Hemidactylium scutatum

Introduction

The Four-Toed Salamander is a small plethodontid with a widespread, fragmented distribution ranging from southern Canada southward to the Gulf of Mexico and westward to Oklahoma, Missouri, and Wisconsin (Petranka, 1998). Adults of this species are terrestrial and inhabit forested areas with fishless swamps or vernal pools that serve as breeding areas. Females often nest in moss clumps near the water’s edge, and may utilize communal nests. Joint nests in Michigan have been recorded containing over 1,000 eggs from 30-35 females (Blanchard, 1934). Unlike most other plethodontids which are capable of caudal autotomy only when grabbed by a predator, Hemidactylium scutatum may voluntarily autotomize its tail (Petranka, 1998).

Characteristics

Diagnosis 

Hemidactylium scutatum is readily identified by having only four toes on each hindfoot, a constriction around the base of the tail, and a boldly patterned venter that is white with black blotches.

Detailed Characteristics of the Subfamily Hemidactylinae

Characteristics are summarized from Lombard and Wake's (1986) phylogenetic analysis of major plethodontid lineages based on 30 morphological characters with special emphasis on the hyobranchial skeleton and musculature of the feeding system. These characteristics are useful in combination for distinguishing Hemidactylium from other plethodontids, although the characteristics listed are not synapomorphies of the subfamily Hemidactylinae.

Tongue and Hyobranchial Apparatus

The tongue is projectile (attached). The hyobranchial skeleton includes a urohyal, an expanded basibranchial, and radii that are discrete, rounded elements tapered towards their tips and distinctly separated from and articulated with the basibranchials. The epibranchial is relatively longer than the basibranchial or first ceratobranchial. The first ceratobranchial is larger than the second ceratobranchial and constitutes the main force-transmitting element in movement of the tongue. The rectus cervicis profundis muscle is folded dorsally near its anterior end. The rectus cervicis superficialis has a lateral slip. The omohydoideus, circumglossus and basiradialis muscles are present. The genioglossus muscle is also present, and its origin is shifted posteriorly along the mandible. The intraglossus is attached to the anterior end of basibranchial, lingual cartilage, or equivalent. The anterior section of the hyoglossus muscle has been lost. The posterior fibers of the hyoglossus are oriented posteriorly. The suprapeduncularis muscle is well developed. The ramus hypoglossus bifurcates distally, near the tip of the basibranchial.

Epibranchial Number

Larvae have three or four epibranchials (Rose, 1995, 2003).

Tail Autotomy

Cutaneous wound healing occurs, there are 3 caudosacral vertebrae, the first and second caudal vertebrae are specialized, and tail breakage is localized.

Brain Stem Motor Column

There are two distinct classes of cells in the motor column of the neck and trunk.

Jaws, Cranial Osteology and Structure of the Inner Ear

The tooth row lies anterior to a bony shelf of the preorbital process. Parietal bones have a distinct ventrolateral shelf. The facial lobe of the maxilla is located in a position near the anterior end of the maxilla, with a distinct section of the pars dentalis extending anterior to it. The premaxillae surround an intermaxillary gland that lies directly behind the pars dentalis. In the inner ear, the periotic canal forms a ventral loop immediately after leaving the periotic cistern. The bore radius of the otic semiarticular ducts has a negative allometry with respect to body weight.

Chromosome Number

The diploid number of chromosomes is 28.

Development

Hemidactylium scutatum has a biphasic life history featuring gilled, aquatic larvae and fully metamorphosed adults.

Phylogenetic Position of Hemidactylium scutatum

The phylogenetic placement of Hemidactylium is currently uncertain, although it does not appear to be closely related to other members of the former tribe Hemidactyliini (Eurycea, Gyrinophilus, Haideotriton, Pseudotriton, Stereochilus) or nested within any of the other major plethodontid clades (Bolitoglossinae or Plethodontinae [including the supergenus Desmognathus]; Chippindale et al., 2004; Mueller et al., 2004; Macey, 2005; Min et al., 2005).

Classification

Based on their phylogenetic analysis of morphology, mitochondrial DNA and nuclear DNA sequences, Chippindale et al. (2004) recommended elevating the tribe Hemidactyliini (sensu Wake, 1966; Lombard and Wake, 1986) to the subfamily Hemidactylinae, reserving the name Hemidactylinae for Hemidactylium and recognizing the subfamily Spelerpinae to represent remaining members of the Hemidactyliini (Eurycea, Gyrinophilus, Haideotriton, Pseudotriton, and Stereochilus). Similarly, Macey (2005) recommended placing Hemidactylium in its own subfamily (Hemidactylinae) and recognizing remaining taxa previously assigned to Hemidactyliini in their own subfamily, based on his analysis of whole mitochondrial genomic DNA sequences.

Other Names for Hemidactylium scutatum

• Hemidactylinae
• The Four-Toed Salamander

References

Blanchard, F. N. 1934. The relation of the female four-toed salamander to her nest. Copeia 1934:137-138.

Chippindale, P. T., R. M. Bonett, A. S. Baldwin, and J. J. Wiens. 2004. Phylogenetic evidence for a major reversal of life-history evolution in plethodontid salamanders. Evolution 58:2809-2822.

Lombard, R. E. and D. B. Wake. 1986. Tongue evolution in the lungless salamanders, family Plethodontidae. IV. Phylogeny of plethodontid salamanders and the evolution of feeding dynamics. Systematic Zoology 35:532-551.

Macey, J. R. 2005. Plethodontid salamander mitochondrial genomics: A parsimony evaluation of character conflict and implications for historical biogeography. Cladistics 21:194-202.

Min, M. S., S. Y. Yang, R. M. Bonett, D. R. Vieites, R. A. Brandon, and D. B. Wake. 2005. Discovery of the first Asian plethodontid salamander. Nature 435:87-90.

Mueller, R. L., J. R. Macey, M. Jaekel, D. B. Wake, and J. L. Boore. 2004. Morphological homoplasy, life history evolution, and historical biogeography of plethodontid salamanders inferred from complete mitochondrial genomes. Proceedings of the National Academy of Sciences USA 101:13820-13825.

Petranka, J. W. 1998. Salamanders of the United States and Canada. Smithsonian Institution Press, Washington.

Rose, C. S. 1995. Intraspecific variation in ceratobranchial number in Hemidactylium scutatum (Amphibia: Plethodontidae): Developmental and systematic implications. Copeia 1995:228-232.

Rose, C. S. 2003. The Developmental Morphology of Salamander Skulls. Pp. 1684-1781 in Heatwole, H. and M. Davies, eds. Amphibian Biology Volume 5. Surrey Beatty & Sons, Chipping Norton, NSW.

Wake, D. B. 1966. Comparative osteology and evolution of the lungless salamanders, family Plethodontidae. Memoirs of the Southern California Academy of Sciences 4:1-111.

Information on the Internet

• AmphibiaWeb: Hemidactylium scutatum