Spiny eels: they do breed in captivity!

When I bought my Macrognathus maculatus almost three years ago (they were sold to me as M. circumcinctus), I had no idea that one day I would breed them. It is amazing how little information is available about this group of fishes, since they have been established in the hobby for so long. The notion that they would not breed in captivity, at least without hormones, was well known to me. But soon I was to be taught a lesson!

I acquired my Frecklefin Spiny Eels in a group of three because of my deeply held view that no animal should be kept completely without company, unless they tear each other apart. I did not know at the time that I had bought two females and one male. At first, there was great friction between the larger female and the much smaller male. The female chased him continually, and eventually I separated them for six months until the male had grown to equal size. From that moment on, the secret love affair between the two began.


Sometime in June of 2010, I was doing my usual morning inspection rounds. As I sat in front of the eel tank, suddenly I saw a 0.2-inch (5-mm) wormlike something swimming through the current, and my first thought was “Oh, no! Planaria!” The creature stopped and hung onto the side of the tank, where I could get a better look. First doubts: a planarian flatworm with a dorsal line? I had never seen such a pattern before. I got myself a magnifying glass and looked at the thing very closely. It dawned on me very slowly that this might be an eel. No, it could not be true! But the more I inspected it, the more certain I became: it could only be a spiny eel larva.

I removed the tiny creature from the tank and moved it into a hurriedly furnished breeding tank. Meanwhile, five more larvae appeared in the aquarium, so I moved them, too. Now I could observe them more closely and take the first photos. Honestly, I have been stunned at how cute the juveniles of other fish species are, but these babies, with their round faces, huge eyes, and always-erect pectoral fins, were enough to make anyone swoon.To date, I have not witnessed a single spawning or found a clutch of eggs. Trying to film the spawning with a continuously running video camera failed, because the fish discreetly withdrew from the field of view.

I know from reports of a breeder of Macrognathus pancalus that spawning always takes place in the early morning hours, preferably in feathery floating plants.

Because my tank lacked floating plants, I suspect that the roots of the large Anubias served as spawning grounds. After I found the first larvae, every Thursday morning another 6 to 15 babies appeared promptly. As far as I could tell, the larvae hatched with yolk sacs and spent three to seven days lying on the bottom until the yolk sac was used up. Then they started to swim free.

The juveniles paddled around the aquarium in search of food. They did not hide and were not very cautious. However, it was striking how slowly they moved. They seemed to act like pieces of wood, drifting through the water without proper propulsion. This probably saved them from being viewed as food by the Synodontis lucipinnis and Dianema urostriatum in the tank. Unfortunately, my orphaned Labidochromis caeruleus “Yellow,” who was watching closely what I was chasing with my net, got the idea that this “driftwood” might be something edible—so he had to be moved.Problems with glass bottoms

Initially, rearing the eels posed no major difficulties. The babies ate anything that fit into their mouths, but only after extensive investigation: Artemia, Cyclops, Grindal worms, even retracted Hydra. A spiny eel baby encountering a new prey item is a spectacle without equal. The animal is suspiciously viewed from all angles, tentatively sniffed, and then finally snapped up.

The first feedings with glass worms (larvae of the Chaoborus plumicornis fly) triggered gales of laughter because the sniffing of the larvae, of course, induced the typical spasmodic flight of the glass worm. The confusion among the spiny eel babies was great, and it took some time until they finally captured the first prey.

The fry grew with an uncanny speed. Within a week they doubled their length; after two weeks, they were already0.6-0.8 inch (15-20 mm) long. Within four months they were 2.5-3 inches (6-8 cm) in length. At the beginning they lacked the typical rostrum. In the case of Macrognathus maculatus this starts to grow at the age of about six weeks, when the animals are starting to lose their immature appearance.

I now had about 70 juveniles, but when I relocated them to a tank without substrate I encountered problems. The symptoms were always the same: a necrosis started around the anus, then continued toward the tail. Within days, the affected animals died. This necrosis did not appear to be contagious. It turned out to be a bacterial infection that was apparently due to contact with the bare glass tank bottom. After the introduction of a sandy substrate, the dying stopped immediately.

Eventually, I realized that the young animals were in no danger in the parents’ tank. The parents did not eat their fry and, fortunately, the catfishes were not after the little ones either. So

1 left the next fry in the tank and they grew up very quickly and vigorously. Up to a size of about

2 inches (5 cm), they lived in the pores of a Poret Foam filter. Feeding time was always amusing. As soon as food hit the water, suddenly there were heads looking out from all the pores of the filter sheet. In the parents’ tank the babies were also much less anxious and shy.

Husbandry on demand?

Again and again, I am asked how to breed spiny eels. I cannot answer that question because I have never really done anything to get it started. I can only report my observations.

Probably the most important factor is to keep them in a group that contains both sexes. The distinction between the sexes is extremely simple. In males, the ventral side forms a straight line from the head to the body, whereas in females there is a distinct edge or angle. I always say that the females have a double chin. This sexual dimorphism is already clearly visible in juveniles from a size of 3 inches (8 cm). Even if the animals are quite fat (which happens at times in my tanks), in males the straight line remains, while in females the double chin becomes even more prominent.

The water parameters seem to play a minor role. In the literature, medium hard water is recommended for M. maculatus, but my animals live in very soft water. However, the influence of the water temperature is striking. Courtship begins whenever the temperature rises. I must say that I keep this tank without a heater, so the temperature is usually around 75-77°F (24-25°C). In the summer months, the temperature increases to 81-84°F (27-29°C) and the spiny eelsbecome very active. During the day, they often swim restlessly throughout the tank. The adult pair constantly plays hide and seek, and there is a lot of excitement. The food is probably important and the animals must be well fed. The female soon develops eggs, which can be seen clearly through the abdominal wall. Just over two weeks later, the first larvae emerge, so the adults do not waste any time.Aggression

The myth that spiny eels won’t breed in the aquarium probably has several explanations. First, many retailers and hobbyists think they should be kept individually because they are aggressive toward each other. I cannot confirm this at all. On the contrary, in a group they are far more outgoing and seem to feel safer.

The rumor that spiny eels are aggressive may have various origins. In my experience, socialization of M. circumcinctus with M. maculatus leads to the smaller M. circumcinctus being constantly chased and bitten. Unfortunately, both M. maculatus and M. circumcinctus are often offered as “Belted Spiny Eels.” They look quite similar, but M. maculatus have no stripes on the abdomen and are up to 11 inches (28 cm) long compared to the 7-inch (18-cm) M. circumcinctus. Of course, such errors lead to socialization problems that promote the view that spiny eels are aggressive toward each other.

Mastacembelus armatus is supposedly quite aggressive, but I have not determined yet if that is also the case in sufficiently large and well-structured tanks. However, if animals differ significantly in size, there is always friction and the smaller ones get pressed hard. You can avoid this with the purchase of a group of similarly sized animals.

A fellow spiny eel keeper has successfully kept Macrognathus pancalus, M. zebrinus, M. meklongensis, M. lineatomaculatus, and M. aculeatus in large groups without any significant aggression.

In addition, spiny eels probably only breed if all the parameters are met to some degree. The combination of higher temperatures and lots of good food is important. They are greedy carnivores, not hard to please, and they will take bite-size meaty foods, insect larvae, crustaceans, pieces of mollusk, and the whole gamut of live worms. But sometimes the right weather seems to play a role as well, because during bad summers there are far fewer offspring.

I would argue that one should keep spiny eels in groups, because they are basically social animals. If species are carefully and correctly identified, interspecific socialization and aggression can be avoided. Under the correct circumstances, I hope that many owners are experiencing the spontaneous reproduction that has happened in my tank. The husbandry of spiny eels is really a special experience!

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