Marine Institute

Marine Vertebrate Questions & Answers

Marine Vertebrates are animals that have a backbone. The following questions and answers are about marine vertebrates including fish, birds and mammals.

How many fish species are there?  

There is an estimated 20,000 different species of fish known to science; however there may be many more fish species that have yet to be discovered.

What is the world’s largest and smallest fish in the world?

The whale shark is the largest fish, growing over 15meters in length. The smallest fish is the tiny dwarf goby that seldom grows longer than 1cm.

Do fish sleep?

Yes, fish do sleep, but not as we know it. Most fish with the exception of sharks do not have eyelids so they do not close their eyes.  They do however, spend a portion of their day in an energy-saving state of rest, which constitutes as sleep. This allows them to rest while being alert for danger at the same time. While in this resting state most fish will remain motionless and will usually find a hiding place, such as under a log or rock. Others will just float in place. Some sharks however can never stop swimming as they need to continuously push water through their mouths to breathe, and therefore will sleep while still swimming.    

Is a blue shark really blue?

Yes, the blue shark displays a brilliant blue colour along the upper portion of its body which then fades to a white colour along its belly.

Do sharks have good eye sight?

Yes, sharks have very good eye sight. They also have very good night vision and can see colours too.

Are sharks a fish?

Yes, sharks are a fish as they live in water and use gills to breathe. However, they are a special type of fish as their body is made out of cartilage instead of bones like other fish. They are therefore classified as elasmobranchs. Other fish in this group include rays and skates.

Do sharks only eat meat?

No, sharks are known as omnivores which mean they eat both meat and vegetation. So if there is not enough meat to eat they will turn to sea vegetation for nutrition.

What is the fastest fish in the sea?

Some experts believe that the short finned mako shark is the fastest fish in the sea reaching estimated speeds of up to 95 kilometres per hour.  However the fastest recorded fish to date is the sailfish which can reach speeds of 110 kilometres per hour.

Are there sharks in Ireland?

Yes, there are indeed sharks in Ireland. There are at least 30 different types of shark found around our shores, some of which reside here all year round and others that only seasonally frequent our waters.

What is the largest shark in Irish waters?

The largest shark in Irish waters is the basking shark, which in fact the second largest fish in the sea reaching lengths of up to 36ft and weighing up to 7 tonnes. Rest assured however, this huge shark poses no threat to humans as it only feeds on tiny plankton.  

What do fish eat?

Fish have a varied diet, depending on the type of fish and where they are in their lifecycle, they can eat anything from tiny planktonic plants and animals to larger fish depending on their position in the food chain.

Do fish get thirsty?

Yes, fish sure do get thirsty. Fish take in water through their skin and through their mouth when they open it or take in food. Because sea water contains so much salt, saltwater fish have a special mechanism on their gills that enables them to extract the excess salt from their body so they don’t get dehydrated.

Can one shark eat another shark?

Yes, some sharks do indeed eat other sharks. For example the grey shark can eat any shark that is smaller than two and a half meters or the great white shark can eat anything smaller than itself.

Do sharks have scales or what is their skin made of?

No, sharks do not have scales. Instead they have dermal dentricals which are literally “skin teeth”.  Their body is covered in rows of tiny teeth made out of dentine which is the same stuff our teeth are made of.  All these teeth point backward so when we stroke a shark they are smooth one way and rough the other.