From the "midnight zone"

Well, I have to interrupt "The Cedars Series" briefly.  Eric and I found some unusual hydromedusae washed up on Salmon Creek Beach on 26 April 2015.  While this species might be found offshore, it's rare for them to strand on beaches, so I thought you might want to know about it.

As we walked, we started noticing these bright yellow blobs on sand:

 

The blobs ranged in size from ~2-3 cm across, with most being ~3 cm (1.2 inches).

Each one had 4 long tentacles:

 

We counted at least 50 of these hydromedusae on the beach, and we knew we'd never seen them before.  Many of the jellyfish and hydromedusae that wash up on local beaches are clear or have hints of red, pink, or purple.  This amazing yellow color is unusual and was a clue that this was something different.

Although they were in rough shape after coming through the surf zone, we wanted to take documentary pictures.  I put a few of them in a small aquarium:

 

The upright tentacle position is typical for this group of hydrozoans (Subclass Narcomedusae).  Also note that the tentacles leave about mid-way up the umbrella (rather than from the margin).  Meet Aegina citrea!

A side view highlights the bright yellow stomach pouches along the periphery of the bell:

 

And a view from below shows the mouth opening in the center:

 

I liked this view because it closely matches a diagram prepared by Eschscholz who first described Aegina citrea in 1829:

 

From Mayer, A.G. 1910. Medusae of the World Volume 1: The Hydromedusae. 

Carnegie Institution: Washington, D.C.

So the crazy thing about finding Aegina citrea on a beach is that it's generally known as a bathypelagic species.  The bathypelagic zone is sometimes called the "midnight zone."  It's 1,000-4,000 meters below the ocean surface — that's 3,300-13,000 feet!

It has been windy lately, which can cause the upwelling of deeper water to the surface.  Is that what happened here?  Once again, I don't know the answer, but can only document the observation and consider possible causes.  I'd rather think about Aegina swimming through the depths offshore, but nevertheless it was exciting to learn about an intriguing deep-sea neighbor.

P.S.  To see what Aegina citrea looks like in better condition, check out these pictures from the Monterey Bay Aquarium.  (They call them "lemon jellies.")