Sap-sucking sea slugs do exactly what they sound like they do: They go up to seaweed and suck the sap out of it, and they do it fastidiously. But it’s the way that they do it that makes them so unusual.
Each sap-sucking sea slug has a whole row of teeth, but it doesn’t use them all. One at a time, a tooth descends, pierces one single seaweed cell, and withdraws. The slug then sucks the contents of the cell up into itself through a little tube. This is how it eats.
Their fastidiousness is important, because most sea slugs confine their diet to one type of seaweed only, and that seaweed isn’t always available. When the slug can’t get its (metaphorical) hands on actual seaweed, it turns its (metaphorical) gaze on the components of seaweed.
By eating seaweed, it has ingested chloroplasts, the little engines in seaweed cells that perform photosynthesis. The slug spares these components, warlord style, so they can work for it. During lean times, the chloroplasts provide energy for the slug that ate their creator.
