ON a couple of days every fortnight, just for an hour or two, a secret world appears along our local coastline. It is the rocky seashore at extreme low spring tides, and to anyone interested in wildlife it is one of the most fascinating places on the planet.

On one of those scorching days during the first week of July, I was fortunate enough to be on the seashore at Runswick Bay, helping a group of A-level biology students from Fulford School in York with the field-work part of their course. We had spent the morning walking towards the sea, measuring how seaweeds and animals change as we moved down the shore.

By 12.30pm, with the tide at its lowest level, we had reached the edge of the ocean and were struggling to keep our feet on the slithering mass of kelp covering the floor. Every rock lifted drew oohs, aahs and wows from the students as they uncovered and recorded six different species of crab, squat lobsters, butterfish, sea-squirts, sponges, saddle oysters and sea anemones. Emma’s sudden exclamation was more animated than most – followed by, "I’ve found a big fish, and I think it’s dying!" We all hurried over to find her pointing at a male lumpsucker.

As its name suggests, this is not the most attractive of fish, but it is impressive and has a fascinating breeding biology. The adults are about the size and shape of a knobbly melon, with both sexes in the main a slate grey colour, except for the belly of the male which is a bright, almost fluorescent, pinky-orange. The body feature which gives them their name is a gripping mechanism on their underside, formed from a pair of pelvic fins, which allows them to suck onto rocks for security in rough water, a useful trick in the not-so-calm waters of the North Sea.

In early spring, lumpsuckers move inshore where the females lay up to 400,000 eggs in two or three bunches, glued to the sides of extreme low-tide rock pools. With her admittedly crucial job done, she flits, leaving all the rest of the work to the male.

First he fertilises the eggs, and then for the next six weeks tends and guards them until they hatch, even oxygenating them by fanning water over them with his fins if the pool becomes too still and warm at low tide on a sunny day.

The fish that Emma had found was a particularly dedicated father. As the tide receded he had sat tight, doggedly refusing to abandon his eggs whatever might happen – even to the point of all the water around him disappearing, leaving him to suffocate and die. Which is, of course, what would have happened had we not saved the day by moving him to a small rock pool a few inches away, and splashing his precious eggs with seawater, until the tide came back in.

In a few days time the eggs will hatch and this determined dad’s job will be done. He will leave the young lumpsuckers, which look remarkably like tadpoles, to their own devices in the inter-tidal zone. They will move off-shore as they grow but will not go too far, returning to the rock pools after five or six years to breed themselves.

You don’t have to study A-level biology to enjoy rocky seashore nature, by the way; just get yourself down there with a net, a bucket and an identification guide – or alternatively, take photos of what you find and identify them on the internet later - ispotnature.org is particularly good.

If you would like expert guidance, then join in with a Yorkshire Wildlife Trust Seashore Safari day at Flamborough (see ywt.org.uk for details).

For your diary, our next spring tides are here on the weekend of July 17 to 19.