Glossary of Terms for Crayfishes

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.

Cervical Groove    The joint line between the "head" part and the "thorax" part of the carapace.  There can sometimes be spines near or an interruption in this line that may be important.

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 who 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 contains the CHELA (an 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.

Telson    The last, center segment of the tail.  It is flanked on either side by two pairs of segments that form a fan-like uropod.

Tubercles    Raised bumps (if they are sharp they are called SPINEs) on the surface of some part of a crayfish.  Often the spines on small crayfishes (young) are very sharp, whereas in older crayfishes the spines are worn down to tubercles.

Walking Legs    The four pair of legs used for walking.  Does not include the "legs" with pinchers (CHELA).  The second and third pair of walking legs (count from the head to the tail) have hooks (ischial hooks) near their base in male crayfish.  The first two pairs of walking legs have small chelae.