Life’s Secretive Pioneers in the Sandy Pits
Many animals love to explore or spend most of their lives in the gaps and spaces created by piles of stones or wood: some cats like confined spaces, mice certainly do, and ants just love them. But ants can, of course, make use of much, much smaller crevices than bigger animals can. However, what about the sand on the beach? It is fine and seems compact, but there is always a small volume of interstitial space between the sand grains: could anything possibly be living in there? I bet no beach-frolicking folk ever think about that, but sure, further down the beach towards the sub-littoral you can step on shells buried in the sand and may get bitten on the toe by a crab that has sheltered in the substrate while further up, along the zone of decaying algal debris, you may encounter beach-hoppers and sand flies. But in between these two horizontal layers, would there be any life at all in the sand?
Well, think about a box of oranges or apples and imagine sand grains as big as these fruits: a little more than 20% of the space inside the boxes filled with oranges or apples would actually be empty, i.e., filled with air. Anything small enough to fit into these spaces could eke out a living there. Therefore, the gaps between the sand grains can indeed be a habitat for some specialized life forms that are too weak to actively burrow into the substrate and displace the sand. These animals that live in the gaps have to be small, preferably worm-like and tough with regard to temperature fluctuations, oxygen availability and salinity.
Elongated protozoans, minuscule roundworms, the always microscopically tiny rotifers (commonly known as wheel animals), and so-called archiannelids, unsurprisingly, make up a considerable portion of the interstitial fauna. More surprisingly is the richness of this biotope in the microscopic gastrotrich animals (sometimes called hairybacks) , which slide along on their bellies propelled by a coat of minute motile hairs and the fact that miniaturization has allowed numerous waterflea-like crustacean species to exploit this habitat as well. The most surprising sandgap dwellers, however, must surely be a kind of elongated, motile, dwarf polyp, furthermore some 1 mm long shell-less snails and a few species of solitary, non-sessile moss animalcules (the Bryozoa). The presence of some cute bear-animalcules is almost taken for granted in this extreme habitat, but in reality the tardigrades, as these organisms are scientifically known, are not actually very common.
What amazes me is that so many different major invertebrate groups are represented. The virtually guaranteed small amount of interstitial space always available in amongst the sand grains, even if the sand itself is unstable, and the relative scarcity of predators, must have represented too attractive an environment to remain uncolonized. That the niche of the interstitial sand dwellers does have its positive points for those that occur there, becomes only too evident when you try to extract, spot and identify some of these elusive and pioneering creatures. The next time you are on the beach, go ahead and try finding some: Good Luck. You’ll need it.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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