macrocritters

A freshwater planarian flatworm (Polycelis coronata) crawling across the surface of a submerged leaf

I’ve got planaria!

No, that doesn’t mean I need a shot of penicillin—planaria are freshwater flatworms in the class Turbellaria, and I’ve got a jar full of them. If you have ever taken high school biology, you will know what kind of critters I am talking about…and if not, well you don’t know what you have been missing! Of course most people, even most zoologists, aren’t as enamoured with planaria as I am. I blame my sister Patricia. Allow me to explain…

The first book I was ever interested in was a classic textbook about invertebrate biology entitled Animals Without Backbones by Ralph Buchsbaum. I discovered it when I was three years old when my sister was in college and left a copy of the book lying around the house. I couldn’t read the text of course, but I fell in love with the photos and drawings. I found the entire book fascinating, but it was the two chapters that discussed planaria that really caught my attention. First of all, the planaria pictured in Buchsbaum’s book are the familiar Dugesia that are supplied to the typical biology class. They are incredibly cute with their arrow-shaped heads and crossed eyes (eyespots actually). And I thought it was neat that they didn’t have a mouth and instead fed through a tube (the proboscis) that stuck out of the middle of their body. But what really caught my attention were the pictures showing that if you cut a planarian in half, the head end will grow a new tail and the tail end will grow a new head! How cool is that? Even better, if you cut the head of a planarian in half, it will grow two new heads! Seriously, what three year old boy could fail to be impressed by that?

I have had a soft spot for planaria ever since, and I have no doubt that my early interest in Buchsbaum’s book was a major reason for my lifelong fascination with invertebrates and why I decided to become a zoologist when I was only eight years old. Actually, it was a toss-up whether I became a zoologist or a mad scientist…my two great loves have always been animals and monsters.

Over the years I have kept and cultured planaria many times, but they were always sourced from biological supply companies. Up until this past week, I had never actually collected planaria myself! Looking back, I find that to be a surprising revelation given how much time I have spent playing in ponds and streams with a net and jar. But for whatever reason I just never encountered planaria.

This summer—partially due to the need to write and take pictures for this blog—I decided that it was time to go out and bag a wild planarian! Many years ago, when I still lived in Victoria, BC, I was told that the ditches around agricultural land were good places to find planaria. So for the past couple of months I have been looking for these little critters in the murky, slow moving agricultural ditches and sloughs in Delta (part of Greater Vancouver). I have poked around ditches with my net and pulled-out and examined sticks and plants looking for the elusive beasts. I even tried luring planaria with raw beef liver: according to many invertebrate biology books you can wrap up a chunk of liver in cheesecloth, tie a string to it and throw it in the water to attract them. Supposedly when you pull it back out of the water there will be planaria on the outside of the cheesecloth trying to get at the yummy liver. Sadly, none of these attempts were successful at finding any planaria.

Last week I went for a hike to a local salmon stream to see what I could find. The environment in this stream is drastically different from the aforementioned ditches with fast running, clear, cold water over a stony bottom. I looked for planaria under some of the stones, but didn’t find any. I did however see lots of interesting freshwater amphipods, so I took a sweep of the bottom with my net, dropped the contents into a jar of water and left. When I got home I sat down to sort out the amphipods from the bits of water-logged wood and bark, leaves and other detritus. And that was when I saw it [queue dramatic music]: my first planarian! It’s a little embarrassing how exciting that moment was for me…

Needless to say I was back to the stream again the next day and I now have a couple of dozen Polycelis coronata in a jar in the fridge. The biggest is about 6mm long, but most of them are much smaller. These planaria look quite different from the Dugesia I mentioned above (and that the typical reader may be familiar with) mainly because instead of two prominent eyespots in the middle of the head, they have numerous (100 plus) tiny eyespots scattered along the leading edge of their head.

A close-up showing the head of a Polycelis coronata: the tiny black dots along the front edge of its head are eyespots (primitive eyes)

From a technical standpoint, they are not difficult to photograph. After all, they just slowly creep along on the bottom of whatever container you put them into. They are so flat that depth of field isn’t much of a problem. The real challenge in photographing planaria is to make the picture look interesting and/or attractive to anyone other than a bio-geek like myself! After all, they aren’t exactly the most photogenic little critters. One of the things I really wanted to do was to photograph planaria on natural backgrounds. The photos on this page are my first attempt…each picture was taken looking directly down into a shallow tray of water. What do you think? Did I succeed in making tiny, flat aquatic worms look at all interesting?

Polycelis coronata as it crawls upside-down under the water surface: its triply branched intestine and (white) proboscis in the middle of the body can be clearly seen

So now what? I expect you will see more pictures of planaria on this blog in the future. Maybe I’ll succeed in finding another species or two…

…and don’t be surprised if a two headed planarian shows-up someday soon [queue evil mad scientist laughter].

Cheers,
EC

The technical stuff:
Camera: Olympus E-620 digital SLR
Lens: Zuiko 35mm macro
Settings: manual exposure (F8-11 @ 1/125 sec)
Lighting: Olympus RF-11 ring flash (manual; 1/4 power)
ISO: 100