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The frigid Antarctic waters pack an especially diverse and vibrant array of sea spiders, marine arthropods that grow as large as dinner plates and use their spindly legs for breathing and digestion. Researchers fear these critters (like the giant Antarctic sea spider Colossendeis megalonyx, pictured above) could suffer as the oceans become warmer and more acidic, but an international collaboration has shown modern sea spiders come from a long line of staunch survivors.

In the study, scientists genetically sequenced all living sea spider families (spanning 89 species, all distantly related to land spiders) to create the first comprehensive family tree. They also incorporated information from ancient fossils.

The results showed which body parts evolved when, and revealed that the ancestors of modern sea spiders are much older and hardier than previously thought. This group has thrived and diversified for almost 500 million years, the team reports this month in Molecular Biology and Evolution. That’s despite the dramatic changes in ocean temperature and chemistry that accompanied the end-Permian mass extinction, which eliminated nearly all marine life.

What makes sea spiders so tough? The answer remains unclear; it helps that they don’t depend on calcified armor like clams and oysters, which dissolves in acidic water. But only time will tell how today’s aquatic creepy crawlers will fare against modern-day challenges, like microplastics, oil spills, and habitat destruction.