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Tuesday, March 18, 2014

Below the water line

On 14 March 2014, we encountered about a dozen By-the-Wind Sailors (Velella velella) on Salmon Creek Beach. 

It's been a long time since large numbers of Velella have been observed on local beaches.  I only saw one in 2012, and I'm trying to remember if I saw any in 2013.  (Did you?)  Most of the Velella we noticed last week were quite small, less than 2 cm long (although one was ~3 cm long).  In the photo below, the Velella on the right is lying on its sideit's tiny, only ~3 mm long!

Because I hadn't seen Velella in a while and these weren't going to make it (they were shipwrecked on the beach), I brought a couple of animals back to the lab for documentation.

You may be familiar with the parts of Velella that are exposed above the water line the sail, the float, and the mantle (see diagram below).  But many people haven't seen the parts below the water line, where a lot of the action happens!

Modified from Siphonophores and Velellids by Kirkpatrick and Pugh (1984)

Here's a view of Velella as seen from below (under water).  It might seem confusing at first, but study the diagram above.  Velella is a colony made up of units called zooids with different functions. We're going to look for the gastrozooid, gonozooids, and dactylozooids.

The following closer view shows all three.  In the upper right, the dactylozooids are long, smooth, and tentacle-like.  In the lower left, the gastrozooid is a large, pleated cone.  And all of the other blue, rough-textured, spindle-shaped structures in between (many with white tips) are the gonozooids.

That picture still doesn't do these structures justice, but now I'm going to focus on the dactylozooids and the gonozooids.  In the next image the dactylozooids are on the left and the gonozooids are on the right.  All of those beautiful shiny patches are clusters of nematocysts that Velella uses for stinging and capturing prey.

The nematocyst patches on the dactylozooids may also be used for defense.

The gonozooids have both reproductive and feeding functions.  I'll talk more about feeding another night.  But I can't resist sharing a few more images related to reproduction in Velella because it's the first time I've seen it!

At the base of the gonozooids there are blastostyles that produce buds that grow into tiny medusae.  Here's the diagram again, with an added inset to help visualize the structure of the gonozooids: 

Modified from Siphonophores and Velellids by Kirkpatrick and Pugh (1984)
and Schuchert, P. 2010. The European athecate hydroids and their medusae
(Hydrozoa, Cnidaria): Capitata Part 2.  Revue Suisse de Zoologie 117: 337-555.

Well, a couple of the Velella I was looking at had medusae buds!  In the photo below, look for the grape-like cluster in the center the largest buds are loaded with golden, symbiotic algae.

And then we noticed that some of the medusae had just been released!

Velella medusae will eventually mature and produce eggs or sperm (depending on whether they're male or female).  The fertilized egg will develop into the floating hydroid colony you're more familiar with. 

It's fascinating that the medusae are pre-loaded with symbiotic algae.  Here's an even closer view of the algae under extreme magnification:

Part of the reason I bothered to show so many of these pictures is that images like thisof Velella's zooids and medusaeare so hard to find.  I hope they are useful!


Serena Caplins said...

Amazing!! This was really nice to see.

beachmama said...

Fabulous resource! Norma sent me here after I inquired about them so early on the beach this year. Thanks so much for your always informative posts.

Krista B said...

Wow! Thank you. I have seen many Velella velella stranded on Ocean Beach at times, and I am reading about them, as well as many other ocean invertebrates, in Drew Harvell's Sea of Glass right now (based in part on the glass models created 150 years ago by the Blaschkas). I wanted to understand and see more details about this species than she provides in her beautiful book.

Jackie Sones said...

Hi, Krista!

Thanks for writing! I've heard about Drew's book, but haven't seen it yet. Sounds like you're enjoying it! And it's wonderful that you were curious enough about Velella to follow up with some of your own research. I hope the pictures in this post have been useful!