Why giant Antarctic sea spiders are surprisingly good dads (2024)

Any schoolkid will tell you seahorse dads carry their babies. But sea spiders?

There are 1,500 species of these long, spindly-legged denizens, found in oceans worldwide, and most are doting fathers that care for their unborn young. They range from tiny creatures roaming intertidal pools to behemoths stalking the polar depths.

The fact that male sea spiders play such a big role in raising offspring just adds to their smorgasbord of strangeness. Though they’re arthropods—the phylum that includes spiders, insects, and crustaceans—sea spiders are not closely related to spiders.

In fact, these animals also have a tiny body and store all their internal organs in their limbs. “They’re all legs,” says Felipe Barreto, an evolutionary biologist at Oregon State University. “They’re kind of their own thing.” (Read how sea spiders “breathe” through their legs.)

Now, a recent study on male giant Antarctic sea spiders, the world’s largest, offers a new clue on how their fathering behavior may have arisen.

In the past, male sea spiders may have attached eggs to a safe place, like a rock. Over the eons, the animals likely started gluing eggs on themselves to help their young survive—providing them with an evolutionary advantage.

It’s all in the legs

In most sea spider species, males use ovigers, an extra set of little arms that aid in grooming and to attach fertilized eggs onto their bodies, says Claudia Arango, a sea spider expert at the Queensland Museum in Australia.“It’s quite a big job,” she says.

Methods of egg-carrying vary across species. For instance, some sea spiders package eggs in a mucus-y blob and carry it on their torso. Others roll their offspring into little balls. Some attach them directly to their legs.

The males can carry these eggs for several weeks or months. When the eggs hatch, baby sea spiders, or larvae, often continue to cling onto the caring father for a period of time. They eventually crawl away when they find a food source like an anemone.

Previously, no one knew if male Antarctic giant sea spiders also provided care. Males of these dinner plate-size spiders lack ovigers, and scientists had also never observed a male Antarctic sea spider carrying eggs. (See photos of excellent animal dads.)

In the latest study, scientists captured Antarctic sea spiders from the wild and observed their mating and reproduction in the laboratory. The researchers found that, unlike other sea spiders, Antarctic giant sea spiders don’t attach eggs to their bodies.

Instead, they meticulously glue their eggs to a rock or other substrate at the bottom of the ocean. For two days, “the male just kind of walks around the egg mass and seems to be grooming it,” says study leader Amy Moran, a marine physiological ecologist at the University of Hawai'i at Mānoa.

This behavior may provide a possible explanation on how male sea spiders evolved to be the primary parent. “We [now] have evidence of a species that does something in the middle. They're not quite attached to the body, which is very costly, but they still provide some care,” says Barreto, who wasn’t involved with the research.

Adds Moran: “It sort of opens up this sort of new view of how that might have happened.

‘Full of mysteries’

Some research shows carrying eggs can be a huge sacrifice for males. Eggs can weigh him down and make him less able to clean or take care of himself, says Barreto.

Sometimes males carry so many egg masses, his body is no longer visible. That can make them more susceptible to threats like parasites. "It can take a lot of energy," Barreto says.(Read about the pesky hitchhikers that make life miserable for Antarctic sea spiders.)

Why male sea spiders invest so many resources into their offspring, however, is unknown. The females, in this case, are often only present during mating, when she lets go of her fertilized eggs.

Regardless, learning about sea spider males reveals how little we know about the marine arthropods, and how the animal kingdom can subvert expectations.

“Every expedition to a remote island or remote seamount, scientists come back with new species with something new that we didn't, we didn't know,” says Arango.

For instance, recent research has found that sea spiders can regenerate segments of their bodies.

“There’s a lot of excitement that comes with finding such fascinating creatures,” says Arango. “They’re full of mysteries."