adshepard:
Disregarding the rude and surprising comments concerning my "know-it-all postings on marine life that are annoying and sometimes wrong", you appear to have your observations of sea pens backwards. Having my older copy of Barnes' conveniently sitting right beside me, let me quote something most marine invertebrate zoologists are quite familiar with.
From pp. 137-137 of Barnes Invertebrate Zoology (5th edition).
The "primary polyp" is also commonly referred to as the "stalk" or "axial polyp". You can have absurdly short stalks like in Renilla, very long ones like in Umbellula, but most commonly you'll get the intermediate sized ones like in Virgularia and Stylatula. I'm only commenting on genera I'm well familiar with from our research and teaching collections, mind you. Pennatulaceans are not my taxonomic specialty.
From Florenty's photo, the axial polyp is not visible. And while from an ecological sense I'd rather go with it being an egg case, we'll argue the sea pen for the sake of accuracy.
In this case what you're seeing are the secondary polyps coming off from the upper primary polyp, also referred to as the "quill of the pen" in Virgularia forms. For these and related genera your secondary polyps will not assume a cylindrical shape from the upper stalk, but more like the one in the photograph. However you can most assuredly have a "cylindrical" form. An internet search of Stylatula should show this quite clearly... looks like stick candy. And if want to see a wholly different type of sea pen morphology, look up Umbellula, which looks like a pinwheel. Glows in the dark too, although I've never been able to get my trawl specimens to do this. A multiple hour ride through several thousand feet of water tends to put animals in a black mood.
As for pennatulaceans requiring soft sediments for attachment, that's dead on and can be referred to in my first post (#3 I think). The lower end of the axial polyp is technically referred to as the "physa" or "peduncle", and works by digging into the bottom and then becoming swollen with actively pumped seawater. In essence it performs much like dredge anchors. And as far as I know ALL pennatulaceans are epipelic (dwelling atop sediments), not "most all cases". If you know of an exception please post it, as I would like to append it to my notes.
EDITED ******* The egg case theory for instance is very good and most likely is what the unidentified feature is. Not from direct visual observation mind but from the reported depth and location of the photo, and that gorgonian (a plexaurid of some type) sitting right next to it.
I back up most if not all of my statements regarding marine life on this board, and do not take offense if they turn out to be incorrect. I expect other posters to return the courtesy.
