Tiny pawns

Well, this has been an interesting week for cnidarians.  Do you remember the golden planulae (larvae) of the hydroid Abietinaria from a few days ago?

 

Eric is teaching a summer class right now, and he kept a few planulae around to show his students.  But when he checked on them today, this is what he saw:

 

Tiny yellow discs attached to the bowl — the planulae had undergone metamorphosis!

Below is an even closer view.  I chose these three individuals because they're in different stages of development.  Can you guess which is youngest and which is oldest?

 

The youngest juvenile is on the far left, the oldest is in the middle (lowest), and the juvenile on the far right (highest) is at an intermediate stage.  You can tell by the number of divisions in the basal disc, and by the development (height) of the stalk growing up from the center of the disc.  

Remember that these juveniles are going to become tall, extensively branched colonies (reminiscent of a fern or a miniature tree).  These are the youngest hydroids we've ever seen.  They would be too small to see in the field, so we just got lucky that they metamorphosed in the lab.  In the next image you can see the central stalk extending upward.

 

Amazingly, when I looked around, I noticed a few juveniles that had the first developing zooids!  It did not appear that these zooids had formed tentacles yet, but two developing zooids are clearly visible attached to either side of the central stalk:

 

These little hydroids have grown very quickly.  It will be interesting to see how the branching pattern proceeds, and when the first feeding tentacles appear.

A fun side note: When I first saw the these juvenile hydroids as in the last photo, one of the first things that came to my mind was that they looked like tiny chess pieces.  And then later I read an article that described a stage similar to this as the "pawn" stage!  

P.S.  To review what an Abietinaria colony looks like when it's older, see the post from 27 June 2016.

Golden -- Part 2

Okay, remember the mystery photo from last night?

 

I received some great guesses about the identity of these small organisms — and I need to congratulate Marni for submitting a correct answer! 

These are indeed planula larvae, in this case from a hydroid called Abietinaria.

Hydroids are colonial cnidarians (related to jellyfish, sea anemones, and corals) with many connected polyps.  Colonies often have a branching structure, and because of this they are often mistaken for plants.

Most of the polyps in the colony are feeding polyps — they can extend a ring of stinging tentacles to capture small animals from the water.  Other polyps are reproductive and produce the gametes that will form the next generation.

In this case, we were fortunate to have collected a female colony.  Within small capsules (gonozooids), there are attached medusoids that produce eggs which are retained and, once fertilized, develop into planula larvae.  When they're ready to disperse, the planula larvae emerge from the gonozooids and swim to a new location to begin another colony.

Here's an expanded view of the gonozooids filled with developing planula larvae:

 

And two more views so you can really see the individual larvae:

 

 

Intriguingly, I might have also found a male colony.  I'm not sure about this, but it's possible the white, flat-topped structures in the photo below contain male medusoids that will release sperm.

 

This picture is also helpful because you can see some of the tiny feeding polyps arranged along the branches.  I've circled a few in the next image, but I'm sure you'll be able to spot others:

 

So there you have it!  Planula larvae are so tiny that you don't often get to see them, so it's fun to be able to share them with you!

P.S.  On 30 January 2014, I showed a fun sequence with a planula larva (from a different hydroid species) emerging from a capsule.  To review those pictures, click here.

Little narwhals

Remember the Giant Green Anemones (Anthopleura xanthogrammica) spawning in late June?  (You can review that post here.)  Well, there's more to the story.

I mentioned that you don't see anemones spawning in the field very often.  Eric's in the middle of teaching a summer class about marine invertebrates.  So when I noticed the anemones releasing eggs and sperm, I ran up to the lab and called him to see if he wanted to take advantage of the rare opportunity.

He appeared at the site a few minutes later with a flask and pipette.  He obtained a few eggs from a female anemone (look for all of the orange dots in the flask in the picture below).

 

 

Then he sucked up a tiny bit of sperm from a male anemone and released it into the flask (also note the nice upside-down reflection of the Bodega Head cliffs and waves at the bottom of the flask) :

 

A couple of days later there were healthy, swimming, Giant Green Anemone larvae!  This is what they looked like when 5 days old:

 

This is called a planula larva.  You can see that the edges of the larva are covered with short hair-like structures (cilia).  These are used to propel the larva through the water — they are very fast swimmers! 

Note the very long apical tuft.  This long, pointed structure is sometimes twisted (it's made of several strands held close together)...and the planula larva swims with the tuft pointed forward.  Perhaps it's my wild imagination (is that possible?), but they remind me of little narwhals as they swim along!

 

Arthur Siebert did a study on these larvae in 1974 at the Friday Harbor Laboratories.  He tried to get the larvae to settle on various substrates (and metamorphose into little anemones), but they never did.  And as far as we can tell, no one has ever been able to get them to settle in the lab, so no one knows how long they stay in this larval stage before becoming the anemones we know so well.

 

I hadn't seen Giant Green Anemone larvae before, so it was quite an experience to see how different they look in the larval stage.  Who knew that Giant Green Anemones look like little narwhals when they're young?