01-13-2008, 12:47 PM
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This is the common bristle worm, and even though you didn't see it, it has probably been there for quite some time. Bristle worms often come in with live rock, they live in the rock and substrate of the aquarium. Bristle worms feed on excess food going into the tank, fish waste, and other forms of plant and animal life, and as the food source increases, so do the number of bristle worms.
Are they good or bad? That all depends on how many of them are present and how big they are. Any bristle worm over 3 inches long is best removed so that it isn't able to damage animals in the tank. There are many species of bristle worms, and some are more dangerous to the aquarium than others. The fire worm is a good example of this. A typical fire worm has an orange/pinkish body with white tufts along both sides. These can be harmful to the aquarium and should be carefully removed. The fire worm is not to be mistaken with the bi-colored bristle worm, which is pretty harmless. (the bi-color worm is orange and purple, with white tufts along both sides)
Most species of bristle worms can be beneficial to the aquarium because they are good scavengers, and help to maintain a natural balance in the environment.
Bristle worms got their name because of the tufts of bristles along each side of their body. These bristles are sharp, and will penetrate skin if you come into direct contact with the worm. If you get a bristle in your hand or finger, it will burn... much the same effect as if you had handled a thistle. The bristles are almost clear, and thus very hard to see for removal, and can cause a lot of pain and irritation.
The best way to remove a large bristle worm or fire worm from a tank is with a net, which can be a challenge. Bristle worms move pretty fast and have the ability to get into the tighest of places. Careful watch and planning is then needed to achieve removal.
The worm in the picture is a standard bristle worm, and doesn't appear to be very large. I would not be concerned if I found this worm in my own tank, but I would take note that it is there and watch for more... and for growth rate. When the time comes that it becomes 3 inches or more, then it's time to remove it.
Dottybacks, longnose hawk fish, some juvenile angels, arrow crabs, and some coral banded shrimp will all eat bristle worms, so if in the future you find you have a huge population of them and you wish them decreased in number, adding one of the above animals (if it's safe in your tank) can help to control the population naturally.