I FIRST snorkelled with spawning toads in a side-pool of the River Devon last year – and the temptation to do so again this spring proved impossible to resist.

It is unusual for toads to breed in a river, but this large backwash pool of the Devon is placid and languid, providing the perfect conditions for them.

As I slipped into the water, it wasn’t long before I saw my first toad lying on the muddy bottom. Another kick of my flippers, and a pair of mating toads appeared before me, the male tightly clasping the back of the female in a position known as "amplexus".

There was other life about, too, including caddisfly larvae which scuttled across the muddy bottom encased in their protective coverings made from various bits of river detritus, stone and sand particles, glued together by a silk lining spun from special glands around the mouth.

A stickleback also briefly flickered ahead of me, using its side pectoral fins to scull through the water. It, too, showed superb adaptation, the three spines on its back acting as a deterrent from being gobbled by trout and water birds.

After a while, I pulled myself out the water and sat on the bank to watch the spawning toads below. A sinuous movement caught my eye. I looked closer and realised it was a brook lamprey, an eel-like fish that is seldom seen.

Most species of lamprey are migratory and spend part of their life cycle in the sea. They also represent some of the most primitive vertebrates alive today, being over 360 million years old.

Technically, they are not true fish at all since they possess no jaws and have only a cartilaginous skeleton. They live for several years as larvae in streams and lakes, burrowed in silt beds, where they filter feed on micro-organisms. In this juvenile stage they are known as ammocoetes. Adult lamprey species have a toothed sucker mouth used to latch onto other fish to feed upon their bodily fluids.

The diminutive brook lamprey, however, is different as it spends its whole life in a river, and after several years in the mud as an ammocoete, it metamorphoses into a non-feeding adult, which then spawn in groups on gravel or sand beds before dying.

I imagine that the brook lamprey I glimpsed by the spawning toads had just emerged from its ammocoete larval stage in the mud in the backwash pool and would soon migrate, or drift downstream, to a nearby part of the river to spawn where the bottom substrate was more suitable.