Giant Antarctic sea spiders reveal their parenting styles

The giant Antarctic sea spider (Colossendeis megalonyx) has a parenting style that has puzzled ecologists for more than a century.

But after diving under the ice to investigate, US researchers think they’ve solved the mystery.

Sea spiders are marine arthropods, and are related to, but not the same as, land spiders (they belong to the order Pantopoda, while land spiders are from the order Arachnida).

Among their many bizarre traits, including having between 8 and 12 legs which breathe for them and guts that pump blood, sea spiders exhibit the “rarest type” of parental care in the natural world.

“In most sea spiders, the male parent takes care of the babies by carrying them around while they develop,” says Amy Moran, a professor at the University of Hawaii at Mānoa’s school of life sciences and lead author on a paper published in Ecology.

“What’s weird is that despite descriptions and research going back over 140 years, no one had ever seen the giant Antarctic sea spiders brooding their young or knew anything about their development.”

Sea spiders are found all over the world’s oceans, and most species are a few millimetres in size. But giant Antarctic sea spiders run to about 30cm in diameter, mostly thanks to their super-long legs. This is an example of “polar gigantism”, where organisms get much larger at the poles.

Moran and colleagues dived under the ice near McMurdo Station during an Antarctic expedition to collect sea spiders that looked like they were mating.

They put them in tanks to observe, and saw two mating groups produce gelatinous clouds of eggs.

Then, one parent – the researchers think this is the father – glued the sea spider eggs to rock at the bottom of the tank.

The eggs stayed there for months, during which time algae grew over them – providing camouflage.

“We could hardly see the eggs even when we knew they were there, which is probably why researchers had never seen this before,” says co-author Graham Lobert, a PhD student at the University of Hawaii at Mānoa.

Eventually, tiny larvae hatched from the eggs.

“We were so lucky to be able to see this,” says co-author Aaron Toh, also a PhD student at the University of Hawaii at Mānoa.

“The opportunity to work directly with these amazing animals in Antarctica meant we could learn things no one had ever even guessed.”