U is for Urchins.

Urchins, more commonly referred to as Sea Urchins, are part of the Echinoderm phylum. Echinoderm literally translates as Spike/Hedgehog skin. By looking at the picture, it is not hard to see why some people refer to Urchins as Sea Hedgehogs. Other Echinoderms include starfish and sea cucumbers. There are about 950 different types of Urchin like animals and can be found in every ocean. They can be found in depths ranging from normal, coastal levels to as much 5000 metres deep!

Sea Urchins come in a variety of shapes and colours. They range from small, 2 inch grey ones, to large, red 14 inch urchins.  They also have special pentameric symmetry which means they have a 5 fold symmetry. This means they have 5 repeating segments which make up the whole organism. This isn’t initially apparent in a living, mature Urchin but can be seen when they become dried out. Their young larvae have bilateral symmetry which is like all mammals. Sea Urchins move about by tiny ‘tube feet’ that are found at the bottom of the 5 sections. They move in a very similar way to that of octopus limbs; by hydrostatic pressure caused by the movement of water. This makes the feet ‘firm’ can allow them to stick to and push away from sediments, allowing them to navigate the ocean.

Although it is the most striking feature of Urchins, not all of them have the sharp spikes. Those that do, use them as a means of protection against predators. They penetrate the skin and can become implanted in any predator. They range in size with the smallest being a little under half an inch long, whilst the largest can reach a whopping 12 inches long!! It is unknown whether or not the spines contain venom and more tests are being carried out on more diverse species. The spines can sometimes penetrate human skin although, if removed properly, they are not harmful. However, if they are not removed properly, then further infections can take place.

Another defence mechanism of the urchin is a strong outer ‘shell’. The proper name for the shell is the test. In order to protect the soft, squishy organs, the urchins develop a tough outer shell which, in some urchins, the spikes rest on. The shell is made of tough calcium carbonate and then has an dermis and epidermis on top of it. In order to build the test, the sea urchins are able to convert dissolved CO2 from the water and create calcium carbonate. With the test being formed from calcium carbonate, the sea urchin is negatively effected by global warming. As more and more carbon dioxide is dissolved in the ocean, the ph of the ocean is likely to become lower, making it more acidic. This will dissolve calcium carbonate and therefore the urchins hard test may disappear. The same is happening to coral reefs.

Urchins themselves can have a negative impact on ecosystems. For example, in California where there a large kelp forests’, urchins are becoming a persistent pest. Their natural predators, the sea otter, has reduced in numbers meaning that more urchins are able to survive. This is bad news for the kelp because the urchins grab onto the bottom of the kelp and gnaw away at its root structure. This can cause the whole kelp to become dislodged and washed away. Kelp forests are a vital place for biodiversity and therefore the destruction of this habitat by an increase in the number of Sea Urchins is a severe environmental problem.

Perhaps one answer to this urchin problem is to eat them. Sea urchins are becoming quite the delicacy because of their extremely tasty ‘roe’. A ‘roe’ is a more polite name for their sex organs. Apparently, if you can pry open the strong exterior, then the inside of the Urchin is a culinary pleasure is “out of this world”. If you are interesting in finding out more about this, please follow this link.


Whilst writing this I remembered one of my favourite jokes and it actually won an award:

Hedgehogs: Why can’t they just share the hedge?


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