Flatworms. I have (had) four in my aquarium; two that came long ago as little ones, two that arrived recently, barely visible then, but now about 1/4 inch long. The older two stretch out to about two inches now.
Marine flatworm, as photographed a few hours ago.
These two patrol the aquarium, sliding around and around, under and over everything, looking for edibles. I know they eat mussels, and I've caught two working on snails. And they don't mind a taste of leftover fish. But that was it, I thought.
I came home this afternoon to find one dragging one of my medium-sized hermit crabs up to the water's edge. The flatworm was wrapped around the hermit's shell and body, leaving the sharp pincers and hard legs untouched. The antennae and legs dangled, unmoving; poor Hermit was already dead. Not much I could do about it.
I kept checking back. A few hours later, the remains of the hermit were on the bottom, being investigated by Nassa snails. The flatworm was sleeping it off in a corner.
The worm's mouth is in the centre bottom of his body. When he finds prey, he engulfs it and holds it tight against a hard surface. Then he extrudes his pharynx (throat) from the mouth and inserts it into the soft tissues. He injects digestive enzymes into the unlucky victim and sucks out the juices, along with a few torn pieces.
If you enlarge the photo above, you can see a few chunks of the hermit, still undigested. Half an hour later, they were gone:
The digestive system extends right to the edges of the worm. Mouth and intestines, and a meager brain; that's all he is.
Snails and mussels are one thing; my hermit crabs are another. Maybe my value system doesn't quite match the seashore codes, but hermits are cute! And they have eyes that look at me; they pay attention to the big critters beyond their glass walls.
No. The flatworms can't have my hermits.
I removed one worm from the tank. Did him a bit of damage in the process, but he'll recover. The hermit killer slid down into the sand and disappeared. But I'll be watching; next time he's out hunting, I'll catch him. The two flatworms can have a canning jar all to themselves. Maybe I'll give them a mussel or two ocassionally. No hermits.