I have at least three fair-sized polychaetes, probably sea nymphs, living in the sand at the bottom of the aquarium. I don't usually see them, unless I empty the tank and comb through their hiding places. In the last couple of weeks, I've done that twice; once in a periodic cleanup, and once because Big Green, the crab, made such a mess of the place. Each time, the nymphs have had to make themselves new burrows, and, as luck would have it, a couple ended up right beside the glass.
I caught the biggest sticking his nose out of the sand, stretching and waving around, as if sniffing for food. When I moved, and my shadow fell on him, he shrunk back into cover instantly. I polished off the glass and waited; after a bit, the cirri (long tentacle-like face decorations) poked through the sand, then the face followed. Slither, slither, left, right, straight ahead, over a rock, always leaving the bulk of his length securely inside the tunnel. When a hermit crab came near, he reacted even faster than he had before; a quick convulsion and he was gone.
Face of the sea nymph.
Worming his way across the bottom.
This was when I decided to break out the goodies. I have some tiny dried minnows I bought at a Chinese food store. (For myself, but I don't mind sharing.) I broke off a little head, crumbled it, and dropped it near the place where the worm had emerged. The crabs and hermits dashed over to collect, but after a bit, here came Mr. Worm, reaching, retreating, stretching out again. He found a piece of fish. I didn't see exactly how he did it; he was far too fast, but back he went into the hole, the fish trailing along in his wake.
He was greedy, and came back for more several times, enough for me to get these photos.
Friday, when I re-organized the tank again, three much smaller worms ended up near the glass. These are ones I had not seen before, except maybe as tiny red hairs. Now they are about as thick as a pencil lead. And they are out looking for food.
The biggest of the three
These look green, unless they come out into full light; then they are as pink, shot with blue, as the big ones. And the one face that I got a good look at is the same; long, hooked jaws in front, and those eight waving cirri.
I was wondering, earlier, what they eat. Kozloff says they are algae eaters, but other researchers call them predators. These ones, at least, are meat eaters, whether as scavengers or predators on live animals, I can't tell.
These aren't the only worms in the tank; doing the last tidying up, I removed a shell where one of the flatworms was hiding. After I refilled the aquarium and all was at peace, he was wandering around on the glass.
Flatworm and orange-striped anemone.
This is the underside, seen through the glass. The worm's head is to the left. Its mouth is towards the front of that long central tube. There is a simple eyespot on top that discerns changes in the light, but does not capture images. It travels on thousands of tiny cilia. And, fragile though it seems, it is a voracious predator.
When I brought these home, six weeks ago, they were about 6 or 7 mm., just over 1/4 inch, long. Now this one is as long as the first joint of my thumb, almost 3 cm, or over an inch. It must be getting enough to eat. I wonder what that is.
He slip-slid all the way around the tank, roaming from the top to the bottom, then back again. Eventually, he came across this big black-tailed barnacle. I watched as he left the glass and flowed over the rough shell. ( I wondered how that felt; didn't it scratch his tender flesh? Guess not.) He came to the lip, flowed over it, covering the mouth, then slid farther down between the plates and completely disappeared.
Very scratchy barnacle
Later on, I saw the flatworm on the glass again. It was heading down, towards the sand. When it got there, it flowed down between the sand grains and disappeared without leaving a trail or hole. A few seconds later, a green polychaete burst out of the sand an inch away, squirmed across a pile of shells and went to earth again.
Do these flatworms eat polychaetes? Do they eat barnacles?
Hermit Rex, out of harm's way, on eelgrass.
Some interesting flatworm sites: MuseumVictoria (Australia), Wikipedia. And see "penis fencing".