UNDERWATER SEA SPIDER

Following positive comments about the marvels of the ants, I thought I would continue the theme by drawing your attention to underwater spiders. God is not just a God of infinite greatness, but also a God of infinite smallness. As someone said maybe you should turn your telescopes into microscopes and reignite your sense of wonder at the incredible power of the God you serve.

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Gibber! Gibber!

Chugley:

Underwater spiders

Engineering marvel and evolutionary enigma

by Shaun Doyle

The diving bell spider (Argyroneta aquatica) is a unique spider. It is the only spider to live its life almost completely underwater. However, like all other spiders, it has ‘lungs’¹ and can only breathe air.

How does it manage to survive? It takes air with it underwater! When it goes to the surface, it sticks its abdomen into the air and, as it drags it back down into the water, millions of rough, water-repellent hairs on its abdomen, each with a waxy coating, trap air all around its abdomen.² This thin layer of trapped air is called a plastron (figure 1). It takes this air underwater and weaves an underwater web to create its own little air bubble ‘cave’ underwater (figure 2).³ This ‘cave’ houses much more air than the plastron, so it takes several trips to create the ‘cave’. It can stay in this air bubble ‘cave’ for up to several days without going back to the surface. But when the oxygen begins to run out, it simply uses its abdominal hairs to move air from its ‘cave’ and exchange it with fresh air from the atmosphere.The diving bell spider is a lone genetic andecological outlier.

The hairs on the diving bell spider (and some other insects and spiders) create an air bubble through a mixture of both mechanics and chemistry. Mechanically, the surface the air bubble sticks to must be extremely rough. For the spider, the millions of hairs on its abdomen do this. This makes it easy for the air and hard for the water, with its high surface tension, to latch onto. But the surface also needs to have a low surface energy. In other words, it needs to be slippery to water. In the diving bell spider, this is achieved by the waxy coating on the hairs. This combination of factors allows the air molecules to form a stable, intact structure next to the spider’s surface without the water breaking up this air bubble.

An evolutionary enigma

The diving bell spider is not the only invertebrate that can form a plastron on its abdomen.⁴ However, it is the only known spider species to live almost its whole life underwater. Its uniqueness is underscored by being the only species in its genus. There has also been long debate over which spider family it belongs to.⁵ It has been assigned variously to Cybaeidae⁶ and Dictynidae.⁷ Because of its uniqueness, some researchers have even assigned it to its own family, Argyronetidae.⁸

Nonetheless, since its mitochondrial genome was sequenced, researchers have generally placed it in Dictynidae.² Even that, however, highlighted its uniqueness. It was found to have an unusually long control region in its mitochondrial DNA compared to similar species.^9,10 This suggests it has some unique genetic specializations.

Moreover, how did the spider come to live an underwater lifestyle? Some of the other spiders and insects that can form plastrons could in theory breathe via these indefinitely.¹¹ This is because the air in the plastron is not static; gases flow across the barrier between the plastron and the water. For this to enable ongoing breathing, the plastron needs to be big enough and have the right shape. In addition, the colder and more disturbed the water (e.g. running water), the better. This is because cold water holds more dissolved gases, and faster water brings more of these gases into contact with the plastron.

Similar issues would apply to the diving bell spider breathing via the air bubble ‘cave’ it forms underwater;² however, it lives in stagnant ponds. Thus, both its plastron and its ‘diving bell’ cannot sustain it indefinitely.² This is why the spider must make regular trips to the surface to replenish its O₂ supply, as described earlier.

It would be a difficult challenge for evolutionists to try to construct a path via natural selection to this underwater lifestyle. Everything the spiders do in their ‘cave’ could just as easily be done above water. All the spider’s actions require atmospheric O₂ to do. The regular trips to the surface cost energy, and are risky because they alert predators and prey to its presence.¹²The plastron of the diving bell has also served to inspire new technology.

The diving bell spider is a lone genetic and ecological outlier. It may thus have been created this way. But if not, it would be difficult to see how undirected mutations and natural selection alone could explain how the lifestyle of this species arose from the first members of its kind. Rather, designed adaptability in the genome probably played a part.¹³

An engineering problem difficult to solve

The plastron of the diving bell has also served to inspire new technology. Surfaces that can maintain plastrons for long periods could have many technological uses. Because air separates the surface from liquids, it can help stop things like bacteria and barnacles from growing on the surfaces. This could help with protecting materials, ranging from underwater devices to medical tools, from biological damage or fouling.

Moreover, researchers have for decades well understood how these surfaces work. But they could not copy what creatures do. They could not make plastrons stable in the long term. Why not? First, rough surfaces are mechanically weaker than their smooth counterparts. They are easily ground down. Second, the non-wetting properties of the surfaces degrade over time. Air rests in the pockets of the rough surface, making the water ‘slide off’ easily. But as water gets into the rough surface, the air pockets disconnect, which wets the surface. This is worse when the surface stays underwater for a long time because the air trapped within the rough surface dissipates over time. Without the air within the rough surface, the plastron it supports falls apart.

However, researchers have recently found a way to overcome these problems and preserve a plastron for months on an engineered surface.¹⁴ They had to consider several more parameters than previously. That enabled them to more accurately predict how the plastron would behave. This led to developing a relatively cheap and reproducible process to create a surface that forms a plastron for months at a time.

Conclusions

The diving bell spider is an amazing enigma. The intricate engineering of its plastron was well known for decades before we could copy it effectively. And it is hard to see how it could have simply evolved such a unique lifestyle without careful design embedded in the creature. It is one more testimony to the ingenuity of its Maker.

Posted on homepage: 6 May 2024

References and notes

1. These are of a special type, called ‘book lungs’. They consist of layers of thin folded membranes (lamellae) stacked like the pages of a book with hemolymph (‘blood’ equivalent) inside the membranes, alternating with pockets of air. This gives more surface area for the atmospheric gases to exchange with those dissolved in the hemolymph. Return to text.
2. Neumann, D. and Woermann, D., Stability of the volume of air trapped on the abdomen