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Saturday, March 17, 2018

Hello, hydrocoral!

I've always been drawn to our local hydrocoral, Stylantheca papillosa.    It appears as bright pink patches growing on rocks in the low intertidal zone:

Now that I have a waterproof camera, I thought it might be interesting to photograph Stylantheca under water.  During a very low tide in February, I found a patch that was submerged, plunged my camera under water, held the camera in front of the patch and took some pictures.  (Note that I couldn't really see what I was photographing at the time.) 

This was one of my first pictures.  (Each pore is ~1 mm across.)

Interesting!  And can you see those very slender thread-like things in front of the colony (especially in the lower right corner)?

I thought there might be something drifting in the water, so I moved the camera and tried again, this time a little closer:

With this view I could tell that those "threads" probably weren't random debris.  They looked like they could be tentacles associated with the colony.  At this point I realized how little I knew about this species.  Were those really tentacles?  They're so long!

I took more pictures, then the tide came in and I had to leave.  Here's another example of what I captured:

I was really puzzled about these tentacle-like structures and decided to do some reading about Stylantheca to learn what they were.  Well, it turned out this was not an easy question to answer!  I had trouble finding good descriptions of Stylantheca anatomy.  Eventually I found a couple of older papers from 1879 and 1938 that helped.

Hydrocorals (a type of hydrozoan) are colonial animals made up of different units with specialized functions, e.g., defense or feeding.  These long tentacle-like units are involved in protecting the colony and are called dactylozooids.  Note that their tips are slightly but noticeably swollen:

Since they have a defensive function, you would expect the dactylozooids to have a high concentration of stinging cells (like jellyfish or sea anemones).  To confirm this, we clipped one of the dactylozooids and looked at the tip under a high-powered microscope.

Check it out (below)!  The entire surface is packed with stinging cells (oval shapes).  At the lower right are three stinging cells dislodged from the surface.  When triggered, the cells rapidly fire tiny coiled harpoons called nematocysts, which appear as the long dark threads emanating from the battery of stinging cells.

Note that the dactylozooids are not always expanded.  Often they're retracted and look like short tentacles tucked inside the pores:

While learning about the dactylozooids, I read that the hydrocorals have feeding units (called gastrozooids) in the center of their pores.  So I went back and took more pictures, hoping I could capture an image of a gastrozooid:

And there they were!  In the center of the pores, can you see the little rounded polyp with tiny tentacles (nubs) around the perimeter?  The gastrozooids can withdraw to the bottom of the cavity, but they can also extend upward to the rim.  The gastrozooids have little mouths they'll open up to ingest food (either caught by their own tentacles, or possibly passed to them from the dactylozooids).

It took me a while to sort out all of this.  In the process, I decided to try to sketch what I was seeing and learning.  In case it's helpful, here's an example from my notebook to summarize:

For such a striking species, I was surprised how hard it was to find out more about it.  I hope this makes information about Stylantheca papillosa a little more accessible!

P.S.  I first wrote about Stylantheca in 2012 — see "The hydrocoral and the worm" on 22 May 2012.

P.P.S.  We have more stories to tell about Stylantheca and its associates, so stay tuned.  


Russell said...

Totally fascinating. Thanks for all the detail! I'm never going to look at pink rocks while freediving the same again.

Hollis said...

Thanks for this! So beautiful and so interesting and your difficulty finding information about these little guys shows how much there is yet to discover and learn. Looking forward to updates!

wendy in port townsend said...

I love how you looked closer and closer and kept finding more fascinating details.

Cassowary said...

another wonderful post! beautiful photos

Jeanne Jackson said...

Amazing photos and a fascinating subject. Thank you, Jackie! So glad you got an underwater camera.

Jackie Sones said...

Hi, everyone! Thanks so much for the feedback! Indeed, this was a fun post to put together. A very nice combination of a beautiful, fascinating animal along with some intriguing mysteries and new discoveries!

And, yes! This underwater camera with a microscope function is a gift! I'm amazed at what it can do. And in this case, I was thinking about how taking these types of pictures wasn't possible in the late 1800s/early 1900s. Most of those scientists were describing preserved specimens (which potentially led to a lack of descriptions of the extended dactylozooids?). It's insightful to photograph living animals in the field.

:) Jackie

John W. Wall said...

Very nice image results. What model of camera are you using? I probably would have mistaken that coral for a coralline algae and walked right past it.

Jackie Sones said...

Hi, John,

It's an Olympus Tough TG-4. When my father passed away a couple of years ago and left a little money behind, I decided an underwater camera would probably be something he'd support. He gave me my first camera when I started college (way back when)...which started my photographic journey.

And yes, this hydrocoral can definitely look like coralline algae at first. It's a different color -- generally a deeper, more intense pink. And the pores (with slightly raised edges) are noticeable -- so the surface of the hydrocoral looks perforated throughout, rather than smooth as in many species of crustose coralline algae.

The hydrocoral is also a lot less common than coralline algae. We tend to see it in wave-exposed sites (with good water flow)...and it seems to occur on vertical walls. You need a very good low tide to see it as it lives quite low in the intertidal zone. (Divers might see it in the subtidal zone.)

:) Jackie