Phronima: a plankton organism that came to Hollywood

Today, I want to introduce you to some small crustaceans, the Phronima. They belong to the order Amphipods and have a rather curious life; in addition, they are very famous, but we should not advance events.

The Phronima do not exceed four or five centimeters (a couple of inches), have long legs, very thick compound eyes, and are semi-transparent, with some red spots. So far, very common for a crustacean. What makes Phronima unique is its life strategy, as it swims across the deep ocean until it finds a salp, a doliolid, or any other gelatinous planktonic tunicate — free-living gelatinous, barrel-shaped and semi-transparent organisms-. Once it finds a suitable one, it moves in, like someone moving into a summer house.

Painting of Phronima sedentaria. Author Miquel Alcaraz

Equipped with terrible claws, the Phronima cuts the inside of its guests to leave an empty barrel structure. Although the final form bears little resemblance to the original host, it still keeps some cells alive. The Phronima then navigates the sea from the inside, feeding when it finds suitable prey. Its transparent shed serves as protection and to lay the nearly 600 eggs a female can produce. The eggs hatch in this sort of nursery and develop inside it until they reach pre-maturity when they leave the house and seek life in the oceans.

Very curious indeed. However, I’m sure you’re wondering why I said Phronima are famous. Well, it is said that the terrifying creature that killed almost the entire crew of the Nostromo (except Lieutenant Ellen Ripley and her cat, Jonesy) from Ridley Scott’s Alien (1979) was inspired in a Phronima. If you look closely, you will see the similarities.

Picture of Phronima from Solvin Zankl. Alien from the movie, 20th Century Fox.

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