Puzzling pigment and points

 

With so many Velella washing ashore recently, I have been wondering why other pelagic animals haven't appeared also.  I was thinking about it again tonight, when I looked down to see this animal near my feet. 

Do you remember it?  Doug found one that was similar last fall — see the post from 20 November 2014.  At that time I mentioned that I'd never seen this stage of this species, so needless to say, I was very intrigued to encounter it tonight.

Here's a picture with a ruler for scale:

 

This is the solitary stage of a salp (or pelagic tunicate) called Thetys vagina.  You can read more about salps in previous posts (e.g., see the link above).

Tonight I'm going to show two close-ups, because this is what caught my attention while observing the animal:

 

There is a pair of pigmented posterior projections.  I know they look like "horns" at first, but keep in mind that these are trailing off the posterior end.  I looked very hard to find a description and explanation for these projections, but I came up empty.  Why do they have them?  Why are they green (and blue)?  It appears as though a portion of the salp's body extends into the middle of these projections — what is that and what is its function?  Do the projections have something to do with streamlining while swimming?  Are they involved in camouflage/distraction?  What do you think?  Do you have any ideas?

 

And here's another close-up, this time of the middle portion of the salp's body:

 

Did you notice all of the small gelatinous spines?  Why do they have these raised points?  Do they deter predators?  They're so small.  I found one paper that suggested a texture like this, rather than a smooth surface, might cut down on reflections, making it more difficult for predators to see the animals.  Let me know if you have other ideas!

(I've always wondered about the role of surface texture in Corolla spectabilis, too — see pictures from August 2012 and November 2012.)