Where do I start?!

There is so much to know about sharks, so many questions - and yet we still hardly know anything.

A friend face to face with a gorgeous wobbegong

(Image: Tony Isaacson)

Biology

Sharks are amazing creatures, and they ARE FISH.  They share many features in common with other fish, but there are major differences outlined in the table below:

 

Some of these attributes may not be self-explanatory so I'll go through them here:

  • Skeletons: Bony fish have calcium skeletons just like we do, but sharks, rays and their cousins, the chimaeras, have skeletons made of cartilage.  Our noses and ears are made of cartilage, and it is found in our joints.  It is relatively flexible and therefore weighs less than bone, helping to keep sharks and rays bouyant.
  • Fertilisation: this is about reproduction.  Most bony fish have to put their eggs and sperm into the water outside their bodies and hope that they mingle and create fertilised eggs.  Sharks and rays have to mate to enable the male to insert one of his claspers into the female to fertilise her eggs.
  • Operculum vs gills: bony fish all have a bony plate called an operculum covering their gills, which are soft and easily damaged.  Sharks have no such protection for their gills and you can see the (usually) 5 gill slits on each side of the head, or under the body in the case of rays.
  • Scales: the scales of bony fish are made of keratin, the same substance as our hair and nails.  Sharks and ray scales (denticles) are actually small teeth, they have the same layers of pulp, dentine and enamel on the outside.
  • Tails: Bony fish have tails that are usually symmetrical, so that the top half is the same shape as the bottom half.  There is no shark species that has a symmetrical tail, the top half is always larger than the bottom half.  In thresher sharks this has been taken to an extreme where the top tail lobe is as long as the body!
  • Bouyancy: Most bony fish (but not deep sea fish) use an internal organ called a swim bladder to aid in their bouyancy, to help them rise and sink in the sea.  Sharks and rays have no such organ, so as well as their cartilaginous skeletons they have an extremely large liver (up to 1/3 of their internal body space) filled with an oil called squalene, which is lighter than water and therefore aids in keeping sharks and rays bouyant.
  • Ampullae of Lorenzini: This is something unique to sharks and rays, bony fish do not have an equivalent.  These are small pores situated around the head and mouth of all sharks, and around the mouths of rays underneath the body.  They are filled with a special jelly, which is able to detect extremely small electrical vibrations from living creatures.  Sharks and rays are able to use this special electrical sense when they are very close to a prey animal to detect its presence.  For example, some rays cruise over muddy and sandy sea floors, underneath which are hiding many potential prey animals such as crabs and molluscs (snails, bivalves).  Even though the rays cannot see these animals, they can detect their electrical vibrations using their ampullae of Lorenzini, so they can go back and dig out their lunch.

Close up of a grey nurse shark showing the ampullae of Lorenzini as small black spots. (Image: Tony Isaacson)

Shark diversity

For a group of animals with relatively few species, sharks have diversified to an extraordinary extent.  Here is just a sampling to demonstrate this diversity:

  • they can be found in all parts of the global ocean, in fresh and sea water
  • they range in length from 15 cm to 18 m
  • they eat everything from plankton to dolphins
  • some are hunters while others are scavengers
  • some lay eggs (e.g. Port Jackson & epaulette sharks)and some give live birth (e.g. great white & whale sharks)
  • some are flat (e.g. angel sharks), some are pointy, some have big mouths (megamouth shark), and some have extra long tails (thresher sharks)
  • some sit on the sea floor (e.g. wobbegongs), some are eternal swimmers (e.g. makos) 
  • they come in a range of colours - grey, black, green, pink, yellow, brown - and can include spots, stripes, blotches, and frills
  • some are migratory (e.g. porbeagle)
  • some are nocturnal

I hope to put up photographs soon to show this amazing diversity.

In the meantime, you can see some of these sharks by visiting the links at the bottom of the page.

Shark conservation

Sharks are under threat from:

  • Illegal and legal finning
  • Fishing (as bycatch and when overfished as the caught species)
  • Coastal development
  • Habitat degradation
  • Pollution
  • Climate change

Sharks also have life history characteristics that make them particularly vulnerable to these threats:

  • Generally long-lived (greenland sharks are estimated to live over 200 years, making them the longest-living vertebrate on the planet!)
  • They become sexually mature relatively late in their life history
  • Low fecundity (tend to have few offspring)
  • Whether they are fished
  • Whether they are considered dangerous to humans

For a list of sharks species listed in the IUCN Red List, go here.  The IUCN is the major international body that assesses and lists species that are considered to be becoming endangered.  The current list contains 47 shark species from around the world, more than half of which are found in Australian waters.

There is much to be learned about sharks and there are a few shark researchers around the world trying to explore basic information about reproduction, growth rates, migratory patterns and much more. Here are some of them:

  • Fox Shark Research Foundation
  • Greenland Shark and Elasmobranch Education and Research Group
  • South African White Shark Research Institute
  • Shark Research Institute
  • Undersea Explorer - Shark Research

There are many groups around the world advocating for shark conservation, some are listed here:

 

Frequently asked questions 

How many different species of sharks are there?

Believe it or not, the answer to this question is not so easy.  Depending on where you go for your information, the estimates vary between 375 - 450 species!  That's a large variation for so few species.  New species of sharks are still being found, so the number is climbing, but I think the best current estimate is around 400 species.

What's the biggest shark?

The Whale Shark (Rhincodon typus).  It is also the largest fish in the world (because sharks are fish).  It can grow up to 18 m in length and it's mouth can measure up to 1.2 m wide.

What's the smallest shark?

There are a few deep sea sharks that only grow to about 15-20 cm long. That's half the length of a school ruler!

What's the fastest shark?

Many sources of information agree that the mako is the fastest shark, but they can't seem to agree about how fast it can swim! One source claims that makos can swim up to 50 km/hr for short bursts, whilst another one claims only 35 km/hr.

How many sharks live around Australia?

According to 'Sharks and Rays of Australia' (2009, Last & Stevens), there are currently 182 known species of sharks found in Australian waters.  There are 125 known ray species (in the same group as sharks).  Both of these figures are likely to increase in the future, given that new species are still being found.

Shark links

 

 
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