What is this worm??

[kjstallan]

Hello everyone, I am new to the saltwater aquarium hobby. I have been a freshwater hobbiest for over 10 years now and have finally converted to salt. My tank is 2 months old and currently has a bicolour blenny and PJ Cardinal with live rock (no corals yet). I have a few of these worms in my tank and I am unsure what they are or if they are harmful. Any help would be hugely appreciated!!

 

[taytay_1119]

Could be spaghetti worm. They’re generally harmless.

 

[ISpeakForTheSeas]

Welcome to Reef2Reef, and - as mentioned in the other thread - that's a female spoon worm (it may or may not have tiny males living inside it):

That's a female Bonelliid Spoon Worm/Echiuran - most spoon worms are harmless detritivores (good CUC), but a few (likely including yours) have a toxin (Bonellin) on their proboscis (the forked "mouth" mentioned above) and/or skin; these eat things like small pods that succumb to the toxin when crawling on/over the proboscis.

These would probably be fine to keep in the tank as long as you think your critters are smart enough not to touch the proboscis, but it may carry some risk (especially for smaller critters, and especially if your female has one or more males - which are tiny - living with it so that it could reproduce):

be aware that it is toxic (the toxin is called Bonellin, and I’m not sure about it’s effects on people), and please handle it with care and a good pair of gloves.

If you decide to keep it, I'd probably suggest running carbon and doing regular water changes to be safe.

Bonelliid spoon worms exhibit extreme sexual dimorphism - the females are large while the males are extremely tiny and often/always live literally inside the female. So, by seeing the worm's regular-sized proboscis extended, you can tell it's female (no idea if it has a male with it or not though).

Unfortunately no idea how harmful the toxin is - as a general rule, the physically greener the worm, the more toxic I would expect it to be, but I don't know how much of the toxin it would take to harm a small fish.