CR 16 is ok.
Were we going to drop split?
Anyone know if there's reason for another organization? These are so big that only solitary kind of makes sense.
Severing a tentacle makes sense. Let them grow back in 8 hours?
I've already said we should drop Split.
In real life, Sabella often live in colonies with a large number of fan worms, in the "hundreds and thousands" kind of large. So, I would think we should allow for encounters with more than one. Say organization solitary or grove (2-12).
Challenge rating 16 seems a bit high to me. Sure, it's got a lot of hit points, but it's an immobile target. Maybe 14-15?
EDIT: The SRD Kraken is Challenge Rating 12 and is roughly comparable (AC 23 vs 20, hp 400 vs 290, 60' reach with tentacles) It's got a much worse BAB & Grapple, but the jump from +28 melee & +44 grapple to +32 touch and +56 grapple isn't going to make much difference, it's most likely to change a 90% hit to a 95% hit or something. More important is that a Kraken is mobile, does damage with its attacks and has a good Will save and some magic.
Comparisonwise, that makes the giant Sabella look more like a 13-14 to me.
Any thoughts? Is it just that the kraken is woefully under-CR'd?
:END EDIT
Finally, I wanted to have a "severing the tentacles" bit, didn't I mention it? Using an adaptation of the Kraken or Giant Squid seems the easy way to go, something like:
A giant sabella has a number of tentacles equal to its hit dice, but can only use 8 of them in a full-round attack. An opponent can make sunder attempts against the sabella's tentacles as if they were weapons, each tentacles has 20 hit points. If the giant sabella is currently grappling a target with one tentacle, it usually uses another tentacle to make its attack of opportunity against the sunder attempt. Severing a giant sabella’s tentacle deals damage to the sabella equal to half the limb’s full normal hit points, the severed tentacle regrows in 1d6+6 hours. A sabella usually withdraws from combat if it loses more than half its tentacles.