Evolution Trims the Useless

Parasites often lose traits their host can replace. Tapeworms lost digestive systems because they absorb nutrients directly through their skin. Some parasitic wasp lineages have reduced or lost wings when crawling through a host or nest matters more than flying.

Evolution is ruthless about maintenance costs. If a trait no longer earns its keep, mutations that shrink it can spread.

Sacculina takes simplification to its limit. It starts life as a barnacle, but once it invades a crab it abandons everything that made it a barnacle: no shell, no legs, no eyes. It dissolves into a web of root-like tendrils threaded through the crab's body, even tapping into its nervous system to steer the host's behavior. What remains is barely an animal at all, just a machine for feeding and breeding inside someone else.