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.
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.)