Most deceptive of all – indeed probably holding the record for animals not looking remotely like the thing that zoologists know them to be – are the parasitic barnacles, such as Sacculina. Sacculina is not what it seems with a vengeance. Zoologists would never have realised that it is in fact a barnacle, but for its larva. The adult is a soft sac that clings to the underside of a crab and sends long, branching, plant-like roots inside to absorb nourishment from the crab’s tissues. The parasite not only doesn’t look like a barnacle, it doesn’t look like a crustacean of any kind. It has completely lost all trace of the armour plating, and all trace of the bodily segmentation that nearly all other arthropods have. It might as well be a parasitic plant or fungus. Yet, in terms of its evolutionary relationships, it is a crustacian, and not just a crustacean but specifically a barnacle. Barnacles are indeed not what they seem.
And it gets even better….
By the way, Sacculina‘s branching root system is not indiscriminate in its invasion of the crab’s tissues. It heads first for the crab’s reproductive organs, which has the effect of castrating the crab. Is this just an accidental by-product? Probably not. Castration not only sterilises the crab. Like a fat bullock, the castrated crab, instead of concentrating on becoming a lean, mean, reproducing machine, diverts resources towards getting larger: more food for the parasite.Richard Dawkins. The ancestor’s tale : a pilgrimage to the dawn of evolution (Boston: Houghton Mifflin, 2004), 434-35
In action (from Hans Hillewaert, via Wikimedia Commons):