algae on my horse

Dear Sandy:

I’m very sorry to hear that you lost two of the pipefish. Because they are still collected from the wild, pipefish in general tend to be a little more delicate and disease prone than domesticated seahorses that have been born and bred for life in the aquarium for dozens of generations. All my condolences on your loss!

Yes indeed — algae often grows on the exoskeleton of seahorses, typically on their head and neck which are closest to the light source. That’s perfectly normal and nothing at all to be concerned about. In fact, seahorses often encourage algae to grow on them as a protective device to enhance their camouflage, and it’s best to ignore any such growth.

That being the case, I always caution seahorse keepers against trying to physically remove such a coating. Although the algal growth can be unsightly, it is part of the seahorse’s natural camouflage; it can certainly be brushed away, but doing so risks removing the protective slim coat along with the algae which can have harmful consequences. Allow me to explain a little more about the seahorse’s skin and mucosa and the important purpose they serve:

We are all well aware that seahorses can change color to blend into their backgrounds, and that Hippocampus is capable of growing or shedding dermal cirri, which are long filaments and branching extensions of its skin, as called for in order to match its environment. To complete its disguise, the seahorse allows algae, bryozoans and other encrusting organisms to grow on its body, thereby rendering it all but invisible in its natural habitat. In fact, its skin contains polysaccharides which are believed to encourage algal growth, thereby helping it disappear into its surroundings. This vanishing act is so convincing that, even when they are collected by hand seining, seahorses often go undetected amidst the plant matter that accumulates in the net. Unless the collector is really diligent, he will lose a large proportion of his catch simply because many specimens will be overlooked and thrown back with the debris. In short, it’s perfectly normal for algae to grow on seahorses.

The seahorse’s pliant skin or integument is of course its first line of defense against disease. It contains mucus glands, and the slime covering the skin acts as a barrier to ectoparasites and infection. The protective slime can contain antibodies and antibacterial substances, and excessive mucus production is often the first sign of an infection or parasite problem. On the other hand, healthy seahorses often have small bright dots on their heads and torsos, which are actually mucus deposits. These beads of mucus glisten like little diamonds and are a sign of vibrant good health. When handling a seahorse, it’s important to wet your hands first in order to avoid removing too much of the protective slime coat.

Marine fish are always in danger of dehydration because the seawater they live in is saltier than their blood and internal body fluids. As a result, they are constantly losing water by diffusion (osmosis) through their gills and the surface of their skin, as well as in their urine. The mucus layer acts as a barrier against this, waterproofing the skin and reducing the amount of water that can diffuse across its surface.

So, all things considered, it’s best just to ignore any algae that happens to grow on your seahorses. Removing it means removing the slime coat and mucous barrier as well, leaving the seahorse susceptible to infection and dehydration. It will secrete more mucous and repair its slime coat over the next day or two, but it will be vulnerable in the meantime.

Best of luck getting your rampant algae growth under control, Sandy! When it comes to your seahorses, it’s only a concern in so far as it indicates there may be too much nutrient loading in your aquarium at the moment.

Happy Trails!
Pete Giwojna