Saturday, October 30, 2010

Mother of thousands

Ma Crab is skinny again!

Two weeks ago, she was carrying a belly-full of eggs. (If you missed it, the video is "In Berry".) I've been watching her closely these days; every day her egg mass grew, until it was at least as large as her body. She looked awkward and uncomfortable, and didn't move around much. She didn't show any interest in feeding; too busy fanning and cleaning her brood of thousands of embyros.

This morning she had climbed into the top of the sea lettuce, and hung there, spread-eagled, with her apron open as wide as it would go, palpitating gently. She looked miserable. (I had kids; I could relate.)

This evening, I caught her as she cleaned her empty brood chamber. The babies were gone, hatched. And she's got her figure (and appetite) back.

Belly flap, up tight against the body. 

She picked at the inside of the flap for a few minutes, expanding and closing her pleopods, the feather-like appendages that the eggs were attached to. Then she closed up shop; the flap sealed itself close against her body, with a gasket of fine hairs. And off she went to find supper.

Empty belly, showing pleopods.

I've been trying to find the gestation period for these crabs. It's difficult. King crabs can carry the eggs for 11 months. For the green shore crab, I've found sites giving lengths from a few weeks to 8 months. In my aquarium, I saw the eggs barely two weeks before she released the zoea.

But she would have been carrying eggs before I saw them. They would be too small to see, at first. And until the embryos have developed eyes, they are transparent; the pinkish red of the mass came from baby crab eyes.

And the times vary; on the east coast of Canada and the northern US, the crabs have one reproductive cycle per year. Here on the Pacific Coast, as they do in Europe, they produce two broods, in spring and winter. (National Shellfisheries Association, Inc., 2008) The time and length of gestation seems to vary with the temperature of the water, which is warmer here.

And the babies, the zoea? They're swimming. They look sort of like amphipods, but they're far too small for me to see. In the wild, they would drift away to join the plankton off-shore, feeding on tiny particles of organic matter until they have undergone several molts. Most of them wouldn't survive, which is a good thing, since there are several thousands of them. I don't think any will make it in my small tank, surrounded as they are by anemones and barnacles. But I'll be watching for them, just in case.


  1. Aquariums are wonderful to me...I once had a couple of huge cichlids who paired up and had babies. They were amazing to watch, since they would carry their live babies in their mouths.

    Unfortunately, I had another fish (I can't remember what it was)in the same tank who loved eating baby cichlids. He did manage to eat them all...but the cichlids, who had tolerated him before, killed him after he ate their babies. I was boggled by the whole thing! It taught me a lot.

    I really hope some of the baby crabs survive. How exciting this was...I loved the video. Thanks so much, Susanna, for posting about this event!

  2. Having an animal successfully reproduce is one of the surest signs that you have provided the most natural conditions for that animal.

    How is filtration managed? You'll literally need to make sure you don't throw out the baby with the bathwater. Congrats!

  3. Congrats on your babies, I loved hearing (and watching) about how crabs give birth.

  4. Marion, That's fascinating; the cichlids punishing the baby-eater.

    Tim, I have a small pump with a sponge filter. I clean it every second day, and change 1/8 of the water. I usually wash the filter under the tap, but this time, I washed it in salt water first, then examined the water under a bright light. There were some tiny swimmers, visible only with the light against them; I collected them with an eyedropper and put them in a separate container. They're probably just copepods, but maybe there's a few zoea, too. We'll see.


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