Shark in the Pool!

Explanation: Everyone out of the pool!

Welcome to Punta Moreno on Isabela Island off the coast of Ecuador. We are less than one degree latitude south of the equator, exploring the rugged lava coastline of this largest of the Galapagos Island archipelago.

Before us is a salt water pool apparently isolated from the coast a short distance away, and swimming in the clear cold waters are a green sea turtle, pufferfish, blue-chin parrotfish, salema fish, and not one but two whitetip reef sharks patrolling the shallows.

This isolated pool was a good half kilometer from the actual coast
on Isabela Island of the Galapagos Archipelago. Not a safe place
to take a dip, although if you look closely, the two tiny pink dots
in the upper right are two wading lesser flamingos.

These whitetip reef sharks have reached their usual maximum size of a bit over 5 feet long, and seemed well fed and healthy. I strongly suspected that they, along with the turtle and fish, had reached this inland pool by way of underground lava tubes connecting this water body to the ocean.

Note the shape of the tail, typical for sharks -- called a heterocercal tail in which the the upper, longer lobe contains the vertebral column.

This is an ancient anatomical form found in most Paleozoic fishes.

The function of this more extreme tail shape in sharks has been debated but seems to play a role in steadying the fish's horizontal and angled swimming.

Whitetip reef sharks are widely distributed in the Pacific Ocean, although it is reported that they are mostly active at night and rarely come to the surface, making these observations most special and uncommon.

Although not known to be particularly aggressive to humans,
this is still not the best swimming buddy!
Whitetips feed on a variety of benthic creatures including
fish, octopus, lobsters, and crabs, and even feed cooperatively.

Whitetip reef sharks are fished for their meat, and are denoted as "near threatened" by IUCN.

Information:
     Kim, S. H., K. Shimada, and C. K. Rigsby. 2013. Anatomy and evolution of heterocercal tail in Lamniform sharks. Evolutionary Biology 296(3):433-442.
     Wilga, D.C., and G. V Lauder. 2002. Function of the heterocercal tail in sharks: quantitative wake dynamics during steady horizontal and vertical maneuvering. Journal of Experimental Biology 205(Pt16):2365-2374.
     Thomson, K. S. 1976. On the heterocercal tail in sharks. Paleobiology 2:19-38.

EPOW - Ecology Picture of the Week