Michoacan Stream Salamander

Ambystoma ordinarium measures between 70 and 75 mm SVL at sexual maturity, reaching a maximum size of 86 mm SVL for terrestrial adults. This salamander has a narrow head, and generally bears 16-24 tooth-rakers on the 3rd arch. Pattern and color may vary. Adult individuals generally sport a uniformly dark to black dorsum, but mottling may also be present. Some adults retain the larval coloration, consisting of slight ventral, lateral, and dorsal rows of light silver-yellow specks from axilla to groin (Anderson and Worthington 1971; Shaffer 1984). Larvae of Ambystoma ordinarium resemble those of Ambystoma opacum, and have sparse, but evenly distributed melanophores in addition to the rows of light silver-yellow specks. The larval fins are well-developed, and darken and mottle at maturity. Larval gills are reduced in size but bushy. Ambystoma ordinarium larvae reach a maximum size of 100 mm SVL and 191 mm in total length. Ambystoma ordinarium possesses features of both pond-dwelling and mountain brook-dwelling species. The dorsal and ventral fins of hatchlings are fully developed, as is typical of pond-type larvae, while gills are both diminutive and bushy, as is typical of mountain brook-type larvae. (Anderson and Worthington 1971; Anderson 1975).Ambystoma ordinarium is a member of the Ambystoma tigrinum complex, a group of salamanders which has undergone rapid lineage diversification and radiated throughout North America, from southern Canada to central Mexico. Ambystoma ordinarium is thought to have split from Ambystoma dumerilii 7-10 million years ago. Recent work has shown that Ambystoma ordinarium is genetically distinct but that mitochondrial introgression has occurred from Ambystoma dumerilii, which is a paedomorphic species found at nearby Lake Patzcuaro (Weisrock et al. 2006).

Also known as the Michoacan Stream Salamander or Puerto Hondo Stream Salamander, this amphibian is native to the Cordillera Volcánica within the Mexican state of Michoacan. It is found at high elevation (above 2200 meters). Both larvae and neotenic individuals inhabit high mountain brooks or spring pools at the headwaters of streams, with water temperatures remaining between 11.8 and 12.4 *C. Metamorphosed adults can be found hiding under debris in pine and fir forests as far as 30 meters away from streams (Alavarado-Diaz et al. 2002).

Neoteny is prevalent in this polymorphic species, but not uniform within populations. Metamorphic adults are terrestrial and have been observed to occupy the high-elevation woods and streamside habitats alongside juveniles. Paedomorphic adults and larvae are found in the montane streams (Anderson and Worthington 1971). An average of 109 eggs are laid singly or in clusters of 2-5 eggs onto the undersurfaces of roots, branches, and rocks, hanging free in the current. Eggs are pigmented and measure 9.9 mm including all layers of egg capsules. Each egg is encased in two inner capsules plus a third, thicker outer capsule. Egg clusters are observed not within an envelope, but consisting of individual eggs stuck together (Anderson and Worthington 1971). Ambystoma ordinarium appears to have a prolonged breeding season which may extend throughout the entire year, unusual for a temperate salamander. In the wild, eggs, embryos of various stages and larvae of different sizes have been found in both July and January. The testis and cloacal glands of males undergo no periodic changes once mature, suggesting the capability to reproduce year-round. Periodic changes do occur in females, but reproductive cycles are asynchronous within a population, evidence also of the species’ ability to breed throughout the year. Opportunistic breeders are able to continuously add small numbers of larvae to colonies throughout the year. Since environmental conditions are fairly consistent year-round, the staggered breeding may serve to reduce intraspecific competition for resources in the small mountain streams (Anderson and Worthington 1971). Stream-breeding is unusual among ambystomatid salamanders, as most of these species breed in ponds. Only two Ambystoma species, Ambystoma rosaceum and Ambystoma ordinarium, breed in moving water in a natural setting. It is thought that Ambystoma ordinarium has invaded stream habitats relatively recently, in evolutionary time. As would be expected for stream-dwelling salamanders, older larvae and neotenic adults show reduced fins, possess no balancers, develop limbs and digits early, and have a reduced gill size. However, new hatchlings have fully developed fins and well-developed ypsiloid cartilage, as is typical of pond-breeding salamanders. The hatchlings of Ambystoma ordinarium are thus more mature than those of typical pond breeders, but less mature than those of stream breeders. Adults and larvae alike walk on the bottom more than they swim (Anderson and Worthington 1971). Ambystoma ordinarium is perhaps the only primarily diurnal species of Ambystoma. Individuals generally remain hidden under cover of stream banks or underneath logs in the early part of the day, coming out by late morning (Anderson and Worthington 1971). It is possible that this behavior can be attributed to the slight rise in cold mountain water temperatures that facilitates daytime feeding. The lack of natural predators also encourages diurnal activity (Alavarado-Diaz et al. 2002). Ambystoma ordinarium feeds on both terrestrial and aquatic prey. Terrestrial prey items for this species include grasshoppers, as well as ants, leafhoppers, scuds, earthworms, and nematodes. Aquatic prey items include aquatic insect larvae (especially caddisfly larvae), small aquatic beetles, and clams (Alavarado-Diaz et al. 2002; Duellman 1961).

The declining quality of surrounding forests does not pose an immediate threat, because the amphibians are able to utilize cleared pastures and watering holes as a breeding environment. However, desiccation of streams combined with habitat fragmentation due to human development continues to threaten the livelihood of this species. This species is protected in Mexico (Alavarado-Diaz et al. 2002; IUCN 2006).

The Puerto Hondo stream salamander or Michoacan stream salamander (Ambystoma ordinarium) is a mole salamander from the Trans-Mexican Volcanic Belt within the Mexican state of Michoacán.