Ashwyn Davis is a visitor guide at the Two Oceans Aquarium. As a part of his day-to-day work, Ashwyn teaches members of the public about the interesting, but tiny ocean life that can only be seen under the microscope in our Skretting Diversity Gallery. Manning this station, Ashwyn has seen some pretty unusual animals - and would like to share a few with you!

Today's microscopic marvel - Barnacles! Well, specifically, barnacle penises!

Barnacles are crustaceans - this means that they are in the same sub-phylum as many of the animals you know well - crabs, shrimp, lobsters, etc. But, barnacles have made one major lifestyle change - instead of moving around, they have evolved to be sessile, meaning that they remain anchored in one place.

Whether they anchor onto coastal rocks, the backs of whales, the carapaces of turtles or even the hulls of boats, barnacles rely completely on the movement of water around them to bring them their food, remove their waste and transport their eggs. To overcome the challenge of being sessile, barnacles have evolved a whole host of adaptations - for example, their feet have been modified into featherlike "cirri" which they use to filter food fragments out of the passing water (the barnacle on the left in the GIF above can be seen pulsing its cirri).

All barnacle species rely on modified feet called "cirri" to filter feed. Credit: pshab [CC BY-NC 2.0]

Being sessile creates another problem - finding a mate. Most sessile organisms, from trees to mussels, rely on some or other form of "broadcast" mating, where one or both sexes release their eggs or sperm into the water or air in huge amounts with the hope that by chance, they will encounter the opposite gamete and fertilisation can occur. It's the ultimate form of blind dating - a.k.a. "blind mating".

For barnacles, there was just one evolutionary problem - their common ancestor was an Arthropod (the group of animals that includes spiders, insects, crustaceans, etc.) and this group of animals already had sexual organs evolved for specialised forms of copulation, usually with a male passing a waterproof sperm package directly to a female in some way. For barnacles, it was easier to "upgrade" these organs, than to go back to a simpler option.

So, what did barnacles do? Well, as you can see from the image above - if you can't go to your mate, you need to grow a penis long enough to reach them! In fact, barnacles have the longest penis-to-body size ratio of any animal. But, as any barnacle will tell you, length isn't everything. Longer barnacle willies have a chance of being ripped off by strong currents, so barnacles in more turbulent waters, such as the intertidal zone, generally have shorter and thicker penises. Even though they can't reach as many mates, their risk of failure is lower. In other words, it's not about the size of the barnacle penis, it's about how they use it.

Most barnacles have also evolved another tactic for dealing with the problem of mate selection - they are hermaphrodites. They cannot fertilise themselves like many plants can, but most barnacles can choose to be either a male or female at any given time and can send out chemical signals to other barnacles in the area about their status. In general, the way barnacles "decide" which sex to be is based on their size, food supply and the local population density.

Recently a new trick was discovered in the barnacle's arsenal - a technique called "spermcasting". Unlike broadcast spawning, which is random, some barnacles are actually able to fire their sperm directly at a female from quite a distance, and she is able to catch it with her cirri.

Nature is full of surprises, and barnacles (and their amazing penises) are a terrific example of how evolution finds unique ways to solve problems when given time. Sometimes we miss these small details when we just look at the large, iconic animals - so remember to lean in for a closer look at the Two Oceans Aquarium!

Barnacles mating at the Two Oceans Aquarium, spotted under the microscope. Credit: Ashwyn Davis / Two Oceans Aquarium

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