That should be enough to keep you awake at night.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:
The Current State of Eunicida (Annelida) Systematics and Biodiversity
In this study, we analyze the current state of knowledge on extant Eunicida systematics, morphology, feeding, life history, habitat, ecology, distribution patterns, local diversity and exploitation. Eunicida is an order of Errantia annelids characterized by the presence of ventral mandibles and...www.mdpi.com
Which of these jaws would you like to encounter while working in your tank?? I can attest, Eunice worms aggressively defend themselves!
