to watch. They are not shy at all and the male usually bumps my syphon hose inlet when I do a water change. Great fish and definitely the best looking hoplo.

ethan    BROWNSVILLE, TX

7/17/2010 7:57:44 PM

so many catfish everywhere lol

sal    la, CA

12/31/2009 5:35:26 AM

This look like a really exotic little fish to try and breed in the home aquarium!

Olivia    c-town, CA

9/24/2009 7:57:45 PM

Love learning new stuff on here! Thanks!

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