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The meaning of pectoral fin position part II

The meaning of pectoral fin position part II

Exception: The hammerhead sharks – a guestpost by Pascal Gospodinov

In part I we discussed the fundamentals of pectoral fin position. Pectoral fins as ruder; its a rule that counts for all species of sharks. But as we know every rule has his exception, here it is the genus of the hammerhead sharks (all species of the hammerheads).

Why are the hammerheads not using (or mostly not using) their pectoral fins as ruder?

Hammerheads are evolutionary speaking the latest model – a very modern shark.

His appearance is very different from other sharks. Yes talking about the hammer, in the scientific world we talk about the hydrofoil. The hydrofoil or hammer is the new ruder, very sophisticated and more sensitive, with very small movements of the head, hammerheads are able to steer. The bigger the hammer the better the function of steering. Species with smaller hammer will still use the pectoral fin, but still less than sharks without hammer (see picture 1 and 2).

It is very difficult or even impossible to recognize when a hammerhead wants to turn, because its pectoral fins will stay in the same position.

The pectoral fins of hammerheads are used to stabilize and they always have an angle of more or less 45° in regard of an horizontal line.

In most of the publications about hammerheads, the hammer is described as an enlarged area that serves to accommodate more lorenzinian ampullae. Those lorenzinian ampullae are used as electrosensitive organ, every sharks posses them, but the higher amount in the hammer enable the hammerhead to be more sensitive to electrical stimuli. So hammerheads are better in searching buried prey in the sand or even able to use the magnetic field of the earth to navigate during their migration trough the pelagic areas of the oceans.

So what is the trues: hydrofoil as ruder or as upgraded electro-sensor?

The answer is trivial: both are true! Evolution most of the time has more than just one reason to evolute an organ. Or the change of an organ or his morphology suddenly brings new advantages that eventually speeds up the adaptation. The hydrofoil brings massive advantages!

Picture 1. Scalloped hammerhead turning to his left without using his pectoral fin – hammer is bigger

Picture 2: Bonnet hammerhead turning with the help of pectoral fin – hammer is smaller

 

 

 

 

 

 

 

 

 

 

 

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