Tetrapods: Out on a limb

Amphibians are unique among tetrapods for their "double life," a term reflected in their name, which comes from the Greek words amphi (both) and bios (life). Unlike mammals, birds, or reptiles, amphibians undergo a dramatic transformation that allows them to thrive in both aquatic and terrestrial environments.

What Defines an Amphibian?

Most amphibians share several key characteristics that distinguish them from other vertebrate groups:

• Metamorphosis: This is their most defining trait. Most begin life as aquatic larvae, such as tadpoles, breathing through gills. As they mature, they develop lungs and limbs to live on land.

• Moist, Permeable Skin: Their smooth skin must stay damp to assist with breathing and the absorption of water and oxygen. This requirement ties them closely to water sources or damp habitats.

• Ectothermic (Cold-Blooded): Like reptiles, they rely on external heat sources to regulate their body temperature.

• Vertebrates: They possess a backbone, placing them within the same broad category as humans.

The Three Main Groups of Amphibians

The class is divided into three distinct groups:

Group Common Name Key Characteristics

Anura Frogs and Toads The largest group, recognised by powerful jumping legs and a tailless adult form.

Caudata Salamanders and Newts Elongated bodies with tails; they resemble lizards but lack scales.

Gymnophiona Caecilians Elusive, legless, worm-like amphibians primarily found in tropical regions.

Amphibians in the Sid Valley

The waterways, ponds, and heathlands of the Sid Valley provide essential breeding and living spaces for several native UK species.

• Common Frogs and Toads: Both are native to Sidmouth. Frogs have recently colonised a new pond in the Roncombe Valley.

• Palmate Newts: These thrive in the Lower Knapp Pond Reserve, favouring the acidic environments often found in heathlands.

• Smooth Newts: This is the species you are most likely to find in local garden ponds or the Knapp nature pond.

The "Star Jelly" Mystery: In wet areas, you may occasionally find clear jelly lumps known as "star jelly". These are actually masses of frog spawn without fertilised ovules, regurgitated by predators like herons or grass snakes when the eggs swell in their stomachs.

Why Amphibians Matter

Despite their small size, amphibians are critical bioindicators. Their permeable skin makes them incredibly sensitive to pollution, meaning their health directly reflects the health of the local ecosystem. They also provide vital pest control by eating vast quantities of insects.

Threats and Conservation

Amphibians are currently among the most threatened animals on Earth.

• Habitat Loss: Deforestation and urbanisation isolate and eliminate populations.

• Pollution: Pesticides and chemical runoff contaminate breeding sites.

• Disease: The chytrid fungus is a deadly pathogen responsible for significant population declines worldwide by disrupting skin function.

• Climate Change: Altered temperatures can desynchronize breeding cycles and dry up essential ponds.