March 13th, 2007
a crayfish primer
I don’t know how apparent this is, but I almost never have any idea what I’ll be writing about each time I sit down to work on my blog. This morning, I thought I’d dig up some aquatic insect photos and write about them, but instead, I came across some crayfish shots, so that’s today’s topic. They’re creatures about which I’m sadly lacking in knowledge, but I’ll share what little basics I know. I’m determined to learn more this summer, hopefully while photographing them while wandering around the countryside.
Okay, first, what is there to know? Crayfish are freshwater crustaceans that resemble lobsters. They live in streams, creeks, lakes and rivers. While growing up, I often encountered them while spending summers at our cottage on the Ottawa River. One of my fondest memories is of building small stone “corrals” in the shallow water, then catching a dozen or so crayfish to put inside and pretend that they were horses — and yes, I know, I was a weird little kid. At the time, I wanted a horse so badly that anything that remotely looked like one (anything with four or more legs) was a suitable substitute. After rounding up my herd of crayfish horses, I’d “open the gate” and turn them all out to pasture once more.
More recently, my playing with crayfish activities have been related mainly to stream surveys, or to time spent along creeks photographing aquatic creatures. As it happens, many of the local creeks are filled with crayfish — unfortunately, of a species that we would rather not find. Just as we’re seeing invasive plant species cropping up all over, we are also seeing a host of aquatic invertebrates appearing in watersheds around the country. Perhaps I’ll write about some of them later this season, but for now, I’ll just make a brief mention of one of the most frequently encountered species in our area, and that’s Orconectes rusticus, often referred to as the “Rusty Crayfish”. It’s an aggressive and prolific species, and rapidly annihilates the native species. So, how did this species manage to get around the country? As is the case with many invasive species, it received a helping human hand. Much of the spread is attributed to these crayfish being used as baitfish, and moved from one body of water to the other, escaping or being released from bait pails. It doesn’t take long for this species to become established once they’ve got a clawhold. Here’s a fact page on the Rusty Crayfish if you’re interested in knowing more.
As you can see in the above photo, a female crayfish produces an abundance of eggs, which she carries around in a mass on the underside of abdomen until they hatch. After hatching, the young crayfish may cling to her for some time before beginning life on their own. The day that I shot the above and below photos, I was out on a stream survey workshop with a bunch of high school science students. We saw a good number of egg-laden females crawling about in just a small section of river. We also found huge numbers of crayfish of various sizes under and between the rocks. It’s safe to say that this population of crayfish has become very well established.
I’m going to jump around a little bit and say just a little about species identification of crayfish. Later this summer, I’ll write more if I’m at all successful in finding and photographing more species while out and about. However, before continuing, here’s an excellent set of diagrams of crayfish anatomy and a glossary for those who are interested.
When studying crayfish, the shape of the rostrum (see the photo at the top of this post), chela (large claws on front pair of legs), and the areola (the hourglass shaped lines on the crayfish’s “back”), are some of the parts that are used to “key out” (differentiate and identify) species. Of course, there’s more, but just knowing there is a difference helps us to look at crayfish a little more critically and realize that they are not all “the same”. Coloration is not all that useful as there’s quite a bit of variation, and also, when you find crayfish, they are often covered with algae or discolored from the water.
Male and female crayfish may be quite easily distinguished by examining the under (ventral) side of the body. Females have a small opening (seminal recepticle) between the latter pairs of legs. Males have small gonopod legs used to transfer sperm to the female.
In summer, crayfish may often be found by searching in areas of streams where there are rocks among which they hide. Carefully turning a few rocks may reveal a crayfish hiding beneath. In streams that are becoming somewhat dry by late summer, the crayfish may burrow down beneath a rock to remain moist (see above photo). In areas where streams tend to dry out, some species of crayfish dig deep burrows into the earth to survive until the water returns.
Crayfish are considered opportunistic feeders and eat a wide variety of foodstuffs, including aquatic insects, snails, minnows, and other crayfish. In the above photo, you can see a very feisty crayfish that was accidentally captured in a net while we were out doing a fish survey. It’s not certain, but we think this crayfish actually latched onto one of the minnows that had been caught alongside of it. As you can see, it’s got a viselike grip on the minnow and had little intention of letting go. What you don’t see in this photo, is that a few seconds later, the crayfish tossed the fish and managed to latch onto my finger with its claws. In an illustration of the power and sharpness of said claws, it managed to snip into my finger enough to make it bleed. In a rare moment of lack of self control, I shouted something not too nice and it was all I could do to keep from flinging the crayfish to the ground as I tried to pry it loose from my throbbing finger. So, yes, be at least a little wary of those claws if you decide to pick up a crayfish to take a closer look. The safest place to hold them is by the sides of the carapace as you can see me doing in the third photo from the top.
Tags: crayfish, Orconectes rusticus