Telling crayfish from crawdads
Some tips for identifying crayfish
The verdict is in: it’s “crayfish”
Well, the results are in, and it looks like we (59.6% of us) prefer to call these crustaceans crayfish. Of the 55 people who responded, everyone used one of the four most common terms (crayfish, crawdad, crawfish, crawdaddy, in that order), except for one person who apparently calls them “shoo-lobs,” which I quite like. This fascinating heat map shows how common each term is across the country.
As adults, weÂ rarely consider the humble crayfish, other than, perhaps, in wistful musings about how those summer hours during the carefree years of our youth spent flipping over rocks. Perhaps if we lived in the south, our interest would have shifted as we grew older, though not much past our stomachs. Somehow in the foodie northeast, the localvore movement seems to have swept right on past these tiny shellfish (thoughÂ Sal VitaglinoÂ is trying to change that). We’re left then, trying to make sense of a group of organisms mostly overlooked. It’s pretty hard to find good, detailed, and reliable scientific information on crayfish in the northeast.Â I found it hard to even find consistent information on which species we have here in Vermont. And within the lists, there’s often conflicting information about which species are native and which have been introduced.
Well, lets put an end to that, eh? In theÂ previous issue, I showed you how to tell males from females.Â In this newsletter I’ll point out some key features that astacologists (scientists who study crayfish) use to identify various species of crayfish by looking at two common species in Vermont, the northern clearwater and the virile crayfish.
Rostrum: this is the beak-like projection that sticks out past the eyes. The sides of the rostrum (A) can be straight, parallel, concave, or convex. On crayfish in the genus Orconectes, the rostrum has 2 projections (B) just behind the central point. Northern clearwater crayfish have a slight ridge, called an ocarina or keel, in the middle of the rostrum (C).
Size: Most crayfish are born in the spring and mature by that fall, so many of the crayfish are of adult size this time of year. Two measurements are commonly taken:
- Body size: measured from the tip of the rostrum to the joint between the abdomen (tail) and the cephalothorax (the fused head/torso).
- Largest claw: is also taken (limbs can regenerate if they’re removed, so the claws are not always the same size).
While there’s some variability within a species and overlap between species, size of adults can rule out regularly rule out options. Check out this field guide to get a sense of sizing.
Thumb shape: The “thumb” or dactyl of the claw is either straight or slightly s-shaped. There’s some variability in this characteristic, but the feature becomes more obvious when the the claw is closed. S-shaped thumbs leave a gap, while straight thumbs clamp firmly shut.
Areola: The areola is the space between the two plates on the sides of the carapace. This is either nearly touching (as in the virile crayfish) or with a gap, as with most of our crayfish, including the northern clearwater.
Color patterns/markings: Overall color can be highly variable, depending on how recently the crayfish molted (males molt 2x/year so are typically less covered in algae and therefore lighter) as well as its diet, age, etc. But there are specific markings, regardless of overall color, that can be diagnostic:
- Rusty crayfish: Red patch on the sides of the cephalothorax, black band behind orange tip of claws
- Virile crayfish: Paired dark markings on abdomen, blue chelipeds
- Northern clearwater: Dark band down the middle of the tail (see image above in the Size section)