Scuba diving is a popular activity in Hong Kong and around the world🤿, allowing us to bring the oxygen that we are unable to absorb directly from water😮💨. Just like us, many terrestrial organisms are limited by this vital constraint and thus unable to exploit some of the resources available under the water surface🌊. Many species, however, have evolved particular adaptations to carry with them the precious air necessary for extended journeys under water😮. This is the case of the diving beetles, Dytiscidae, which keep air bubbles between their body and elytra (the hard wings present on their back), and with great success as this family now includes over 4300 species🙀! But in the absence of specialized morphological features, how to remain within water without drowning🤔?
Our species of the day has found an original solution to live underwater despite belonging to a group not particularly known for their love of water or diving performance 💦. 𝘈𝘳𝘨𝘺𝘳𝘰𝘯𝘦𝘵𝘢 𝘢𝘲𝘶𝘢𝘵𝘪𝘤𝘢 (Clearck, 1758), known as the diving bell spider, use its talent of silk designer🧵 to live underwater and create its own air supply while submerged. This species is widely distributed across Europe, central and northern Asia, ranging from the British Isles to Japan!
The diving bell spider spends almost all its time underwater and is the only species of spider that has gone fully aquatic 🕷️. How does it get its oxygen supply if it does not leave the water at all🧐? Its name may give you some clues🔍. Have you ever heard of a diving bell? A diving bell is a large, bell-shaped chamber 🔔 that gets lowered into the water to trap air inside. It allows divers to enter through a small hatch and use the trapped air to breathe while they work or explore the underwater environment.
Spiders usually use their book lungs and tracheal system for gas exchange. Contrary to the spiders living on land, the air in water is limited and the diving bell spider will get its oxygen supply in a unique way. On the abdomen of the diving bell spider, hydrophobic hairs allow it to trap air at the water's surface for breathing. Since the book lungs of the spiders are located at the bottom of their abdomen, trapping the bubble around their abdomen would allow them to breathe through it. Moreover, the spider will build silk webs 🕸️on the underwater vegetation 🌱to create an air-filled bubble, the ‘diving bell’. Therefore, it is typically found in freshwater environments containing aquatic vegetation, such as lakes, ponds, marshes, and slow-flowing streams🏞️. The air in the ‘diving bell’ is held by the surface tension of its silk fibers🧵, and the chamber is open at the bottom. The bell can be filled by carrying bubbles from the surface with the help of the hairs on its abdomen and legs😟. When carrying a bubble, the spider has a silvery appearance, which gave its genus name, 𝘈𝘳𝘨𝘺𝘳𝘰𝘯𝘦𝘵𝘢, meaning "silvery" in Greek. By living in the water, the diving bell spiders have fewer predators and are able to hide inside the aquatic environment and vegetation😶🌫️. There is also quite a lot of food in the water, and they prey on aquatic insects, including mosquito 🦟 larvae, crustaceans or even small fishes🐟.
Other than acting as a “physical gill”, the bell also serves as a base for resting 🛏️💤and reproduction🚼. Both male and female diving bell spiders produce diving bells🔔, however those of females are larger than the males. Male diving spiders will often seek for a female and build its bell next to hers. He will also pierce an opening between his bell and the female’s bell🕳️. Once there is enough space, mating will occur, and the female will lay around 30-70 eggs in a cocoon within the bell. Later on, she will need to maintain the bell to make sure the eggs can survive. Sexual dimorphism can be seen in this species, with males larger than females, which is unusual😕, as for most spiders, females are larger than males. Why is it then reversed in this species? Studies suggest that it is due to the behavioral differences between the sexes of diving bell spiders. Smaller females may be selected as they can invest more time in the construction 👷♀️and maintenance of a large bell (e.g. caring for the eggs) 🚧rather than hunting prey actively. On the other hand, the males spend less time maintaining their bell and thus have more time to search for prey 👀with their larger size giving them a better diving ability🤿. From these behavioral differences, this rare sexual dimorphism is created unlike in other terrestrial spiders🤯, where smaller size favors terrestrial locomotion🏃♂️.
For more fascinating spider specimens, come and visit the Hong Kong Biodiversity Museum🤩!
Text: Kiko Wong
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