A few years ago, Carl Zimmer and I ran a workshop on science writing, where we talked, among other things, about explaining science without talking down to your audience. It apparently left an impression on Craig McClain, a marine biologist and blogger who was in the audience. “I made a comment about how I always wanted to write a post on how giant squid sizes are bullsh*t,” he recalls,” but that those always come off as an arrogant scientist telling the world that it’s wrong. And you said: You should write it, but you just need to find the right tone. That kicked me off.”

Rather than an angry blog post, McClain decided to put together a scientific paper that would accurately answer a simple yet slippery question: How big do the biggest animals in the ocean get?

The oceans are home to giants: blue whales and great white sharks; giant squids and giant clams; elephant seals and Japanese spider crabs. These creatures have no trouble capturing the public imagination, but scientists often have trouble capturing them. Many are rare, elusive, or live in inaccessible parts of the sea. Some are only measured when they wash ashore, after dry land distends or deflates their bodies. Some are so big that they are just plain hard to measure. And so, oceans are also home to exaggeration.

Take the giant squid. Umpteen media report claim that this nigh-mythic animal can grow up to 60 feet (18 metres) in length. That’s absurd, McClain thought. The vast majority are less than half that length. Many individuals are measured when they wash ashore, after decomposition loosens their muscles and eager humans stretch their tentacles. One size estimate even came from someone counting his paces next to a beached squid! Shoddy data had been unleashed upon the kraken.

McClain, together with Meghan Balk from the University of New Mexico, went after better sources. They recruited five keen undergraduate students and large team of colleagues, who divided a list of target species between them. They trawled the scientific literature for measurements. They combed through books, newsletters, and newspapers. They asked colleagues at museums to measure specimens in their collections. They contacted networks that rescue stranded turtles. They reeled in eBay records to find the measurements of giant clams and snail shells. And together, they found the best possible estimates for the maximum sizes of 25 ocean giants.

For some species, widely quoted figures were outrageously wrong. The largest verifiable giant squid was 12 metres long—giant, sure, but a damn sight smaller than 18 metres. The biggest known walrus weighed 1,883 kilograms, a far cry from the 2,500 kilogram titan that a hunter supposedly shot, and clearly embellished.

For some species, estimates were outdated—giant clam sizes all date to the 60s and 70s. For others, like the lion’s mane jellyfish and Japanese spider crab, the team found that accurate data just doesn’t exist.

And “some animals may just not be getting as big as they used to get,” says McClain. In 1885, fishermen in the Aleutian Islands caught a Giant Pacific octopus that was 9.8 metres from one arm tip to the next. “The two octopus experts who were with me on this paper say that they just don’t get that big any more,” says McClain. “It could be pollution or climate change.”

Hype and decline aside, the stats from the paper still tell of oceans that are full of impressive leviathans. There are giant barrel sponges, whose bodies are just two layers of cells sandwiching a jelly filling, but can nonetheless grow to 2.5 metres wide. There are 2-metre-wide Nomura’s jellyfish that can weigh 200 kilograms. In the depths, 3-metre-long giant tube worms thrive near hot, belching, volcanically heated vents, and giant isopods—a kind of undersea woodlouse on steroids—can reach 50 centimetres. Seven treacherous metres exist between the tip of a great white shark’s toothy snout and the end of its powerful tail.

The largest mammal, the blue whale, grows up to 33 metres long, and can swallow half a million calories in one mouthful. The longest fish—the star-backed whale shark—gets to 18.8 metres. The longest bony fish—the bizarre, serpentine oarfish—can reach 8 metres. The heaviest bony fish—the ocean sunfish, which looks like the decapitated head of a much larger fish—grows to just 3.3 metres long but weighs up to 2,300 kilograms. That’s much heavier than the biggest turtle (leatherback, 650 kg), a little heavier than the biggest walrus (1,883 kg), and not a patch on the heaviest seal (Southern elephant seal, 5,000 kg).

These measurements are all as accurate as possible; finding them often involved a labyrinth of references and phone calls. For the Australian trumpet—a beachball-sized monster of a snail—McClain found an issue of Hawaiian Shell News, in which a collector named Don Pisor holds up an enormous and supposedly 90-cm shell. But McClain also found a copy of the Registry of World Record Size Shells, which said that the record-holder was just 72 centimetres long. It was also attributed to Don Pisor. Were these the same specimens? Was this guy a charlatan? Or just phenomenally good at catching escargods? There was only one thing to do: McClain tracked down Pisor and asked him. It was just one specimen, he said, and he donated it to the Houston Museum of Natural Science. McClain called the museum. 72 centimetres, they said. Another data point for the list.

In a few cases, the team discovered new things about the giants. Andrew Thaler, who collected the data on giant isopods suddenly realised that males were, on average, much bigger than females. “That’s not something either of us knew before and both of us know everything there is to know about giant isopods,” says McClain.

And for several animals, the team managed to plot out the distribution of their sizes, rather than just the maximum. Scientifically, that’s more useful. People love to know how big animals can get, but that tells us very little about their typical lives. The biggest known giant squid was 12 metres long, but their average length is 7.3 metres, and most individuals are shorter than 9.2. Its archenemy, the sperm whale, has a recorded maximum size of 24 metres, but 95 percent of these whales are shorter than 15 metres.

As McClain—himself a bear of a man at 6’ 2”—points out, “individuals may reach these extraordinary large sizes through developmental or genetic defects.” The tallest human ever, Robert Wadlow, was 8 feet and 11 inches in height; he also needed leg braces to walk and died at 21 from an infection aggravated by an autoimmune disease. The tallest woman, Zeng Jillian, reached her lofty 8 feet and 1 inch because of a tumour in her pituitary gland; she died at 17. We are fascinated by extremes, but life mostly plays out in the middle.

Reference: McClain, Balk, Benfield, Branch, Chen, Cosgrove, Dove, Helm, Hochberg, Gaskins, Lee, Marshall, McMurray, Schanche, Stone & Thaler. 2015. Sizing ocean giants: patterns of intraspecific size variation in marine megafauna. PeerJ http://dx.doi.org/10.7717/peerj.715