Harry Potter Science # 6: Harry Potter and the Hypertonic Cephalopod

Continuing the Harry Potter science series leading up to the release of Harry Potter and the Deathly Hallows (just two days away!), here is a recap of what we've covered so far, in case you are just tuning in: genetics of magical ability, real-life fossil dragons named after Hogwarts, conservation biology, the botanical principles behind wands, and kin selection.

Today, we turn to something that bothered me the first time I read the the Harry Potter series, and I immediately thought of it when I was brainstorming science-related topics for this series. The issue is that there is apparently a giant squid living in the Hogwarts Lake. It is mentioned at least once in almost every book in the series:
Philosopher's/Sorcerer's Stone: The Weasley twins and their friend Lee Jordan tickle its tentacles
Prisoner of Azkaban: It is sighted crossing the lake.
Goblet of Fire: It rescues the ever-clumsy Dennis Creevey when he falls into the lake, and later someone feeds it toast as a treat.
Order of the Phoenix: Students relax after exams by taking a swim with the creature.

So apparently Hogwarts is home to a benevolent cephalopod, kind of like a school-bus sized class pet. The fact that the squid lifts little boys to safety and eats toast is not what bothered me, however. The point of concern is that cephalopods are supposed to be strictly marine animals: they don't tolerate freshwater environments such as lakes. Despite various cryptozoological musings and even excellent hoax websites (Save the Northwest Tree Octopus! Celebrate the Festival of the Freshwater Squid!), there are currently no known species of freshwater cephalopods in the world, except of course for the mysterious denizen of the Hogwarts lake.

And so, being the intrepid zoologist-in-training that I am, I decided to look into the matter and figure out what is going on here.

First, some background on the giant squid (Architeuthis spp). These animals are thought to grow up to sixty feet long, and although there are claims that they can weigh over a ton, biologists have analyzed all the known specimens and estimate that the maximum mass is 275 kg (606 lb) for females and 150 kg (331 lb) for males . Like all cephalopods, they are relatively short-lived, and reach sexual maturity at around three years old. Taxonomy is, as always, controversial, with some sources claiming there are up to eight or nine different species and some arguing for a single species.


Evidence of the giant squid has been found in all of the world's oceans, especially around Norway and the British Isles (although the only video of a live individual was recorded several hundred miles from the coast of Japan).

Since Hogwarts is located somewhere within a train ride of London, this is one factor that could make it fitting for the animal to be in the area, at least, since it is most abundant in the oceans of this region.

Giant squid are thought to live at depths of 900-3,000 feet, which is a factor that could exclude them from lakes regardless of the salt content, but then again this is Hogwarts, where the ceiling of the dining hall changes to correspond to the weather and where paintings act as guards to the dormitories. In the Goblet of Fire we are told that the lake is a half mile in diameter, but who knows how deep it goes?

But what about the salinity issue? It is well-known that none of the 1000-1200 currently identified ceph species can tolerate freshwater, and many even have seasonal migration patterns that correspond to fluctuations in the salinity of their habitats. Some think this is because cephalopods use hemocyanin as their circulatory fluid. It functions as an analog of hemoglobin in humans, binding oxygen for transport into tissues, and while some claim that it binds oxygen less efficiently at lower salinities, there is not a full consensus on the issue. Also, in animals that are adapted to marine environments, their cells have a lower concentration of salts and other materials than their surroundings, so if they are put into freshwater environments their osmotic regulation goes all out of whack. Freshwater and marine fish groups show separate adaptations to achieve water balance in their respective environments, and although some migratory species move between ocean and river systems during their lifetimes, this requires some unique adaptations. I couldn't find much literature on salt regulation in cephs, but it is probably safe to assume that it would be an issue to their survival even if oxygen-binding was not a factor.

There is a middle ground, however: some species can survive in brackish waters, such as estuaries, where rivers meet the ocean and result in a mixture of fresh and salty water. So, while cephs can't tolerate lakewater, they can sometimes inhabit areas less saline than the sea.

This, combined with the implied depth of the Hogwarts Lake, could lead us to a loophole that would allow Hogwarts to house a squid. It is thought that some Scottish lakes made contact with the sea around the end of the last ice age (about 15,000 years ago). Apparently the glaciers were massive enough to depress the Scottish land enough to allow sea water much farther inland than it is in modern times. Since the melting of the glaciers, the land has gradually risen again, so the sea no longer reaches the lakes, but some, such as Loch Lomond (see picture below) still maintain high salinities to this day. These salty lakes (aka meromictic lakes) have layers that don't mix, due to being very deep (up to 310 m in some cases, putting us into the range of giant squid depths) and having little disturbance. So as long as the Hogwarts squid doesn't go through the wrong layer, it could potentially survive in the lake.

Another alternative is that the lake is indeed so deep that it goes straight below the continental shelf, allowing it to contact saline sea water. There are examples of these "sea lochs" in the British Isles too: Loch Morar and Loch Etive, for example.

There you have it, once again the magical world of Harry Potter allows us to explore the world of science, which, lucky for us mere muggles, doesn't need spells and wands to be fascinating and awe-inspiring.

(Hat-tip to R. H. for information on the sea lochs!)