Lights, cilia, action!

 

If you liked learning about the atlantid heteropod last night, perhaps you'll enjoy this, too.  

I didn't mention that the heteropods have a larval stage that looks very different than the adult.  And although I've talked about veligers (the larval stage of most marine snails) before, e.g., see post from 11 September 2014, this veliger is noticeably different.

 

Check out those lobes!  Many veligers have a velum (a lobed, highly ciliated structure surrounding the head that is used for swimming and feeding), but it's unusual for the velum to have 6 lobes.  When we saw this veliger, we knew it was something different right away, just because of the spectacular multi-lobed velum.

Here's a nice diagram of an atlantid heteropod veliger:

 

Modified from Lalli, C.M. and R.W. Gilmer.  1989.  Pelagic Snails: The Biology of Holoplanktonic Gastropod Mollusks.  Stanford University Press, Stanford, California.

Note that the shell of the veliger is different than the shell of the adult.  It's much more rounded (see below):

 

But just like the adult, the larva can withdraw completely (velum and all) inside the shell: 

 

This veliger was discovered by Emily after a plankton tow off Bodega Head on 9 September 2014.  Eric has kept it alive in the lab since then and we've seen it developing and growing.

Thiriot-Quiévreux (1973) provided a helpful table with three stages of development in an atlantid heteropod.  As development proceeds, the shell, velum, eyes, and foot all become more complex (see below).

 

 Modified from Thiriot-Quiévreux, C. 1973.  Heteropoda.  Oceanogr. Mar. Biol. Ann. Rev.  11: 237-261.

When we first looked at it, the veliger was in Stage II — two of the lobes were much smaller than the others, the eye spots were relatively small and rounded, and the foot wasn't that noticeable.

However, within 9 days, we saw dramatic changes as the veliger transitioned to Stage III.  The smaller lobes grew substantially so that all 6 lobes on the velum are now of equal size.  The eye spots have become more visible and are now teardrop-shaped.  And the foot has grown as it develops into a swimming fin.

Below are two images so you can see the changes in the velum (the growth of the smaller lobes).  And then you'll get to watch a video with the veliger in action!

Velum with two smaller lobes (they look like the thumbs on a pair of mittens):

 

Velum with all 6 lobes of equal size:

 

And here's the highlight video — it shows the atlantid heteropod veliger actively swimming.  Watch for all of the things discussed above — the long cilia beating along the edges of the velum, the rounded shell, and the transition from having 2 smaller lobes to all 6 lobes of equal length.  By the way, this veliger is less than 0.5 mm long, so we're thankful that a microscope video camera can provide such nice views of this tiny whirling wonder!

atlantid_heteropod_veliger_sanford_2014 from Jackie Sones on Vimeo.

 

P.S.  We'd like to thank Jeff Goddard for helping us confirm this as a heteropod veliger.

Say hello to Atlanta!

 

Don't worry if you're not familiar with this animal yet.  They're microscopic, so not easily observed without special equipment.  But they're fascinating, so I'm sure you'll want to learn more about them soon! 

Thanks to Emily, we had our first chance to photograph and film an atlantid heteropod (Atlanta sp.) that was caught in a plankton tow off Bodega Head on 17 September 2014.

Heteropods are pelagic snails.  They spend their entire lives floating and swimming in the open ocean.  You can see several adaptations to a planktonic existence — a mostly transparent body and shell, a foot that's been modified into a swimming fin, large eyes, and a radula (basically a tongue) with sharp teeth for grasping and tearing prey.  Heteropods are active, visual predators.  [Facts from Roger Seapy's excellent introduction to heteropods on the Tree of Life web page.]

Here's the video.  [This is pretty special, as we can't seem to locate many heteropod videos on the Internet.]

atlantid_heteropod_sanford_2014 from Jackie Sones on Vimeo.

The diagram below illustrates the main body parts visible in these pictures and the video.

 

Modified from The Light and Smith Manual (2007)

Note the coiled shell with the unusual keel that extends from the edge of the outermost whorl.  The keel apparently helps stabilize the animal as it swims with back-and-forth undulations of the swimming fin.

The animal can withdraw completely into its shell.  When it does, it can close the aperture (opening) of the shell with its operculum, just like many intertidal snails that you're probably familiar with.

The fin sucker is used to secure prey, e.g., other pelagic gastropods.

The proboscis is extensible, like an elephant's trunk.  It's transparent, so you can see the radula inside — the radula looks a bit like a small zipper.

The eyes are large and dramatic.  Atlantid heteropods are known for interesting eye-scanning movements, which you saw in the video.  Look for the rounded lens (it looks silvery).  It's thought that the eyes can pick up reflected light off stationary objects.

Here's another picture to look for all of these structures:

 

This heteropod was swimming, so it was difficult to photograph.  Below, look for the keel at the lower edge of the shell.  The gill is also visible inside the largest shell whorl.

 

Although many atlantid heteropods are pictured as I've shown them in the first two images, their normal swimming position is with the swimming fin held up towards the surface.  So it's helpful to think about them like this:

 

P.S.  If you need an explanation of the video: There are a few different things going on.  It starts with an overview of the entire animal.  The eye-scanning behavior will be obvious.  A close-up reveals the radula moving inside the proboscis.  Then you'll see the gill and a close-up of the heart pumping.  (Amazing!)  After that, the heteropod extends its proboscis and its tentacles, then swims away.  The video ends with a few more images to help you appreciate this little-known animal. 

P.P.S.  Most atlantid heteropods are found in tropical and subtropical latitudes.  One species, Atlanta californiensis, is found along the West Coast up to British Columbia.  The Light and Smith Manual (2007) says that Atlanta californiensis is abundant is southern California, but we're not certain how common it is in northern California.  We're trying to get help identifying this individual to find out which species it is.