Back in 2015, an MBARI research team working off Mexico's Gulf of California steered their remotely operated vehicle (known as Doc Ricketts) through the murky depths hoping to glean more info about the biology and distribution of the area's deep-sea inhabitants. At a depth of 2,566 metres, the team spotted a squid drifting through the gloom, a cluster of eggs cradled in its arms. A decade or so later, researchers suspect the egg-carrying squid is a brand new species.

"The deep sea is the largest living space on Earth and there is a lot left to be discovered," explains marine scientist Steven Haddock of the Monterey Bay Aquarium Research Institute (MBARI). Haddock was the chief scientist during the expedition that discovered the squid. "Our unexpected encounter with a squid brooding giant eggs caught the attention of everyone in the ship's control room. This remarkable sighting underscores the diversity of ways that animals adapt to the unique challenges of living in the deep."

While many species of octopuses have been recorded hanging onto their eggs until they hatch, this behaviour is not normally associated with squids. Most squid species deposit their egg clusters on the seafloor or leave them to drift along water columns until they hatch. But that alone wasn't what made this sighting so striking. It was the size and quantity of the eggs that stood out.

At around 11.6 millimetres (about a half inch) in diameter, the eggs are twice the size of those reported in previous sightings of brooding Gonatus squids. There were also far fewer of them. Researchers estimated that the squid was carrying around 30-40 eggs, far fewer than the 3,000 or so seen in other documented cases.

Biologists from MBARI, GEOMAR’s Helmholtz Centre for Ocean Research Kiel, and the University of South Florida carefully reviewed the footage and compared it to other specimens of similar-looking squids collected in the area. They concluded that the 2015 squid is most likely an unknown species of the family of Gonatidae, a finding which they shared in a research publication in Ecology.

Brooding was documented in the squid Bathyteuthis berryi back in 2012 suggesting this behaviour may be more common among deep-sea squids than originally thought. Image: © 2005 MBARI

“Squid play an important role in the ocean – they’re fierce predators and a vital food source for lots of animals, even humans – but we still have a lot to learn about the squid that live in the deep sea,” said Henk-Jan Hoving, who heads up the deep-sea biology working group at GEOMAR, and was lead author of the study describing the new species. “Advanced underwater robots are helping us better understand the lives of deep-water squids, revealing fascinating new information about their biology and behaviour. Each new observation is another piece of the puzzle.”

For the Gulf of California squid, this maternal act was likely her last. “Brooding takes a lot out of a mother squid. She won’t eat while carrying her eggs and ultimately dies after her eggs hatch,” Hoving explains. “But her sacrifice improves the chances that her offspring will survive. It’s just one of the many remarkable adaptations that may help cephalopods to survive in the deep sea.”

MBARI’s ROVs have only observed brooding squids on 17 occasions in the organisation’s 37 years of deep-sea exploration, so there is still much to learn about the biology of deep-sea cephalopods. Researchers have theorised that the giant eggs of the newly revealed Gonatus squid could be beneficial in the relatively stable conditions of the deep sea, as squid moms can invest more effort into fewer offspring possibly increasing their chances of survival. For squids that deposit more numerous, smaller eggs in environments where predation is high, there’s a good chance several of the eggs will wind up on the menus of other species.

Deep-sea squids are an important part of ocean food webs as both predators and prey, but we still know very little about their reproductive biology and natural history. ROVs are helping to uncover the secrets of these deep-sea molluscs.