Wednesday, April 17, 2013

Ten legs and a hard shell

kw: book reviews, nonfiction, natural history, crabs, arthropods

Meeting a crab this size would take the edge off my desire for crab cakes. What must it be like for this fascinating creature to live in the depths of the sea, without enemies (except humans, of course)? The Japanese Spider Crab, Macrocheira kaempferi, can get 12 feet (3.7m) from claw to claw and weigh 20 lbs (9 kg) or more. There is a crab species that can weigh twice as much, but is nowhere near as spectacular as the 9-footer (2.7m) "Big Daddy", seen here in Blackpool, England (image from You can see from the small size of the claws that Big Daddy has little to fear in his native habitat.

At the other end of the scale, little, semiparasitic Pea Crabs grow up to measure about 2cm (females) or 0.8cm (males). They live in the gill chambers of mussels and other shellfish, mainly in the northern hemisphere (Image of a species found in California, Fabia subquadrata, from the RaceRocks website).

With almost 6,800 known species, the crabs number about a tenth of crustacean species. As a child I was surprised to find out that the little pill bugs in the garden are more closely related to crabs than to millipedes. They are formally called woodlice, and are also crustaceans.

Not all crabs walk sideways, but the familiar ones do, so Walking Sideways: The Remarkable World of Crabs is an apt title for a new book by Judith S. Weis. The ten chapters present the full range of crab knowledge and lore, and 23 vignettes scattered throughout add associated subjects, such as lobsters, festivals related to crabs (mainly as food), their use in medicine, and as pets.

Presenting crabs and inducing us to sympathize with them is a harder sell than telling us what life is like for birds (see this review from last month). Crabs are just too strange. Compared to ours, their internal body plan is upside-down: The main nerve bundle, comparable to our spinal cord, runs down their front, while the digestive organs are in back. Our skeleton is internal, theirs is external, and needs to be replaced frequently. Hmm, I'd sure like to replace my skeleton, with some of its creaky joints! We have two jaws; they have two or three pairs of claw-like mouth parts, and can taste their food before it gets into the mouth. Their eyes are usually on stalks, and are more closely related to the eye of a dragonfly with its wonderful geodesic-dome shape, than to our liquid cameras. They have gills, that take up at least as much of their body cavity as our lungs do of ours. Their blood is blue, and they have arteries but no veins: the blood just leaks out of capillaries and sloshes back toward the heart.

For all that, they are intensely fascinating. Many years ago, when it was allowed to visit tide pools in California, I got the biggest kick out of finding some crabs and watching them. Starfish and snails are cool enough, but they move slowly. Crabs scuttle right along, and aren't afraid to challenge an intruding human child, ready to give a sharp nip if needed. I wasn't afraid of a nip from a crab up to perhaps an inch across the shell, but any bigger, and they could Hurt!

The author packs so much information into the book's 214 pages that it has to be read rather slowly. It is dense reading, and I hope it does not put people off. The author would clearly like to tell us everything about crabs, but had to limit herself. There are so many, which occupy so many different habitats, that a large encyclopedia would not suffice. And new things are being
learned constantly. At the current rate of a new species found every few months, they ought soon to surpass 6,800 known species. It is thought that we have found only a tenth of them. This purple crab from the Philippines (image from Mother Nature Network) is one of four new, purple crab species discovered just a year ago.

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