Monday, June 27, 2016

When purple is green

When they're babies, purple shore crabs come in all colours from creamy yellow to green to red or even bluish. Most adults are purple or green.

Green shore crabs are mostly in shades of green, although some are dark red or purple. Otherwise, they look like purple shore crabs and live under the same rocks. It's usually easy to tell them apart, though, because the purples have polka-dotted pincers. The greens have plain pincers.

But then, there are the colour morphs. Some purple shore crabs are all over green, and they don't wear polka dots.

Green purple shore crab with a cracked carapace.

(I don't know how she got hurt, or how old the injury is. One of my students, a 9-year-old boy, brought her to me as is. He says that's how he found her. She was quite lively, in spite of the damage.)

So how do I know she's a purple? Because green shore crabs have hairy legs; purples are "nude". Their Latin name is Hemigrapsus nudus.

Side view. Clean-shaven legs.

She only stayed on my hand for a few seconds, then she scrambled off and dropped into the rocks. But before she went into hiding, she stopped to warn me off:

"Go home, huge monster!"

... thereby showing me her green, non-polka-dotted pincers.

These crabs lose pincers and legs with no noticeable disruption of their lives. The cracked carapace won't heal, but at next molt, she'll have a nice, new one; she should be ok.

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