Aegids are "bugs from hell" as far as the aquarist is concerned. They are like predatory Cirolanids, only more so. Large Aegid isopods in the North Eastern Pacific have been seen to wait on the bottom until an acceptable fish, such as a small salmon, swims overhead. The isopod then swims rapidly up and fastens on to the fish, and proceeds to eat its fins and tail. The bug then slices open the fish and eats all its blood, proceeding then to eat the lateral muscle bands and, when they are done, they discard the guts and skeleton.
Parasite, indeed!
Figure 4. This is an Aegid similar to those collected in aquaria photographed in nature in the waters of northern Puget Sound, Washington, USA. This animal, Rocinela belliceps, was about an inch long and is capable of killing small salmon, and making the lives of some of my students miserable (see below).
The same species will fasten onto larger fish and eat its way into a major blood vessel where it will remain for some time sucking blood and eating tissue. When sated, it will excavate its way out of the host and swim away.
Tropical species show up somewhat frequently in reef tanks either riding on a fish or in a piece of live rock. Often the first the aquarist knows of them is when they see the isopod on a fish. Murphy's Law is active here; the bug will never be on a cheap or expendable fish. The problem is how to remove the isopod from the aquarium. If the bug stays on the fish, the fish needs to be captured. This happened to me several years ago, and the fish it was on was a Mandarin dragonet. Imagine trying to catch this fish in a fully set up 100 gallon aquarium! Of course, I couldn't catch the fish until the next day, and the isopod was still on him. There wasn't much left of the fish. Even I, with my notable lack of coordination and dexterity, can catch a fish that is half eaten.
If you can catch the fish, the isopod may be removed with a pair of forceps. Carefully!!! Upon removal, the fish should be isolated in a hospital tank, and treated with antibiotics until the wound heals. The bug may be disposed of. Carefully!!! About 15 years ago, I had a student who was holding in her clenched hand a 1.5 inch long Aegid. The bug cut through the flesh of her palm, dug in, and started to eat HER. Her vocal response was rather impressive. So was the tenacity of the isopod, it was HARD to remove!
If you notice one of these animals in your tank, and it leaves the fish, there is almost nothing that may be done to catch it. They are very fast and quite capable of avoiding a net. And if it is a pregnant female (and remember, all female isopods have brood pouches), and the brood hatches, you have REAL problems. There are only three courses of action in this situation; and I truly am not jesting about these responses. The first is to remove all the fish from the tank and wait the two or three months until you are certain that all the isopods have died from starvation. The second solution is to effectively nuke the tank. Remove all live rock and discard it as the isopods may hide in it and, as some of the isopods bury in the sand, you should also remove and discard the sand.
You may, of course, take the third option and do nothing. The most likely outcome in this situation will be that the isopods will kill your fish one by one. These isopods are masterfully designed predators. Hope fervently that you never have to deal with them.
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