Under Cover: Ecosystems in the Sand

If you run a quick search for ‘ocean’ or ‘beach’ quotes online, a majority of them will talk about sand. Whether you find it beneath your feet, slipping through your fingers, or caught in your hair, sand is believed to have healing powers for anyone in search of peace and happiness. I agree. I also believe that sand is alive! Ocean sands might appear to be plain, unending, and monotonous, but by providing homes to an extraordinary wealth of animals and plants, they are also some of the most interesting living spaces on earth.

Sand forms a significant component of the ocean floor, starting at the seashore, going into shallow lagoons and coral reefs, all the way down to the deep seas. A closer look just at the colour of sand in any of these habitats tells us the story of their origin. Tan-coloured beaches that line much of the Indian coast are predominantly made from different forms of silica that have eroded and run off from mineral rocks over evolutionary time. When the sand is pearly white, as humans often prefer it to be, there are coral reefs close by. Corals, shellfish, calcareous algae, and other calcium-building organisms make their way to the ocean floor after they die or are broken down by fish. Black sands induce a mysterious and haunting feeling; perhaps it comes from knowing that they originate from lava and ash from rumbling volcanoes somewhere.

But who goes there?
If we were to play a game of word association and ask people to name an animal that comes to mind when they hear the word ‘sand’, the answer is usually ‘crab’! Crabs have, in a way, become the unofficial mascots of sandy areas.

Sand bubblers are small crabs of the Dotillidae family that inhabit the sandy intertidal zone of the ocean. During high tides, these crabs live in a burrow of their own making, waiting for up to six hours at a stretch for the water to recede. Once the beach has been exposed, the crabs have only another six hours to find food. This involves scooping up moist sand, ingesting organic matter and spitting out the remains into neat, round sand ‘bubbles’. They move and deposit these pellets radially around their burrow resulting in exquisite art installations. These are especially remarkable when thousands of sand bubblers work side by side. The tide eventually creeps in erasing all signs of their communal feeding frenzy and the crabs must retreat into their burrows. Each will take with it a trapped air bubble to breathe from until the next low tide.

Adapting to a shifting life

Life in a ‘sandscape’ can be a constant challenge and thriving in it requires nothing short of ingenuity. Let us look at the three basic survival needs of any living being— food, shelter and mates. Now imagine having to search for any one of these in a constantly moving environment composed of layers of loose particles. Three-dimensional structures like rocks are few, making the space appear featureless. How would one look for food or a place to hide?

Many animals have found at least a part of the solution underground. Burrowing is a strategy often used for protection and a sense of home. Some animals, like the vibrant and diverse group of tube worms (Polychaeta), live inside calcified tubes in the sand only exposing a dense set of feathery tentacles to filter-feed on plankton drifting in the water. Lugworms on the other hand live out their lives drilling and funnelling through the sand in search of food. These beige-coloured relatives of the earthworm are not usually easy to find. What we do get to see plenty of are the swirls of sand castings they deposit on the seabed while feeding; something like noodles of poop! Even less is known of the millions of micro-invertebrates hidden under the seabed waiting to be explored by science.

Sandy habitats have also been a laboratory of evolution for ambush predators, all of which face the same problem of ‘seek but don’t be seen’. There are predators that can completely camouflage with the sand to hide from prey (flatheads), predators that mimic other sedentary animals to fool their prey (frogfish), and camouflaging predators that also use venom for their own protection (stonefish and scorpionfish). Flounders take it up a notch further. By concentrating and diluting specialised pigments in their tissues, they are able to adapt to their surroundings in real time. By blending in with just about any colour and texture of sand, flounders also use their extremely flattened bodies and modified fin-rays to crawl out of sticky situations involving their own predators.

Having spoken so much about plain backgrounds and the need to blend in, one must not assume that sandy habitats are entirely devoid of brightness and contrast. Vivid colours in this setting are on the contrary quite useful in communication. They signal ‘stay away, I am toxic’ to predators of the (extremely) flamboyant cuttlefish, which appears to be fashioned by layers of rose petals. Colour changes in seahorses help mates find each other when it is time for courtship and parenting.

Perfect for partnerships

Life in the sand can sometimes get lonely, especially when the going gets tough. Partner shrimps (Alpheus sp.) spend their entire days trying to keep their burrows in place. After spending hours shovelling out sand from its burrow, another one of its walls could easily cave in. These shrimp are tasteful interior designers who like to fortify their burrows with bits of coral and shells sourced from its vicinity. But this work requires undivided attention which, for a shrimp with poor eyesight in a world full of predators, is very risky. Enter goby (Gobiidae), the watchman. Gobies have good vision, but are not builders. Instead they assist their partner shrimp by being on the lookout in return for a shared space to live in the burrow. Using various kinds of contact signals, the goby tells the shrimp when to ‘go back in’ and when ‘the coast is clear’!

In an unprotected environment, even when there is no opportunity for a 50-50 partnership, there is always room for tolerance. In the absence of boulders and crevices, the number of hiding places and support structures are limited in the sand. Smaller and more vulnerable animals have little choice but to piggyback on larger animals for shelter without particularly giving anything in return. For a crab, covering under the arms of sea star might be the most secure environment it is likely to get for miles. Sea pens (Pennatulacea) are quill-like animals loaded with stingers and well rooted in the sand, but invariably they become safe havens and vantage points for animals — brittle stars, hydroids, shrimps, slugs — that might otherwise be swept away in a strong current.

There are many lessons we can learn from time spent around sandscapes, from adapting to extreme conditions, to understanding the value of interactions. But for starters, this story should hopefully bring a new perspective to the old saying: “the world looks different when your toes are in the sand”.