Blue blood and duplicate legs

Hermit crabs are peaceable critters. They are content, mostly, to trundle about, picking at bits of algae or scraps of yesterday's lunch. They don't mind carrying several of their friends on their backs; they take turns trying on interesting shells; they tidy up their neighbour's shells for them.

But - there's always a but - they have no patience when it comes to special treats. The first one they find is the one they absolutely must have. Even if someone else found it first.

So, Hermie A (we'll call her Eve) finds a yummy shrimp pellet and grabs it. Hermit B (Bob) comes along, sees the pellet, and tries to take it away. Eve retreats into her shell, taking the pellet with her. Bob grabs that shell and pokes away with his smaller pincer, trying to get the food. Eve shrinks back even farther, holds on tighter. Bob pokes and yanks. Usually, he gives up after a while, and Eve comes out still holding her dinner.

Sometimes things get out of hand, and Eve loses a leg. Better that than losing her food, she says. Legs grow back; treats don't.

An Eve was standing by the wall this evening, showing off her brand new leg.

The new leg is a rich blue.

As the leg grows, it will develop the nodules and hairs of a mature leg, but for now, the skin is smooth and transparent; the blue colour showing through is the hermit's blood. Our blood contains iron, which binds oxygen and carries it to our cells. It gives our hemoglobin a nice red colour. Crabs, including the hermits,* have hemocyanin instead; the oxygen-binding metal is copper, which turns the blood blue.

*So do some spiders, such as the tarantula.