Sea spiders as a remarkable example of stasis: 450 million years?

Found in all oceans, these pycnogonids are not actually spiders, though most have eight legs.

About 1,300 species of sea spiders have been identified, and more are found every year. Fossils suggest that they’ve existed on Earth for at least 450 million years. Back then, pycnogonids and other arthropods likely dominated the oceans. Few animals lived on land.

Sea spiders lack organs for breathing, instead expelling carbon dioxide and taking in oxygen directly through their outer layer, or cuticle. This gas exchange primarily occurs via the legs, as they have the most surface area by far. Even more fascinating, it seems that pycnogonids primarily transport oxygen within their bodies via peristalsis, involuntary constriction and relaxation of the gut. Peristalsis moves hemolymph, or blood, throughout the body. “Shared digestive and respiratory functions may save energy,” the researchers who uncovered this strategy said. “Legs function as the gills used by other arthropods, and the gut functions as a heart.”

Ross Pomeroy, “The Strange, Unsettling World of Sea Spiders” at RealClearScience (February 2, 2022)

Their biology doesn’t sound like it would work — yet no big changes seem to have been needed for nearly half a billion years.

Pomeroy also notes,

As far as reproduction goes, males carry fertilized eggs then care for offspring. Females produce and lay eggs, which males then externally fertilize. Subsequently, males pick them up and store them with specialized body parts called ovigers.

Ross Pomeroy, “The Strange, Unsettling World of Sea Spiders” at RealClearScience (February 2, 2022)

That’s a complex sort of behavior, isn’t it? Unfortunately, we can’t know if it is an ancestral behavior or a recently developed one. But if there is no design in nature, how would it have developed at all? There is no clear path via random mutations.

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You may also wish to read: Stasis: Life goes on but evolution does not happen

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