An amazing transformation

Earlier in December I wrote about finding the first larvae of an unusual hydroid, Candelabrum fritchmanii, in California.  Well, we have continued to follow some of these larvae in the lab, and I have a few more fun photos to share.

First, here's a nice image showing a newly hatched actinula larva next to an empty egg case (the egg case is clear and partially torn).  I'm guessing the larva had emerged from this case very recently.  (If you look closely, you can see that the egg case is being held from below by a slender clasper tentacle.)

 

Next, two of my best pictures of a young Candelabrum fritchmanii larva.  These images are of the same individual, but they show how extensible the larva is — stretching outward and pulling back in.  This larva is ~1 mm long.

 

 

When the larvae are about a week old, the posterior region elongates and develops an attachment disc at the end.  Here are two different individuals, the first with the attachment disc at the bottom of the photo (a bit blurry), and the second with the disc at the top.

 

 

The next stage in the transformation is dramatic.  Once the larva attaches to the substrate (in this case the substrate is kelp), it basically collapses into a sphere.  During this process of metamorphosis, the long primary larval tentacles are resorbed, so all you see are short stubby tentacles:

 

And then it very quickly becomes a tiny polyp!  Two days after I took the picture above, here is what this polyp looked like:

 

It was amazing to see the process from hatching to a crawling actinula larva with long outstretched tentacles to an attached rounded sphere and then a tiny polyp, all within about a week and a half!

Prepare for landing

 

Tubularia sp., a beautiful hydroid found in the low intertidal zone on Bodega Head on 3 August 2012. 

The flower-like portion of the polyp is ~4 mm across.

This one caught my eye because it looked so full or swollen.  A closer look revealed clusters of small orange structures inside the outer ring of tentacles (see below).

 

These orange structures are called gonophores (or medusoids in some references) and they're involved in reproduction.

Here's a basic diagram of the Tubularia life cycle.  The adult polyp develops gonophores, which either produce sperm (males) or eggs (females).  The eggs are retained within the gonophores and develop into actinula larvae (with long tentacles).  The actinula emerges from the gonophore and then settles on the bottom.  It attaches and undergoes metamorphosis into a juvenile polyp.

Modified from Invertebrates by Brusca and Brusca (2003)

After reviewing the life history, I checked the hydroids again and, Voilà!  It was relatively easy to find developing larvae in the gonophores.  A couple of days later, I was a little surprised to peer down and see actinula larvae that had emerged from the gonophores (see below).

 

 

 

Now that you know what the actinula larvae look like, you'll be able to see them developing inside the gonophores.

Below is one that's very early.  The center gonophore has four long, clear tentacles pointing downward.  Above them, you can just make out short orange tentacles of the developing larva inside the gonophore.

 

The next photo shows a larva in a more advanced stage.  Now you can see very long tentacles arching downward inside the gonophore.  One rounded tentacle tip is visible at the bottom of one tentacle.  (Remember that hydroids are cnidarians, related to anemones and corals, so these tentacle tips are armed with stinging cells).

 

One last photo — an actinula larva that has recently emerged.  Somehow they look like little rocketships or lunar landers, so it seemed an appropriate post to follow Curiosity's successful landing on Mars last night.