New Year's Eve ball drop?

  

Very young Red Sea Urchin (Mesocentrotus franciscanus), about 8 mm across, on 31 December 2024.  Best wishes for 2025!

Convergence

  

The underside of a Pacific Sand Dollar (Dendraster excentricus), showing the amazing food grooves leading to the mouth (close to the center of the sand dollar).

The grooves look like tree branches or tributaries:

 

 

You can also see the tiny white knobs that the sand dollar's spines would be attached to when alive:

 

 

One more, showing a close-up of the convergence of the food grooves at the mouth:

 

 

 Photographed in Crescent City, CA, on 27 October 2022.

 

P.S.  For a little more info about food grooves, see the post called "Petals and grooves" from 11 September 2012.

Tubercles and tube feet

 

Yesterday I took a few close-up photos of a Purple Sea Urchin test (or skeleton).

You can see the rounded and raised tubercles to which the urchin spines are attached:

 

Also noticeable are the podial pores, or the holes from which the tube feet emerge.  

Note that the pores are paired (two are placed side-by-side).  One tube foot emerges from each pair of pores:

 

To provide some context for these anatomical features of the skeleton, here are two images of live urchins (an adult and a juvenile), highlighting the urchin spines and the extensible tube feet:

 

 

A little leather

 

We don't encounter juvenile Leather Stars (Dermasterias imbricata) that often, but here's a nice example.  This sea star was only ~4 cm (~1.5 inches) across.  It was photographed in Del Norte County on 26 May 2017.

 

Reds among the purples

Red Sea Urchins (Mesocentrotus franciscanus) among Purple Sea Urchins (Strongylocentrotus purpuratus) in tidepools along the Mendocino Coast.  The abundance of red urchins this year is notable.

Photographed in Mendocino County on 10 June 2016.

 

 

Three "seas"

We were doing field work on the Mendocino coast today (10 June 2016).  In between surveys, we documented a few nice invertebrates:

 

It might be tricky, but can you find three different species of echinoderms in the photo above?

There's a Purple Sea Urchin (Strongylocentrotus purpuratus).  Next to the urchin is a Six-armed Sea Star (Leptasterias sp.).  And just below the sea star, looking a bit like a brownish blob, is a sea cucumber — still to be identified...stay tuned!

Mystery uncovered

Okay, I know you've been waiting for the answer to last night's mystery photo, so here's another hint — a zoomed out version:

 

Perhaps you can get a sense that this is an animal with a circular shape?  In the picture above, it's mostly buried below the sand.  

Sometimes, we'd see a vaguely circular outline followed by a faint trail:

 

Below, I'll show the entire animal, no longer covered by sand.

So this is a warning — the next photo will give away the answer:

 

Did you guess sand dollar (Dendraster excentricus)?  This was probably difficult, because you couldn't see much of the animal itself.  The mystery photos only revealed the outer rim with short spines and slender tube feet.  Sand dollars live partially buried in the sand where they gather and consume organic material from the sediment (deposit feeding).  In some situations, they capture food particles from the water (suspension feeding).

We saw individuals of various sizes and colors:

 

The smallest were less than ~1 cm across, indicating they were probably quite young (less than a year old?):

 

Have you been seeing sand dollars, too?  If so, we'd love to hear about it.

P.S.  For some microscope views of a small sand dollar, see the post from 12 January 2013.  And for information about sand dollar skeletons, review the post from 11 September 2012.

 

Small change

 

Last week we noticed quite a few small sand dollars — or perhaps they should be called sand quarters or sand dimes? — washed up on Salmon Creek Beach.  

So this was intriguing.  Eric and I guessed that the sand dollars we saw ranged in size from smaller than a dime, to as large as a quarter, with the average size being about dime-sized.  The true range we measured was 10 to 23 mm in diameter, with an average size of about 17 mm.  

How well did they match the measurements of the coins?  I had never measured the coins before, but tonight I measured them — a quarter is 23 mm across and a dime is 17 mm across.  Amazing!  Our estimates were pretty darn good relative to the coins!  But we've both measured a lot of things for many years, and used a lot of coins, too, so I guess that shows you the value of practice and repetition (...or we just got lucky).  

I wrote about sand dollar skeletons a few years ago, so if you'd like to learn more about them, you can review the post called "Petals and grooves".

In a pinch

The predator approaches from below:

 

The prey reacts:

 

In this case, the predator is a Sunflower Star (Pycnopodia helianthoides) and the prey is a Purple Sea Urchin (Strongylocentrotus purpuratus), photographed on the Mendocino coast on 22 August 2013.  

In the photo above, note that the sea urchin has flattened its spines and extended its pincer-like pedicellariae.  The pedicellariae look like little white stars among the purple spines and tubefeet.  

Although small, the pedicellariae can be very effective deterrents against larger predators such as Sunflower Stars.  The pedicellariae pinch and they contain toxins!  [The toxins in Purple Sea Urchin pedicellariae are not harmful to people.]

The next image shows a better view of the urchin's pedicellariae. 

 

This urchin was in "full defense mode", which made it easy to photograph the exposed pedicellariae.  In these images you can clearly see the three-parted pedicellariae spread wide open, ready to fend off the approaching sea star.  In the still photo below, the pedicellariae look delicate and flower-like, but it's more appropriate to think of them as menacing and dangerous!