On a beach in Malaysia I saw small balls of sand in patterns around some holes (see photo, right). I couldn鈥檛 see an animal inside any |of the holes. So what makes these patterns, and how are they formed?
Clearly patterns of small balls on the beach is a common sight. So too are their creators. We鈥檝e received reports from northern Queensland in Australia, Borneo, Goa in India, and Zanzibar. Here are two possible culprits 鈥� Ed
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鈥� The patterns are characteristic of the intertidal crab Dotilla (see photo, below right). These very small crabs live in burrows, emerging at low tide to feed around their burrow鈥檚 entrance by scooping up sediment with their claws and passing it into their mouthparts. Organic material is filtered out to be eaten, while rejected material builds up into a ball at the top of their mouthparts. Once this ball reaches a certain size, it is removed by the claws and passed underneath the crab鈥檚 body. The crab then moves forward and repeats the process, dropping the sand balls as it goes and creating the pretty patterns seen.
Christopher Allen, School of Ocean and Earth Science, National Oceanography Centre, Southampton, UK
鈥� The photo shows a number of fiddler crab burrows. Worldwide there are more than 90 species of comprising the genus Uca. Generally they are small warm-water crabs that thrive in countries such as Malaysia, though they can also occur in temperate areas such as the Algarve coast in Portugal. Their common name derives from the male of the species, which waves a single enormously enlarged claw, rather like a violinist鈥檚 bow, to attract females. Their other distinctive feature is their eyes, which are mounted on long stalks, allowing them to spot other crabs or potential predators from a distance.
鈥淢ale fiddler crabs have enormously enlarged claws and eyes mounted on long stalks鈥�
Fiddler crabs forage for decomposing organic matter by scraping off the surface layers of the sand, which are rich in bacteria, fungi, creeping diatoms, a variety of microscopic protists, plus dead organisms and faeces. The crabs ingest the valuable organic material, disposing of the unwanted sand by creating the numerous small balls that can be seen in the photo. Broadly speaking, the larger the crab, the larger the sand balls that it produces. In the photo it is evident that there were several crab sizes present.
Fiddler crab burrows are quite deep. This is necessary to allow access to water at any stage of the tidal cycle, even if they are some distance from the water鈥檚 edge. When the sun is strong, and temperatures too high for comfort, the fiddlers retreat to the bottom of their burrows, emerging again once temperatures fall. They can spot people from many metres and respond to their presence by diving deep into their burrows, making it very unusual to see fiddler crabs at the burrow entrance.
From the photo one can see that fiddler crabs don鈥檛 scrape sand in a 360-degree circle around the burrow. Instead they forage over 180 degrees or less. Fiddlers are highly territorial and need to keep their eyes on each other at all times; if they scraped a whole circle they would have to turn their back on a competitor. If foraging zones overlap, as they do for the two burrows at the right of the photo, much time is wasted in displaying and fighting, so burrows are usually spaced out.
Each time the incoming tide covers the burrows the sand balls are normally washed away and the organic-rich surface layers of the sand renewed. However, some years ago in , I saw a trail of forest ants marching out of the forest onto the sand and collecting the sand balls from the fiddler crab foraging ground, presumably as useful building material.
John Davenport, Professor of Zoology, University College Cork, Ireland