Bobbit worm?

[BristleWormHater]

Pretty sure this is a bobbit worm, in a clear image I count 5 tentacles on the head which matches the Eunice genus description. Polychaete ID is still a weak spot for me though. The only hang up I have is the behavior, its been hunkered down under that coral for months and it pulls in debris and secures it to the rock using some glue-like substance. Thats what its trying to do with the caulpera in this clip.

 

[BristleWormHater]

@ISpeakForTheSeas

 

[JoJosReef]

That's a Eunice worm alright. From what I've read, most are detritivores, but I imagine all are opportunistic. They all have mandibles. And they know how to use them! Nabbed one with tongs once and it turned it's head and began attacking the tonga like a cobra. Scared a bit of pee out of me and I released the worm, which instantly escaped into its hole.

 

[BristleWormHater]

That's a Eunice worm alright. From what I've read, most are detritivores, but I imagine all are opportunistic. They all have mandibles. And they know how to use them! Nabbed one with tongs once and it turned it's head and began attacking the tonga like a cobra. Scared a bit of pee out of me and I released the worm, which instantly escaped into its hole.

Tried to grab it earlier, they are super fast!
I think ill make a trap for it.

 

[Crabs McJones]

Took me a minute to see it, I was focused on the worm on the left. But yes I would agree with your ID

 

[BristleWormHater]

Took me a minute to see it, I was focused on the worm on the left. But yes I would agree with your ID

Gonna add that to the original post

 

[JoJosReef]

Tried to grab it earlier, they are super fast!
I think ill make a trap for it.

Good luck! I have yet to find a trap that works. They like to remain anchored to their holes.

 

[ISpeakForTheSeas]

@ISpeakForTheSeas

I'm late, but yeah, 5 tentacles on the head indicates a Eunicid worm of some variety. That said, it's not a true bobbit (sorry, the info from the quotes is a bit messy):

I'm late, but yeah, five tentacles on the head tells us it's a Eunicid worm; the white collar around the neck is found on several Eunice and Leodice species - the bad news, these species are generally not reef-safe (white collar + 5 tentacles on head = bad at this point), as they tend to be more predatory.

For anyone wondering, the Bobbit Worm (Eunice aphroditois) lives burrowed in the sand to ambush prey; a lot of these other Eunice/Leodice species live in the rocks/tubes they've made in the rocks and seem to be willing to either ambush prey or just take food when aquarists feed their tanks. They are known to be very fast and very aggressive when people try to remove them with tongs.

With just two tentacles visible, I'd be inclined to think it's not a Eunicid - I'd be inclined to think it's probably a Dorvilleid with four tentacles, two of which are just difficult to see. Eunicids have the five tentacles (the "five prostomial appendages" listed below), so I really doubt yours is a Eunicid worm:

"3.2. Morphology​

Eunicidae is defined by two synapomorphies, dorsal buccal lip fused to the dorsal side of the prostomium and dorsolateral fold anterior extensions medially connected [11] (Figure 5A–E). Additional diagnostic features are present at prostomium, peristomium and jaws. These are ventral buccal lips with median transverse groove, one (single median antenna) to five prostomial appendages (a pair of palps, a pair of lateral antennae and a single median antenna), double ringed peristomium (Figure 1A and Figure 5A–E), and asymmetric eulabidognath maxillae (Figure 3G). The family encompasses great length variation, bearing species from few millimeters to few meters long. All species have a variable number of chaetigers."*

Regardless, as mentioned, your worm is most likely completely harmless/beneficial (even if it is a Eunicid species):

*Source:
https://www.mdpi.com/1424-2818/13/2/74

Generally speaking, Dorvilleids are harmless/beneficial. Eunicids, on the other hand, can be either harmless/beneficial, mildly risky (think opportunistic predators eating things like snails, small crustaceans, fish larvae, etc. when starved or when the prey is weak enough), or outright predatory. Some are predators while others are literally herbivores (fun fact, even some of the predators - such as the infamous bobbit worm - will still take algae when given the chance). With that in mind, many (possibly most) Eunicids are also harmless/beneficial, so even if it's a Eunicid, it may not be a bad thing.

TLDR; it's probably harmless, but closer up pics (particularly of the head and tail) under white light may help with an ID.

For anyone who's curious, the article below is a fantastic resource on Polychaetes (the supplementary material that details family by family diet, habitat, and more is especially good IMO, though it is 340 pages long - I haven't gotten through all of it yet):
https://www.annualreviews.org/doi/10.1146/annurev-marine-010814-020007

 

[JonoH]

Looks very similar to one I had in my old tank!


Have some pics around of when it was finally removed - I could never trap it, had to break the rock apart that it was inside.