Some shark species show up with greater regularity every summer along the Maine coast. The movement of the White Shark Carcharodon carcharias is probably of the greatest interest to the casual swimmer.
First, yes, people swim in the cold waters of Maine.
Second, shark attacks are rare; however, that means little when an attack hits close to home.
White sharks tend to move into northern waters, probably searching for prey. Typically the observations of White Sharks in the Gulf of Maine start in August and peak in October. However, as with so much else, the arrival of sharks is earlier with global climate change.
This year, 2023, by late July a White Shark was observed feeding on a seal off Rockland Harbor. In 2020 a swimmer was attacked and killed off Bailey Island (near Portland) in late July by a White Shark. This death was the only one ever recorded from Maine waters. It is unlikely to be the last.
My goal here is not to spread fear, far from it, but to spread awareness until I go off on some rant or other in later paragraphs. White Sharks are glorious creatures, and I would love to see one swimming near me as I explore the Maine coast this summer.
Shark anatomy has features of great interest, from the large liver, which aids buoyancy, to the salt-excreting rectal gland; the function was discovered not far from where I’m writing this on MDI Maine at the MDI Biological Lab (MDIBL) to the Ampullae of Lorenzini, the electro-reception organs which enables them to hone in on prey when in close quarters.
Back to the rectal gland. The function was determined in the dogfish shark, Squalus acanthias. The rectal gland’s presence and function were hypothesized by Homer Smith in 1930 because dogfish sharks were not excreting excess salts in their urine. He figured those excess salts had to be dealt with somehow. Wendell Burgess discovered that the rectal gland did the excreting, removing the excess sodium (Na) and chloride (Cl) ions common in animals living in the sea that maintain differing osmolality. This work, as mentioned above, was done at the MDIBL. It was Wendell Burger who identified the rectal gland at the MDIBL.
A side note of sorts, if an animal conforms to the ocean’s osmolality (saltiness if you wish), like many invertebrates, it will suffer the consequences of changes in the outside environment. Sharks are not the only animal with special organs that deal with salinity. Many marine birds, for example, herring gulls, have specialized glands in the beak, and marine iguanas have salt-excreting glands around their head.
I almost forgot about the White Shark: Linnaeus described the Great White Shark as Squalus carcharias in 1758. The description from fishbase.org of this animal is something like this: Possibly to 8 m, the world's largest predator. Generally in the open ocean, but does swim into shallower waters. Most attacks occurred in estuaries. Usually, swims alone or in pairs but can be found in feeding aggregations. A versatile predator with a broad prey spectrum. Feeds on sturgeon, tunas, sea lions, and other large animals and fish. Presumably ovoviviparous. Reported by some experts to attack humans, which they mistake for their normal prey of seals.
Let’s end this odd, off-kilter rant with the images:
Burger JW, and Hess WN. 1960. Function of the rectal gland in the spiny dogfish. Science 131: 670–671.
Curtis, T.H., Metzger, Gl, Fischer, C., McBride, B., McCallister, M., Winn, L.J., Quinlan, J., Ajemian, M.J. 2018. First insights into the movements of young-of-the-year white sharks (Carcharodon carcharias) in the western North Atlantic Ocean. Scientific Reports 8(1):10794.https://doi.org/10.1038/s41598-018-29180-
James, R.C., Curtis, T.H., Galuardi, B., Metzger, G., Newton, A., McCallister, M.P., Fischer, G.C., Ajemian, M.J. 2022. Overwinter habitat use of young-of-the-year white sharks (Carcharodon carcharias) off the eastern United States. Fisheries Bulletin 120:68-73. doi: 10.7755/FB.120.1.6
Franks, B.R., Tyminiski, J.P., Hussey, N.E., Braun, C.D., Newton, A.L., Thorrold, S.R., Fischer, G.C., McBride, B., Hueter, R.E. 2021. Spatio-temporal variability in white shark (Carcharodon carcharias) movement ecology during residency and migration phases in the Western North Atlantic. Frontiers in Marine Science 8:744202. doi: 10.3389/fmars.2021.744202.
Merly, L., Lange, L., Meÿer, M., Hewitt, A.M., Koen, P., Fischer, C., Muller, J.,Schilack, V., Wentzel, M., Hammerschlag, N. (2019). Blood plasma levels of heavy metals and trace elements in white sharks (Carcharodon carcharias) and potential health consequences. Marine Pollution Bulletin 142:85-92. https://doi.org/10.1016/j.marpolbul.2019.03.018.
Skomal, G.B., Braun, C.D., Chisholm, J.H. and Thorrold, S.R. 2017. Movements of the white shark Carcharodon carcharias in the North Atlantic Ocean. Marine Ecology Progress Series 580:1-16. https://doi.org/10.3354/meps12306