And. . . we have mouths!

Finally! At long last I have evidence that my juvenile urchins have mouths and are feeding. A week ago I put a batch of seven teensy urchins onto a scuzzy glass slide and have been watching them daily ever since. And yesterday, just as I was beginning to worry that they’d never be able to eat, I saw that some of them had eaten little tracks through the scuzz on the slide.

Here’s an example:

Juvenile urchin (Strongylocentrotus purpuratus), age 73 days, 3 April 2015. © Allison J. Gong

Juvenile urchin (Strongylocentrotus purpuratus), age 73 days, 3 April 2015.
© Allison J. Gong

The little urchin still has a test diameter of about 0.5 mm, so it hasn’t really started growing yet. However, see the squiggly dark paths? Those are areas of the slide that have been eaten clean. The scuzz is algal in origin, giving the slide an overall brownish-green color, so the scuzz-free parts of the slide are clear–or dark, actually, given that I took this photograph against a black background–having been munched clean by the urchin’s teeth. And the other bit of evidence that I saw? Poop! Yes, there were fecal pellets on the slide, which proves that the little urchin has a complete functional gut.

And those small round golden objects you see on the slide? Those are big centric diatoms of the genus Coscinodiscus. They are the only local diatoms that I know of that are big enough to be seen with the naked eye.

Lastly, because I just can’t seem to stop myself, here’s a video of the little urchin:

I love the sculpturing of the spines. And do you see that three-pronged structure at about 9:00 on the urchin? That’s a pedicellaria. On adults of the genus Strongylocentrotus there are four types of jawed pedicellariae, three of which, in my experience, are easy to distinguish on a living specimen. But in this young an animal I can’t yet tell how many types of pedicellariae it has. I suppose that the formation of pedicellariae might be the next event for me to follow as these urchins continue to grow and develop.

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2 Responses to And. . . we have mouths!

  1. PamelaR says:

    Poop is good…really, really good. 🙂 What’s the next step? Or are they fully formed now and just need to grow?

  2. algong says:

    They’re pretty much fully formed at this point, but as I mentioned they may still need to develop their complete arsenal of pedicellariae. As they get bigger they will add spines, tube feet, and pedicellariae, but their underlying body plan has been completely established. And now we know that their guts are working!

    Poop tells us a lot!

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