a crayfish primer

Rostrum of an Orconectes virilis

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.

Female crayfish – arrow points to seminal receptacle between legs

Male crayfish – arrow points to gonopod legs used to transfer sperm to female

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.

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11 Responses to “a crayfish primer”

  1. Susannah (Wanderin' Weeta) Says:

    Great post!

    “yes, I know, I was a weird little kid.”

    The best kind to be. I used to tease crabs until they stood up and fought me back. Then I let them go.

    (I wouldn’t do that now; I try to avoid stressing the creatures I meet these days.)

  2. John Says:

    I wondered where you get your ideas for posts…now, I’ve decided it’s just magic. Great post! I’ve had an interest in crayfish for a long time, mostly because they are promoted as wonderful Cajun food (I can’t verify that). As always, an excellent, informative post!

  3. threecollie Says:

    I too loved horses as a kid and wanted one SO bad.. little invertebrates that I could find around the house and surrounding fields had to substitute for me too..and I always loved crayfish. We caught a couple this summer and put them in the fish tank just to enjoy watching them. Thanks for all the enlightening information about them.

  4. Wayne Says:

    Very nice post, Bev. A great intro to crayfish.

    I love the kid story and the crayfish horse corrals. I spent a lot of time as a kid, 40 miles north of here in Hall County, Georgia, capturing crayfish (crawfish, crawdads), but never thought to make little dude ranches.

    Looking through your O. rustica suggestion, and a few Georgia sites, it looks like we don’t have that species here (yet). We do have a large number of highly localized species, mostly confined to one or two counties. While that page isn’t exactly worthless, out of all those species it doesn’t recognize Cambarus latimanus, which is apparently (one of the most?) common around here. So we’re left with a checklist of a coupla dozen species over three genera, and they’re all rare :-)

  5. Ontario Wanderer Says:

    Another great and interesting blog post! I have never really looked very closely at crayfish. Come on summer!

  6. pablo Says:

    But did you ever get a horse?

  7. burning silo Says:

    Susannah – Thanks! Yes, it’s good to be an inquisitive kid, and also very good to have the opportunity to see crayfish.

    John – I don’t know where the ideas come from. Perhaps it is magic. So far today, I haven’t figured out what I’ll write about, so the magic better kick in pretty soon! (-:

    threecollie – It’s interesting to compare notes with other adults and find out how many of us wanted a horse. I did have horses for years, so eventually, my wish was fulfilled – I’m sure the crayfish all breathed a collective sigh of relief!

    Wayne – Thanks. I think playing with crayfish may be one of those universal experiences of children raised in the country or near creeks. When I was very young, I was totally blown away by them. I can still remember the thrill of finding my first crayfish as I was *sure* it was a baby lobster.
    I’ve been looking at your crayfish checklist this morning and it’s quite a lengthy list compared to what we might see here in Ontario, but then that seems to be the case with many other groups of organisms as well. That could have much to do with climate as much as anything. Interesting in any case!

    OW – Yes, come on summer! I always have so many projects at the outset of summer. I should really make a list. Perhaps that’s a good subject for today’s blog post.

    pablo – Yes, indeed, I did get a horse – also a good subject for a blog post!

  8. FC Says:

    Me too.
    I sit down and think, what shall I post about? And then it just happens.

    Me too on crawfish also. I’m a rock flipper from way back. We have a good variety of crawfish here in FL too, and I enjoy catching them for aquariums at school. I usually bring in a few dozen live ones for a crustacean behavior lab with the kids. It’s always a big hit.

    Have you folks thought of eating this invader to help slow it’s spread?

  9. burning silo Says:

    FC – A few months ago, I did make the suggestion that we could establish an Ottawa Valley Rusty Crayfish Festival as a combination tourist attraction – invasive species eradication event, but as one of my biologist friends quickly pointed out, the struggling surviving crayfish species might accidentally be harvested along with O. rusticus. Still, it’s not a bad idea as there are large numbers of these crayfish in a couple of the local watersheds. It probably wouldn’t take much work to round up a few thousand crayfish for the event.

  10. Therese Says:

    Great photos! As a Cajun, I feel compelled to add that crawfish are really delicious! I’ve often wondered how I would have felt witnessing the mass exodus of crawfish from the Southwest Louisiana marshes the day before Hurricane Audrey hit in 1957. People gathered them up for a crawfish boil that would never take place. I think when the critters leave the coast, it’s also time for me to go!

  11. Mike Mills Says:

    link
    http://www.crayfishworld.com/idguide.htm
    fairly helpful, but you can easily become overwhelmed. I don’t know of any good info on Orconectus in Ontario.