Saturday, June 20, 2009

The pod critters

(Another dishpan aquarium post.)

Clams are monopods. So are these, ...

... but I haven't seen any around here lately.

We and the birds are bipeds, the raccoon and the squirrels are tetrapods, the leafhopper in a jar on my desk is a hexapod. Octopuses and squids are cephalopods as well as octopods; that they have to hog the numbering word, and leave the equally eight-legged spiders orphaned is unfair.*

Next, we have the decapods, with ten legs. That covers the crabs, the shrimp and lobsters, and a bunch of other shrimpy things.
As their name implies, all decapods have ten legs; these are the last five of the eight pairs of thoracic appendages characteristic of crustaceans. The front three pairs function as mouthparts and are generally referred to as maxillipeds, the remainder being pereiopods. In many decapods, however, one pair of legs has enlarged pincers; the claws are called chelae, so those legs may be called chelipeds. Further appendages are found on the abdomen, with each segment capable of carrying a pair of biramous pleopods, the last of which form part of the tail fan (together with the telson) and are called uropods. Wikipedia
Simple and logical, isn't it?

The amphipods, (meaning they have feet on both - "amphi" - ends, as if they were unique in that respect) are decapods, with eight pairs of limbs, three of which are used for other purposes, eating and grasping, leaving five pairs for getting around on.

Orange-eyed amphipod from my dishpan.

Twelve legs? Who has twelve legs? That would work out to "dodecapod", but that's a geometric shape.**

After that, isopods are easy. "Iso" means "equal"; all fourteen feet are similar and used for the same functions; swimming, crawling, and grabbing. Some of them are good at the grabbing bit.

Idotea resecata, the eelgrass isopod.

Holding on tight.

This little guy was dead. He died holding onto an eelgrass root. I tried to shake him off, brush him off, wash him off; he held on as if he were part of the plant. I had to cut the root to get him out of the water.

Back to the monopods again? A clamshell and a snail both fit that description. But here are our fourteen-leggers; two small isopods trying to hide under the snail.

Gnorimosphaeroma oregonense, Oregon pillbugs, forgetting about contrast.

These look almost like the "normal" pillbugs that live under my flowerpots, and are about the same size. But they zip around in the water, swimming on their sides, their backs, their bellies indiscriminately. Then they hide in the seaweed or the eelgrass, not very effectively because of their colour. On the sandy bottom, they are almost impossible to see. When I touch them, they roll up in a little ball, just like some of their land cousins.

Shell of an Oregon pillbug, a bit dismantled, with eelgrass isopod and ruler (metric) for comparison.

And we'll leave the myriapods for another day.

* Update: Steve Daniels, blogging at Crete Nature, remarks that "... cephalopods are always putting their foot in their mouth."***

**Update 2: Barnacles have the twelve legs, so should qualify for the term "dodecapod".

*** Ditto for the barnacles. 



  1. I love your beachy blogs, always informative, and always inspiring.

  2. Thanks for the photos and rundown on all things pod not beginning with 'i'.

  3. Thanks, Huckleberry.

    Tim, they taught us, long ago in elementary school, never to begin anything with "i".

  4. wow, i wish you could bottle up the amphipods and ship them to me


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