Seen close up by the camera, urchins can be surprisingly colourful.

WHAT DO YOU THINK WHEN YOU SEE ONE BORING SPIKY OUCH They make me imagine underwater hedgehogs already curled up in preparation for their predators, which is ironic, because the word urchin is the Old English word for hedgehog!
Few divers seem to be interested in these creatures. They dont move much, do anything particularly outwardly interesting and they dont even have brains! However, as you can see from these pictures they can be engaging subjects, as well as having some interesting biology.
As humans, we should be saying a big thankyou to sea urchins, rather than bemoaning their existence as we attempt to tweeze out the spines in our hand or foot (delete as appropriate), because they play an important scientific role in human reproductive biology. If not for the sea urchin, we might still think that a sperm is actually a tiny human being, and the egg merely there to give it a comfy home!
Sea urchins are echinoderms, related to starfish, featherstars, brittlestars and sea cucumbers. All echinoderms are characterised by radial symmetry, and have bodies made up of five equal segments.
These segments are visible in urchins if viewed from the top, and also in their empty cases when the animal is dead.
Urchins have many spines, which are used for protection, movement and feeding. They have many feet on their underside, too, all with strong suckers to allow them to move and also to hold on to rock or substrate.
Amazingly, this is not a muscular action but an hydraulic one - urchins use water by pumping it around their many feet, allowing them to walk.
They feed on both plants and animals, scavenging dead fish and decaying plant matter as well as feeding on sponges, mussels and barnacles.
They also play host to other animals, many of which seek shelter among the urchins spines. Shrimps, cardinalfish, small cuttlefish and the beautiful mandarinfish often live close to urchins, and use them as a retreat should a predator approach.

DESPITE THEIR DANGEROUS SPINES, urchins have many predators. Certain starfish will happily eat their spiky cousins, while wolf-eels munch them whole as their staple diet.
Trigger and pufferfish push or blow them over to devour the sperm or eggs inside, which is also what attracts humans. Urchin roe, known as ebi, and their reproductive organs are a delicacy in Japan.
It was humans eating these eggs 140 years ago that sparked the interest of German biologists at a biological research centre in Naples. They watched local fishermen eating them, and began to take samples from urchins to study.
In 1875 Oskar Hertwig, a German biologist, was the first person to witness reproduction on a cellular basis.
This was possible because urchin eggs and sperm are relatively transparent, so he could watch as the dark nucleus of the sperm met the nucleus of the egg and fused into one.
He then saw this one cell dividing and growing into many cells, discovering the theory of reproduction and blowing the birds and the bees out of the water!
Since his discovery, scientists have found that urchins have similar genomes to humans, making them perfect for cancer and stem cell research, which they are used for today.
So the next time youre cursing your dive guide for making you soak your spiked appendage in vinegar, boiling water or urine, remember how much urchins have taught us.
If you happen to be the unfortunate dive guide, try adding insult to the injury by mentioning that sea urchins are also practically immortal, some living more than 200 years with virtually no signs of age-related disease.
That should stop their moaning!

Sea urchins come in a wide range of shapes, sizes and colours - theyre not all black.
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Treating urchin injuries
A quick foray onto Google looking for urchin remedies will leave you wincing, even if you haven't trodden on one! Fancy soaking your feet in a mixture of urine and pineapple juice
If you do get any spines in your hands or feet, try soaking them in water as hot as you can bear for 60-90 minutes. Try to remove only any spines that you can get out easily. Do not stab or push them, as this can break them and cause infection.
If the spine is stuck near any joints, or you already have symptoms of infection, see a doctor as soon as possible. You may require surgery to remove spines that are deeply stuck.
Soaking in hot vinegar is said to remove spines quickly by dissolving them, as well as helping to prevent infection, though you may end up smelling like a fish & chip shop. Your choice!