Circumstantial evidence

"Give him enough rope," I said, dumping a flatworm suspected of mussel slaughter into a canning jar with a fresh mussel as bait.

The questions I was attempting to find answers to were;

1. What does this marine flatworm I picked up on the beach eat?
2. Did it and its friend eat all those 24 mussels in my aquarium?
3. Will it eat my barnacles and clams? What about the other worms, the polychaetes?

On the way to answering those questions, I discovered much more ...

There had been two flatworms, but I had only captured one. It sat in the jar for a couple of days, doing nothing, ignoring the mussel attached to the clamshell it had chosen as a resting spot. I used the time to search for more info about flatworms; maybe I was doing something wrong.

I found a "tree" to be helpful in understanding what I was looking at.

• The flatworms belong to the phylum Platyhelminthes, which could be simply translated from the Greek as "Flat Worms". A nice, simple descriptive term.
• The class is Turbellaria, referring to the turbulence created by the cilia that they use for locomotion. These flatworms are mostly free-living; the other classes are parasitic. Mine are free-living. They are usually, but not always carnivorous. They are all hermaphrodites, with both male and female reproductive organs. Some of the larger ones mate through "penis fencing". I'm wondering about mine; this I've got to see. There is a great video of a pair of fencers on the Shape of Life.
• The order is Polycladida. "Poly", as in "many"; "clad", meaning gut. Their intestine branches out through the whole body. The mouth is not on the head, but towards the center of the belly.

Underside of a large polyclad flatworm. The intestines extend almost to the edges. The mouth is in the center.

• The family is Leptoplanidae, which means "thin and flat". Nothing new here.
• The species, I think, is Phylloplana viridis. "Phyllo", a sheet, and "plana", flat. And "green". Which it is, sometimes. My "Marine Life" says it eats bivalve mollusks. There go my mussels!

Back to the live beasties. On the morning of the 28th, I found the second of the flatworms in the aquarium and captured it. While I was doing that, I discovered a hermit crab eating the remains of a good-sized mussel. Hmmm... Was it the hermit, the flatworm, or something else that killed the mussel? Looking at the worm, I noticed that there was an extra something in the center of its gut; see the photo above. Mussel meat?

I put the second flatworm (and now, I'll name the two, for convenience. #1, because it is almost leaf-like, is called "Lief". #2 is quite a bit bigger; it's "Long".) in the canning jar with Lief and the maybe-doomed mussel. And Lief woke up and started to roam, sometimes following Long, sometimes wandering about alone.

(These photos were all taken through the scratched, warped glass of the canning jar. I must get a container of perfectly clear, new glass.)

Long, between the glass and the clamshell. The food is breaking up now. This worm, at least at the moment, is much darker than Lief. Still, if you look closely, you can see the algae pattern on the clamshell, and the dark mussel, right through the worm flesh. The two eyespots are visible at the front end. They distinguish light, but not much more.

Lief. Note the tidy central gut area. Nothing in it but flatworm, and maybe some green algae. The white branching "leaf" is the pharynx; it unfolds through the mouth and wraps around the prey.

Long checked out the mussel, sliding over and around it, then went away. It couldn't be hungry, anyhow. After a while, reading that some flatworms are scavengers, I decided to test that. I broke a head off one of those dried minnows and dropped it in the middle of the jar. When I came back after a bit, here's what I found:

Lief, with fish eye. I checked. The fish head was now missing one eye.

By 1PM the next afternoon, the fish had been digested.

Ok. One thing is established; my flatworms are scavengers. Whether they are killers, I don't know. Yet.

At 10PM, I checked the jar again. Here's Lief:

Almost pure white, except for a pinkish tinge around the edges.

And Long:

Long, with more or less empty mouth area, but lots of colour in the rest of the intestine.

At 5 the next afternoon, I noticed that both flatworms were sluggish. And Lief was almost as dark as Long:

Lief, sleeping it off inside the clamshell.

I worried; maybe they were sick, maybe the water was not cool enough, or the light was too strong. I poured off some of the water, stirred it up a bit; maybe they missed the current of the aquarium. I looked at them closely under the light. And discovered the mussel. Open, dead, about 2/3 eaten. Oh. No wonder they're sluggish!

So that question is answered, more or less. I think they do eat mussels. Not that I've caught them in the act, but the circumstantial evidence does add up.

They're back in the aquarium with the other critters. Six mussels are still alive; I'll keep tabs on them. But it looks like I'll have to supply them with new prey occasionally, if I'm to keep them from extending operations to an attack on the clams.