Why Are These Sea Urchins Sporting Cowboy and Viking Hats? There's Science to Their Hot Looks

After noticing his urchins carrying rocks, shells and even hermit crabs around in his 120 gallon saltwater aquarium, a Colorado aquarium enthusiast started making them custom hats.

Wilson Souza's first design was meant to be functional rather than stylish. Souza compared it to a wok cover. Since then, he's had to become a fashion designer of sorts for the echinoderms (a phylum including sea urchins, starfish and sea cucumbers), tailoring cowboy, viking and top hats with an arched bottom to give the urchins enough purchase for their spines to hold on.

A sea urchin wearing a 3D printed cowboy hat. Wilson Souza

Emma Verling, who holds a doctorate in marine ecology from University College Cork, described to Newsweek how sea urchins mount hats, shells, rocks and other objects atop their spines, manipulating objects with hundreds of tube feet—flexible stalks with suction cups.

"This army of tube feet essentially grabs on to the upper surface of the objects," Verling said, in response to emailed questions about urchin hats. "Hundreds of stiff, needle-like (but brittle) spines really spring into action, manipulating the object like little levers. Once sufficient of their number can get underneath the object, they then appear to push the object from below as the tube feet maintain their positions on top, and the object is 'flipped' onto the urchin itself."

Verling described the process as "quite entertaining to watch," as the dextrous echinoderms coordinate their appendages to lift objects onto their spines, sometimes making multiple attempts when their coverings slip off.

Souza's sea urchin is ready for the opera. Wilson Souza

Souza's behatted urchins were an immediate hit on saltwater aquarium forum Reef2Reef, where users brainstormed rhinestone caps, shared memes ("Pardner, this tank ain't big enough for the two of us" one user captioned a photo of a sea urchin in a cowboy hat) and imagined trying to justify buying a sea urchin viking helmet to their spouses.

After Souza started getting requests from other aquarium owners, he began printing and shipping out hats, largely at-cost, "for the pleasure of having other people enjoy the experience too."

Wilson Souza's multiple sea urchin hat designs. Wilson Souza

Souza was initially prompted to craft urchin hats by scientific curiosity, after observing behavior that pointed to a possible explanation for why his urchins kept picking up objects to cover parts of their spiny bodies.

"I noticed at night they would let go of some of the stuff they carried during the day," Souza told Newsweek. "I started wondering if it has anything to do with the full spectrum lights that we use for corals. I shared it with my wife Sylvia, who said, 'Well, if they are trying to protect themselves from the UV rays, then why don't you design and 3D print some hats for the poor little things?'"

Souza's hypothesis has been borne out by scientists, including Verling, now a post-doctoral researcher at MaREI, the Science Foundation Ireland Research Centre for Energy, Climate and Marine—coordinated by the Environmental Research Institute at University College Cork in Ireland—who conducted research in 2000 demonstrating one reason for "covering" or "heaping" behavior among Paracentrotus lividus, or the purple sea urchin of the Mediterranean and eastern Atlantic.

"I remember sitting for hours with only my flask of tea for warmth and the urchins for company," Verling recalled for Newsweek, describing the experiment she conducted at Lough Hyne Marine Nature Reserve on Ireland's southern tip, where a saltwater lake connects to the Atlantic Ocean, creating an oxygen-rich, warm water environment for a variety of species, including the purple sea urchin observed by the area's first marine biologist researchers in the 19th century. Urchins there preferred covering themselves with the widely available shells of saddle oysters.

Lough Hyne was designated Ireland's first Marine Nature Reserve in 1981. © Superbass / CC-BY-SA-4.0 via Wikimedia Commons

In her paper, published in the journal Marine Biology in 2002, Verling recorded covering behavior under different lighting conditions and found that purple sea urchins covered up most in response to UV light—the same kind of light keeping Souza's aquarium corals healthy.

As with human sunburns, UV light can be damaging to marine species, potentially conferring an evolutionary advantage to urchins who engage in covering behavior to shield themselves from the sun.

But Verling cautioned against a simple explanation for the surprising and complex behavior, especially since multiple sea urchin species cover themselves, including deep sea species who aren't subject to UV light. Instead, she described the behavior as likely "multifactorial," citing other potential explanations validated by research into different urchin species, including protection or camouflage from predators and ballast against surging ocean currents.

"I saw them covering with all sorts of things over the years: shells, stones, bits of macroalgae, bottle tops and pieces of wood," Verling said. "It would appear to be an energetically costly thing to do, and that's why it's so interesting to study it."

Verling has seen many variations on sea urchin covering behavior, but has probably never observed a sea urchin wearing a top hat and carrying a back-up top hat in the wild. Wilson Souza

Designing hats for sea urchins turned out to be a recent bright spot in an otherwise challenging hobby for Souza, who sought out help from the forum in 2019, when a disastrous experience with aquatic parasites—including a protozoan known in aquarium circles as ich and a dinoflagellate algae that causes a deadly disease known as "marine velvet"—killed all of his fish within a three-day period.

Out of 20 fish, only two pajama cardinalfish survived. Souza's tank is now populated by shrimp, hermit crabs, anemones, snails, emerald crabs and his hat enthusiast urchins, which include tuxedo urchins, a black sea urchin and a white shortspine sea urchin named "Spiky."

"I am very close to quitting for the sake of the fishes," he shared at the time, describing saltwater aquariums as "the most frustrating hobby" he's ever had. Ascribing the deaths to fish dealers subjecting their stock to chronic copper exposure—used to control algae and parasites, but suppressive to fish immune systems and damaging to gills over time—Souza urged anyone considering getting into the hobby to test water from their supplier for copper and consider shopping elsewhere if fish stock is being subjected to the metal preventively.

While sea urchins can spend their days sporting hats in Souza's aquarium, they fulfill less fashionable roles in their wild marine environments, grazing on seaweed (macroalgae) and opening seafloor habitats for other species (Verling describes them as the "beavers of the ocean"), balanced by keystone species like otters keeping them in check and preserving kelp forests.

"The macroalgae-sea urchin-otter balancing act is a beautifully simple example of how ecosystems are a web of intricate relationships, not a collection of independent species doing their own thing," Verling, who currently works on scientific policy related to improving environmental conditions in European waters, told Newsweek. "That is one thing I would like more people to understand, so that they can see that we humans are also part of this web of relationships and that keeping our planet in balance is in all of our interests."

This web of relationships also means we may someday develop cowboy hats for humans, as well as sea urchins.