BARNACLES

Barnacles are a species of Crustacean I like a lot :3
I can't quite say what I find so interesting about them. Probably just how strange a sessile arthropod is going to be on principle. Watching their feeding reflex is also just kind of mesmerizing.
Arachnophobes beware the right scroll panel, but also maybe check it out if you're curious :3

UPDATE: I SAW A BARNACLE IRL ON VACATION (i live in Nebraska so.) AND IT WAS SO COOL

OKAY SO BASICALLY BARNACLES ARE REALLY COOL
They have shrimp-like larval forms that can freely swim and look for a place to anchor themselves. They judge every spot, but grow less picky as their energy depletes.
Once they find a spot they like, they use CEMENT GLANDS on their antennae to cement themselves upside-down to the spot.
As they grow, they will add more cement to their 'mound', as well as molt their actual organic bodies like all other crustaceans.
Those feathery structures that peek out of their shells? THOSE ARE THEIR LEGS!!
They basically strain the water looking for plankton/whatever the fuck is floating around.
If they sense a drop in light levels or otherwise sense a threat, they retract their cirri (feeding legs) and close their plates.

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