MadSci Network: Zoology

Re: How many species of animals live in the ocean?

Date: Sun Sep 11 08:50:06 2011
Posted By: dave armstrong, Faculty, Biology, Cedars Tutoring, Qatar
Area of science: Zoology
ID: 1315655147.Zo

Dear Jenna,

This is an amazing question. Since Noah counted (about 40 land) animals into pairs, everybody has been cataloging living things. In the oceans, we have all of the first animal groups we know, as well as most of the more recent evolutions. So simple one-celled creatures join those with a ball of cells and the animals we class with the Jellyfish. The only big group thatís missing is the Insects, who nearly all live on land and have six legs. Their cousins making up the rest of the Arthropods make up huge numbers of species in the sea, both large creatures and tiny plankton.

As far as I remember, Tennessee hasnít got much water to swim in, but I hope you can count some animals when you visit a marine environment in the US. At your age, you must try and see a natural rock pool in the Pacific or Atlantic and lose your fright at getting slimy seaweed in between your toes. Remember, itís the home for animals in that zone, forming forests and meadows on which they can feed and hide.

Ignoring the plants such as these, though, we have probably at least 10 million animals on Earth, with a million or so discovered every year to be really new (not just an animal that was described way back when). If you look at my last reference below here, youíll find an equally good estimate of 8.7 million. You can check this estimate on Of those 10 million or so, we have only NAMED a mere 1.7 million, say 20%!

Now in the oceans, there is a greater problem than the one we have on land with huge tropical forests. The forests hide thousands of species of quite large animals so that they have only been discovered in the last decade.
Deep in the ocean trenches, no one has ever swum. Some people assume there must be 10 million species in the sea alone! We only have limited knowledge of any of the dark depths - less than that we have of the Moon! So your question is obviously left unanswered to some extent. We have less knowledge of these species than the few we know well enough to name on land (and sea).

Another estimate from the 10 million figure is unforgiveable. This final figure is unscientific and is a definite guess. We think that of the millions of animals that now exist, maybe one quarter will be marine, or at least spend most of their lives in water that is now classed as fresh water, like a lake or a stream. This leaves us with a guess at 2.5 million and more animals living in the oceans, but only 220,000 have been described. If the same attention to naming creatures had been applied, we would have 10% of the animals, so our guess using this source leaves us at 2.2 million. To be honest, these researchers agree with me :

Now you can see what a guess that was! I believe we need to leave the numbers game permanently. We need to know how many animals there were before the extinction rate became so high. As well as ďgainingĒ new species, we lose animals and plants every minute of the day. If one in five animals is decreasing in numbers significantly, our prospects are very poor. Soon we will have none of the uncommon species left. Extinction is one of the major threats to Earthís environmental stability, but we succeed in preventing it very rarely.

I hope you grow up into a biologist, Jenna, who can take part in the rescue of this planet. Sadly there are many chaotic places, on land and at sea, where nothing original has survived and the only evidence of life is decay organisms.

Mora C, Tittensor DP, Adl S, Simpson AG, Worm B. (2011) How many species are there on Earth and in the ocean? PLoS Biol.9(8):e1001127.

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