Deep-sea video captures giant sea spiders mating for the first time

In a landmark moment for marine biology, scientists have successfully documented the mating behavior of giant sea spiders.

This species, belonging to the group of marine invertebrates known as Colossendeidae, has been recognized by science for over a century and a half. However, the details of their reproductive cycle have remained mysterious.

The groundbreaking observation occurred during a 2022 expedition conducted by the Ocean Exploration Trust (OET) aboard the Exploration Vessel (E/V) Nautilus. A remotely operated vehicle (ROV) dove to the impressive depth of 5,525 feet to capture this never-before-seen behavior. 

The focus of the expedition was to explore unsurveyed deep-sea habitats around the Kingman Reef and Palmyra Atoll, including within the Pacific Remote Islands Marine National Monument.

The significant findings have been recently outlined in a study published in the scientific journal Marine Biodiversity.

Sea spiders — understanding the basics

Sea spiders, or pycnogonids, aren’t actually true spiders, but they are part of the arthropod family, along with spiders, insects, and crustaceans. These marine creatures hang out in oceans all over the globe, from shallow waters to the deepest trenches.

Some sea spiders can get pretty big, especially in polar regions, thanks to a cool phenomenon called polar gigantism.

This means that species living in cold and deep-sea areas tend to grow larger than their relatives in warmer parts of the world.

The biggest sea spider out there is the Colossendeis colossea, which you can find in the Southern Ocean around Antarctica.

These giants can boast leg spans of over 70 centimeters (27 inches) — that’s pretty massive compared to most other sea spiders, which usually have leg spans of just a few centimeters.

Sea spiders have a unique look compared to other arthropods. They have tiny bodies and long, thin legs that make them resemble spiders, even though they’re not closely related. They also have a proboscis that they use to suck up nutrients from their prey, which includes various soft-bodied invertebrates.

Mating secrets revealed

Study lead author and zoologist Georg Brenneis from the University of Vienna, expressed the importance of the event: “This is the first time humans have ever witnessed this behavior.” 

Despite sea spiders being extensively sampled during numerous historical field surveys, their reproductive biology had remained elusive, said Brenneis.

“At this stage, people believed they may have a completely different reproductive biology to their relatives. But this video unmistakably shows that at least their mating follows typical sea spider fashion.”

Fascinating footage

Using morphological analysis, the scientists determined that the mating pair belonged to the genus Colossendeis, a group known to include the largest sea spiders.

Some of these creatures can boast leg spans stretching up to 2.5 feet. The captured video showcased a mating ritual wherein two sea spiders were seen on top of each other, with the female manipulating an egg mass using a specialized pair of legs.

The role of technology in enabling this monumental discovery was highlighted by Daniel Wagner, OET’s Chief Scientist.

“While humans have sampled the deep sea using nets and dredges for well over a century, advanced technologies like high-definition cameras on submersible vehicles have only recently allowed us to see deep-sea animals in their natural environment,” Wagner said.

These expeditions equipped with sophisticated technology have exponentially expanded our baseline knowledge of deep-sea habitats in the Central Pacific. Still, vast tracts of this remote region are yet to be explored. 

Future of deep-sea exploration

Wagner shared the exciting news that E/V Nautilus will embark on an 8-month-long field season this month.

The focus will be to investigate unexplored sites in the Pacific, including within the Pacific Remote Islands Marine National Monument, currently under consideration for designation as a national marine sanctuary.

Adding an interactive aspect to the upcoming expeditions, the deep-sea footage will be live-streamed, giving everyone with an internet connection an opportunity to join the journey of discovery. 

“Given that the vast majority of our ocean remains unexplored, these upcoming expeditions will undoubtedly lead to many more discoveries, and we are thrilled to be able to share these explorations with everyone,” said Wagner. 

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