Update: Thanks to Fabio Russo on the Underwater Macro Videography group, I have now been able to identify this as a Peanut Worm!! It is a bilaterally symmetrical, unsegmented marine worm. The most interesting (make that weirdest) fact about them is that they are actually a delicacy in the town of Xiamen in Fujian province of China. I will not be moving there!!
It's not every day, or even every dive, that we see something that we have no idea what it is. This week, though, saw one of those days and dives.
What on earth (or underwater) is this? I found it on a black sand site, about 16m (50ft) deep. It was moving horizontally through the surface on the sand and was about 5cm (1.5") long when contracted and about 10cm (3") long when fully extended. It moved with a peristalsis-like movement, contracting and expanding it's body like an accordion.
The really strange thing was evident when I saw a close-up of it's 'head'. Even though it resembled, a little, an earth worm, it's head opened up in a very weird way. You can see the front of this thing is just freaky!!! Even though I've watched is many times now, I still can't quite work out if the front tube that extends rolls out from inside, or just stretches out.
Once is it extended, then a quite substantial part of it's insides then moves through the new front opening...you can see organs (?) moving forwards...it really is very very weird!!
So, what is it? I have no idea. I have no idea where to start looking, I don't even know of it's a worm, juvenile Sea Cucumber, or what...so please, if you know what it is, or know someone how might know, let me know :)