I picked my way across the little patches of grass and stone, the few areas that aren’t submerged, till I reached the point where the water began to deepen. I squatted down on my heels, peered into the water and saw……
At least, not at first. The water looked still and quiet and empty. I can’t say this surprised me a whole lot; if I lived in a pond year-round, I certainly wouldn’t come out of wherever I was spending the winter until the water was a civilized temperature. Some bubbles floating on the surface cast some interesting star-shaped light patterns on the pond bottom, but that was about all I saw.
…a bright red water mite zipped across a little depression in the mud. It was the only one, and it didn’t stay out in the open long enough for me to study it or get a good photo. But it did tell me that things were actually awake in the icy water.
So I carefully studied the pond bottom with a bit more scrutiny than the casual scan I’d given it initially. The first thing I noticed were tiny little organisms moving about suspended in the water column (the short amount of it there was). I couldn’t make out a whole lot of detail on them. As I was considering these, a larger movement caught my eye. I also couldn’t make out enough to say what this was, but it was brown and seemed to have a shiny silver eye.
I tried taking some photos of the creatures in the pond, but it was hard to get a good clear shot while they were swimming around, over and under vegetation and detritus. So I ended up getting a bucket and tall yogurt container from the house and scooping out a few containers’ worth of water and pond muck. I couldn’t tell if I’d gotten the creatures of interest or not, but there was really only one way to find out.
The water was very cloudy for the first little while. Gradually as the evening progressed the silt settled out to the bottom of the bucket, but it was still difficult to see the bottom even by the time I went to bed. I could see the little creatures swimming about in the water column, but not the silver-eyed things. I wondered if maybe I hadn’t scooped any up.
When I got up this morning I was delighted to see that the silt had all cleared and I had in fact caught a number of the silver-eyes. The challenge was then how to get them somewhere where I could study them, since I had six inches of water sitting atop the muddy bottom. I ended up using a turkey baster, which was large enough that I could aim the end over the creature and suck it and a bit of water up and deposit it in a little white measuring cup that would allow me to see more details. I set the cup under a bright lamp and poised my camera, firmly attached to a tripod, directly above so I could get some macro shots.
The silver-eyed creatures resembled tiny shrimp, once I got them out of the muck and against a clean background. Less than 2mm wide and no more than a centimetre long for the largest, they had long antennae and many long legs, and curled their tails under their bodies. They scooted about quickly, on their sides just as often as on their “feet”.
This locomotive habit gives them one of their two common names, “sideswimmers”. Their other name is “scud”, which comes from the Norwegian “skudda”, meaning to push. They’re a type of crustacean, belonging to the same family as crabs, lobsters and shrimp, and in fact are sometimes called “freshwater shrimp”, even though they belong to a different group than the shrimp you eat.
They come in all sorts of colours: orange, brown, green, and even silver. These individuals are all likely of the same species, despite the colour differences. There are a few different species of sideswimmer around here, representing a range of different aquatic habitats. I suspect these to belong to the genus Hyalella, which are one of the most common groups. They are so common, in fact, that their conspicuous absence is sometimes used as an easy indicator of lake acidification below pH 6.5 (their tolerance limit). In some streams with ample cover and food it’s possible to record up to 10,000 of these little guys in just one square metre. They eat primarly detritus and help to tidy up pond bottoms. They’re also mostly nocturnal, hiding in the mud during the day, which explains why I didn’t see any while I had the bright light suspended over the water to warm it yesterday evening.
This is the other creature I saw in the pond. If the scuds are tiny, then it’s itsy-bitsy. Only 2mm long, it’s hard to distinguish much detail without a microscope, which I don’t have. The macro lens on my camera allowed me to get a bit closer, but you still can’t make out much detail.
This is a copepod, another type of crustacean. The name means “oar-foot”, and reflects their use of their long antennae as a means of propulsion. This one belongs to the suborder Cyclopoida, the “cyclops” part of the name referencing their “eyespot” (actually two close together when looked at under high magnification), which is used for detecting light. You can just see the small dark dot at the front of the head. The above individual is a female, identified by the two prominent egg-sacs on either side of the body. When she lays the eggs they drop to the bottom of the pool. Some may hatch right away, but depending on conditions, others may settle into the mud and wait. They can survive long periods of drought, and scientists have even discovered and successfully hatched 300-year-old eggs.
I believe this is a male of the same species (although it could be a different species altogether). Males don’t carry the egg sacs and so can look fairly different, particularly when you can’t get a good sense of the whole body shape without a microscope. I’m not sure why the dark upper body; it may be food that it’s ingested recently (the organisms are somewhat transparent and their inner contents can usually be seen fairly easily; for instance, the dark-coloured eggs the female is carrying can clearly be seen through the sacs).
Different species can be either predators of smaller plankton in the water column, or grazers of algae on vegetation or other surfaces. Copepods in general will undertake daily vertical migrations, usually coming to the surface for the night and returning to the bottom during the day.
As I was poking around with the turkey baster sucking up little creatures to put in the cup and examine, I spotted something else moving slowly, worm-like, along the top of the mud. I stuck it in the cup with all the rest to have a closer look.
I recognized it right away from my Invertebrate Zoology classes as a member of the group Platyhelminthes, and observed that it was a flatworm, but I couldn’t get much further than that without additional reference. There’s a number of different types of flatworms, many of which have triangular heads. This one didn’t, which helped narrow down the options. I think it may be a member of the genus Hymanella or Phagocata, but who knows, really; my Invert Zoo text is packed away somewhere at the moment, and the other guides I have on hand don’t give enough detail.
Flatworms are incredible contortionists. This one went from being stretched out, perhaps nearly a centimetre long but only a millimetre wide, to short and squat, nearly round and almost 3mm across. They don’t actually swim, but rather move on hair-like cilia on their underside. They’re typically predators or scavengers of microorganisms or other protein sources. They don’t have teeth, but instead use an extendable mouthpart that acts as a suction tube to suck fluids from their prey, or ingest small ones whole. One of the neat things about these guys, that you can actually see in the first picture, is their “crossed eyes”. Like with the copepods, these spots are actually simply photo-sensors, used for detecting light. Another cool factoid: they’re regenerative, and if you cut one into multiple pieces, nearly every piece will regenerate into a new complete flatworm.
That seemed to be about it for my haul, and after I’d taken a few pictures I returned everything to the bucket and then took the bucket back out to the pond where I’d gotten it. I’ll go back in a little bit, once the spring peepers start to peep, to see if the fairy shrimp are out yet, and perhaps to look for vertebrate life.