Ocean Watch

Both men emerged from the water with bites. Dad's bites were on his leg, and Nathan's on his arm, even though he was wearing a long-sleeved Lycra shirt. A year earlier, Nathan had similar bites doing the same thing at the same spot. He wonders if I know what bit them.

At first I thought these were more likely stings than bites. But with so many possibilities around here -- jellyfish, anemones, hydroids, corals, fire worms, fire sponge, stinging limu -- in so many stages of development, without a good clue, I couldn't venture a guess.

Nathan's father, however, did have a clue. He heard that a shellfish larva with a pointy shell is responsible for bites like his and Nathan's. These creatures, Dad was also told, are especially common in areas where fresh and salt water mix. Freshwater springs, it turns out, are abundant throughout the area the men fished.

Shellfish with pointy-shelled larvae? The image that came to mind was a zoea (embryology is loaded with jargon), a stage of crab development.

When crab eggs hatch, the youngsters don't look at all like crabs. They look like miniature monsters. A zoea body is roundish with huge compound eyes, a long tail and spikes. One spike sticks out the back of some species, and two even longer spikes poke out the front. These sharp-tipped spines discourage small fish from eating the developing crabs.

An average zoea is about 1 millimeter long, and during successive molts grows to about 3 millimeters long. (One millimeter is about the smallest we can see with the naked eye.) The next molt, called a megalops, looks more like a crab but still has a ways to go.

Nowhere in my reading could I find any suggestion that spiky zoeae irritate human skin. And even if they do get inside shirts and poke us, these little creatures bear no toxin. The injury, if any, would be minuscule.

I did, however, come across a paper by researchers in the Canary Islands, who discovered that baby octopuses (which have no larval stage, but hatch looking like tiny octopuses) seize and externally digest the soft parts of three species of crab zoeae, leaving only intact, empty shells. The scientists surmise the baby octopuses use their toxin to paralyze this prey.

Another study showed that octopus babies drift as plankton for several weeks before settling down on the bottom.

That means the mystery bites might be from wandering baby octopuses.

Or not. It's impossible to know. But it doesn't really matter, because there's little or no effective treatment for octopus bites or any other marine-related stings.

Once, after a day of snorkeling at Hanauma Bay, I peeled off my swimsuit to find several itchy, quarter-size bumps on my side. I called my physician-husband, co-author of our marine injury book, "All Stings Considered" "What should I do?"

"Take a picture," he said, "and then leave them alone."

It worked. The bumps went away all by themselves. Whatever caused Nathan and his dad's mystery bumps, I hope they did the same.