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Saturday, March 5, 2016

Purple pelagic predators

Remember the Purple Sea Snail (Janthina umbilicata) that I introduced in January (see post from 20 January 2016)?  I mentioned how this pelagic snail makes a bubble float and drifts at the surface of the ocean (see below):

Eventually we also observed some feeding behavior.  First, here's an orientation to their basic anatomy.  Remember that you're looking at the snail from above as it drifts upside down.  Prominent in this photo is the extensible proboscis that the snail uses to reach for nearby prey.  Note that the tip of the radula is also visible.  (The radula is like a rasping tongue.)  Many predatory marine snails use their radula teeth to tear the flesh of their prey:

You might be wondering what a Purple Sea Snail eats.  Various types of prey are mentioned in different accounts, e.g., By-the-wind Sailor (Velella velella), Blue Button (Porpita porpita), and Portuguese Man O' War (Physalia physalis).

When we encountered these Purple Sea Snails, there were many juvenile By-the-wind Sailors in the wrack line, and some of them came in with the snails.  So, of course we were curious would the snails eat the Velella? 

Here's a hint: 

Above, you can see the snail's proboscis and radula approaching a tiny juvenile Velella.  The radula of a Purple Sea Snail consists of two bundles of sharp, slender teeth that grasp the prey.

And then the snail inhaled the Velella!

Because some of the snails weren't alive, this was a rare opportunity to see their radula teeth up close.  Eric took this picture under high magnification (100x).  Note the slender teeth, with a slight hook at the tip.  And they're abundant!

If you're curious, Eric was exceptionally fortunate to film one of the snails probing with its proboscis and revealing a hint of the radula teeth.  In this video, watch for a few different things: an introductory image, the snail using its large black foot to maintain its bubble raft, and then the proboscis with the arching, glistening radula teeth.

It was amazing to see these purple pelagic predators in action!


Hollis said...

These photographs and video are amazing! I have a special fascination for radula of both carnivores and vegetarians and seeing one in action is a rare treat. Thanks so much for sharing!

Anonymous said...

Amazing! I await TNHofBB every day. Question: How does the snail get to its prey? By accident? Stealth? Speed?

Jackie Sones said...

Good questions! I'm not 100% sure, as I haven't observed the snails in the open ocean. They can't swim towards their prey, so I would guess they depend on drifting close enough. Visualizing that, you can see the benefit of having a long, flexible proboscis, as well as strong, grasping teeth. One article states that Janthina can detect (and respond to) prey within 10 cm.

Leth Benz said...

I was reading through my Hawaii inverts book and coincidentally came across Janthina janthina (pūpū poni) - I will be keeping my eye for these violently predaceous violet creatures when the Portuguese Man-of-War are out and about in Kaneohe Bay! My book says "Beachcombers on windward shores can find these snails and their prey washed up in numbers after a period of strong tradewinds!" Casey and I are well positioned for quite a treat! :D

Thanks, Jackie!

Anonymous said...

Absolutely stunning. Thanks so much for sharing these and all the information on them. Carol