Ronald Altig 1 , Roy W. McDiarmid 2 , Kimberly A. Nichols 3 and Paul C. Ustach 4
1 Department of Biological Sciences, Mississippi State University, Mississippi State, MS 39762-5759, USA
2 USGS Patuxent Wildlife Research Center, National Museum of Natural History, 10th Street & Constitution Ave., NW, Washington, D.C. 20560-0111, USA
3 Biodiversity Programs, National Museum of Natural History, NHB W314A, 10th Street and Constitution Ave., NW, Washington, D.C. 20560-0180, USA
4 Department of Biology, Utah State University, Logan, UT 84321-5305, USA
Abstract . A key for the tadpoles of the United States and Canada features a different format and approach to identifying frog larvae. More details of ontogenetic variation are included than in many keys, and more attention is paid to using characteristics of living tadpoles. A tutorial examines morphological traits, and color photographs are included to simplify the identification process.
Keys are written with the goal of providing accurate identifications with a minimum of effort. A truly dichotomous strategy (i.e., one that divides the original and subsequent sets of taxa into equal units) is the most efficient (shortest number of steps) approach. Unfortunately, the distribution of useful characters among taxa rarely allows realization of this method. Instead, authors usually present a choice between two alternatives, each defined by one or more characters. This often results in long keys that are cumbersome to use, not especially enlightening, and frequently mask relationships (similar appearing forms come out together but closely related ones may not). Except for those distinctive forms that are separated early in the identification process, a user often has little assurance of being on the right track, and, if the trail reaches an obviously incorrect endpoint, frequently one has no idea which choice led to the wrong conclusion. Such keys are poor pedagogical tools because they fail to show patterns of character distributions among forms and can result in closely-related taxa being widely separated in the key.
Anuran larvae have little in common with their much larger, better known adults. Although tadpoles typically are in specific aquatic habitats for longer periods than their adults, they sometimes are more difficult to find and nearly always more difficult to identify. Most tadpoles are drab in coloration and pattern and even distantly-related species have very similar appearing tadpoles. Even though the tadpoles of many North American frogs superficially appear very similar (How often have you heard: "It's a tadpole but who knows which species?"), they have certain morphological traits that can be used to distinguish among species. We believe that with some practice an assiduous, inquisitive naturalist can learn to differentiate among most forms. To encourage more interest in tadpole biology, we offer this key to the larvae of taxa found in the United States and Canada ( APPENDIX ) and are convinced that it will afford the interested person the opportunity to identify tadpoles from their area. We are hopeful that this key also will increase the inclusion of tadpoles in inventory and monitoring projects.
Those who are not familiar with the general morphology of tadpoles should first familiarize themselves with the information in the TUTORIAL . This material, or any other division of the key can be reached by clicking on the preferred topic in the menu at the start of each Division. To be most successful at using this key, one must have a live or well-preserved tadpole and data on locality and ecological setting. A knowledge of local seasons relative to temperature and rain patterns is helpful in some cases.
The first of six self-contained Divisions allows one to key specimens to family, and families with single representatives in this key end here. Larger families continue in separate Divisions. One can work through the key from the beginning, or if the family of the tadpole at hand is known, one can go directly to that family by clicking the family name in the menu at the start of each Division. Sections within Divisions include all pertinent choices at that point and start with a heading of the characters to be used in the following choices in that Section. After that list of characters, there are three other sets of information: a list of Divisions and Sections that allowed one to get to the present position from the start of the key, a list of characters within that Division that allowed one to get to a given position, and a bracketed number. That number represents our estimate of the probability that the reader will be able to make the correct choices in that Section: 5 = always correct, 4 = almost always correct but may err when geographic or ontogenetic variations complicate the situation or character evaluations are difficult, 3 = about a 50% chance of being correct, proceed with caution, 2 = low likelihood of being correct, use as much additional local information as possible, and 1 = essentially impossible to differentiate with the present data. Notations of relevant cases of sympatry are presented each time a species is identified; those marked with an asterisk are likely to pose problems, and those with double asterisks present severe problems. A letter in brackets after each species name indicates [A] adequate (but seldom exemplary) descriptive information available, [B] some descriptive information available, or [C] descriptive information inadequate or lacking. A low average for these scores is one reason for a low probability value for that Section; attempting to identify a series of well described tadpoles that are very similar morphologically is the other alternative for a low identification probability for a Section. The first case can be remedied by further study, but the second situation will always persist. We commonly use geographical limits to avoid the inherent problems of trying to distinguish among closely related and morphologically similar allopatric species with subjective evaluations of certain traits (e.g., coloration and shape) that we suspect have considerable variation. Differences in elevation, habitat, and breeding phenologies are included, and parenthetical notes are added non-uniformly throughout the key. A larger image of the tadpole can be accessed by clicking on the small (thumbnail) image within the key. If two images are shown, clicking on either will bring up larger ones of both, one above the other.
Staging is by the system of Gosner (1960), and all measurements are in millimeters. The key applies to free-living, feeding tadpoles and emphasizes individuals in the middle of larval ontogeny (ca. 30-36). A version of this key (and similar ones for amphibian eggs, salamander larvae, and metamorphic anurans) will appear in an up-coming book on the biology and identification of larval amphibians of the United States and Canada. We welcome immediate input and particularly solicit ideas on better ways to distinguish among the tadpoles in several genera or groups: Bufo , Pseudacris , all members of the Rana pipiens complex, and Spea . ALTIG: TEL: 662-325-7561, FAX: 662-325-7939; E-MAIL: RAltig@biology.msstate.edu ; MCDIARMID: TEL: 202-633-0731, FAX: 202-357-1932, E-MAIL: firstname.lastname@example.org.
Gosner, K. L. 1960. A simplified table for staging anuran embryos and larvae with notes on identification. Herpetologica 16: 183-190.
Acknowledgments .-Several persons helped us locate tadpoles in the field: in Canada, Alberta - L. Powell; in the US, Alabama - G. W. Folkerts; Arizona - T. Wood & J. W. Wright; California - A. McCready & S. Sweet; Florida - D. G. Hokit & J. B. Jensen; Hawaii - A. Asquith; Illinois - R. A. Brandon & M. K. Redmer; Mississippi - M. A. Robertson; Missouri - E. Britzke, R. B. Thomas, & R. F. Wilkinson, Jr.; Nevada - E. Simandle & D. Spicer; New Mexico - W. C. Akers and D. Lynn; Oregon - J. M. Kiesecker & E. Wildy; South Carolina - P. Marino; and Texas - A. Price & F. L. Rose. Other colleagues sent us live specimens for photography: Arizona - E. W. A. Gergus; California - J. K. Reaser & H. H. Welsh; Florida - P. Delis, R. Franz, S. A. Johnson, W. E. Meshaka, Jr., P. E. Moler & D. Wilson; Illinois - R. A. Brandon & M. K. Redmer; Kentucky - A. Sih; Louisiana - T. Ostertag; Mississippi - M. A. Robertson; Nevada - D. F. Bradford & E. Simandle; New York - C. K. Sherman; North Dakota - R. A. Newman; Ohio - R. L. Saunders; Oregon - J. M. Kiesecker; South Carolina - S. Bennett; Texas - A. Graybeal & L. A. Fitzgerald; and Virginia - S. W. Gotte & A. H. Savitsky. The following persons sent slides for our review: G. Grall (National Aquarium, Baltimore), D. M. Dennis (Ohio State University), R. W. Hansen (Clovis, CA), T. R. Johnson (Missouri Department of Conservation), W. P. Leonard (Washington Department of Fish and Wildlife), B. W. Mansell (Jacksonville, FL), K. R. McAllister (Washington Department of Fish and Wildlife), A. Price (Texas Parks and Wildlife Department), M. Redmer (Southern Illinois University-Carbondale), N. J. Scott, Jr. (US Geological Survey, San Simeon, CA), S. E. Trauth (Arkansas State University), and R. W. Van Devender (Appalachian State University). The following curators, collection managers and others provided loans or access to preserved specimens in their institutions collections: R. C. Drewes, California Academy of Sciences (CAS); L. S. Ford, American Museum of Natural History (AMNH); H. W. Greene, Museum of Vertebrate Zoology, University of California-Berkeley (MVZ); D. C. Cannatella, Texas Memorial Museum, University of Texas-Austin (TNHC); R. W. McDiarmid, National Museum of Natural History (USNM); and C. K. Sherman (Ithaca, NY). Lois Loges and others at Patuxent Wildlife Research Center helped in various ways. To all of these friends and associates we extend our thanks.
DIVISION 1: FAMILIES
SECTION 1. MOUTHPARTS, EYES, VENT, ORAL DISC EMARGINATION, AND LABIAL TOOTH ROW FORMULA (LTRF): -  -
|A.||oral disc and keratinized mouthparts absent; eyes lateral; vent medial (oral disc emargination and LTRF not applicable)|
|B.||oral disc and keratinized mouthparts present; eyes dorsal; vent medial; oral disc not emarginate; LTRF 2/3, 4/4, 6/6 or 3/9-12|
|C.||oral disc and keratinized mouthparts present; eyes dorsal; vent medial; oral disc emarginate; LTRF 2/2 or 2/3 (most commonly)|
|D.||oral disc and keratinized mouthparts present; eyes dorsal; vent dextral; oral disc emarginate, weakly so in some stream forms; LTRF commonly 2/3 (1/3 and 3/3 are uncommon variants) or larger (5/3, 2/4, 3/4 or 6-7/6)|
|E.||oral disc and keratinized mouthparts present; eyes dorsal; vent dextral; oral disc not emarginate; LTRF 2/2|
|F.||oral disc and keratinized mouthparts present; eyes lateral; vent dextral; oral disc not emarginate; LTRF 2/2, 2/3 or 2/4|
SECTION 2. SPIRACLE, ORAL STRUCTURES, NARES, AND RANGE: Division 1, Section 1-A - oral disc and keratinized mouthparts absent; eyes lateral; vent medial -  -
|A.||spiracle single, midventral and near vent; paired, semicircular oral flaps overhanging mouth separated by inverted U-shaped notch; barbels absent; nares absent until near metamorphosis; widespread east of Sierra Nevada range|
|B.||spiracles dual and lateral; oral apparatus absent; long, mobile barbel at each corner of transverse, slit-like mouth; nares transversely elliptical; many kinds of lentic sites, exotic in southern California and Arizona (sympatric: none)|
PIPIDAE - Xenopus laevis [A]
|C.||spiracles dual and lateral; transverse slit-like mouth, usually with several short barbels, 1 at medial symphysis of lower jaw always present (some cohorts of some populations lack barbels or may be reduced in older tadpoles); nares transversely rounded; temporary pools in southern Texas (sympatric: Gastrophryne olivaceaand Hypopachus variolosus)|
RHINOPHRYNIDAE - Rhinophrynus dorsalis [A]
SECTION 3. LTRF, ORAL DISC, MARGINAL PAPILLAE, ORAL DISC EMARGINATION, SPIRACLE, AND RANGE: Division 1, Section 1-B - oral disc and keratinized mouthparts present; eyes dorsal; vent medial; oral disc not emarginate; LTRF 2/3, 4/4, 6/6 or 3/9-12 -  -
|A.||LTRF 3/9-12; multiserial A-1 about equal to length of biserial A-2; large, ventral oral disc stands open even in preserved specimens; marginal papillae complete; oral disc not emarginate; spiracle medial on chest and without dorsal wall; mountain streams of Pacific Northwest (sympatric: number of taxa but none with unique suite of characters)|
ASCAPHIDAE - Ascaphus truei [A]
|B.||LTRF 2/3; uniserial A-1 about same length as uniserial A-2; small anteroventral oral disc that folds shut when not in use; wide dorsal gap in small marginal papillae; oral disc not emarginate; spiracle sinistral near longitudinal axis; phytotelmons in forests on Oahu, Hawaii|
DENDROBATIDAE - Dendrobates auratus [A]
|C.||LTRF 2/3; uniserial A-1 about same length as uniserial A-2; small anteroventral oral disc that folds shut when not in use; wide dorsal gap in marginal papillae; oral disc not emarginate; spiracle sinistral near longitudinal axis; temporary pools in southern Texas|
LEPTODACTYLIDAE - Leptodactylus labialis [B]
|D.||LTRF variable, but usually either 4/4 or 6/6; uniserial A-1 much shorter than uniserial A-2 and about equal to width of narrow dorsal gap in marginal papillae; small, almost terminal oral disc that folds shut in preserved specimens or when not in use; marginal papillae with very narrow dorsal gap, sometimes appears complete; oral disc not emarginate; spiracle low on left side well below longitudinal axis; widespread, usually in temporary pools|
PELOBATIDAE - Division 6
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