Sea Urchins: Vermin or victim?

The vermin argument:

Here in Tenerife some of our underwater volcanic rock surfaces have been colonised by black long-spined sea urchins which are believed to have originated from the Caribbean and inadvertently been transported here in trading vessels of the 19th century. They drastically alter the landscape that they inhabit by feeding on the algae that grows on the rocks, leaving the rocks scraped bare and depriving other species of both food and habitat. As they are not indigenous it is argued that they upset the natural balance and that their success is detrimental to native species.

An experiment at one of our dive sites, El Puertito, does seem to confirm this. For several years now this site has been cleared of sea urchins by one of the local dive centres (Ocean Dreams Factory). The University of La Laguna has been monitoring the resulting increase in both flora and fauna which indicates that in the absence of sea urchins, other species do indeed thrive.

This site is in fact now one of the most popular dive sites here as it is home to several green turtles, which may be due to the fact that they are mostly herbivores and their diet includes algae (though this may also be due to some surreptitious feeding by divers!)

The victim argument:

Sea urchins just get bad press because divers don’t like the nasty sting their spines can deliver. In fact they have every right to be here and have to take their chances with a number of predators, most notably starfish. The sight of a starfish attacking a sea urchin is something to watch: the starfish swings into action with its arms going in all directions while the sea urchin tries to make a run for it, usually unsuccessfully, though his close neighbours all make a getaway – not much solidarity there! If there are too many of them in places maybe it’s because the fishermen are catching too many of their predators such as the spotted burrfish – so it’s not the sea urchins’ fault, it’s the fishermen!

In some countries they are considered a delicacy on the seafood menu of restaurants, but for some reason they are not found on menus here. If we all just enjoyed eating them their numbers would be limited and our attitudes towards them would be very different.

Finally, they are actually rather splendid to view up close. An iridescent blue shines out from between the spines contrasting beautifully with the shiny black of the spines. Frequently tiny damsel fish of the same colours are darting in and out of the spine forest presumably enjoying the protection it affords. Symbiosis in action!

So what do you think? Vermin, or victims of bad press?

Some facts about Sea Urchins

• Sea urchins are found all over the globe in depths from 1 to 5000 metres. There are around 800 different species, some of which date back 450 million years.
• They are echinoids, which means that they have a skeleton composed of calcitic plates, usually bound together to form a solid structure called a test. This is the shell that can be frequently found in seaside gift shops.
• Their mouth is on the underside and they have teeth, so don’t get your fingers too close!
• They have tube feet with a suction disc at the end. These are mostly used for hanging on to surfaces, whilst the spines are used for motion. The tube feet also have an important role in feeding and respiration.
• They reproduce by releasing eggs and sperm into the water. It is very difficult to tell the difference between males and females.
• The young start out as larvae, feeding on microscopic plankton. After several weeks they undergo a metamorphosis into the adult form.
• They may live for 15 years or more.
• They can be eaten raw and are considered a great delicacy, particularly in Italy. The edible part is the organ that produces the eggs: it has a salty taste and light, soft and almost frothy consistency.