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Saturday, March 30, 2013

A delicate touch

Today I was watching a pair of ravens foraging on Salmon Creek Beach.  They were delicately picking along the drift line left behind by the receding waves.

The ravens were very focused and were spending a lot of time searching for and apparently finding quite a few small food items.  

At first I thought they might be eating beachhoppers.  But their method of feeding didn't match the abundance or behavior of beachhoppers.  They seemed to be picking up prey quite regularly and in a narrow search area.  If they had been feeding on beachhoppers, I would have expected them to find fewer and to be more sporadic and random in how they caught them.

Also, the ravens would lean down and pick up the food from the surface of the sand with the very tip of the bill (see next images). 


Unfortunately, even with binoculars, I couldn't see what they were finding.  But eventually I had been watching long enough that I was really wondering what could be so interesting to the ravens.  So I walked up to a similar tide height, knelt down, and started looking at the sand.  Here's what I saw:

Can you see the small pinkish shrimp-like animals?  It's the best I could do in the field, as most of these animals were very small (less than 1 cm long).  Below is another example.  Note also the very large dark eyes.

I didn't have much time, but decided to bring a few back to look at them under the microscope.  This is the animal that the ravens were spending so much time looking for and eating on the beach:

I'm not an expert, but I'm pretty sure this is a type of euphausiid, or krill.  The ravens were feeding on krill!  [I know there are other people out there who have a lot more experience with these crustaceans, so if you can identify the species (or genus), or tell us anything more about them, please do.]

Here's one more picture of a different individual:

And I can't resist showing a close-up of the compound eye.  Although I've known about krill for a long time, I've never looked at their eyes under a microscope until today.  Beautiful!

I'm so thankful to the ravens for leading me to the krill on the beach.  It's made me realize that there's a lot I don't know about krill in this area, so I have some research to do.  And who knew that ravens ate krill!


Asian Mussel said...

I sent a quick email to Robin Ross, who authored the section in Light's Manual on Euphausids (pages 631-632), with your post Jackie! From the pictures, she suspects the krill you're finding is most likely Thysanoessa spinifera, a species commonly found close to the nearshore and can wash up on the beach. In Light's manual it says this species has a large, essentially round eye, but this can also mean oblong and bulbous.

Jackie Sones said...

Excellent! Nice to know which species of krill this might be. This is one of the great things about the blog -- it enables much greater progress on local species identifications thanks to group participation! Brian and Robin -- thanks so much for your help!

victor said...

I worked with euphausiids in the California current, this is not Thysanoessa spinifera, the eye is not elonged, it is composed (check the link for see the diferences) and it doesnt have spines neither.
It is most probably Nematoscelis difficilis

sometimes It can be confused with Thysanoessa gregaria in early stages
of Thysanoessa gregaria
Its nice to see the interest of the people in this lovely litle animal

Jackie Sones said...

Hi, Victor,

Thanks so much for the feedback! I had wondered about that extra little bulge on the eye. It's very helpful to see the illustrations of different eye types in krill, and your experience with these species is even more helpful. Thank you for sharing your expertise!