No Horsing Around Cowfish

These Hippocampus kuda are large seahorses. They can grow up to 17cm in adulthood. They are friendly and outgoing. However, even these large seahorses' behaviors are affected by cowfish tankmates. Photograph by James R. Ulrich. Copyright March 2000.

By Theresa Ulrich

Reprinted from Freshwater and Marine Aquarium Magazine MAY 2000

Cowfish and seahorses are two amazing and unusual species of fish. Their most notable feature is that they resemble farm animals. They are fascinating in their own special way, yet they maintain many similarities. After analyzing numerous features and behaviors of each fish, it would be easy to conclude that they could be compatible tank mates.

First of all, both cowfish and seahorses are found together in sandy sea beds. Living together in the wild appears to make a convincing argument for keeping them together in an aquarium. Of course, each species also has a similar diet. Both cowfish and seahorses enjoy assorted grass shrimps, as well as other foods along grassy shores.

Both have a protective outer shell or skeleton. They have a similar looking form of locomotion as well. Their fins appear invisible as they convey the allusion of being suspend by strings, much like a puppet. Their unusual mobility makes them slow swimmers. In fact, both creatures are considered to be calm and docile fish.

Even more, both cowfish and seahorses have independent eye movement, allowing them to look in two different directions at the same time. This is an adaptation for keener hunting. Both species are ever hungry and on the prowl for food.

These two completely different species are amazingly alike. With all that they have in common, they seem to be perfect cohabitants for an aquarium. Indeed, this may be true when cowfish are very young. Boston beans, or peabodies for those from the land down under, are considered to be a complementary tank mate for seahorses. (Boston beans and peabodies are names given to tiny cowfish and trunkfish, measuring less than one inch. They are named for their minute size and round shape, causing them to resemble beans or peas.)

Boston beans are appealing creatures that will add excitement to a seahorse tank. However, young cowfish don't remain small for long. Once cowfish reach more than two inches, they may become little terrors among seahorses. Anyone who buys juvenile cowfish to keep with seahorses should be prepared to move them eventually.

As cowfish grow, they become stronger, more aggressive, and more competitive. The progressive changes cause several problems for their seahorse companions. Both cowfish and seahorses have a predation response triggered by movement. However, cowfish, unlike seahorses, are not intimidated by food that is larger than their mouths. They may nibble on seahorse tank mates whenever the seahorses move.

Their natural grazing behavior also causes cowfish to nibble on seahorses. It is not uncommon for seahorses to grow a film of algae on parts of their bodies. The algae provides a protective coating against parasites and acts as additional camouflage. Cowfish graze on algae, and will even nibble it off of seahorses.

Cowfish nibbling is not in itself an immediate danger to seahorses. Seahorses have a tough, bony body, so they make very poor meals for any creature. It is unlikely that a cowfish would kill a seahorse by merely chewing on it. However, seahorses may receive wounds caused by the nibbling. These wounds tend to develop into bacterial infections very easily. Seahorses can die from quick spreading bacterial infections. So cowfish bites can kill seahorses indirectly.

Cowfish can also intimidate seahorses easily. When seahorses become irritated or even fearful of constant cowfish bites, they remain hidden much of the time. When this happens, the seahorses lose their natural color and sometimes their livelihood. If the situation is allowed to progress, seahorses will eventually stop feeding and die of starvation. This danger amplifies as cowfish continue to grow. Cowfish typically grow much larger than seahorses. Their formidable presence alone can subdue normal seahorse behavior.

Even large, bold seahorses will eventually develop conflicts with cowfish tank mates. As cowfish age, they swim faster and capture prey more easily. Cowfish are insatiable and rarely turn down food, even after having apparently stuffed themselves. They gobble food so quickly that seahorses may not be able to receive their share. Cowfish even take food from seahorses' mouths! It eventually becomes impossible to provide enough food for the seahorses without polluting the tank.

There may be some who feel they have kept cowfish and seahorses together successfully. However, there are more risks to consider beyond those already mentioned.

Although both cowfish and seahorses are constant eaters, cowfish can consume and hold much greater portions of food. Because of their high food intake, they also have an extremely high waste output. Of course, the high waste load affects the water quality. In particular, nitrates remain slightly elevated. Both fish require pristine water conditions. They are sensitive to nitrates; however, the seahorse tolerance for nitrates is much lower than the cowfish. It is difficult to keep the nitrates under 10 ppm in an aquarium containing cowfish. Anything above that value puts a seahorse's health at risk. Seahorses are much more likely to contract disease or host parasites in even a slightly weakened state, and it is very difficult to treat seahorses successfully once they become ill.

Disease concern can become even more complicated. Cowfish contract cryptocaryon irritans (saltwater ich) very easily. Because they appear to naturally harbor the ich cysts, they will often heal themselves with time, good water conditions, and a varied diet. However, seahorses in the same tank will also contract the disease. Their resistance to ich is much weaker. If contraction of cryptocaryon is not treated immediately, the seahorses will succumb to the disease quickly.

There then becomes an additional concern of medication use. It is typical to treat a display tank in response to an ich out break, so a safe medication and proper dosage need to be sought for both the cowfish and seahorses. Finding a suitable cure for the two different species can pose a problem. Sometimes the stress of treating recurring disease can be an overwhelming task for an aquarist. Cryptocaryon is stubborn and difficult to kill off completely. Seahorses will constantly be at risk of contamination.

Another risk factor to consider is the cowfish's ability to poison the tank. Cowfish can exude a toxin in the aquarium when they are upset or stressed. If this potent toxin is released into a closed system, it will kill all the tank inhabitants, including the seahorses and cowfish themselves.

A final consideration of keeping cowfish and seahorses together appeals more to the advanced aquarist. Anyone trying to breed fish should never keep more than one marine fish species in an aquarium. Cowfish or seahorses may be less interested in breeding if another species is kept in the tank with them. Even if the fish's desire to breed is present, they may physically be inhibited by interruptions. This is especially true for seahorses. They need to rise up the water column together in a precise manner in order to mate. Any interference by another fish can cause a seahorse couple to miss their mark, forcing the female to spill her eggs on the substrate. Should seahorses manage to successfully mate in a community tank, their newly born brood would be immediately gobbled up by the cowfish.

It is easy to succumb to temptation. Seahorses and cowfish would agreeably make an attractive and unusual display. However, as much as seahorses and cowfish hold in common, they are kept most successfully in separate aquaria.

Best of luck!

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