Crayfish Identification





As is true of many invertebrates and at least some vertebrate animals, the identification of juvenile crayfishes is difficult, and frequently, without locality data, can not be made.  Furthermore, in localities in which two or more closely allied species occur together, it is oftentimes almost impossible to distinguish between the younger members of populations.  Compounding this difficulty among the male members of the subfamilies Cambarinae and Cambarellinae is the existence of a cyclic dimorphism associated with the reproductive cycle that, in the more northern representatives, occurs typically in a circadian rhythm.  Among those species that have an annual reproductive cycle, the breeding  ("Form I," or "first-form") males of the population, at the end of their first season, molt and are transformed to essentially a juvenile morphology ("Form II," or "second form") that is retained until the advent of the next breeding season when the second semiannual molt returns them to the adult form (Form I).  Thus between each breeding season there is a regression to the quasi-juvenile (Form II) stage which may have a duration of three to perhaps as long as six months.  For those species that have a seasonal reproductive cycle, the entire male population may be in the juvenile or quasi-juvenile (From II) stage throughout most of the summer months.  In many of the species occurring in the lower temperate latitudes, there is no well defined breeding season, and, with staggered molting periods, the male members of a population consist of juveniles, quasi-juveniles (Form II), and adults (Form I) throughout the year.

Inasmuch as an individual may have three or more actively reproductive periods during its life span, and, with few exceptions, there is an increase in size of the individual with each molt, it is to be expected that quasi-juveniles (Form II) following their first or second breeding seasons are larger than an adult (Form I) male in its first.  Consequently size alone cannot be used in distinguishing between first and second form males.

Except for member of the genus Pacifastacus, only breeding (Form I) males may be identified using these keys.  These first form males may be distinguished from juvenile and second form males by the presence of one or more corneous, or horny, terminal elements (projections) on the distal ends of the first pleopods.  These rigid pleopods (Fig. 2), is their usual position, extend from the base of the abdomen forward between the bases of the pereiopods and lie against the sternum of the cephalothoracic region.  The first pleopods of the juvenile and second form males have no corneous terminal elements; the projections are more bulbous and much less clearly defined than in the first form males, and the entire pleopod is of similar texture.  (Compare Fig. 3d-j).  In females these pleopods are small and flexible or lacking.

Hopefully, in the future, it will be possible to construct a key to the female crayfishes, but until a satisfactory terminology can be developed fro designating the elements of the secondary sexual characters, their identity must be based on correlation with the male or comparisons with specimens of which the identity is know.




Fig. 1.  Dorsal view of generalized male crayfish illustrating structures and measurements referred to in keys. Fig. 2. Ventral view of generalized male crayfish illustrating structures and measurements referred to in keys. 



Fig. 3.  First pleopods of members of Cambarinae and Cambarellinae.  a,b, Mesial and lateral views of terminal portions of generalized pleopods illustrating arrangements of terminal elements;  c, Methods of measuring first pleopods (mesial view) in Orconectes (e, central projection; mp, mesial process);  d-i, Lateral views of left pleopods of first and second form males, respectively, with corneous central projection shaded - d, Cambarellus;  e, Cambarus;  f, Orconectes;  g, Procambarus;  h, Hobbseus;  i, Fallicambarus);  j, Caudal view of left pleopods of first and second form males, respectively, in Faxonella (corneous central projection shaded).


The essential characters and methods of measurements utilized in the keys are summarized in Figures 1-3; in addition, most couplets are augmented by additional illustrations.  Because of the extensive use of the first pleopod (unless otherwise indicated, all drawings are based on first form males) and the standard terminology that has been adopted for its several elements, Figure 3 has been prepared to provide an understanding of these terminal structures as they exist in the several genera.  The central projection (shaded in Fig. 3d-j) of the first pleopod of the first form male is always corneous; the other terminal elements may or may not be so sclerotized.  In the genus Procambarus (Fig. 3a,g), all four terminal elements are often present; in most members of the genera Cambarus (Fig. 3b,e), Fallicambarus (Fig. 3i), and Orconectes (Fig. 3c,f) and in all those of Faxonella (Fig. 3j) and Hobbseus (Fig. 3h) only two (mesial process and central projection) are present, while in Cambarellus (Fig. 3d) there are three -- only the cephalic process is lacking.

For convenience of description, the first pleopod is considered to hang pendant from the abdomen.  Toward the attached end is proximal; toward the free end is distal; the side toward the head, cephalic; that toward the telson, caudal; that facing the corresponding pleopod of the pair, mesial; and that facing away from the midline of the body, lateral.

Regeneration. -- A large to moderate proportion of the crayfishes in a population loses one or more pereiopods during their life span, and, if the loss does not occur too late, the appendage is completely regenerated.  The new appendage, however, seldom, if ever, is a perfect replica of the original one, and certain characteristics of the cheliped that have been utilized in the keys may become so altered as to lead one to an erroneous determination.  Unfortunately, to my knowledge, there is no specific alterations that would enable a novice to recognize a regenerated chela; consequently, it is suggested that if one of the chela of the specimen being identified is markedly different from the other and that of other individuals from the population, the characters of the chela that is more similar to those of other individuals should be used.

Equipment and Procedures that may be Helpful in Identification. -- For many crayfishes, a hand lens is adequate for observing the necessary characters.  In attempting to identify members of the genus Procambarus, however, it will be discovered that the terminal elements of the first pleopod are often obscured by "subapical setae."  Frequently, to make these elements visible, the setae must be removed, and it is suggested that this be done with the aid of a stereoscopic microscope in order to avoid dismembering one or more of the delicate terminals.  To accomplish this, it is recommended that the pleopod be removed from the crayfish by severing the articular membranes at its base and transferring it to a dish of alcohol.  By holding the pleopod at its base with a pair of forceps, the setae may be removed with a fine needle or slender forceps.  Then if the pleopod is oriented with the flattened mesial surface against the bottom of the dish, it is in a position to be compared with most of the illustration of the pleopods included in the keys.

All of the illustrations of the pleopods have been made in lateral, mesial, or caudal aspect from the left member of the pair.  Because of certain asymmetries, it is essential to examine the left pleopod.  (NOTICE !  -- REFERENCE IS MADE TO THE ANIMAL'S LEFT, not that of the observer.)


Last Updated:  10 September 2004