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The meaning of pectoral fin position part III

Hunched back: threat- or feeling threatened display – a guestpost by Pascal Gospodinov

First we will discuss how a hunched back is created, talking about general appearance and than in a second part we will try to enlighten what the meaning of this hunch display might be.

The general appearance of the hunch display is lowered pectoral fins, upward pointing snout and the resulting hump and as result an arrhythmic swim motion.

Here both pectoral fins are lowered. Why? The reason is simple the shark is preparing himself to turn in every direction, because he probably don’t know exactly which direction is gonna be the safest. The result of lowering both pectoral fins is an upward pointing snout. We discussed in part I that the pectoral fins are also used to counteract the downforce. as we don’t have this counteraction when both pectoral fins are lowered, the shark try to get more water under his belly, to compensate the missing fins. The result of the upward ponting snout is the hunchback and the result of the hunchback is the arrhythmic swim motion. All the hunch display is just the result of both pectoral fins being lowered, not more not less. Again both lowered pectoral fins will enable the shark to turn left or right.

As discussed in part I lowered pectoral fins increases the lateral surface and enables the shark to turn. In the hunch display the pectoral fins are mostly lowered to the maximum, to give the shark the possibility to turn with the narrowest bow possible.

Sharks will show the hunch display when pressure of any kind is put on them; for example if divers hassle them, or in very closed environment or both… but all kind of scenarios are possible. So back to our question what can be the meaning of the display? The general assumption is that the hunch display is a threat display.

Why should an animal prepare himself to escape, because thats what the shark do in lowering his pectoral fins if he wants to threat, show dominance, aggression or even attack? It makes no sense. An animal that prepares himself to flee feels threatened. The shark feels stressed and is looking for an escape. At that point its up to the diver that is confronted to that kind of behavior to react correctly and to relax the situation whilst giving the shark more space to maneuver and not hassle even if it wasn’t deliberate… When sharks show hunch display its because they feel threatened and its not a sign of aggression, dominance or any kind of agonistic display.

Picture 1. On the right side: hunch display. On the left: cruising posture

It exists another scenario in which the pectoral fins are lowered and the shark will show a hunched back. A scenario triggered by Remoras a.k.a sharksuckers that sucks in themselves to sensory areas of the shark. The shark will try to force the Remoras to move by shacking his body or bending his skin in the area in which the sharksuckers put suction pressure. This bending and shacking can look very similar to the hunch display explained before…

As asked at the beginning: „is the hunch display a threat- or a threatened display?“, what would you say?

A behavior is caused by a need. Why would a shark create a behavior against a person, because they never show that in regard of other sharks. We were not part of their evolution so how can a behavior against humans be created in less than 100 years (the time in which humans are swimming in the oceans frequently), its simpley impossible.

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.

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!












The meaning of pectoral fin position part I

Fundamentals of pectoral fin position – a guestpost by Pascal Gospodinov

When we see sharks, we are impressed, amazed, overwhelmed and during first encounters we never have the time or mindset to really observe behavior details. But with time and after several encounters, we will still be extremely amazed to dive with sharks, but we will begin to see different things that we couldn’t realize due to first astonishment.

This is gonna be a little introduction in the meaning of pectoral fin position. It will help you to know and to understand when a shark will turn and in which direction. The comprehension about it can help to feel more than comfortable during shark diving, it can help to understand a situation or it can be a great „tool“ to make the picture you want. All in all it is always good to know when and if a shark is gonna turn or not. We are not going to discuss WHY a shark is turning, this can have a multitude of reasons, but when and how you can recognize it.

As the title already shows it have something to do with the pectoral fins (breast fins), the pectoral fins can be defined as ruder and the tail fin as propeller. So the shark steers with his pectoral fins.

Knowing that it becomes very easy. If a shark wants to turn to his right he simply pushes his right pectoral fin down (see picture 1), if he wants to turn left he pushes the left fin down. The lowering of a pectoral fin increases the lateral surface and creates a speed burst, so than its all about physics.

Picture 1: Oceanic Whitetip turning to his right – He is lowering his right pectoral fin

In a result of lateral surface increase and speed burst on one side the shark is turning.

So depending on the inclination of the pectoral fin the bow that the shark is gonna describe is smaller or bigger. If the shark pushes his fin completely down (90° from starting position) the bow will be small, if the angle is smaller the bow will respectively be bigger. How fast the shark is veering depends on how quick he is lowering his fin and how fast he is (due to tail fin frequency). If he wants to veer quickly the shark will have to increase his speed, quick speed burst, 2-3 or more rapid tail beats.

So if we see a shark lowering one of his pectoral fin we know now that he will veer in the respective direction. As discussed it can happen quickly or slowly depending on his speed and the bow can be narrow or wide depending on the angle of the pectoral fin. Beside being used as ruder the pectoral fins can have other purposes!

When sharks are cruising the pectoral fins are used as kind of wings, to stabilize and to counteract the slightly negative buoyancy (downforce). This is the case especially when the shark don’t use his tail fin for a short while, than it really looks like the sharks is flying and the pectoral fins works as wings, because of their morphology (slightly concave). The water stream has a longer way on the upper side of the fin due to the structure and so the water is accelerated on the upper side and kind of „sucks“ the shark upwards.

If sharks want to stop cruising because an object is suddenly in their way, they try to use the pectoral fins as brakes… Sharks are awfully bad in putting on brakes! To brake they put up the pectoral fins (see picture 2) that will slow them down but sharks are not really able to stop like bony fishes do and if you are the suddenly appearing object and they don’t have the time to turn anymore they will crush into you.

Don´t panic because the shark is probably more afraid to have crush into you than you are.

Picture 2: Tiger shark trying to „brake“