The Eastern Narrowmouth Toad (Gastrophryne carolinensis) is a species of microhylid frog. It is a relatively small, toad-like amphibian found in damp, shady habitats. The species is highly fossorial, and feeds primarily on ants. These North American microhylids (Family: Microhylidae) are distinguished from true toads (genus Bufo), and other anurans due to their moist, smooth skin, their lack of eardrum or tympanum, it’s distinguishable squat body shape, and the unique fold of skin superior to their eyes.^[1][2] It is found in the United States, from southern Maryland to the Florida keys, west to Missouri and Texas. While not a true toad, they are so called because they are terrestrial.

Taxonomy and Nomenclature

Members of Gastrophryne carolinensis belong to one of the largest anuran families, the microhylids.^[3] No subspecies are currently recognized.^[4]

Physical Characteristics

Description

Eastern narrowmouth toads are comparably small, growing to a maximum length of approximately 5.33 centimeters (2.1 inches). They have an oval shaped body and a narrow head with a pointed snout. They vary in color from brown, grey, to green, often with black and white spotting. Gastrophryne carolinensis toads have a typical teardrop-shaped body that is characteristic to members of the Microhylidae family. Though it’s morphology lacks a tympanum (eardrum), the subgular vocal sac is clearly visible.^[5] The Eastern narrow-mouthed toad can grow to approximately 3.8 centimeters (1.5 inches) long. They have a distinguishable pointed, narrow head with a small mouth, and a fold of skin that runs immediately behind their small eyes, and when viewed from above, the species appears ovoid.

An uncommon characteristic among other varieties of toads is their skin. Though most toads have rough skin that is usually lumpy in appearance and texture, the skin of Gastrophryne carolinensis is smooth and without ridges or warts. Yet, the skin is extremely tough, which most likely protects themselves from the ants they feed upon.^[6][7]

The color of the toad varies from brown, red, light tan, black, grey, to olive green, and can change its color depending on its activities and environment.^[8] The colors on its sides are generally faded, but its dorsum is dark and broad with various light or dark spots, mottling, or patches that cover the light strip along its middorsal area. Its stomach is generally highly mottled, but its ventral surface is nearly light or unmarked. The venters of certain Narrow-mouthed Toads in the Great Plains are generally unmarked, or virtually so, while the venters of eastern toads are strongly pigmented.^[9]

Its legs are generally stubby with short, heavy legs, and the tips of the toes are round and tapered.^[10] Like the Leptodactylidae, the toes are absent of webbing or toe pads.^[11] However, the heels on the back foot have one “spade” or tubercle for digging.^[12]

The sexual orientation of the Eastern Narrow-mouthed Toad can easily be determined due to its characteristic sexual dimorphism. The coloration of the throat region for males is dark and highly pigmented, while the females throat is absent of this discriminate coloration.^[13]

Ecology/Habitat

Altitudes

The narrow-mouthed toads can reach elevations up to 550 meters (1,800 feet). They are generally absent from most Blue Ridge Mountains and the Appalachia's.^[14]

Geographic location

G. carolinensis is only one of three species belonging to the Microhylidae family that is native to the United States. The species resides in the southeastern quarter of the United States, extending from west to Missouri and eastern Texas, and from southern Maryland to the Florida Keys, as well as in an isolated colony in Iowa.^[15][16] Many have also been introduced to the little and great Bahamas, as well as the Cayman Islands.^[17]

Habitat/Range Description

Although they live in a broad range of habitats and are rather ubiquitous, their most important environmental variables must include moisture and shelter.^[18] They reside in either freshwater or terrestrial systems, and are able to utilize both temporary and permanent waters, as well as human modified habitats.^[19] The members of this species tend to discount dry areas and mountains, though some may live in near desert-like conditions.^[20][21]

General habitat types include:

• Borders of swamps
• Cypress-gum swamps
• Bottomland hardwoods
• Live-oak ridges
• Pine-oak uplands
• Sandy woodlands and hillsides
• Open woods
• Pine forests
• Coastal secondary dune scrub forest
• Maritime forests
• Small streams
• Beneath logs^[22]
• Near ponds with fallen logs and other debris^[23]

• Urban habitats such as:
  • Woodland lots
  • Sandy pinelands

• Suburban Habitats such as:
  • Sandy soils around the lawn^[24]

Population

The species population is currently large throughout most of its range, and also in many suburban locations. Additionally, the species introduced to Grand Cayman is equally abundant. Though this particular species of microhylid’s experienced a severe population bottleneck effect in the past, its succeeding expansion has allowed its current population trend to remain stable.^[25][26]

Reproduction

Size/description at reproductive maturity

Adults range from 22 to 35 mm (0.9-1.4 inches) in body length; females usually attain larger sizes than males and exceed a little over 1.25 inches. Adult males are at an inch or less in length.^[27] Males can be distinguished from females during breeding season due to the visibility of a dark-pigmented vocal sac on their throat. Breeding males also exhibit enlarged tubercles on the chin, as well as a visible anterior edge of the lower jaw.^[28]

Breeding habitat

G. carolinensis reproduce in aquatic habits, and may breed in either temporary or permanent waters. The toad species will breed in deep water only if it is covered with a dense mat of floating vegetation/debris.^[29] They may also breed in ephemeral aquatic habitats such as temporary ponds, roadside ditches, borrow pits, deep wheel ruts in dirt roads, and shallow drainage ditches.^[30]

Breeding migrations

The male toad species call for mating at reproductive maturity. They generally call from the edge of the water when concealed by plant debris. Though more rare, they have also been observed to call while floating in the water with their forelimbs resting on stem or twig.^[31] Many times, they will also extend their hind limbs.^[32] They can also call the female while sitting on a bank, or on rainy nights while buried in the damp sand with only their head and vocal sac exposed.^[33]

The mating call sounds like a high-pitched, penetrating, nasal sheep-like bleat. It may also sound like a buzzing quality, and lasts for approximately 1 to 1.5 seconds.^[34] Unlike other frog and toad species, G. carolinensis does not exhibit the typical trill sound in their mating call.

In the south, April to October rains initiate their breeding season, or midsummer in areas farther to the north.^[35] They congregate to breeding ponds depending on the heavy spring and early summer rains. The G. carolinensis species is particularly successful at breeding in ephemeral bodies of water.^[36] When the male amplexes the female, special glands in the sternal region of males secretes a sticky substance to allow adhesion of breeding pairs, and also presumably to help him resist other males’ attempts to dislodge him.^[37] Breeding grasp (amplexus) for the toad species is axillary (behind the forelimbs).^[38]

Egg deposition

The female spawns clear marble-like eggs on the surface of the water, which is deposited as a mosaic structure.^[39] The egg mass is round or squarish, with approximately 10 to 150 eggs deposited as clusters in each mass.^[40] The female deposits over 800 tiny eggs that hatch within 1.5 to 3 days.^[41]

Larvae Metamorphosis

Tadpoles transform in approximately 23–67 days, and unlike other anurans, the larvae of G. carolinensis filter-feed on plankton.^[42][43]

Post-metamorphic

The tadpole’s head has a pointed head with lateral eyes, and appears dorso-laterally flattened when viewed from above. Their bodies are dark in color (almost jet black), and are flecked with blue. Its belly is marked with lateral whitish blotches, and the intestinal coil is not transparent through the skin.^[44] A lateral white stripe extends from the body to the tail, and a single spiracle is visible from the anus.^[45] The tadpole’s head is slightly pointed, and its tailfins are dark dipped and dark-flecked. They lack beaklike mandibles and the specialized labial tooth generally associated with most tadpoles. The newly transformed toads range from approximately 7–12 mm (0.3-0.5 in) in body length. Like adults, they are found to share the same habitat, and the older stages of the larvae are also toxic to many predators. The males reach sexual maturity in a year, while the females reach sexual maturity in approximately 1–2 years of age.^[46]

Feeding behavior

These toads feed predominantly on ants, termites, small beetles, and other various arthropods, and have been found feeding immediately at the opening of anthills.^[47][48] These various arthropods make up to 75% of their diet, though other prey include small snails, spiders, mites, collembolans (collembola), and lepidopterans.^[49][50]

Life History and Behavior

• Movement

The G. carolinensis toads are weak jumpers, and thus, either run or move by short and rapid hops.^[51] In general, the males generally move more than the females.^[52]

• Sleeping patterns

These toads are mainly nocturnal, and generally remain underground or hidden in debris during the day.^[53]

• Predators

A few species are reported to prey on the "Gastrophryne carolinensis" toads such as the Copperheads (Agkistrodon contortrix) and the common garter snakes (Thamnophis sirtalis). Other known predators include the glossy watersnakes (Regina rigida), the Eastern cottonmouths (Agkistrodon piscivorus), and the Cattle egrets (Bubulcus ibis).

• Anti-predator methods

This particular toad species avoids predators by burrowing, seeking cover, and/or nocturnal activity patterns. Chemically, they are also capable of producing mucous secretions that produce a violent burning sensation, irritate membranes. The toxins secreted from their skin deter predators, but they also plays a role in inhibiting ant attacks when individuals forage near ant mounds.

• Other behavior

The G. carolinensis toads are accomplished burrowers, and are seldom seen on the surface of the ground. Though they may be present in large numbers, their existence is hardly suspected.^[54]

• Territories

Though they are not known to establish territories, their mating calling positions can be as close as 2 cm, or as distant as several meters apart.

• Fossil records

Though little is known about its fossil records and evolutionary history, a few fossils belonging to the Miocene Epoch have been found in Florida.^[55]

The ilia of Gastrophryne is very distinctive and different from other small anurans. Based on observances of the different developmental features of the ilium between G. carolinensis and G. olivacea, it appears that the two species could have differentiated from the early Miocene.^[56]

Conservation status

According to the IUCN Red List category and criteria, Gastrophryne carolinensis is listed as a species of least concern due to its presumed large population, wide distribution, and ability to adapt to various habitats. Thus, no conservation actions are currently needed, though the population in northern Maryland is protected as an endangered species.^[57][58]

References