biology zoology blog benno meyer rochow sand fish

Having a Helping Hand in Making Sand: Fish, Foraminifera, Worms and others

I had a brother in law who was an engineer, controlling and supervising the production of different types of glass, a material that is based on sand, lots of it. For that reason, he often had to make business trips to Yemen, a country that exports high-quality sand. Well, my brother always came back with a smile on his sunburnt face and full of praise for the sand’s quality and the locals’ hospitality and friendliness. But one day he returned with fever, feeling dizzy and weak: he had contracted yellow fever and had to spend several months in the isolation ward a hospital. I was wondering, isn’t there enough sand on our beaches? Couldn’t he have obtained his raw material for the glass production elsewhere without risking to get sick?

Sand, after all, we learn at school is abraded, gradually crumbled, pulverized rock – the forces of erosion aided by temperature and humidity extremes, culminating in tiny fragments we call silica (or simply sand). Well, as always when you probe more deeply, it isn’t as simple as that and even amongst the beaches of temperate climes you’d find pieces of varying sizes that stem from the hard shells of shell fish (molluscs). Wave action can break and grind this material into sizes and shapes that become almost indistinguishable from the sand pieces of non-biological origin. And it’s not just shellfish that contribute to the volume of beach sand: there are the broken down calcareous homes of tube worms, the shells of barnacles and even the skeletal elements of sponges that make contributions to beach sands in temperate climatic zones.

In the tropics, however, vast areas of beautifully white beaches with soft and almost powdery “sands” owe their existence to sands of purely biological origins. A Japanese student once gave me a small tube with sand as a souvenir from Guam and said that the sand contained lots of tiny shells of Foraminifera, often simply referred to as “forams”. The shells of these single-cell amoeboid organisms remain after the animal has died. Currents then can wash up billions of these shells, thereby creating tropical beaches with them known from Bermuda, Brazil, Indonesia and many places in the Pacific. About 50,000 species of foraminiferans are known and for geologists, they can serve as important indicators of oil deposits.

The most important creators of tropical beach sands, however, are fish. Yes, fish like parrot fishes that break off pieces of living corals with their very strong jaws, crunch them up with their teeth, ingest the pieces to extract and digest the contents of the living coral, and finally excrete the non-digestible and finely broken down material as beautifully white and clean calcium carbonate poo. So efficient are these coral eaters that it has been calculated that a single parrot fish will produce 1 to 2 tonnes (not ‘tons’) of sand per year. Given that parrot fishes aren’t exactly rare on tropical coral reefs, you can imagine the amount of coral sand that even a population of just 1,000 fish would add to the sand volume on tropical beaches. It’s mind boggling when you further consider that there are at least 90-100 species and that individuals most probably live at least 20 years (total life span isn’t known). What makes these fishes interesting apart from their appetite for corals, is that sex changes (first female and then male) occur in most species and that the juveniles may look very different from when they are adult males or females.

We hear a lot of coral bleaching and the dramatic and worrisome deaths of reef corals, but we don’t hear much of the danger that beach loving humans could lose their sand producers (viz. the parrot fish), when there are no longer healthy corals around for them to munch and crunch on. I think for beach-loving folk this has to be a worry and should turn them all into conservationists and parrot fish lovers. Remember up to 2 tonnes of sand per parrot fish per year (but only if there are still living corals to munch on)!

© Dr V.B. Meyer-Rochow and, 2019.

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