My son has had the same spade since he was three. When I first agreed to let him loose with something bigger than himself with sharp metal on the end it was something of a risk. Since then, it has been his favourite possession, enjoying frenzied use on beaches all around the Cornwall and in all weathers, creating dams, pits, castles and ‘sand volcanoes’. The blade has been wobbling for some time now, but today Junior has plans for a tide fort at Millendreath, so we hope for the best.

We cross the rocks towards the sandy beach, stopping on the way to explore the pools. Many of the seaweeds growing at the base of the rocks are covered in a dense thicket of Dynamena pumila hydroids.

They look like pale plant stalks, each just a few centimetres long, but up close I can that each ‘stalk’ is made of a stack of downward-pointing triangle shapes.

When they are submerged as the tide comes in, a circlet of delicate stinging tentacles will emerge from each side of every triangle to catch passing food. Hydroids are fascinating animals, and are also a favourite food of some other species, including sea slugs.

Among the hydroids are a few spots of jelly, just a few milimetres long. They are very hard to see, especially while the seaweed is stranded out of the water, but these are sea slugs. In places I find the hydroid stalks are entangled with a fine strand of white – the sea slug spawn.

I try various ways to get the hydroids into water so that I can see the slugs better, but nothing works. I don’t want to harm any of the animals by removing them so I give up.

Further down the beach towards the sea, the gulls are making a huge racket, screaming and splashing. Where the rocky gully we are in opens into a wide sandy pool, we come upon a scene of complete chaos. Scores of herring gulls and some greater black-backed gulls are jostling for space: some swimming on the pool, others flying down and yet more perched on the rocks all around. Many are dunking their heads in the water, reaching for something. There must be food here.

We try not to bother them but most of the birds fly up as we clamber over the last rocks to the beach. I take a quick look in the pool and find it is strewn with dead sand eels. There are so many that they have drifted into heaps against the rocks and some have tangled themselves into balls in their efforts to escape.

These mass strandings of sand eels happen sometimes. Perhaps it is the warm weather and low tide combining to starve them of oxygen as they hide in the sand, or perhaps a large shoal became trapped here and were an easy target for the seabirds. There is nothing to do but leave the gulls to their feasting.

While Junior is shoveling sand with his dad, I return to the hydroids. After much searching, I find a slug that is only loosely attached to its prey and manage to wash it into a small tub. As soon as it is in the water, it transforms from a featureless blob into a magnificent structure of wobbling towers and waving rhinophores.

This is a Doto sea slug, but the species is not so clear. Most Doto slugs feed on very specific hydroids. My old books suggest Doto coronata can feed on Dynamena, but now it seems that they eat other things and that this is likely a different species, perhaps Doto onusta. Whatever it’s called, it is a true leader in the field of jelly architecture.

I have no idea what purpose the towering protrusions topped with dark spots fulfill – maybe camouflage, maybe just housing to its digestive organs, but they are incredible.

I find a sheltered pool where I can photograph and watch the little Doto for a while, before gently returning it to the exact same place I found it.

Junior has just about finished his sand fort when his spade finally parts from the handle with a wet crunch. We lovingly assemble all the bits and make sure to pack them into our bags, hoping that we can somehow repair it later. We share stories of all the happy times Junior has enjoyed with his spade over the course of the last nine years. It feels like saying goodbye to a family member, but the tide is coming in and Junior perks up to defend his fort from the waves, standing atop the sand until the sea starts to flood his wellies.

Back at home, Other Half disappears into the garage and rummages for a while before emerging with the spade firmly fixed to a new shaft. Blue spade lives to build again!

Other finds…