To mark World Oceans Day, taking place today (June 8), we’re delighted to feature a guest piece from marine biologist, diver, surfer, broadcaster and writer, Dr Helen Scales.

Seventy per cent of the earth's surface is covered by water. This vast aquatic realm is inhabited by a multitude of strange creatures and reigning supreme among them are the fish. Fish are the most abundant and diverse vertebrates on the planet, but many of the extraordinary things they get up to remain hidden away out of sight, down beneath the waves. Dr Helen Scales, author of new book Eye of the Shoal, lifts the lid on this mysterious world and gives a rundown of some of the most surprising and thought-provoking fish facts.

Find out more facts in Eye of the Shoal, available to buy now

1. Great white sharks have enormous oily livers that provide them a store of energy, similar to a camel’s hump, to see them through long migrations across whole ocean basins when there’s little to eat. The liver of an average adult great white weighs almost half a tonne and contains 400 litres of oil, with the equivalent calories of 9,000 Mars Bars.

2. During the Second World War, fish were so noisy they set off seabed mines that were designed to be detonated by the sounds of passing submarines.

3. Manta rays might recognise themselves in mirrors. This classic test has been used on birds, chimps, elephants and dolphins and suggests that they’re self-aware — an important sign of higher intelligence and sentience. Results in manta rays remain controversial. The animals swam in front of their reflections and blew bubbles (from air trapped in their gills). The individual mantas might have thought they were hanging out with other mantas, but perhaps they realised that they were the manta in the mirror.

4. Some herring can hear ultrasound (frequencies up to 180kHz; an average human hears up to only 20KHz), possibly to eavesdrop on dolphins when they use beams of high-pitched sound to find prey, including these schooling fish.

5. Guppies show off their better side. When a male guppy dances to impress females, hoping to secure a mate, he preferentially displays the most colourful side of his bright, blotchy body.

 Male guppies tend to be more colourful than their female counterparts

6. People used to think that fossilised shark teeth were the petrified tongues of snakes or dragons that fell to earth during eclipses of the moon. They were used as an antidote to snake bites or worn as lucky charms.

7. Migrating salmon change the colours they can see throughout their lives. While they’re living in the sea, their retinas are most sensitive to blue light. Then, when they swim inland into freshwater, an enzyme adjusts the photosensitive pigments to a version that’s more sensitive to red and far red light. In murky, muddy rivers and streams, light underwater is shifted more towards the red end of the spectrum. This change in the salmon’s eyes lets them see better as they race upstream to mate and breed.

8. The world’s rarest fish is the devil’s hole pupfish, a species which occupies a single lake in an underground limestone cave in North America’s Death Valley. Since scientists began counting them in the 1970s, the numbers have ranged from 35 to 500. At the latest count there were 115 of them.

9. A whale shark tagged with a satellite tag recently swam 20,000km from Panama to the Mariana Trench in the north western Pacific Ocean, the longest recorded journey in this, the world’s biggest fish species. It’s not clear why it swam this far. It might have been tracking fronts of warm water or aggregations of its favourite food (tiny shrimp).

10. Chalk bass live on coral reefs in the Caribbean where they form life-long partnerships and live together inside old conch shells. They’re hermaphrodites and swap gender roles, male to female and then back again, up to 20 times a day.

 The liver of an average adult great white weighs almost half a tonne and contains 400 litres of oil

Find out more by reading Helen’s book Eye of the Shoal: A Fishwatcher's Guide to Life, the Ocean and Everything

Get 30% off using the code SHOAL at the checkout on Bloomsbury.com