Giant sea spiders have surprising breeding behaviors

The reproductive strategies of giant sea spiders in Antarctica have eluded scientists for more than a century. These creatures have remained a puzzle in marine biology, particularly regarding their breeding behaviors. 

A recent expedition by researchers from UH Mānoa has shed new light on giant sea spiders. These insights could significantly impact our understanding of marine life and ecosystems not just in Antarctica, but globally.

Polar gigantism 

Sea spiders, scientifically known as pycnogonids, are marine invertebrates that bear a resemblance to the spiders found on land. 

Most species of pycnogonids are diminutive, barely reaching the size of a fingernail. However, the sea spiders found in the Antarctic defy this norm, showcasing what is known as “polar gigantism.” 

This phenomenon leads to certain species in polar regions growing to sizes far exceeding those of their relatives in warmer climates. Some Antarctic sea spiders have leg spans that extend over a foot.

Mysterious breeding behaviors 

“In most sea spiders, the male parent takes care of the babies by carrying them around while they develop,” said lead researcher Professor Amy Moran. “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.”

Professor Moran’s lab has been studying polar gigantism for more than a decade. The team embarked on a research expedition to the remote continent in October 2021. The goal was to uncover the breeding behaviors of these colossal sea spiders – a task that had baffled scientists for generations. 

Remarkable discovery in sea spiders

The researchers, including PhD students Aaron Toh and Graham Lobert, collected groups of giant sea spiders that appeared to be mating. Upon transporting the specimens to observation tanks, the team made an amazing discovery.

Thousands of tiny eggs were produced by rwo different mating groups. In this case, one parent (presumably the male) anchored the eggs to the rocky seabed. Here, the eggs remained for several months, developing in isolation before hatching into minuscule larvae.

Unique opportunity

“We were so lucky to be able to see this,” said Toh. “The opportunity to work directly with these amazing animals in Antarctica meant we could learn things no one had ever even guessed.”

Within weeks, microscopic algae had grown up over the eggs, providing the perfect camouflage. “We could hardly see the eggs even when we knew they were there, which is probably why researchers had never seen this before,” said Lobert.

Broader implications 

The study of giant sea spiders contributes valuable insights into the biodiversity and ecological dynamics of marine ecosystems. 

By understanding how these creatures live, feed, and reproduce, scientists can gain a better understanding of the health and stability of ocean environments, which are increasingly under threat from climate change and human activities.

Giant sea spiders 

Giant sea spiders inhabit the depths of oceans around the world. These fascinating creatures, despite their name, are not true spiders but belong to a distinct class of arthropods. They possess long, slender legs that can span up to about 27.5 inches in the largest species.

Sea spider body structure 

Researchers study giant sea spiders for their unique physiological and biological characteristics. One of the most intriguing aspects of these sea creatures is their highly simplified body structure. 

Their bodies consist of a small torso from which their long legs extend, and they have a proboscis that allows them to suck nutrients from their prey, which includes soft-bodied invertebrates like worms and jellyfish.

Circulatory system of sea spiders

Another fascinating feature of the giant sea spider is its circulatory system. Unlike most animals, sea spiders pump blood and oxygen not only through their heart but also through a complex system that involves movement in their legs, a process that researchers are still trying to fully understand. This unique method of circulation helps them survive in the oxygen-poor depths of the ocean.

Diverse habitats

Giant sea spiders thrive in a variety of marine environments, from shallow coral reefs to the dark, cold abyss of the Antarctic waters. Their ability to adapt to such diverse conditions speaks to the evolutionary success of this ancient group of arthropods, which has existed for hundreds of millions of years.

The study is published in the journal Ecology. 

Video Credit: UH Mānoa 

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