Viscoelastic egg sacs of a nudibranch using foldscope

Nudibranch egg sac

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!


Nudibranch egg sacs foldscope


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