Ocean Watch

When I left home last spring to go sailing, I had an e-mail problem.

The boat's satellite phone can't cope with spam, and asking someone to sort my mail for that long was too much to ask. I had to let it go.

My considerate sister, however, knows I like hearing from readers, and for five whole months she monitored my mail, deleting the junk and saving the good stuff. Now, I am enjoying those letters thoroughly.

People sent me a wide range of questions. Some require research on my part and will take a whole column to answer.

Others, though, such as, "What do aama (Hawaii's black rock crabs) eat?" have short answers that I know without peeking in my books.

Aama, and all crabs, eat the same thing: anything they can find.

These scavengers are excellent recyclers, eating both plants and animals, dead or alive. We can thank crabs for cleaning our beaches and reefs of dead fish, invertebrates and seaweed.

Another crab questions comes from Tom of Honolulu: "Is there a crab or other critter that picks up fragments of shell and other debris and glues them on their back/shell for camouflage? I remember seeing this long ago but can't remember details, name, etc., or where they live."

Hawaii hosts two species of crabs that carry sponges on their backs as camouflage. These crabs have a special elevated rear pair of legs to hold the sponges in place.

It's hard to see sponge crabs when diving because the disguise works so well. I did spot a big one, 8-10 inches across, once because it was walking. The brown crab's sponge was bright yellow.

Another Hawaii creature that gathers debris on its top is the collector urchin. This is a round, black, short-spined sea urchin that grows to about 5 inches in diameter. I've seen seashells, fishing lures and even plastic silverware perched on the tops of collector urchins.

No one knows why collector urchins collect. Since the items are often white, it's not a camouflage technique.

In the laboratory, collector urchins cover their "heads" when a bright light is shined on them.

"Do turtles shed their shells?" a reader wonders after finding an empty shell near her backyard pond.

No. Neither land tortoises, freshwater turtles nor sea turtles can walk out of their shells. If you find an empty turtle shell, the animal died and something ate its insides.

A reader from Vancouver, B.C., wants to know the scientific name of shipworms.

"Some people employ these worms for the design they make in the wood, making furniture out of what they leave behind, believe it or not."

I absolutely believe it. The shipworm-eaten driftwood I've found on beaches is beautiful with its convoluted tubes and tunnels.

The wood-borers are clamlike bivalves called Teredo navalis.

Like termites, teredo worms have bacteria in their guts that digest wood.

Thank you, dear sister and thoughtful readers, for providing me with such a fat file of letters to read and ponder. It's a fine welcome-home gift.