Serpents and octos seem to have an interesting relationship. I call them the octo "maids". Sometimes you will see them both in the den and if the serpent gets too touchy feely, the octo will evict it.
Miss Broody, my tank raised female mercatoris (dwarf) went directly to her the brooding den of her mother when she was released from the breeder net (at about 5 months old). I only saw her leave the den once. She move to a lower barnacle, allowed the serpent to come in and clean and then returned when the star left. Her male suitor would come visiting and station himself in one of several adjoining barnacles. Eventually he mated with her while she was still in that same her den.
We have red brittle stars named Pesky and PeskyToo in the current occupied octo tanks. We rarely saw PeskyToo before putting an octo in the tank, now he is frequently visible during the day. Pesky is featured in several SueNami videos and photos but so far has remained living in the overflow since Kaysoh arrived. I expect we will see more of him when Kaysoh is better adjusted though. It also seems that the red brittles are a brighter color orange when there is an octo present. With no octo, PeskyToo was almost brown and now is vibrant.
Peps also seem to have a third sense about Octos. There were two in Beldar's tank when she arrived. One was too curious and became snack, the other, still keeping aptasia watch in the tank, would dart to the opposite side of the tank at the first sign of Beldar approaching.
If aculeatus, there is a good chance of it being young (unusual) but abaculus does not get much larger (7" ish arms at adult and just over an inch mantle) Unfortunately, size is not a particularly good indicator of age though.