A profound change

  

For Eric’s class this summer, he raised the early life stages (larvae) of the Bat Star (Patiria miniata).  Eggs and sperm from spawning can be mixed in the laboratory to produce fertilized embryos.  Like many marine invertebrates, the fertilized embryos of sea stars develop into a swimming larval stage that looks very different from the adult form.  The photo above shows the larval stage (called a brachiolaria) of the Bat Star about 25 days after fertilization.  This stage uses cilia to swim through the water as it feeds on phytoplankton. 

By about 45 days after fertilization, the juvenile sea star is starting to form at the base of the larval body.  In the photo below, the golden-white dome-like section is the developing juvenile body (called the juvenile rudiment), which is already forming its skeleton (made of shiny calcareous plates called ossicles) and its tube feet (that it will use to crawl on the bottom).  

 

At this point, the larva is ready to make its way back to the rocky shore where it will attach and metamorphose into a juvenile sea star!  Metamorphosis requires profound changes, as the larval arms and larval body (transparent structures in image above) are completely reabsorbed into the juvenile body.

Now for the best part.  Eric was able to capture some microscope video of the bat stars over the course of their development!  In the video below you will see the larvae swimming about at ~25 days (where the pink structure is their stomach), and at ~45 days (with the developing juvenile body clearly visible).  In the final video clips from about 50 days, you will see the metamorphosed juveniles walking along the bottom using their first tube feet.  These new juveniles are very tiny (< 0.5 mm across)!

[If you can't see the video player in your e-mail, click on the title of the post above to watch the video on the NHBH website.]

You can read more about this amazing process of development in Bat Stars in some previous posts, "A new star(t)" on 12 August 2013 and "First steps" on 8 August 2017.

P.S.  Many thanks to Ellie and Christina for spawning the Bat Stars, and to Eric for the great video!

Home, home on the seaweed

  

Recently Eric noticed these small crustaceans clustered on a seaweed washed up on the beach.

Here's what it looked like when first discovered:

 

 

This is the seaweed Sea Sacs (Halosaccion glandiforme), which consists of water-filled sacs (with a small volume of gas) normally attached to intertidal rocks.  When they're pulled off the rocks by waves, they'll float at the surface of the ocean.  If you look closely in the photo above, you can see the small clusters of crustaceans.  Under the microscope, more details are visible:

 

 

 

 

These are barnacle cyprids, the final swimming stage, of Pelagic Gooseneck Barnacles (Lepas sp.).  These cyprids will attach to floating structures like driftwood or a feather or this seaweed and then undergo metamorphosis into juvenile barnacles.  The appendages they use for swimming in the cyprid stage will morph into feeding appendages (called cirri) in the juvenile/adult barnacle.  Here's a diagram with arrows pointing to the appendages as they change during metamorphosis from their function in swimming to feeding:

 

 

 

What's cool is that the cyprids are partially transparent, so you can see those dark, feathery appendages in the close-up photos:

 

 

 

Fun!

 

New legs and an old tail

  

 A tadpole with hind legs (above)...

 and a frog with a tail (below):

 

 

Sierran Treefrog (Pseudacris sierra) photographed 3 July 2021.

First steps

 

Juvenile Bat Star (Patiria miniata), only a few days after metamorphosis.

I first wrote about beautiful little sea stars like this a few years ago, so you can review photos of Bat Star larvae and newly-metamorphosed juveniles in the post called "A new star(t)" from 2 August 2013.

Eric raised some Bat Star larvae in the lab again for his summer class, but this time he also recorded a short video.  

It's fun to watch these tiny sea stars (~0.5 mm across) exploring and learning to walk with their new tube feet:

 

Bat Star Settlers Sanford from Jackie Sones on Vimeo.

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.

A new star(t)

Remember the post showing the Bat Star (Patiria miniata) larvae on 4 August 2013?  Well, those larvae continued to develop and they are now starting to metamorphose into tiny juvenile sea stars!  Here's how it happens:

The larvae develop additional arms.  At this stage, they're called brachiolaria larvae.  [In the image below, the orange structure on the right side is a juvenile rudiment.  Remarkably, it will become the juvenile sea star!]

 

Here's a better view of the the brachiolarian arms (highlighted by black arrows below).  Look for the two shorter arms reaching towards each other that are covered with adhesive papillae (tiny bumps).  The larva will use these arms to crawl around and hold on to the substrate.

 

It's hard to see in the photo above, but between the brachiolarian arms is a round adhesive disc (see below).  The brachiolarian arms now look like tiny hands on either side of the adhesive disc.

 

When it's ready to undergo metamorphosis, the larva attaches to the substrate with the adhesive disc.  It will then resorb the larval tissue and the juvenile rudiment will transform into a juvenile sea star.  

The next image shows a larva undergoing metamorphosis — the larva is attached to the bottom, most of the larval tissue is gone, the juvenile is forming but it's not fully developed yet.

 

Once the juvenile is fully developed, it will break off from the adhesive disc that's attached to the substrate and crawl away.  A newly metamorphosed juvenile Bat Star looks like this:

 

I know it's hard to tell how small this sea star is, so here's a picture with a millimeter ruler for scale (below).  Each mark represents one millimeter, so this sea star is only about 0.5 millimeters across.

 

For years we had heard about how sea stars attach to the substrate when undergoing metamorphosis from the larval to juvenile stage, but we hadn't ever seen it for ourselves.  It has been quite an experience to watch these wonderful swimming larvae develop arms with "little sticky hands" and an adhesive disc for attachment, and to see them transform into the tiniest of sea stars!

P.S.  If you're wondering, these sea stars are 47 days old (time since fertilization).