This is an episode from the predecessor of the column 5 vragen: QED. This column centred on a single question.
Photo: ImageSelect
Sand may seem like nothing more than a barren surface for summer fun, but in the narrow spaces between sand grains, millions of organisms make their home. What on earth are these strange creatures?
Het strand lijkt op het eerste gezicht een levenloze zandvlakte: at first glance, a beach appears to be a lifeless expanse of sand – a place where, in summer, towels are spread out, children dig holes, and dogs dash frantically after tennis balls. But appearances can be deceptive. Place a handful of sand under a microscope and you discover a hidden world teeming with life. Between the sand grains live millions of tiny animals per square metre: the so-called meiofauna. Biologist Jan Macher calls it a forgotten kingdom, bursting with biodiversity. But what exactly lives beneath a beach towel?
Suckers
‘Most people have absolutely no idea that anything lives there,’ says Macher, a researcher at Naturalis Biodiversity Center in Leiden. ‘Even biologists rarely pay much attention to this group. Yet it is a complete ecosystem, with predators, grazers, detritivores, and everything in between.’ Meiofauna are tiny invertebrates that squeeze their way between sand grains. Think nematodes, rotifers, copepods, flatworms, isopods, and of course minuscule tardigrades. ‘They are often extraordinarily strange-looking animals. Many species have no eyes or pigmentation – which they have no need of in the darkness – but they do have suckers, hooks, or claws to grip onto sand grains. Their bodies are elongated and slender: all adaptations to life in this remarkable micro-environment,’ says Macher.
This lifestyle, in the minute spaces between grains, leads to similar evolutionary outcomes worldwide: convergent evolution. Distantly related species develop comparable body forms because they must adapt to the same mechanical constraints.
‘Most people have absolutely no idea that anything lives there’
The world between sand grains is complex not only in form but also in function. ‘Within the nematodes alone there is a complete food pyramid,’ says Macher. ‘Some graze bacteria from sand grains, others hunt copepods or members of their own species. There are also detritivores that break down algae and organic waste.’ Meiofauna are moreover food for larger animals such as amphipods and sandhoppers, which in turn appear on the menu of birds such as the sanderling. In this way, this invisible community forms a crucial link in the beach ecosystem. Yet this world remains largely unexplored territory. ‘We know more about the biodiversity of tropical rainforests than about our own beaches,’ says Macher. To change that, he and colleagues investigated the meiofauna of 25 beaches in the Netherlands and Germany. Between 2020 and 2022 they took samples from the upper layer of sand, anaesthetised the animals with magnesium chloride, and analysed them in the laboratory. ‘We found millions of animals and hundreds of species. DNA analysis gave us insight into their ecological niches, even though we had never seen many of the species before.’
Disturbance
Macher is now working on a follow-up study into the impact of human activities such as beach cleaning and urbanisation. ‘Mechanical beach cleaning in particular turns out to be harmful: it removes organic material that is needed for dune formation, and with it the habitat of the meiofauna.’ His aim is to translate this knowledge into ecological indicators. ‘Because meiofauna are so sensitive to disturbance, they could help to better underpin environmental policy on beaches. They are a kind of biological thermometer: if these animals disappear, something is seriously wrong.’
So anyone spreading their beach towel on clean sand this summer may well be lying on top of a vibrant but fragile miniature world – one that scientists are only just beginning to understand.
Curious questions from pupils? Send your questions to redactie@bionieuws.nl.
PHOTO: IMAGESELECT