Natures wonders: aquatic spiders (Introduction)

by David Turell , Sunday, April 28, 2024, 19:09 (15 days ago) @ David Turell

Many adaptations:

https://knowablemagazine.org/content/article/living-world/2024/meet-the-aquatic-spiders...

"What scientists do know is that dozens of described spider species spend at least some of their time in or near the water, and more are almost surely awaiting discovery, says Sarah Crews, an arachnologist at the California Academy of Sciences in San Francisco. It also appears that spiders evolved aquatic preferences on several distinct occasions during the history of this arthropod order. Crews and colleagues surveyed spiders and reported in 2019 that 21 taxonomic families include semiaquatic species, suggesting that the evolutionary event occurred multiple independent times. Only a swashbuckling few — not even 0.3 percent of described spider species — are seashore spiders; many more have been found near fresh water, says Nelson.

"It’s not clear what would induce successful land-dwelling critters to move to watery habitats. Spiders, as a group, probably evolved about 400 million years ago from chunkier creatures that had recently left the water. These arthropods lacked the skinny waist sported by modern spiders. Presumably, the spiders that later returned to a life aquatic were strongly drawn by something to eat there, or driven by unsafe conditions on land, says Geerat Vermeij, a paleobiologist and professor emeritus at the University of California, Davis — because water would have presented major survival challenges.

“'Since they depend on air so much, they are severely limited in whether they can do anything at all when they are submerged, other than just toughing it out,” says Vermeij. Newly aquatic spiders would have had to compete with predators better adapted to watery conditions, such as crustaceans, with competition particularly fierce in the oceans, Vermeij says. And if water floods a spider’s air circulation system, it will die, so adaptations were obviously needed.

"But spiders as a group already possess several water-friendly features, suggests Crews. They have waxy, water-repellent exteriors, often covered in hairs that conveniently trap air bubbles. Even having eight legs is helpful, says Nelson: Spiders can distribute their weight nicely while they skitter across a water surface, or use their octet of appendages to row along.

"Some spiders take their aquatic adaptations to the next level, though. Consider the diving bell spider, Argyroneta aquatica, an overachieving arachnid that is the only one known to do it all under water: breathe, hunt, dine on insects and their larvae, and make spiderlings. Found in fresh water in Europe and parts of Asia, it spins a silken underwater canopy and brings air bubbles from the surface to its submerged home via its body hairs. When it goes out, it carries a smaller air bubble, like a little scuba tank, on its back.

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"There’s a splash zone,” she says. “It’s kind of a wild environment.” A spider might be baking in hot sun one minute, drenched in chilly saltwater the next. Some spiders migrate up and down their beaches with the tides; Nelson speculates that they might monitor lunar cycles to anticipate when to move.

"Other seashore spiders spin watertight nests where they hide out for hours while the tide is in. M. marina, for example, seeks seashells with nice, concave spaces in which to spin safe tents. Another spider, Desis marina, hides in holdfasts where bull kelp attaches to rocks, lining the holdfast’s interior with silk to create an air-filled pocket and staying submerged for as long as 19 days. D. marina emerges only when the tide is going out, to hunt for invertebrates like shrimp.

"A spider that’s even occasionally submerged in saltwater or eating briny seafood will also have to maintain proper internal salt levels. “Presumably, they will be able to concentrate the salt somehow and then poo it out,” Nelson says. Scientists don’t know how marine spiders pull this off. And at least one intertidal-zone spider, Desis formidabilis of South Africa’s cape, comfortably maintains an interior salt concentration much like the crustaceans it eats, according to a 1984 study. (Freshwater species also probably require adaptations because their insides must stay saltier than their surroundings or food, Vermeij speculates.)

"When a spider hides out with a limited air supply for days or weeks at a time, oxygen levels also may become a critical issue. Intriguingly, researchers have identified gene variants within the oxygen-guzzling, energy-making mitochondria of aquatic spiders that may help them cope with low-oxygen environments. These changes mirror beneficial changes to mitochondrial genes in birds that live in high-altitude, low-oxygen environments.

"In another study, researchers investigated the genes used in the silk glands of aquatic and land spiders. They found that water-spider silk seems to have a high proportion of the water-repelling amino acid pair glycine and valine — which might also be an adaptation, they suggest."

Comment: this takes us back to explaining the whale series. Why did the spiders go back to water? As with whales many physiological changes had to occur. God must have stepped in as a designer.