Introduction & Physical Description
Ambystoma opacum is a small, robust salamander, with thick limbs, head, and tail. The dorsal coloration and pattern is striking, consisting of a black background overlain with silver or white solid and broken crossbars. The crossbars of males are typically a brighter white color, while those of females are duller, silver-gray. Crossbars number from 3-8 on the body, and 4-8 on the tail, and costal grooves number 11-12 (Trauth et al., 2004). Adults can measure 3.0-5.0 inches in total length. 

Newly metamorphosed young are dark brown or black with white flecks, and develop web-like whitish coloration about the dorsum within 3 weeks of transformation. Adult coloration begins to appear within 3 months after transformation.

Hatchling larvae are pond type, with short, robust bodies, long gills, and high tail fins that extend past the limb insertions. Hatchlings typically measure 10-14 mm (.40-.50 inches) in total length, with more advanced embryos reaching up to 19 mm (.75 inches) (Petranka, 1998). Larvae are black or brown dorsally, with a series of dots along the sides of the abdomen, just below the level of the limb bases (Petranka, 1998). Mottling is sometimes present in older larvae, as well as various amounts of light green or yellowish coloration. The ventral surface is mildly mottled with black coloration, as is the throat.

When disturbed, A. opacum will assume a defensive posture consisting of lowering the head, and raising and lashing the tail. A noxious, milky secretion may also be produced from the tail.

There are no subspecies recognized, but pattern and coloration vary immensely among populations. A. opacum is considered a rather stable taxon, whose life history has been thoroughly examined in previous years.       

Synonymy & Etymology

Vernacular Synonymy: Marbled Salamander, Blotched Salamander, Banded Salamander, Opaque Salamander

Etymology: The genus name Ambystoma is thought to be Latin in origin, derived from the word anabystoma, and translating to "to cram into mouth" (Beltz, 2004). It has also been proposed that Ambystoma is simply a misprint of the Greek term Amblystoma, which breaks down to amlys = blunt, and stoma = mouth (Beltz, 2004). The species name "opacum" is Latin, and means shaded, dark, or obscure, and refers to the dark color pattern of A. opacum (Beltz, 2004).  

Distribution & Habitat

A. opacum may inhabit deciduous forests, tallgrass prairie (in the western range), and floodplain forests. New York populations have been observed in uncharacteristically dry areas outside the breeding season (Petranka, 1998). Habitats may also include swamp areas, as well as sandy woods and rocky hillsides (Trauth et al., 2004).

For the most part, adults and juveniles are fossorial, but often emerge after rains during non-breeding times. A. opacum cannot create a new burrow from scratch, but enlarge and occupy rodent burrows and other suitable sites. A. opacum are territorial, and aggressively attack conspecifics in their vicinity.

Breeding Behavior

Migration to the breeding ponds occurs in later summer and fall, and primarily on rainy nights. Northern populations tend to breed earlier than southern populations, which is reversed in other Ambystomid species. 

Migrating males arrive at the ponds several days, to two weeks prior to females. Adults in North Carolina and Rhode Island were observed leaving the ponds at nearly the same point of entry, indicating homing behavior (Petranka, 1998). The sex ratio varies highly, but males always outnumber females. Evidence shows that males often court females before reaching the nesting site (Petranka, 1998), after which both continue to the breeding ponds.

Courtship consists of the male nudging and lifting up other adults of both sexes near the tail and cloacal region. Females respond by nudging the cloacal region of males (Petranka, 1998). Mating pairs move away from the other adults, and engage in a circular dance, both nudging the other about the cloacal region. After a time, the male will move along the body of the female, undulating the tail and raising the body, and deposit a spermatophore. The male will then attempt to lead the female over the spermatorphore, where she will collect the sperm cap into her cloaca. Females sometimes do not follow the male, instead nosing about until a spermatophore is found, and positioning herself to collect the sperm cap (Petranka, 1998).

Fertilized females select shallow nest sites in the dried or partially dried floor areas of temporary ponds and ditches. Nests may consist of rodent burrows, or shallow constructions just below the leaf litter or other protective cover (Petranka, 1998). In certain habitats, females may create communal nests with several clutches (Petranka, 1998). At a site in Indiana, more than half of adults observed nested communally, a behavior thought to result from the unique aquatic habitat, which lacked leaf litter or other protective materials that would normally cover the eggs (Palis, 1996).

Clutches vary in size from 50-200 eggs, and clutch size is directly correlated to the snout-to-vent length of the female. Eggs are deposited singly, but in clusters (Trauth et al., 2004), and the female remains to brood, coiling around the eggs, and moving and turning them over every now and then. Females may stay with their clutches until the nest is flooded, although some may leave prior to flooding. Fresh eggs measure between 1.9-2.6 mm, and increase in size with development (Petranka, 1998). Larger eggs generally result in larger hatchlings, but at the cost of the female producing a smaller clutch. Eggs are ready to hatch at 9-15 days after oviposition, but hatch only after inundated. The eggs become rather camouflaged as they pick up dirt and debris from the surrounding environment. The thick, sticky outer egg capsule protects the embryos from dehydration prior to hatching, and may shrink or swell markedly depending on the state of hydration (Petranka, 1998). 

Hatching may occur 1-2 days after inundation in advanced-stage eggs (Petranka, 1998). Hatching is stimulated by the reduction in oxygen absorption experienced when covered in water. The lack of oxygen triggers the release of digestive enzymes, from hatching glands on the snout, that dissolve the egg capsules (Petranka, 1998). Hatching is based solely on environmental cues, which can result in larvae hatching in both underdeveloped or advanced states, depending on the amount of time allowed for development.

Larvae spend the winter developing, and reach maximum growth rates in late winter and early spring, when the waters begin to warm. Southern populations metamorphose primarily in March and April, while the central and northern populations metamorphose a few months later, in May and June (Petranka, 1998). The longer larval period of the central and northern range is a result of slower growth rates in the colder climates (Petranka, 1998). Larvae may reach a maximum length of 30-72 mm at metamorphosis, depending on the exact location and seasonal characteristics.

Metamorphosed juveniles can be found around breeding ponds shortly after transforming, before dispersing on land. Sexual maturity is reached within 1-5 years, at which point the salamanders return to the same water source to reproduce.

The chosen depth of the nest in the pond varies among geographic ranges, and depends on the annual rainfall characteristics in the particular area. In ponds that fill slowly, the time of hatching is related to the placement of the nests (Petranka, 1998). Those that are placed in deeper areas hatch first, when early rains fill the bottom of the pond, giving them a size advantage over later hatchlings. However, such larvae run the risk of desiccation if the following rains do not occur before the shallow water dries up. Those placed at higher areas hatch last and will be the smallest larvae in the pond, subjecting them to predation by older larvae. Higher nest sites may also be subject to freezing, or desiccation if the water level does not rise high enough to inundate the eggs (Petranka, 1998).

Studies have shown that females require a minimum moisture level to stimulate breeding, and will not breed below the minimum threshold (Petranka, 1998).   

Again, females may brood clutches until the nests are flooded, although deserted nests are often found. Females may also brood dead eggs normally (Petranka, 1998). In some populations, the survival rate of young is directly correlated to the time the female spent brooding the clutch (Petranka, 1998). In general, the length of the brooding period is related to the seasonal characteristics of the particular habitat, and may last anywhere from a few weeks to several months. Females that brood for several months often incur significant loss of food reserves and energy.