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Just How Big Are the Biggest Sea Monsters, Really?

We have a tendency to exaggerate.

"It was THIS BIG." We have an obsession with size, and a tendency to the extreme—bigger is better.

It's not only hobbyist anglers who like to exaggerate; a new study sets out to determine exactly how big some of the largest ocean-dwelling creatures are. A group of researchers led by Craig McClain of the National Evolutionary Synthesis Center in North Carolina published their results in the​ PeerJ journal in the hopes of settling the score on the biggest sizes that marine megafauna can really reach.

In their introduction, they note that scientists and the public alike get excited about the biggest species; "However, misconceptions about the sizes these species obtain are just as rampant in the scientific literature as the popular media."

Just look at the popularity of pseudoscience around t​he now-extinct Megalodon.

The study reports the sizes of the largest known individuals of some of the most popular marine megafauna species. The biggest in total length included in their study is the lion's mane jellyfis​h (Cyanea capillata) at 36.6m. The blue whale comes in at 33m, and the giant squid at a comparably measly 12m.

Click the image for the full size version. Image: Matthew Maxwell and Pablo Alvarez Vinagre at StudioAM

While it's fun (or terrifying) to try to picture the most monstrous monsters of the deep, measurements are a pretty basic datapoint to help understand these organisms and their biological role. The researchers were also interested in the size variation within species; they point out that the largest sizes aren't necessarily optimal.

But even though the sizes they give for the largest known individuals are more accurate than the figures passed down in dramatic retellings, they might not be entirely correct. The researchers used a wide range of data sources to get their information, from re-measuring museum specimens to collecting fisheries data and official reports, and even checking eBay sales—they found a specimen of the largest gastropod, a sea snail called Syrinx aruanus and commonly known as the Australian trumpet, listed in an old UK eBa​y auction at 72.4cm (the authors put the largest specimen at 72.2cm, however, as this was more reliably confirmed).

Some species in particular were nevertheless tricky to pin down; many are endangered, which means data is limited. For some species like giant squid and oarfish, the researchers only had measurements from dead or dying animals, which might not be representative of the healthy population.

In other cases, data might not be 100 percent reliable. Take that colossal lion's mane jellyfish—the 36.6m measurement comes from ​an 1865 report that may in fact have referred to a different species. The authors note that they are additionally skeptical as the original report does not detail how measurements were taken.

While the top-end of the measuring stick was the focus of the study, the researchers reiterate in their conclusion that for most species, these giants are outliers. The median size of a giant squid is just 7.3m, and the majority of species in the ocean are small.

In a rather poetic admission, they wrote: "By focusing both on the largest species and the largest individuals of them, we concentrated on, ironically, the smallest fraction of life in the oceans."