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Andrew Weisel @_SEV8
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Alright, folks. It's time. Time to get really dang weird.

Time for a thread about parasitic barnacles.

Quick reminder, barnacles are crustaceans related to crabs & lobsters. They're mostly sessile filter feeders and I talked a little about them here:
There are three genera of parasitic barnacle: one weird one, one slightly weirder one, and one *really* weird one.
The Weird One: Anelasma squalicola, the only species in its genus. It's a barnacle that eats sharks.


It most commonly infects lanternsharks, a small deep-sea breed in the Atlantic with bioluminescent markings on its side and stomach.
I should probably give y'all a trigger warning if you don't like gross animal stuff, cuz it's gonna get a little weird. Mute the thread if you have to. If you don't, Happy Halloween I guess.
So A. squalicola (left below) is a great swimmer in its larval stages, able to find sharks and infect them to complete its metamorphosis to adulthood. And when it gets there, it looks mostly like any other goose barnacle (right below). It has vestigial legs, mouth, and gut.
A. squalicola also lost its hard shell. The big change is in that root-like organ on the bottom, called a peduncle. Root-like isn't a superficial description; the barnacle diffuses nutrients from the shark directly into its own body using this organ.
You can see an autopsy here that shows the yellowy peduncle below the leathery body of the barnacle lodged in the shark's body. They also are almost found in pairs, even when they don't infect a shark at the same time. Which is weird, and we don't know why.
The other weird thing is that parasitic organisms often show drastic morphological changes very quickly over evolutionary time.

But A. squalicola still has a ton of vestigial traits despite genetic evolution showing they diverged from other goose barnacles 120 million years ago.
The Slightly Weirder One: Rhizolepas, a genus of goose barnacle with two species, although one is known only from a single specimen. They infect annelid worms, have vestigial legs, but have lost their mouth and butt. They also feed with a peduncle.
OK, hold onto your butts. It's time.

The *REALLY* Weird One: the clade Rhizocephala (meaning root-head). These parasites infect crabs and exhibit the most radial evolutionary shifts of the three groups. They aren't even identifiable as crustaceans beyond their larval stages.
So this looks like a female crab with an egg sac minding its own business, right?




The rest of the barnacle is a network of root-like fibers woven throughout the crab's body siphoning its nutrients.
Their initial larval stage is pretty similar to other barnacles and crustaceans. They have a single eyespot, some legs, and two antennae. They swim about doing their thing until they metamorphose into their second phase.
They form what is called a kentrogon, essentially a shell with two gripping antennae and a sac of undifferentiated cells inside. The males stay free-swimming this way, but the females find a crab host, poke a weak spot in their shell with a needle, and inject cells into the crab.
The shell dies off here, so the entire barnacle is just this small mass of cells in the crab's circulatory system. It starts to grow out like roots, infecting the entire crab's body besides the heart and gills. This prevents the crab from molting and regenerating.
The barnacle diffuses nutrients right into its body from the crab's bloodstream. But it also needs to reproduce. For this, it grows that external egg pouch that contains ovaries and a place for male larvae to deposit sperm.

But here's where things get chemically *weird*.
The barnacle is able to stimulate hormone production in the crab so that it will take care of the barnacle's external pouch like its own egg sac. It will also perform spawning procedures to help disperse the barnacle's larvae when they are ready to hatch.

But here's the thing...
Stimulating hormone product in female crabs is easy. But what if they infect male crabs? The barnacle stimulates the same genes, which broaden the male crab's shell, reduce its chelae, and instigate female breeding behaviors to make it look and act like any other crab mom.
While the barnacle itself isn't fatal to the crab, the inability to regenerate, molt, or sustainably eat eventually kill the host most of the time.

When all is said and done, the drawing below shows an approximation of what the barnacle's body looks like inside the host crab.
There is a fantastic study done a few years back that did CT scans of infected crabs in order to better isolate the barnacle tissues from the crab tissues. You can read and see everything here:…, but the upshot is that the below images are of a barnacle.
Scroll back up and look at the cutaway of the regular goose barnacle at the beginning of this thread. Now look at that CT scan. Somehow, they are both barnacles. This is the perfect example of the extremes to which evolution can push parasitic organisms.
Happy Halloween, because that's basically The Thing. It's Emrakul. It's every invasive parasite from your nightmares, infiltrating and controlling your body in mindless reproduction.

And it's. A globdamn. Barnacle.

Respect. /fin
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