SCUBAHUB
CRITTER CONFIDENTIAL Common heart urchin ( or sea potato )
Echinocardium cordatum
… quickly burrowed in …
… and proceeded to …
… disappear
A heart urchin found out on the sand that …
It may look like an extra from the Gremlins films , but the heart urchin is a real , albeit reclusive , part of the UK marine scene , says Paul Naylor
The attractive live version of this
secretive animal is a rare sight . It is usually its dead remains , in the form of a distinctive casing , that tells you it is in the neighbourhood . The casing , called a ‘ test ’ because it belongs to a sea urchin , has rows of holes that remind me of the stitching on a baseball .
If , like me , you ’ ve ever been tempted to tuck the attractive empty object into your BC as a souvenir , you probably ended up with a crumbly mess in the bottom of your pocket ! The test is very fragile and , in a living heart urchin , is protected by a mass of gold-coloured spines . Because the spines are more delicate than those of the more familiar spiky sea urchins , mostly point backwards and lie fairly flat against the test , they look more like luxuriant fur than like typical urchin spines . Another difference from conventional sea urchins is that the heart urchin ’ s test is indeed heart-shaped rather than round .
The fur-like spines , the more streamlined test and the rare sightings are all explained by the heart urchin ’ s lifestyle . This animal is a digger and spends almost all its time hidden under the sand in a burrow . The spines don ’ t simply allow burrowing , they actively assist the process with a rowing motion , especially the spatula-shaped ones on the urchin ’ s underside .
Heart urchins spend most of their time around 20cm below the seabed , having constructed a breathing channel that runs up to the surface of the sand and forms a small depression there . Special , highly elongated tube feet reach up the channel and are used for respiration as well as collecting organic particles for food . This less than appetisingsounding diet includes material that naturally collects in the little hollow made by the breathing channel . Regular tube feet clustered round the urchin ’ s mouth also help to shovel in sediment .
On the infrequent occasions that I spot a complete heart urchin on the surface of the sand , it usually turns out to be a rather sickly individual , which is presumably why it has
ended up there . Very rarely , have I had the treat of encountering a fully fit individual that burrows quickly out of view ; a wonderful sight but a frustrating one for a photographer .
I have never been lucky or observant enough to see heart urchins spawn but they have separate sexes and , like many echinoderms , shed eggs and sperm into the seawater . Perhaps not surprisingly , these intriguing animals seem more resourceful than romantic . �
An empty heart urchin test
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