First thing to note: These little critters are good guys! If you see them in your tank, leave them there.
You’ve spotted a scary looking black, brown, striped or cream coloured worm poking out of your live rock. It has tentacles that surround it’s mouth, looking like they are waiting for a fish to grab them as they swim by… You’ve watched too many Tremors movies!
There are around 350 marine worm species, but the peanut worm is put into it’s own category, Sipuncula.
Peanut worms. Scary looking, but totally harmless detritus eaters. Some peanut worm species can grow up to 60cm in length, but most will not grow more than 10cm in length.
Their main body, a peanut shape, normally resides in a crevice of live rock, or sometimes they will make their home in an empty shell. They have a long tube which the stretch out for feeding, this will be the part of the worm you can see in your live rock.
How do you control the numbers?
Peanut worms will not take over your tank. They are not prolific breeders. You will need to have both a male and female in your tank for them to reproduce, which is unlikely to happen.
Do they have any predators?
Peanut worms are pretty much bottom of the food chain in the grand scheme of things. Any predatory fish or crab, such as an Arrow Crab, will make a light snack of these detritus eaters.
For a reef tank, things like the more aggressive Wrasses or an Arrow crab, really would make light work of them.