Cockles are easy to recognize with their thick ribbed shells. The beach is usually strewn with loose shell halves. They filter plankton out of the water for food, using two siphons. By sticking these siphons out of the sand, they come in contact with the water. A cockle can filter a half liter of water per hour. All together, it takes only a few weeks for the cockles in the mudflats to filter the entire Wadden Sea. Shorebirds and eiders are very fond of cockles, as are some people.