Ambystoma maculatum (Shaw 1802)

The spotted salamander is a relatively large member of the family Ambystomatidae. Adults reach 15-25 cm total length, and have 11-13 costal grooves. Adult spotted salamanders typically contain two irregular rows of yellowish spots on a black to dark gray dorsum, though some populations contain low frequencies of individuals lacking any spotting in addition to albinos or partial albinos. Certain populations exhibit bright orange markings on the head, a pattern that has not been correlated with any taxonomic divisions. During the breeding season males have very conspicuously swollen vents, and females in breeding condition are typically larger than males. Hatchlings of this species do not contain readily identifiable markings and are characterized simply by a dull olive green color. Hatchlings may measure 12-17 mm in total length (Petranka 1998).The taxonomy for the species remains stable though some researchers point out distinct mitochondrial clades. Phillips (1994) identified two divergent, geographically separate lineages in the Ozarks region differing by a minimum of 19 mitochondrial DNA restriction sites.This species was featured as News of the Week on 19 November 2018: As a disease vector, it is important to control mosquito populations. However, biological control with introduced mosquitofish (Gambusia affinis) has the unintended consequence of altering ecosystems. Watters et al. (2018) explored the effectiveness of using native amphibian larvae in Missouri instead. They found that Leopard frogs (Rana sphenocephala), while consuming a large number of mosquito larvae, ate less than mosquitofish. The Spotted Salamander (Ambystoma maculatum), on the other hand, consumed as much as mosquitofish. Moreover, there was a positive relationship between mosquito consumption and salamander larval body size providing encouragement to assess more native amphibians for mosquito control. However, Thorpe et al. (2018) found a body size-dependent response to varying prey densities. With small African Clawed frog (Xenopus laevis) tadpoles, a type II functional feeding response is shown, increasing feeding rates with prey density until a threshold when the predator cannot keep up with the prey, while larger tadpoles exhibit type III response, characterized by lower than expected feeding rates at low and high densities but increasing feeding rates at increasing intermediate densities. This suggests a need for size diversity in biological control (Written by Ann T. Chang).

Ambystoma maculatum ranges from southeastern Canada, south to Georgia and Alabama, and west to east Texas. The species primarily inhabits mature deciduous forests with vernal pools for breeding sites, in addition to coniferous, mixed coniferous, and bottomland forests and adjoining floodplains. Spotted salamanders may be found at higher elevations in mountainous regions providing there is suitable breeding habitat (Petranka 1998).

The spotted salamander generally breeds only in ephemeral pools that are fish-free. Occasionally they will use permanent ponds despite the reduced hatching success due to the presence of fish. Adults migrate to breeding ponds during late winter to early spring, typically during rainy evenings. They exhibit strong homing behavior by remaining philopatric to their breeding sites (Whitford and Vinegar 1966), often even entering and exiting the pond repeatedly at similar locations. Homing behavior is so strong that when captured individuals were released into foreign breeding habitat, they bypassed this habitat and returned to their home breeding ponds (Shoop 1968). Breeding typically takes place en masse, where males are often known to congregate earlier at the breeding pond. The sex ratio at the breeding pond is often skewed in favor of males by 1.5-3.5 times more. Husting (1965) had shown a ratio of 4.43 males per female after a 4-year study. Beginning breeding times vary geographically, though generally in the southern portion of the range breeding begins as early as December, and in the more northern portions of the range March-April. Length of the breeding season varies significantly with location and may range from 3 days to over two months. Typically, the more northern populations have two or three highly synchronized breeding bouts, often only lasting 2-3 days (Petranka 1998). A fairly elaborate courtship may take place at breeding in which the male contacts a female and engages in a nudging ritual. After the male repeatedly encircles the female, he deposits spermatophores on the substrate for the female to pick up with her cloaca. Males will often deposit their spermatophores on top of other males' spermatophores if encountered during the courtship. The female deposits the egg masses within 2-3 days after fertilization, attaching them to submerged vegetation. The embryonic period typically lasts between 4-7 weeks, and larvae metamorphose after 2-4 months. In some cases slow growing larvae may not transform until the following spring or summer, overwintering in the pond. Within a few weeks after metamorphosis, the newly transformed salamanders disperse into the surrounding upland habitat during moist weather. It is not clear how long the juvenile stage lasts, though time to reproductive maturity is believed to be 2-3 years (slightly longer for females), at which time individuals return to the pool to breed (Petranka 1998).Despite the relative isolation of suitable breeding sites and the high tendency towards site fidelity, migration between ponds does occur. Zamudio and Wieczorek (2007) found that immigration between ponds was common within demes in their Tompkins County, New York study populations. Their data suggested that A. maculatum breeding groups were behaving as metapopulations, such that population clusters were the functional units but with sufficient migration between demes to enable potential rescue and recolonization. High gene flow was also found between A. maculatum breeding ponds in northeastern Ohio, despite landscape fragmentation (Purrenhage et al. 2009).Spotted salamander egg masses are preyed on by wood frog tadpoles (Rana sylvatica) as the embryos near the end of development; in turn, the frog tadpoles are preyed on by larval salamanders (Petranka et al. 1998).This salamander is the first vertebrate reported to have photosynthetic symbionts within its cells and tissues, the single-celled alga Oophila amblystomatis. Previously it had been thought that the algae were external to the salamander embryos, but recent work by Ryan Kerney of the University of Dalhousie shows that the algae are actually within the embryonic cells. The symbiosis does not last through development; algal cells are detectable up through larval stage 44, and fewer algal cells are present in later stage larvae (Kerney et al. 2011). Transmission may take place in the salamander oviduct (Kerney et al. 2011). For a commentary on the initial report of this work at the July 2010 Ninth Internal Congress of Vertebrate Morphology (held in Uruguay) and a photo of the algae-harboring embryos, see Petherick (2010) (http://www.nature.com/news/2010/100730/full/news.2010.384.html) in Nature News. Although general convention says that stress reduces reproduction, a species’ length of breeding season and its lifespan also play a role. Woodley and Porter (2015) recently tested the interaction of stress, length of breeding season, and lifespan in Spotted Salamanders, Ambystoma maculatum, which are long-lived, explosive breeders, by comparing the time it took males to deposit spermatophores (sperm packages that females use to fertilize eggs) and how many spermatophores were dropped in males deliberately stressed by handling and control males. They found that, while there was not a significant difference in how long it took males to drop spermatophores, stressed males deposited significantly more. The authors suspect that this may be a strategy to increase reproductive potential when there is a greater risk to survival.

Since the spotted salamander relies upon woodland vernal pools for breeding sites, protection of the surrounding upland habitat is important for them to complete their life history. The rapid spread of suburban development and other degradative anthropogenic practices is resulting in rapid habitat fragmentation for this and other species of Ambystoma. This increased habitat fragmentation leads to increased isolation of local subpopulations and thus decreases recruitment ability and gene flow among populations. These smaller isolated populations may become subject to various levels of inbreeding depression, and colonists (often juveniles) may not be able to reach nearby ponds to rescue local populations that may have suffered extinction. Thus, it is crucial to maintain connectivity in the landscape in order to protect this species and other species of pool-breeding amphibians. Timber harvesting significantly changes the habitat by reducing forest floor litter (decayed woody debris), understory vegetation, and canopy closure in areas surrounding breeding sites (deMaynadier and Hunter 1999). These changes affect not only the immediately impacted forest area but also affect whether habitat is suitable for salamanders in surrounding uncut forest, at least 25-35 m in (deMaynadier and Hunter 1998).If roads run near breeding sites, salamanders may be crushed by cars. Roads may also serve as a partial barrier to movement, further fragmenting the habitat (deMaynadier and Hunter 2000). In addition to habitat loss and fragmentation this species (especially at the larval stage) may be sensitive to decreased pH levels in breeding pools due to increased acid deposition from weather patterns or road salting (Turtle 2000; Sadinski and Dunson 1992). Low pH (