Three forms of vertebrates are commonly found in caves – fish, amphibians and bats, although they are either accidental or using them for shelter. It is thought there is not enough food for them to survive as troglobites in UK caves.

Fish
Bullheads (Cottus gobio) and blanched brown trout (Salmo trutta) are often found in caves in Britain (especially Wales and Yorkshire), washed in from the surface but there are no recent records from caves on the Mendips.  The Eel (Anguilla anguilla) is a third species that might be encountered underground but again has not been reported from Mendip caves. 
It could be the case that many of the streams entering the caves are often small and near their source and might therefore not be of sufficient length to support a population of fish.  An example of this is the small stream entering Swildon’s Hole which has its source not far from the point that it sinks into the cave.  Although the stream contains numerous insect larvae washed in from the surface, no fish were seen during recent, detailed biological sampling in the underground reaches of the stream. Systems with more substantial streams sinking into them (e.g. Longwood Swallet) are likely to be the best places to harbour fish underground.

Amphibians
Common Frogs (Ranus temporaria) can be found underground, but presumably are accidental visitors and often look thin. They can be found a long way underground, presumably lost and will eventually starve. The Common Frog Rana temporaris and Common Toad Bufo bufo have been retrieved from the bottom of the entrance rift in St Cuthbert's Swallet and a frog has been recorded from Swildon’s Hole.
Newts have also been found in St Cuthbert's Swallet.

Reptiles
Adders Viperus berus have been found on at least two occasions at the bottom of the entrance rift in St Cuthbert's Swallet, but any reptile is a rare sight in a British cave.

St Cuthbert's Swallet seems to attract more than its fair share of amphibians and reptiles. The cave is located in the Priddy Mineries which has a rich wetland fauna and supports all three British newts as well as frogs, toads, adder and common lizard.

The Mineries, Priddy The variety of habitats is a legacy of the lead mining industry.