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Webs under water: The really bizarre lives of intertidal spiders

  • Scientists have discovered a 15th species of intertidal spider, a family of unusual arachnids that live in coastal habitats that are submerged during high tides.
  • The newest species, named after singer Bob Marley, was discovered living on brain coral off the Australian coast.
  • Scientists know that some species create air pockets with their hairs, while others build waterproof webs, but little is known about most of these fascinating spiders.
  • Intertidal spiders face a number of threats, including rising sea levels due to climate change, and pollution.

Spiders are one of the most ubiquitous creatures on Earth, found on every continent except Antarctica. Whether in underground caves in the Amazon or the icy climes of Mount Everest, there is a species of spider that has moved into practically every land habitat. But some arachnids are determined to not even let the oceans stand in their way — and scientists have just discovered a new one.

A spider named for the late reggae legend Bob Marley is the newest member of the 15 known species of so-called intertidal spiders. These weird spiders inhabit the intertidal zone: a stretch of land that is submerged during high tide and exposed during low. Scientists from Australia’s Queensland Museum and the Zoological Museum at the University of Hamburg, Germany, first found Bob Marley’s spider (Desis bobmarleyi) in 2009 and described it last December.

A male Bob Marley’s spider (Desis bobmarleyi), discovered in Port Douglas in Queensland, Australia. Photo by Robert Raven

“The connection to Bob Marley was first through his song ‘high tide [or] low tide’ as these spiders live in the high tide low tide zone,” said Barbara Baehr, a research scientist from the Queensland Museum and the lead author of the paper.

The mix of land and sea in the intertidal zone supports a wildly diverse set of habitats. For instance, Baehr found Bob Marley’s spider on brain corals in shallow reefs on the rocky Queensland coast. But another intertidal species, Desis formidablis, or the formidable spider, lives under boulders on rocky shores and hides in barnacle shells in South Africa. To date, scientists have recorded intertidal spiders along the coastlines of Australia, New Zealand, Southern Africa, the Pacific Islands and India.

Bob Marley’s spider (Desis bobmarleyi) on a brain coral at low tide. Photo by Paul Hoye

The most well-known intertidal spider, the marine spider (Desis marina), a species from New Zealand, has been found to live in the holdfasts of bull kelp, a type of seaweed. Holdfasts are like the roots of plants, allowing the seaweed to attach firmly to rocky surfaces in the turbulent intertidal zone. Intertidal spiders shelter in these hideouts during the high tide, and come out during the low tide to feed on amphipods, tiny microscopic crustaceans with no shells.

Surviving in this landscape is no mean feat, says David Schiel, professor of marine ecology at the University of Canterbury in New Zealand. Schiel, though not an arachnologist, has studied other intertidal organisms extensively. He said that apart from adapting to breathing under and above water, animals in the intertidal zone need to survive constant changes in temperature and impact from wave action as well as extreme weather such as storms and cyclones.

“Generally speaking, organisms have to be pretty tough and resilient to withstand these extremes, which can occur on a daily, seasonal and inter-annual basis,” he said.

While organisms like barnacles, limpets and shellfish have evolved physical adaptations to survive in this wild environment, intertidal spiders are built much like their land relatives. So how do they breathe underwater? In 1967, Bruno Lamoral, an arachnologist from Natal Museum in South Africa, attempted to solve this mystery by studying the formidable spider.

Lamoral found that the spider was able to stay underwater for up to 24 hours at a time, thanks to a remarkable adaptation: tiny water-repelling hairs on its body, known as hydrofuge hairs, that trap a layer of air around it.

Desis formidablis, the formidable spider from South Africa, has tiny hairs that repel water and trap air when they are underwater. Photo by Sally Sivewright, www.scientistinlimbo.com

Lamoral believed the spider had more tricks up its sleeve. During his study, he noticed it managed to stay underwater even after the oxygen in the air film was used up, and speculated that the spider’s entire body acted like a gill, fixing oxygen from the water. Unfortunately, he wasn’t able to pinpoint how exactly this could happen.

More than a decade later, in 1983, Donald Mcqueen and Colin McLay from the University of York in Canada and University of Canterbury, respectively, described more complex adaptations in their study of marine spiders in New Zealand.

In some ways, bull kelp is an even tougher habitat because it is only completely exposed during extreme low tides. So unlike its South African cousin, the marine spider is sometimes submerged for days. Yet as Mcqueen and McLay found, they didn’t rely on physical gills.

Instead marine spiders spun thick webs inside the holdfasts, which trapped enough air for them to survive the submergence. Examining the webs, the scientists realized the spiders chose spaces that could hold enough air for their body size. Those that didn’t, perished.

The root-like holdfasts of the bull kelp in New Zealand can shelter Desis marina, the marine spider, underwater for several days. Photo by Stug Stug via Wikimedia Commons (CC-BY-SA-3.0

To make the most of their stored oxygen, marine spiders also lowered their respiration rates, breathing less frequently than their land-dwelling cousins. They also used up to 90 percent of the oxygen in their nest.

Thanks to these strategies, marine spiders are able to lead very full lives inside these kelp holdfasts. They hunt amphipods that also live in the kelp, move in with potential mates, and even nest under the sea. The spiders only really need the low tide to find mates.

Unfortunately, research into the ecology of these fascinating creatures seems to have come to halt since the 1980s. Apart from the discovery of Bob Marley’s spider, the only new piece of recent information was a 2017 study reporting several new locations of the marine spider in New Zealand.

Cor Vink, curator of natural history at Canterbury Museum in New Zealand and the lead author of that study, said records of the marine spider were very sparse, “but once we developed a good technique to find them they turned up to be in more places and greater numbers than previously known.”

For the other 14 known species of intertidal spiders scattered around the world, even such basic information on distribution and population is missing

Although unsure of why intertidal spiders are so poorly studied, Vink was excited by the discovery of the new species. “It’s interesting that new species are still being found in such unusual habitats,” he said.

But how safe will those habitats be in the future?

According to Schiel, populations of bull kelp are largely stable and grow mostly in places away from human habitation in New Zealand. However, this may not be the case for other spider habitats.

“Because of their position between the land and full marine environment, intertidal areas are subjected to impacts in both directions,” Schiel said, citing a list of threats across the planet that include agricultural runoff, excess nutrients, fine sediments from the land, and rising water levels and temperatures from the sea — all of which could impact intertidal spiders.

Vink believes these unusual spiders will continue to bewitch more scientists in the future.

“There are so many interesting questions,” he said. “How does its web work in salt water? How does it survive submerged for so long? How does it sense when the tide is coming back in?”



Citations

Baehr, B. C., Raven, R., & Harms, D. (2017). “High Tide or Low Tide”: Desis bobmarleyi sp. n., a new spider from coral reefs in Australia’s Sunshine State and its relative from Sāmoa (Araneae, Desidae, Desis). Evolutionary Systematics, 1, 111.

Lamoral, B. H. (1968). On the ecology and habitat adaptations of two intertidal spiders, Desis formidabilis (OP Cambridge) and Amaurobioides africanus Hewitt, at. Annals of the Natal Museum, 20(1), 151-193.

Mcqueen, D. J., & McLay, C. L. (1983). How does the intertidal spider Desis marina (Hector) remain under water for such a long time?. New Zealand journal of zoology, 10(4), 383-391.

McLay, C. L., & Hayward, T. L. (1987). Reproductive biology of the intertidal spider Desis marina (Araneae: Desidae) on a New Zealand rocky shore. Journal of Zoology, 211(2), 357-372.

Mcqueen, D. J., Pannell, L. K., & McLay, C. L. (1983). Respiration rates for the intertidal spider Desis marina (Hector). New Zealand journal of zoology, 10(4), 393-399.

McLay, C. L., & Hayward, T. L. (1987). Population structure and use of Durvillaea antarctica holdfasts by the intertidal spider Desis marina (Araneae: Desidae). New Zealand journal of zoology, 14(1), 29-42.

Vink, C. J., McQuillan, B. N., Simpson, A. H., & Correa-Garhwal, S. M. (2017). The marine spider, Desis marina (Araneae: Desidae): new observations and localities. The Weta, 51, 71-79.