11
Key to PA Crayfishes
By Ted R. Nuttall, Ph.D., Lock Haven University of PA
http://www.lhup.edu/tnuttall/crayfish%20key%20-%20couplet%201.htm
1a Carapace posterior to cervical groove completely covered with raised tubercles such that the surface feels granular (fig A); male with ischial hooks on 2nd and 3rd walking legs (fig B); gonopod terminating in more than 2 elements (fig C); chelae long and slender (fig D) - Procambarus acutus acutus
(Found in eastern and southeastern PA in the Delaware drainage basin. Possibly in extreme northwestern PA. Mostly near standing water, large river banks, ditches, swamps, etc. Can burrow into mud during drier season. Can be quite large (> 12cm).)
1b Carapace posterior to cervical groove smooth, or possibly with a few tubercles, but not extending to dorsal surface; males with ischial hooks on 2nd walking legs only (fig A); gonopod terminating in 2 elements (figs B and C - one type or the other)--> go to couplet 2.
2a Areola linear, obliterated, or nearly obliterated at its narrowest point (ratio of length/width > 24) --> go to couplet 3.
2b Areola not linear or obliterated at its narrowest point (ratio of length/width < 24) --> go to couplet 5.
3a Base of immovable finger of chela with conspicuous tuft of setae and base of dactyl with deep notch (fig A); suborbital angle obsolete (not angular) and cervical groove usually interrupted (fig B) - Fallicambarus fodiens
(This species is not yet reported in PA. It might appear in the extreme northwestern and extreme southeastern corners of PA since it is found in Ohio, West Virginia, and Maryland. Prefers seasonally flooded plains and swampy areas. Mud chimneys present near opening to tunnel.)
3b Base of immovable finger of chela without conspicuous tuft of setae and base of dactyl without deep notch (A in fig); suborbital angle acute (angular) and cervical groove usually continuous (B in fig) --> go to couplet 4.
4a Dorsomesial 1/3 of palm of chela (the area of the chela's propodus below the movable finger (dactyl)) covered with tubercles and gape of chela slight (fig A); eyes are somewhat reduced and postorbital ridges without spine or tubercle (fig B) - Cambarus thomai
(Found in southwestern PA. A burrower, common in roadside ditches and clay banks of larger streams. Sometimes under stones in streams.)
4b Dorsomesial 1/3 of palm of chela not covered with tubercles and gape of chela moderate to large (fig A); eyes not reduced and postorbital ridges ending in a spine or tubercle (fig B) - Cambarus diogenes
(Common in western PA, and reported in southeastern PA. A burrower, it is usually near spring-fed swamps around the drier edge, or near steams in logged areas, and is typically found in lower elevations. Can tolerate turbid water.)
5a Marginal spines or tubercles on rostrum present (fig A);terminal elements of gonopod long and slender, straight or curving gently (fig B) --> go to couplet 6.
5b Marginal spines or tubercles on rostrum absent (fig A); terminal elements of gonopod strongly curved caudally (towards the tail), claw-shaped, usually to 90o (fig B) --> go to couplet 12.
6a Ratio of length/width of areola > 9.6; a relatively narrow areola (uncommon; check ahead for rule out) --> go to couplet 7.
6b Ratio of length/width of areola < 9.6; a relatively wide areola --> go to couplet 8.
7a Dactyl of large chela with a notch at its base on the inner margin ending with a large tooth (fig A); ventral margin of joint between dactyl and propodus with only 1 tubercle (fig B); terminal process of gonopod strongly curved caudally and subequal (approximately the same) in length (fig C); ratio of length of central process to total gonopod length < 0.30 - Orconectes immunis
(Not yet reported in PA. Expected in extreme northern PA. Found in ponds and slow-moving water with soft bottoms and thick vegetation. Can be found in streams under rocks with muddy substrate.)
7b Dactyl of large chela with inner margin nearly straight (fig A); ventral margin of joint between dactyl and propodus with 2 tubercles (fig B); both terminal processes of gonopod weakly curved caudally and the central projection longer than the mesial process (fig C); ratio of length of central process to total gonopod length > 0.30 - Orconectes virilis
(Found in extreme north-northwestern and south-central PA. In ponds, slow moving water with vegetation and debris. Prefers warm, turbid water and can tolerate high silt.)
8a Lateral surface of carapace anterior to cervical groove with 2 or more hepatic spines (fig A); gonopods with distinctly divergent terminal elements (fig B) - Orconectes limosus
(Common in eastern half of PA. Prefers small quiet streams of low-lands, ponds, ditches. Also in streams near muddy banks, weeds, and under rocks.)
8b No hepatic spines; gonopods without distinctly divergent terminal elements --> go to couplet 9.
9a Cusp of mandible smooth (fig A); sides of rostrum slightly concave (fig B); dactyls of large chelae often with distinct subterminal black bands and mesial margin of largest palm generally with 7 or fewer tubercles in a row (fig C); central process longer than mesial process and ratio of central process to total gonopod length > 0.31 (generally 0.38-0.40) - Orconectes rusticus
(Introduced species, found at least in south-central PA. In moderately sized streams with rocks and vegetation. Reported to hybridize with O. propinquus and O. limosus.)
9b Cusp of mandible scalloped (looks like teeth) (fig); sides of rostrum straight; mesial margin of largest palm generally with 8 or more tubercles in a row; terminal elements of gonopod subequal (approximately the same) in length; ratio of central process to total gonopod length < 0.31 --> go to couplet 10.
10a Rostrum usually with median carina (fig A); ventral anterior margin of carpus without median tubercle or low spine (fig B); gonopod without shoulder (fig C) - Orconectes propinquus
(Common in north-northwestern PA. Always under rocks with muddy/sandy bottoms. Note rubbish pile (clean small pebbles, sand) by edge of stone. Not in violent streams with stones being moved.)
10b Rostrum without carina (fig A); ventral anterior margin of carpus with median tubercle or low spine (fig B); --> go to couplet 11.
11a Gonopod with right-angle shoulder (fig) - Orconectes obscurus
(Common in central and western PA. Always under rocks with muddy/sandy bottoms.Note rubbish pile (clean small pebbles, sand) by edge of stone. Not in violent streams with stones being moved.)
11b Gonopod without right-angle shoulder (fig) - Orconectes sanbornii
(Not yet reported in PA. Possible only in extreme western-southwestern PA. Found in riffles in small to medium-sized streams with cobble and boulders. Can be also found in undercut banks and in debris.)
Glossary
Terms for Descriptions of Surfaces, Edges, etc:
Anterior Toward the head end. Same as cephalic. Opposite of POSTERIOR.
Caudal Toward the end of the tail (telson). Also referred to as caudad. Opposite of CEPHALIC.
Cephalic Toward the head. Opposite of CAUDAL.
Distal Toward the free end, away from the point of attachment (of a leg, for example). Opposite to PROXIMAL. (on your leg, your toes are distal and your thigh is proximal)
Dorsal When the crayfish is standing normally (assuming it is alive), dorsal is on top. Opposite side that the legs are on. Opposite to VENTRAL.
Lateral A surface facing away from the midline of the crayfish's body. Opposite of MESIAL.
Left This refers to the crayfish's left, not necessarily your left. (See an old Three Stooges skit where they all point to the left, and naturally all point in different directions since they're facing each other!) Opposite of...well, you get the point.
Mesial A surface facing toward the midline of the crayfish's body. Opposite of LATERAL.
Posterior Toward the tail. Same as caudal. Opposite of ANTERIOR.
Proximal Toward the site of attachment of a structure. Opposite of DISTAL.
Ventral When the crayfish is standing normally (assuming it is alive), the ventral surface faces the ground. Same side that the legs are on. Opposite to DORSAL.
General Anatomy Terms
Acumen The pointy end of the ROSTRUM, the flat "horn" that is between the crayfish's eyes.
Areola The hourglass-shaped lines found on the back (dorsal surface) of a crayfish between the head and the tail. Its length divided by its width (see crayfish figures) is important in keying out a species. Sometimes the two lines of the areola touch, in which case the areola is said to be obliterated or linear.
Berry Females are called "in berry" when they are carrying eggs glued under their tails (in the spring, usually). Be careful with them so that the eggs hatch and become the young of the year (YOY).
Carapace The part of the crayfish that doesn't include the abdomen and tail. It is measured from the beginning of the tail to the tip of the ROSTRUM. Also called the cephalothorax, with the "cephalo" referring to the head part and the "thorax" referring to the part of the carapace with the areola.
Carina From the Latin word meaning keel of a ship. It refers to a raised bump in the middle (median) of the rostrum in certain species. The "median carina" is variable in height and length...in fact, it might not be there at all! It is a source of debate for taxonomists, yet its presence is helpful in identifying certain species.
Carpus The next joint below the CHELA of a crayfish. It normally has a nasty spine sticking in toward the middle of the crayfish. From tip of the leg to the base the segments are: propodus (with dactyl if present), carpus, merus, ischium, basipodite, coxapodite.
Chela The large claw or pincher of a crayfish. It contains a large portion called the PROPODUS containing a non-movable finger and a smaller movable finger called the DACTYL. One chela is often somewhat larger than the other, especially in males, and should be used preferentially in keying a specimen.
Form I Refers to male crayfishes found from fall to spring that are reproductively ready and have cornified central projections of their GONOPODS. Form II males are typically found from late spring to early fall, and molt (shed their carapace to grow) into Form I males in the Fall. Form I males molt in the spring to become Form II males.
Gape The distance between the movable (DACTYL) and immovable (PROPODUS) fingers of the chela when the fingers are closed. In smaller specimens, the gape is small, and in large specimens it can be open. Adults of different species sometimes have characteristic gapes that are helpful in identification.
Gonopod The first pair of pleopods in male crayfish. They are larger and heavily calcified compared to the other smaller pleopods along the underside of the tail. The gonopods are held close to the body and point towards the head, thus their presence immediately indicates a male crayfish. In very young males, they are very small in size and harder to see. In reproductive males (Form I), the longer of the two or three projections from the end of the gonopod is called the central projection, and it is thin, hard and brown (cornified), sort of like bird claws (although they can be straight or curved). In non-reproductive Form II males, the same part of the gonopod is still soft and often white. Only Form I males have the diagnostic features helpful in identifying certain species. Note: Use only the crayfish's LEFT gonopod for the key (see above for the definition of LEFT).
Mandible The heavy crushing "teeth/jaw" of a crayfish. Looking from below (the ventral view), if you separate the maxillipeds (see crayfish figures-- the accessory appendages around the mouth) you'll see two "jaws" that may be smooth around the edge where the brown cusp meets the white mandible proper, or scalloped, looking like teeth. The smooth mandible is helpful in identifying the invading rusty crayfish (Orconectes rusticus).
Marginal Spines Where the ROSTRUM tapers down to become the ACUMEN, there may be spines sticking out. They may be sharp, worn down, or broken off. Broken spines often leave a reddish "scar" behind.
Merus The next segment of a leg after the CARPUS. From tip of the leg to the base the segments are: propodus (with dactyl if present), carpus, merus, ischium, basipodite, coxopodite (coxa).
Pereiopods The five pairs of big legs on a crayfish, with the first pair containing the CHELA (and thus referred to as the cheliped) and the next four pairs referred to as WALKING LEGS. Note: Walking leg pair #1 is pereiopod pair #2...confusing isn't it? The first two pairs of walking legs have small chela.
Pleopods Small leg-like appendages on the tail, also called swimmerets on shrimp. In female crayfishes, all the pleopods look alike, with feathery ends. In males, the first two nearest the walking legs are specially modified for mating. (see GONOPOD)
Regeneration Crayfish can grow back lost legs, chela, etc. during successive molts. Regenerated parts are smaller than normal and may look somewhat different. For key purposes, always use a non-regenerated appendage if possible.
Rostrum The flat "horn" that is between the crayfish's eyes. Its shape and the presence/absence of MARGINAL SPINES are very helpful in keying a crayfish.
Setae Hair-like fibers attached to the crayfish that serve as sensory organs. If you look closely, they're everywhere, but sometimes in big tufts at the base of the DACTYL.