Saturday, June 20, 2009

Beachcombing 2






Three more seashells from Warkworth beach – plus something completely different. The top photo shows the familiar pod razor Ensis siliqua. When it’s alive it lives buried in the sand at the low water mark, drawing itself downwards in the sand with an extendable muscular foot and extending two siphons to the surface to circulate water from which it filters food. It constantly moves up and down in the sand, burying itself deeper when threatened, and sand abrasion would create a lot of wear-and-tear on the shell if it wasn’t for that dull brown material on the yellow shell surface. It’s a renewable protein layer called the periostracum, which stops the animal from sanding its own shell away. The periostracum peels away very easily in old, dry shells on the strandline and that’s what has happed to the Iceland cyprina Arctica islandica in the second photo. Freshly dead specimens of this large bivalve, as big as the palm of your hand, are covered with a black periostracum which has worn away in this one. The Iceland cyprina has a very thick shell, so washed up specimens remain intact on the beach for a long time. The third picture shows a thumbnail-sized striped venus Chamelea gallina, which is very common on this beach and lives buried in the sand on the lower shore. You can locate many of these small bivalves by the holes in the sand left by their feeding siphon which are visble when the tide retreats – dig down and you can find the living animal. The last two photographs show the internal shell, or more accurately the 'test', of another animal that lives buried in sand in the intertidal zone and also in deeper water; the sea potato Echinocardium cordatum. This is a sea urchin that lives in a burrow. Those rows of holes mark the locations of the animal’s extendable tube-feet, some of which reach out from the entrance to its burrow to capture food. In life the whole animal is covered in yellowish spines and you can see a few of these still adhering to the test in the lower photograph. The test, made up of fused skeletal plates of the animal, is extremely light and surprisingly strong, surviving the waves to be washed up on the shore, often in large numbers. You can find some photos of the living animal at http://www.seawater.no/fauna/Pigghuder/sjomus.htm

4 comments:

  1. Interesting info on the shells (the living sea potatoes looks quite ridiculous).
    Pod razors are quite tasty - we had a plate served to us in Spain, though I don't think they are part of the British diet.

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  2. Fascinating Phil.
    Love the markings on the last two. At first glance, it looked like a kind of starfish.

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  3. Hi Kingsdowner, I've never eaten pod razors but someone once showed me how to catch them. If you try to dig them out they bury themselves faster than you can dig, so the trick is to pour salt down their siphon holes in the sand - then they quickly come to the surface.

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  4. Hi Keith, sea urchins and starfish are closely related. If you were to take a starfish and bend all its arms backwards and then join them at the edges, you'd have a sea urchin.

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