New Bull Shark Papers!

This is a BIG Tarpon - or a tiny lady! Source.

I've been remiss in not posting about some recent papers.

But now, thanks to Evan, I got time - that is, for as long as we got power!

But first, watch this!

Yes that would be a big-ass Bull Shark!

And yes they do breach!

But over to those papers.

The first is this one of which you can find a nice synopsis here.

Apparently, Bull Sharks have the strongest bite of all Sharks - and whilst they grow, the bite force increases disproportionately (= allometrically) whilst they are young and then increases in a proportionate way (= isometrically) once they are older.

I must say, I like that!

My obligated question to visiting Shark researchers is, why has Evolution selected for the Bulls to be so massive considering that it appears to be a hydrodynamic disadvantage.
My favorite, and most frequent answer is because they are the apex-apex predator that has evolved to also prey on Sharks - and when they do, they are built in order to get the job done and dispatch their prey quickly and without getting injured!

Here are possible explanations from the paper.

About the initial allometric development.
Positive allometry of bite force appears to be largely associated with the earlier stages of life history for this species, perhaps indicative of a “performance gain” allowing bull sharks to attain higher bite forces and access functionally difficult prey earlier in life.

About the high bite force in general.
It is possible that the “over-design” of the feeding mechanisms of large sharks is simply a by-product of large body size, although it may also be tied to functions beyond the initial penetration of the prey item. For example, bull sharks generally inhabit murky waters (Compagno et al., 2005) in which the relocation and recapture of wounded prey is likely difficult. Therefore, a “bite and grip” behavior in which prey is held with extremely forceful jaws (high bite force and high MA) could be an effective strategy. 

This is in contrast to white sharks which may use a “bite, spit and wait” strategy in which wounded prey is left to exsanguinate and is later consumed (Tricas and McCosker, 1984 but see Klimley and Anderson, 1996; Martin et al., 2005). Although bull and white sharks have comparably high bite forces, they appear to be perhaps more excessive in the latter case. 

A “bite and grip” strategy may also be more important for species that use head-shaking to dismember large prey, where a forceful grasp on the swinging food item is necessary to rip tissue and to avoid considerable damage to the teeth and jaws (Gerry et al., 2010). Combined with the dual articulation between the upper and lower jaws (Motta and Wilga, 1995), powerful adduction of the jaws might prevent damage and dislocation.

Very nice :)

I'm however less convinced about this one.

It's thankfully open access so please, do check it out.

If I understand it correctly, it comes to the conclusion that Atlantic Tarpon tread carefully when around large Bull Sharks (which is pretty much trivial - so do we!) to the point of actively avoiding them and even retreating into fresh water systems in order to escape predation by, specifically, Bull Sharks.

Now nobody disputes that large Bulls prey on Tarpon, see below.

But provided that these fishing videos are in fact indicative for what happens naturally, which is not a given - so do other species, foremost of which Great Hammerheads!

Case in point - and kids competing for a Darwin Award!

Now.

Assuming that evolution has selected for Tarpon to escape predation by Sharks by retreating into fresh water systems, which is certainly plausible - is it equally plausible to postulate this specifically with respect to the one Shark that could well decide to follow them inside the rivers if it chose to do so?

Would it not be more plausible to assert that this retreat happened in order to avoid predation by the other, strictly marine large Sharks like Great HHs and Tigers?

And then I find this.

Tarpon are certainly very well adapted to entering fresh water insofar as they can tolerate euryhaline environments (0-47 parts per thousand) and often enter river mouths and bays and travel upstream into fresh water. In addition, tarpon can also tolerate oxygen-poor environments due to a modified air bladder that allows them to inhale atmospheric oxygen.

But this is not limited to this one species. The genus Megalops comprises two species and the Indo-Pacific Tarpon Megalops cyprinoides is equally known to access fresh-water systems all the way to inland lakes - and even the related Ladyfishes that share of the same Order Elopiformes are equally eurhyaline, at least at the juvenile stage.

Are we to believe that all of this has evolved merely in order to avoid predation by specifically, Bull Sharks?

As I said, I'm not convinced.

It's a nice study with interesting techniques and data showing some Shark-Bay-esque displacement effects - but when it comes to the part where our hypotheses outlined above require significant investigation by increasing tracking efforts and gathering further ecological data for sharks, tarpon and their potential prey, I fear that the results will not confirm the link to Bull Shark, at least not specifically.
My hunch is that the Tarpon enter fresh water because they can - in order to hunt in those rich biota and yes, maybe also to avoid Sharks at the time when those aggregate. But to speculate that they may have developed that faculty because they want to specifically avoid Bull Sharks - don't think so.

But I still love you guys - keep up the good work! :)