Tuesday, May 08, 2012

On miles of sand, one little pink beastie

The supermoon, or more correctly, the perigee-syzygy* moon, when the moon is both full and at its closest approach to earth, causes greater than usual tides, both lower and higher. I checked our local tables, and sure enough, the tide dropped this weekend to well below the bottom line on the chart. And in the afternoons when we're free, to boot.

We went down to Boundary Bay Sunday afternoon. Far, far in the distance, halfway to Crescent Beach on the opposite shore, there was a narrow line of blue water. All the rest was sand, mud, and snails.

Looking from Centennial Beach area south to Point Roberts, and the Gulf islands beyond.

We took our time walking out. I was wading in all the tide pools we passed, looking at sculpins and hermit crabs. Halfway out to the edge, I was bent over trying to take a decent photo of a big skeleton shrimp, when a kid came along, slowed to see what I was doing (she couldn't see anything). She walked on, only a few steps, and stopped. Good thing; when I finally gave up on the amphipod, she called my attention to something pink on the sand. Otherwise, I would have headed off in the other direction, where Laurie was waiting for me.

Something pink.

I'd never seen one of these before, but my evenings of leafing through guides bore fruit; I knew what it was! A ghost shrimp! No, not a shrimp, I told the girl, a ghost shrimp. A Bay ghost shrimp, Neotrypaea californiensis, as I found later, checking the Encyclopedia.

I called and waved to Laurie, and he came over. The girl left.** We settled down to take photos. But what was happening?

He's backing out, away from that big pincer. We had caught him at the tail end of a molt.

Completely separated now, belly up and squirming rather awkwardly.

These animals live in burrows up to a couple of feet deep under the sand, their entrance and exit holes unnoticed among the millions of worm holes here. They rarely come out to the surface, where they are vulnerable; fish love to eat them. This guy must have come out to molt.

He flipped himself upright, and started inching backwards across the sand.

He's about 3 to 4 inches long; it's difficult to be exact, since he was curled up and kept stretching and folding. They may grow to about 4 1/2 inches long, and can live up to 16 years.

The long pincer on one side identifies him as a male. The females' pincers are more equal in length. At this stage, barely finished leaving the old skin behind, his pincer is thin, but he will pump liquid into it until it is bigger than the old one.

...in the males, the "master claw" can make up as much as 25% of the animal's mass – compared to only 10% in females – with the minor claw making up around 3% of the total body mass in both sexes. The enlarged claw is equally likely to be on the right side or the left side. (Wikipedia)

The big claw is not used for capturing prey; this critter feeds on detritus sifted out of the mud. Maybe it's just there to impress the females, or maybe he has to fight other males for her.

The ghost shrimp often shares his burrow with a variety of other animals; scaleworms, pea crabs, a burrowing clam, sometimes even small fish. And closer still, on his body he may have two species of commensal*** copepod and under the carapace possibly a parasitic isopod, Ione cornuta. We examined our photos carefully, looking for these.

And here they are! The little red circles with two "tails" are the copepod, Clausidium vancouverense. The "tails" are egg pouches.

If you look closely at the photos above, you will see many of these copepods, mostly on the legs and pincers. But there is a cluster of them also under the carapace, on the side. Kozloff says this is where they live, although a website calls them ectocommensals: "ecto-" meaning " outside, on the surface. Both are correct, it seems.

There's an excellent photo of one of the copepods on Flickr, showing the egg pouches stuffed with red eggs, here.

The parasitic isopod lives under the carapace, and would produce a distinct swelling on one side. There may be a small swelling where the copepods show through, but I'm not sure of this. Brine Queen has a good photo of a parasitised ghost with a discoloured swelling. On the Beachwatchers page, there's a male-female pair of the isopods themselves.

I picked up the abandoned pincer. And promptly dropped it; detached from its owner, it was still opening and closing, as if to defend itself. I collected it again, and brought it home. It was still moving about when I finally wrapped it in a tissue and put it in my pocket.

Sharp-pointed pincers. One tip is hooked; if it grabs you, you stay grabbed.

*Syzygy: line-up of moon, earth and sun, producing a full or a new moon. Pronounced SIZZ i gee. Here's a mouthful; PER i gee - SIZZ i gee.

**I wished I had realized what was happening sooner; I thought of calling the girl back to see what she'd turned up, but she was too far away.

***Commensal: living together without harming each other. (As far as we can tell.)


  1. what a great story. Thank you for the link to the WSU page.

    My beach knowledge is very small.

    I visited a tide pool this weekend, though the degree of "outness' at this location was not easily appreciated. I thought of you as I wastched the hermits.

  2. Wonderful series of photos and quite a find.

  3. I was going to write a comment that I didn't think it was actually a molt, since the claw was moving after it had fallen off (there shouldn't be any nerves or tissue in the old claw), but on inspecting the last few photos and counting the legs, I do see the new claw (and counted the legs to ensure I was looking at the claw). I then wonder what keeps the claw moving then...perhaps there is still tissue and nerves within?

    Also, a reason for a single larger claw might have to do with the confined burrows...it's easier to take one shovel into the sewers than two. Also, from a defensive point of view, it's easier to block off the burrow with a single claw, and fit the smaller one behind it.

  4. Tim,
    It looks as if there is still tissue inside the claw. I'll disect it later tonight, to see how much.

    I know crabs lose part of their digestive system and their gills when they molt. Possibly these do, too.

    I found this on Wikipedia; " All cuticular structures are shed at ecdysis, including the inner parts of the exoskeleton, which includes terminal linings of the alimentary tract and of the tracheae if they are present." That's under ecdysis, and generalized for all arthropods.

    Good idea about blocking off the burrow. I found one report speculating that maybe the male used his claw to protect the female while she was molting.

  5. What a good capture. Those were some really big tides. - Margy

  6. For a while there I thought that it was a dead crab, then I realized it was a molt. These are great pictures.

  7. This is so neat! I'm going to have to return to show this to my kids.


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