Regular divers at Kerkweg dive site in Den Osse have probably come across them at some point: small star-shaped patterns in the sand, with a shell on top. They look almost too neat to be coincidental — and they are not. They are the work of the sand goby (Pomatoschistus minutus), a small sand-coloured fish you can occasionally spot on the bottom at Kerkweg. The name may not ring a bell, but as children most people knew them well: caught with nets at the beach and kept in buckets as 'sand fish'.
Mountain builders
Early in spring, after the cold winter months, they emerge from their hiding places. The males immediately get to work building a mound, with a shell placed on top. As a diver, the mounds of the sand goby are easy to recognise: seen from above, they look a bit like a star with a small shell in the centre. Although the mound may not seem like much to us, for the sand goby it is quite an undertaking. Using rapid movements and sweeps of his tail, he pushes sand upward with his open mouth, digging small channels that run toward the mound. This creates the characteristic star-shaped form. Once his mound is finished, he takes up position beneath the shell on top.
But why build a mound at all? As a hiding place, a lookout post for intruders, or to impress the females?
A flurry of activity beneath a shell
The answer is most likely the latter. A female that encounters a mound will swim past it several times, then settle on the shell, swim away, and return. This can go on for a quarter of an hour — as if she is assessing the male's qualities. Is he big and strong enough? Would he make a good father? After this inspection, the female disappears beneath the shell. Following a brief flurry of activity, she swims away, and the male resumes his watch.
A few dives later, it becomes clear what happened: the female has laid eggs on the underside of the shell, and the male guards them until they hatch. To the naked eye, the eggs — roughly the size of grains of sand — are almost invisible.
The sand goby reaches a maximum length of ten centimetres, though most individuals you encounter measure around five to six centimetres. It feeds mainly on small crustaceans, worms, and other tiny creatures. In summer, sand gobies are abundant along the coast, in surf zones, rock pools, and pools sometimes no deeper than five centimetres.
Photos: Yoeri van Es Photography