What Makes Sharks Sink? Fresh Water

Sharks may be some of the most fearsome monsters of the sea, but like any good monster, they come with an unexpected weakness. Much like the Wicked Witch of the West, if a shark finds itself surrounded by fresh water, it's in big trouble.

That's at least according to a study recently published in the Journal of Experimental Biology, which details how the physiology of a whopping 95 percent of all elasmobranchs - that's all sharks, rays, and skates - boast outdated oily livers used for flotation and buoyancy.

Many modern freshwater-dwelling fish boast complex bladders that are complete with air chambers, which allow them to regulate buoyancy much like the cuttlefish's unique and porous chemical chambered cuttlebone.

The oil bladder, in comparison, is far more simple, and not nearly as effective at maintaining buoyancy in changing depths. Additionally, objects are naturally more buoyant in salt water because salt water naturally boasts a greater density.

In a study of bull sharks - one of the few sharks that travel from oceans to freshwater rivers and swamps - researchers found that because of a loss of buoyancy they wind up spending about 50 percent more energy on lift.

For most sharks, their inadequate livers would cause them to sink like a stone in freshwater. The researchers theorized that sharks could compensate for this fact by boasting wider and less sleek bodies, sacrificing hydrodynamics for the sake of a larger and more buoyant liver. And yet, even that is not enough.

The data seems to reflect this. Of the five bull sharks and 17 largetooth sawfish (a bottom-dwelling shark relative) involved in the study, all were found to be consistently less buoyant (an average of 27 percent less buoyant) in their habitats, compared to their ocean dwelling counterparts, despite their extra fatty livers.

This inadequacy, the authors suggest, may very well be what has kept elasmobranchs stuck in the ocean, their livers telling them that freshwater is just simply not where they belong.

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