Doing my homework

Last Wednesday, I wrote about some of the animals we found on Crescent Beach at low tide. In that post, I identified two clams as geoducks, but I later had to retract that. (I amended the post.)

The clams, at least one of them, were gapers, aka horse clams, or horseneck clams. Here's how I know:

• First, gaper clams and geoducks have a one-way digestive tube. It has two openings, an intake siphon and an excurrent siphon, both of which travel through the neck. In geoducks, these siphons share an exit "mouth", which can look almost like paired nostrils. In gapers, the two separate; each siphon has its unique, round opening. In all our photos, but one, the "mouths" were of the gaper type.

• Second: a gaper clam has tentacles at the mouth of one of its siphons. Geoducks do not. Our clams did.
  

Click on this to see the golden tentacles at the mouth. Probably Fat Gaper, Tresus capax.

• Third: see that white blur between the tentacles? Here it is again, a bit clearer:
  
  
  
  Gaper Pea Crab, Pinnixa litoralis.
  
  
  It's a tiny white crab. These crabs live inside the mantle of the Fat Gaper. I see no mention of them with Geoducks.
  
  Here's another:
  
  
  
  Female Pea Crab, in Gaper Clam siphon.
  
  
  So I was wrong about the species of a big clam, and here I am giving the sex of a tiny white crab, based on a few blurry photos. That's because these crabs are easy; the female has a space between the pincer tips when they are closed. The male doesn't.
  
  Each clam, when the pea crabs are present, has two, a small male and a larger female. There may be others, but they will all be sexually immature.
  
  The crabs don't seem to harm the clam. They eat food scraps in the cavity.
  
  And here's a clear photo of pea crabs in gaper clams from Washington State University.

More info on these clams:

1. Horse clam. From Wikipedia.

   The horse clam (Tresus nuttallii and T. capax) are related to the Geoduck, though smaller, with shells up to eight inches long (20 cm), weight to 3–4 lb (1.4–1.8 kg). Two similar species of horse clams inhabit the Pacific coast intertidal zones: the pacific gaper, Tresus nuttallii, more abundant south to California; and the fat gaper, T. capax, more abundant north to Alaska.

2. Entertaining instructions for digging out geoducks, with photos, from Washington Department of Fish and Wildlife. (Aren't fish "wildlife"?)

   The clam's name, pronounced "gooey-duck" is of Native American origin and means "dig deep." It is variously spelled goeduck, goiduck, or gweduck.
   

   Also includes photos of siphons, exposed and retracted.
   
   
3. Flickr photo of geoduck siphon.
   
   
4. Photos of horse (gaper) clam siphons, underwater, and with pea crab. From Oregon Dept. of Fish and Wildlife.

By the way, remember those worm tubes sticking out of the sand? Here's one. It's about an inch tall.


And here's another, with the clam siphon and crab pincers for size comparison.

Worm tube and clam siphon

This tube has the worm's tail end sticking out. I'd never seen that before. It's a Bamboo worm, the same as the one we found on Crescent Beach last week.

A few feet away, something else left a tube:

Not a worm tube.

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