Marine biology: Zombies of the sea

By Hannah Amor

I am studying a *master’s in applied aquatic biology at the University of Portsmouth. My post below is about rhizocephala, the parasite that I will be doing my research project on. I really enjoy my course as I get to learn things that not many people know about and I also get to do lots of hands on practical work. I did my first degree in marine biology and had the chance to live in France for three months researching cuttlefish

(*A master’s is more specialised study after your degree)

At the beginning of its life you could be forgiven for mistaking the rhizocephalan parasite for an innocent barnacle larvae, however it is something far more sinister.

When newly hatched and swimming free in the water the rhizocephalan searches for a home, inside a crab host. If the crab is not occupied by another rhizocephalan, the parasite will turn female and attack. It attacks the crab at its weakest point; the gills or a joint on the leg and enters the body. Once inside it spreads through the crab like a slow rot, slowly taking control until the crab is like a zombie. It takes over the brain, stopping the crab from moulting (shedding its shell so it can grow), to steal all the energy for itself. When the parasite reaches the crab’s abdomen it bursts through and exposes its ugly eggs to the rest of the ocean.

The appearance of an egg mass underneath the crab attracts another parasite, this one turns male so it can fertilise the eggs and create thousands more of the horrible parasite. The rhizocephalan is clever, having control of the crab’s brain gives it control of its host. It tricks the crab (even if it is a male crab) into thinking the eggs are its own. The crab looks after the eggs as if they were its babies, tending to them until they are ready to hatch.

When they are ready, the lovingly cared for eggs are released into the sea; the next generation of rhizocephala are ready to infect more unsuspecting crabs. The crab host has used all of its energy nurturing the eggs it believes are its own, meaning it has no strength left. It is unlikely to survive once it is of no use to the parasite, so the crab dies thinking it has created a new generation of crabs and not knowing what it has really let loose in the ocean.

Find out more about what marine biologists do and how to become one.

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