Viscoelastic egg sacs of a hooded nudibranch (melibe leonina)

A few weeks ago, I had the chance to visit the Hopkins Marine station. Some of the students had gone on a dive and brought back specimens from a coral reef. Snooping around their tank, I observed them trying to record the swimming dynamics of this beautiful creature. Upon asking, I learnt that its called a nudibranch, which are soft-bodied marine organisms.

I started to look closely in the tank, and observed egg-sacs of these nudibranchs floating around. Naturally, I sucked it up (literally), and wanted to foldscope them. That’s when I had my first surprise.

If you have a rheology background (like I do), the first thing you’ll notice in the previous video is that the egg sac is extremely viscoelastic. It’s slimy, squishy and quite sticky as seen in the next video.

I had a quite a tricky time to mount the egg sac between my glass slide – as soon as I’d apply pressure, it would slide away – the experience was almost like trying to pinch water-melon seeds!!

Well, here comes the cool part. I naively was expecting to see a gelatin matrix (in the foldscope) which would give me some insight to the viscoelasticity of the material. But to my surprise, here’s what I saw.

First, the matrix is transparent and not visible under the foldscope, which in retrospect makes sense (since the gelatinous proteins would be extremely fine, think egg white). Second, I was pretty amazed by the distribution of eggs in the egg sacs – some had 5, 6, 2, 4 – a seemingly heterogenous distribution, with odd and even numbers.

Here’s a collection of some of the images. I just wish I had brought them back at set a time lapse to watch them hatch. Perhaps, next time!

Egg Sacs of a Nudibranch (Melibe leonina)





9 Comments Add yours

  1. Manu Prakash says:

    @Saad: what a fantastic explanation; and story. I am scratching my head to figure out; why would the egg sac be viscoelastic (and I agree; it is). Many animals release egg sacs in masses; with the hope that they will found a cloud of sperm. I am wondering what’s the advantage of being embedded in this jelly/matrix.

    One idea I am thinking about is how this mesh supports the egg mass in flows (specially turbulent or high flowing waters). Such a flexible mesh would avoid the jelly/egg sac to be broken into shreds. To also share that the idea of an egg sac is quiet universal in marine animals; here is a video that was floating on you tube recently –

    You might want to add the link to your post in the end. Watch till the middle section to see what you are dealing with is not that different from what was seen in the ocean in Turkey.

    Now; another material I would like to highlight is the clear jelly matrix surrounding the eggs. This is the protection that forms immediately after the egg has been fertilized with one sperm; to make sure none of the other sperms can enter. Thus the clear round shaped jelly is protecting all the eggs that are fertilized (a tell tale sign of fertilization).

    Finally; I find the embryos quiet remarkable. Since they are all early stage embryos – they can’t really have odd number of cells (at least till 16cells – and not counting single cell stage). So if you can find a 5 cell stage embryo; something is not right with that little guy. Anyway, a fun math puzzle.

    Incredible post – just fantastic.

    Keep exploring

  2. Saad Bhamla says:

    @manu – Nice video. I don’t know if you intended the pun, but yes – it has been ‘floating’ on youtube : )

  3. Matt.Rossi says:

    @Saad, this is the kind of post that makes me infinitely jealous of people living in places where there are oceanographic institutes and students who can go on dives in coral reefs. We have some interesting plankton swimming around at times, but the north Atlantic becomes an increasingly difficult place to hunt in the winter.

  4. Matt.Rossi says:

    @saad what’s the putty you’re using with the slip cover?

  5. Paul Joseph says:

    @matt, I think saad put the putty so that the eggs don’t get squished.

  6. laksiyer says:

    @Matt ditto your comments here. Envious. @Saad, Being molluscs, I wonder if the slime they put around the egg sac is the same or different as the one they normally secrete. Mucopolysaccharides have fascinating chemical properties. We make them too in our body too in large amounts. Wonder what will happen if you make little mucopolysaccharide spheres and suspend them in water, with that instrument that you guys use (there was a foldscope post on it).. wonder if the spheres will aggregate, or stay like a colloid? They should be quite charged and might have interesting properties or perhaps not.

  7. Saad Bhamla says:

    @matt – i think was playing dough (as you can guess from the bright color : ) and as @paul pointed out – I didnt want them to get super-squished..

    @laks – I’ll check with @manu for the instrument you’re referring to – I regret not putting them under my high-mag lens now : (

  8. Tom Hata says:

    These nudibranchs live in the kelp canopy, and they actually are hermaphrodites that simultaneously fertilize each other when they meet. I imagine they then lay the fertilized eggs on the kelp blade they’re on, so that their young aren’t left adrift at sea (likely why they need a viscoelastic matrix to adhere to the substrate). I had the pleasure of watching these specific Melibe for a week. Saad, with your permission, I can edit your post with additional videos of them foraging and swimming. You also forgot to mention the best part, they smell like melons when you take them out of the water!

  9. Saad Bhamla says:

    @tom – What?? I never got a chance to smell them : (
    Please go ahead and add the videos. Believe it or not, but I waited for so long to post this (month now) so that you would post yours and I would follow it with my videos and images 😛

    Also – I knew I was goofing when I wrote the first sentence. It was kelp not corals.. thanks for pointing it out.

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