Marine biology: Love’s tough for a sea slug

By Sarah Bruck

Sarah is studying a *master’s in applied aquatic biology at the University of Portsmouth.

(*A master’s is more specialised study after your degree)

When standing on land, the sea might look vast and lifeless. But dip below the surface, look very closely and you’ll find hundreds, if not thousands, of weird little creatures. One of them is the brilliantly coloured sea slug, known as a nudibranch. The word “nudibranch” comes from the Latin nudus, which means naked, and the Greek brankhia, which means gills.

Nudibranchs come in a variety of colours and patterns. You’d think they’d be dressing up to impress a potential partner. Not so much. Nudibranchs are essentially blind. Instead of eyes they have a set of antennae, or rhinophores, on their heads, which they use to smell and feel when they are out and about hunting for food and looking for love. As these slugs are extremely slow, extremely small and it’s a big ocean out there, nature has made it a little bit easier for them when it comes to finding a mate.

They are hermaphrodite, which means they are both male and female at the same time. This means that in theory they can mate with any slug of the same species that comes crawling along. With what looks like a little arm on their side, they will ‘hold hands’ and fertilise each other.

Nudibranchs mating






But for one nudibranch species called Roboastra it’s not quite so easy. Imagine a Roboastra is out and about, roaming the reef. All of a sudden it smells another Roboastra close by. The pair meet and reach out for each other. Now, if one is large and one is little, the larger slug will gently widen its mouth and like a slow but efficient vacuum cleaner, it will swallow up the smaller one.

With its belly a little fuller, the nudibranch continues on its way and eventually it meets a slug the same size. Again, the two size each other up but because they are more equally matched will then start to fight and literally try to eat each other. The course of true love never does run smoothly! The fighting can go on for up to half an hour, until they eventually realise they would make a perfect couple. Then they stick their arms out, hold hands and make a whole bunch of cute little nudibranch babies.

So why would they do this, you probably wonder? Why would they try to eat a potential mate before reproducing, just because it’s small? As it happens, the female organs in the smaller individuals are not yet fully developed and are therefore less productive. By mating with her or him, the larger nudibranch would be wasting sperm on eggs that might not produce as many nudi babies. Even if it is able to get its own super good eggs fertilised, it wouldn’t be able to take full advantage of the spreading of its genes.

The moral of this story is; if the one standing in front of you is not quite right, just have a snack and wait for real love to crawl along.

Find out more about watery science careers and about what marine biologists do and how to become one.

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