In previous posts we’ve seen beautiful wasps, morbid butterflies and gorgeous caterpillars. In this one we are visiting a crustacean – a type of shellfish organism, not too different from shrimps, crabs and lobsters. Except that this one is an especially obscene parasite. Much like many other parasites, one of its life stages has adapted to using the body of its host for its own benefit. And in this case – the adult form of the sacculina barnacle (Sacculina carcini) in fact looks nothing like a barnacle…
The host of this crafty shellfish though is an unlucky crab. The larva of the barnacle swims happily in the waters of the South East Atlantic until it finds a suitable host and more precisely – a joint in its exoskeleton.
Crabs have external skeletons, which support their soft tissues instead of bones like in vertebrates. Their sceletons resemble a little bit the Batman or Iron Man suits – plates of armor with gaps where the flexibility is needed so the crab can actually move its limbs and parts of the body in space. These junction places are the joints in the exosceleton the sacculina is looking for. The little larva sheds it’s shell and injects its soft body through the join inside the cavity of the host crab and starts an incredible transformation.
Only that it’s not the parasite that is transforming – it’s the host. The little barnacle starts secreting chemicals which prevent the crab from molting (shedding its external skeleton when it becomes too small so it can replace it with a new bigger one – here’s a timelapse of one). Not only that, but this cheeky larva makes its way to the cavity of the crab’s body where it would normally develop its own eggs when ready to mate with another crab and reproduce.
If a female barnacle has found a male crab host, it starts also producing hormones which essentially emasculate the male host to the extent that it changes his anatomy and behavior to resemble a female crab. That way, the male crab does not look for mating opportunities and all resources (food and energy) are directed to the parasite producing its own baby larvae. The host is so confused that they start performing female mating dances and nurtures the parasites eggs like they are its own. Once the eggs of the parasitic barnacle are ready to hatch, they burst open and release a huge number of new sacculina larvae ready to infect the next crab which has the bad luck to be around.
There are tens of different species of sacculina barnacles and they are not all bad, despite seeming absolutely despicable to their host. In fact, they are used to control the population of some crabs which have infested the seas by naturally limiting their ability to mate. Such biological control mechanisms are very interesting and not uncommon, but can be very controversial and can spiral out of control before we realise what’s going on. It’s not unheard of to release one animal into the wild in the hope to control the population of another. Often enough though, it can happen that an unexpected interaction with a third (and 4th, 5th ect) species in this new environment takes place and knocks the whole ecosystem out of balance. For more info, check the cane toad, small Asian mongooseand Eastern mosquitofish cases – it’s a lesson in biodiversity definitely worth learning well!
In any case, in its own capacity the ability of a relatively simple shellfish to almost entirely change the gender of its host is unquestionably impressive. It also sounds like something out of an X Files episode and for these two reasons I thought that the sacculina barnacles deserved their own spot in the Amazing Parasites Hall of Fame.