The last word–

SHIFTY SANDOn a beach in Malaysia I saw small balls of sand in patterns around some holes (see photo, right). I couldn’t 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’ve received reports

from northern Queensland

in Australia, Borneo, Goa in India,

and Zanzibar. Here are two possible

culprits – Ed

● The patterns are characteristic of the intertidal crab Dotilla (see photo, below). These very small crabs live in burrows, emerging at low tide to feed around their burrow’s 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’s 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

● The photo shows a number of fiddler crab burrows. Worldwide there are more than 90 species of fiddler crab 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’s 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.

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’s 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’t 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 Penang, Malaysia , 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

THIS WEEK’S QUESTIONTransient tasteFor the past few days I have experienced a bitter taste in my mouth after eating or drinking. This persists for a while, gradually fading until I eat something else. In my researches to find a cause, I have learned that for some people eating certain varieties of pine nut can alter the sense of taste for days or even weeks. How can they do this and why does the effect last so long?Melanie Verde

Setúbal, Portugal

“Male fiddler crabs have enormously enlarged claws and eyes mounted on long stalks”

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