Sacculina is a parasitic barnacle and a castrator of crabs, there are more than 100 species of Sacculina currently recognized. When outside of their host, Sacculina are free swimming organisms with sensory organs and the ability to move on their own. When a female Sacculina finds a crab it settles into a joint and waits for the crab to molt it’s shell. Once it does the lady delves into the soft body of the host crab. Sacculina grows in the crab spreading out in fibrous strands to up to 50% of the crabs body. It eventually makes it’s way to the crabs thorax where it’s egg sac would be located and builds an egg sac of it’s own.
This happens regardless of the crabs sex. If the crab is male Sacculina will interfere with it’s hormone balance sterilizing the crab, as well as changing it’s body to resemble a female (widening/flattening of the abdomen, release of female hormones, and even causing it to preform female mating dances.) The crab also loses it’s ability to molt, and to regrow lost claws.
At this point seeking male Sacculina implants itself into the egg sac and fertilizes the eggs. The crab then cares for the egg sac as if it were it’s own, though at this point the crab has been rendered infertile. When the eggs are ready to hatch, the crab hatches these eggs as if it were it’s own freeing a new horde of parasitic larvae into the water.
Sacculina represents a curious case in the field of parasitology, as it was used by scientists such as evolutionary biologist Ray Lankester to promote the idea of devolution or degradation. The fact that this organism, and many other parasites in general, would go from a free living state, to a much more primitive one that depended so heavily on a host was a sign of a loss of evolution.
This idea has been discredited and shows a real misunderstanding of the theory of evolution, as evolution does not necessarily lead to more complex organisms.
The concept of devolution was in part prompted by a surge in social darwinism in the late 1800’s by men such as reverend Oscar McCulloch and sociologist Herbet Spencer. Who used Sacculina as an example of how someone could become a parasite to society by shirking their responsibilities for a simpler life. McCulloch was quoted saying that Sacculina, “has a free, independent life common to it’s family, and is living as a parasite, or pauper.” He went on to suggest that this same degredation could happen in human beings.
Stories like this show why it’s hard not to, but dangerous to mix theories of social interaction with biological concepts. Whether or not people degrade socially is up to debate, but organisms do not ever devolve, they just adapt.
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