Trechalea extensa belongs to a family of semi-aquatic spiders. Still, these arthropods aren’t known to specifically dwell in water, let alone when threatened. It was therefore with astonishment that three researchers noted that a specimen had taken refuge there for 30 minutes!
Go to great lengths to avoid water
Being a semi-aquatic species does not mean feeling like a fish in water. Many species of arachnids use a semi-aquatic environment because they search there for prey while generally remaining dry, on a perch. On the other hand, few species are known to spend a lot of time directly underwater. Thus, most species of spiders “may go to great lengths to avoid submersion“, explains a study published on April 11, 2022 in the journal Ethology. So can water be a refuge for them? Nothing is less certain: immersion can be costly both for breathing and for maintaining body temperature.
The spiders Trechalea extensa, semi-aquatic therefore, are able to hunt insects but also small fish or shrimp. “Trechaloid spiders do this by perching at the edge of streams, with their front legs resting on the surface of the water, sensitive to mechanical cues from potential prey.“, explain the three researchers, authors of the new study. But their affinity with water seemed to stop there.
32 minutes motionless underwater
It was without counting on a funny observation carried out in July 2019, in Costa Rica. First posted on a rock at the edge of a river, a Trechalea extensa tried to flee from the researchers who wanted to capture her. “When we continued the pursuit, the spider quickly moved along the rock and submerged about 25 centimeters below the surface of the water.“, say the biologists. She stayed there for 32 minutes! A record? “Good question, answer to Science and Future Lindsey Swierk, co-author of the study. But the truth is, scientists don’t know! There has been almost no work done on large spiders, especially in this genus, and their diving behavior. Some species of spiders live almost entirely underwater, but they are quite different from the ones we studied. In terms of diving, we know too little to really say that it is a record or not. This is certainly the first documented case of diving in this taxon!“.
While underwater, the spider was motionless and retained a film of air over the entire surface of its body, “probably due to the hydrophobic surface of its cuticular hairs“, underlines the study. She was finally captured and measured: 2.4 cm in length (from her chelicerae to the posterior end of her abdomen) and 14.1 cm when two legs are extended.
For the scientists, the aquatic escape would be for the latter a last chance solution in the face of an imminent threat and which she considers important.
An insulating and breathable film?
If new studies will have to look into the subject, it is possible that the thin film of air around T.extensa can maintain a gaseous layer above its respiratory openings, connected to the trachea and flaky lungs. It is also possible that the gaseous layer surrounding the spider may even allow it to meet its oxygen consumption needs when submerged. Finally, this layer could perhaps also play a role of thermoregulation, by becoming a kind of insulating layer. For now, these are mostly guesses. But knowing more about this phenomenon could lead to the development of new materials.