My favorite unconventional animal — the crab hacker barnacle

30 Apr, 2026

I put off writing this post for the April 2026 Bear Blog Carnival, as I usually do, because there are too many cool animals in the world. So many that I had an incredibly hard time choosing.

The obvious answer, what with being the namesake of this blog, would be the wombat (and other furry brown rotund mammals). They are lesser known compared to cats and dogs, for sure. Still, they’re rather cute and fuzzy — so not quite unconventional.

Naturally, I decided on a parasite instead.

Not the poor crab, the yellow blob tucked underneath it.

That’s Sacculina carcini, the crab hacker barnacle. Part of it, at least. Where’s the other part, you ask?

The sac is only the externa, which is the reproductive part of the parasite that sticks out. The rest of the parasite, the interna, is made up of all those tendrils that have worked their way throughout the crab’s insides. Just thinking about it makes my skin crawl.

And yes, it is a barnacle, despite looking nothing like its hard-shelled, boat-dwelling relatives. However, their larval forms are similar.

A brief overview of their delightfully horrific life cycle:

1. A female S. carcini larva, this water flea-looking thing, swims around in the ocean looking for a victim.
2. She finds one, so she sheds her shell and injects herself through a joint into the poor thing’s body cavity.
3. She continues to grow throughout the host’s insides, her tendrils wrapping around:
   • the hepatopancreas (or digestive gland), for nutrient absorption;
   • the nervous system, for hormonal and behavioral control.
4. Eventually, she pushes out the sac, which contains eggs. The teeny tiny male larvae move into the sac and fertilize them, after which the eggs are released for this horror to begin anew.

The crab hacker barnacle essentially mind controls the crab by messing with its endocrine system, making it act as if it’s carrying its own eggs (which female crabs would, in the same spot as the externa). Except it’s now infertile — hence the term “parasitic castrator”¹ — and fully devoted to caring for its alien babies.

Yes, even the males! The parasite ends up feminizing them. Apart from the maternal behavior, their abdomens grow wider and they start doing female mating dances. Even after the parasite is removed experimentally, they regrow ovarian rather than testicular tissue.

It takes several weeks for the parasite’s eggs to develop. If the crab just lounged around being useless all this time, they’d both perish. That won’t do! So the parasite doesn’t mess with the crab’s lifespan. The crab keeps going about its business, feeding itself (and the parasite). Meanwhile, the parasite breeds for multiple rounds until its host eventually dies in a couple of years.

Basically, imagine getting infected by an alien that takes over every inch of your insides, gives you a sex change (if male), and makes you perpetually happily pregnant for the rest of your life. Once in a while, you’ll push out alien babies, and then get pregnant again. You are a very proud mother. Awwww.

There are many other weird and wonderful parasites. Some honorable mentions:

• Mycocepurus castrator, a species of parasitic ant that diverged from its host species just to ride on top of them and get fed.
• The entire category of parasitoid wasps, which are insanely diverse. It seems there is a parasitoid wasp for just about any insect.
  • Fun fact: apparently parasitoidism is the default state of Hymenoptera (the order with sawflies, wasps, bees, and ants). The non-parasitic species are the weird ones who secondarily lost this trait.
  • A bunch of them are also hyperparasites — parasites of parasites.
• The tongue-eating louse, which replaces a fish’s tongue.

Sources:

• Images from Wikimedia Commons.
• Information from Wikipedia and this article. I am not bothered to properly cite everything; this is my personal blog and not an assignment. Sorry!

#bearblog-carnival #blog