"Fire bodies"

This morning Alex asked us about a mystery animal he had observed on Salmon Creek Beach.  He described it as gelatinous, long and finger-like, and covered with short spikes.  I've spent many years listening to people describe different types of animals and trying to assist with identifications.  I like this game.  My mind immediately starts working on the puzzle.  This time, as I listened to Alex, I wasn't coming up with an answer.

He told me where he had found them and it wasn't far away.  When I got there, it was easy to spot them.  This is what I saw:

 

They varied in size from about 1 cm to 11 cm long.  The one below was about 4 cm (1.5 inches) long:

 

Here's one of the larger individuals in my hand:

 

We had to look at them under the microscope to confirm the identification.

This is a pyrosome, probably Pyrosoma atlanticum.

So now you might be asking, what is a pyrosome?

A pyrosome is a pelagic tunicate, somewhat like a salp.  It's a colony of individual units or zooids embedded in a common gelatinous tunic.  You're seeing the entire animal in these pictures.  It's a rigid cylinder, closed at one end (the rounded end) and open at the other (the "squared" end).

This diagram may help.  It shows the entire colony, how the zooids are positioned along the periphery, and then there's a close-up of an individual zooid:

 

Modified from Invertebrate Zoology, Sixth Edition (Ruppert and Barnes, 1994)

Note that each zooid has two siphons.  The buccal siphon faces out and pulls water in.  The atrial siphon leads to the central cavity.  Once water flows into the large central cavity, it exits via the single large opening at the back end and slowly propels the colony forward.

The gill basket is used for both respiration and filter-feeding.  Below is a view of several zooids under the microscope.  You can see the buccal siphon at the top, the large gill basket, and the endostyle (on the right).  The endostyle produces mucus that lines the gill basket for feeding — and is thought to be the precursor for the vertebrate thyroid gland.  (The red spot is the brain.)

 

Here's another view that allows you to see how the zooids are embedded in the tunic, and it's really just their siphons that extend above the surface:

 

I don't know how often pyrosomes are seen in this area.  This is the first time I've seen them since moving here (10 years ago).  If you've observed them before, I'd love to hear about it.

Here's another interesting fact about pyrosomes.  Pyrosome comes from "pyro" for "fire" and "soma" for body = "fire body."  The name derives from their amazing bioluminescence.  

After we read about this, we took a quick look at them later in the evening.  Sometimes there would be scattered bright flashes, but when Eric picked one up it was so bright and sustained that it looked like a glow stick!

I'm so thankful that Alex noticed these mystery animals on the beach and pursued his curiosity to ask us about their identity.  I had no idea when I woke up this morning that I'd be documenting a pelagic invertebrate that I'd never seen before!