Sharks may have belly buttons – and other facts about their incredible diversity

The conversationThe first thing that comes to most people’s minds when they think of sharks is of large, stupid fish with pointy teeth, which are to be greatly feared. But as someone who spends his days studying these creatures, I know how wrong that view is. For starters, the diversity of sharks is astonishing. The differences between species can even make it seem like some of these animals are barely related.

My new research, conducted with colleagues in Britain and South Africa, shows that significant differences can even exist between male and female members of the same species. Our research on South African great white sharks has shown that females do not always make the transition from eating fish as young fish to eating seals and other marine mammals when they are older. Instead, our findings suggest that some females specialize in fish prey and use different habitats than males.

How on earth did we find this out? We performed a type of chemical study known as stable isotope analysis on small pieces of muscle we collected from free-swimming white sharks aboard a cage-diving ecotourism boat. This gave us information about what the sharks had eaten and what habitats they had used for about two years – the first time this analysis has been used on samples from living South African sharks.

What do the results mean? If females spend more time in coastal areas than males, they are more likely to be caught in inshore fisheries and swimmer traps, and are also exposed to more pollutants. This issue is already impacting South Africa’s white shark population. Thus, a better understanding of the diversity of shark behavior could help us improve conservation efforts.

Sharks swim in shallow water

So many sharks.

Image credit: cbpix/

Differences in the way the sexes use resources is called sexual segregation, and is quite common in sharks. The reasons behind sexual segregation can be complex, sometimes involving different temperature or nutritional requirements or the need for females to give birth in special maternity rooms.

Rough courtship and mating are probably one of the reasons why females grow larger than males (to be fair, males don’t have hands to hold on to, so they use their mouths). Female small spotted cat sharks have been documented to avoid sexual harassment and stay away from romantic (or biting) encounters with males by hiding in shallow water caves during the day.

Differences between the sexes are just one example of the incredible diversity found among sharks. It becomes even more diverse when you look at different species. For example, the sizes of the more than 400 species vary from a dwarf lantern shark of 20 centimeters to a whale shark of 18 meters.

They also come in a surprising variety of shapes, including the flat and ray-like angel shark and the round and eel-like frilled shark. Accordingly, their teeth also come in a beautiful variety of shapes, from crushing plates used to crack crabs in smooth sharks, to circular, blade-like teeth that give cookie cutter sharks their name.

Hammerhead sharks swim

All shapes and sizes.

Image credit: Alex Rush/

Sharks also have different reproductive types. Some lay eggs (which also come in different shapes and sizes depending on the species). Some develop eggs that hatch inside them, creating puppies that then develop in the womb and are later born.

Others give birth in the same way as mammals, with a placenta and umbilical cord. The umbilical cord attaches between the pectoral fins and when the puppies are born, they are left with an umbilical scar, essentially a belly button, which is visible for a few weeks until it heals completely. A number of shark species have also been found to have ‘virgin births’ and reproduce without a sexual partner.

It has recently been shown that individual sharks are just that: individual. In recent years, the study of sharks’ social interactions and personalities has come to the forefront of shark science. Researchers have found that lemon sharks show a preference for interacting with familiar individuals. But just like humans, some sharks are more social than others. Port Jackson sharks seem to have best friends that they enjoy spending time with year after year, and different shark species have been found to be made up of individuals with different behavioral types or personalities.

You can even tell individual sharks apart based on their appearance. NASA developed the algorithm now used to identify individual whale sharks by their spot patterns (originally it was used to map stars). Each whale shark has a unique pattern that can be identified like a fingerprint, and software is now being used to catalog photos of individuals so we can track their numbers and movement patterns. Several other sharks and rays are now being identified by different pattern features. White sharks can be identified by the notches on their dorsal fins and other distinguishing features.The conversation

Georgia French, PhD researcher of white sharks, University of Sussex

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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