The Water Spider: An Enigma Beneath the Surface

By Dr. David Glynne Fox

The Water Spider (Argyroneta aquatica) is, as far as is known, the only truly aquatic spider in the world, living almost entirely submerged in ponds throughout Britain and Europe. Although widespread, the species is very local, but often abundant where found. However, none of the ponds that I knew of held Water Spiders or, if they did, they were never observed. With the loss of so many natural ponds through building development in my immediate area, I had little choice other than to search elsewhere and soon came across the now largely defunct Nottingham Canal at Trowell.

The Nottingham Canal has been disused for many years now and has become a local nature reserve. For much of its length the canal is choked with reeds, with the exception of the odd small pool here and there. During times of low rainfall, much of the canal totally dries up, as it did during the drought of 1976. However, during the mid 1990’s, whilst sweeping subsurface weeds with a net, to my delight and amazement I came across a few Water Spiders. Once out of the water they resemble many another brown terrestrial spider but, once submerged, they take on a whole new appearance. Air bubbles become trapped amongst the fine body hairs, rendering the spider with a metallic silvery appearance.

As a wildlife photographer I desired to take images of this amazing species, but to attempt such in the pond would have proven virtually impossible. This by the way was in the days before GoPro cameras and suchlike which can be mounted on a stem and poked below the surface. I was still using film cameras at this time and as such, even with an underwater housing, one step into the pond filled the surrounding water with swirling mud and debris, rendering photography a complete waste of time. Additionally, it is almost impossible to see the spiders from the surface as they live hidden amongst the aquatic vegetation. Without the aid of the net, I would never have been aware of their existence here. So, the only solution was to retain a pair and take them into my studio set-up. For over a week, I took a long series of images of the spiders and also of their fantastic diving bell, about more of which later. I also photographed some of their behavioural traits.

Many questions still remain, including the type of silk produced by the spider and how it differs, if at all, from terrestrial spiders. It certainly seems to disintegrate upon contact with air, and if carbon dioxide is replaced into the diving bell by diffusion through the silken walls of the bell, as has been suggested to me by a colleague, then why does the surrounding water also not seep through? These are just a couple of the questions that require further study.

During my own study, I spent quite some time observing the construction of the bell, which began with the spiders spinning a flat sheet of silk, suspended from and attached to the surrounding aquatic vegetation. Once this had been completed, the spider made numerous trips to the surface to collect air which became entrapped in their body hairs as they broke through the surface film. The spiders then returned to the web sheet, positioned themselves below and combed the bubbles from their bodies with their legs so that they floated upwards towards the surface, but we were trapped against the silken sheet. Each time they performed this function, the sheet became more outwardly bowed from the centre, with the pressure of the bubbles still trying to reach the surface due to their being less dense than the surrounding water. The silken sheet then became gradually more elongated on all sides and thus formed the unique bell. The bell is completed within a few hours. Additionally, as with terrestrial spiders, silken strands run from the base of the bell and are attached to nearby weeds. These strands perform the same function as trip wires in terrestrial spiders. They are covered in sticky globules which trap small aquatic fauna and, because they are attached to the bell, the spider senses the vibrations from the ensnared victims in exactly the same manner as a typical garden spider in a hedgerow. From my observations at least, it appears that the oxygen inside the bell is replenished in the same manner in which the bell was constructed, by simply course to bow to collecting more fresh air from the surface. I am prepared of course to bow to superior knowledge on this aspect, however, as I currently have no way of detecting gas exchange through the bell itself.

This experiment was proving to be an excellent way of learning about this enigmatic spider and I was soon to discover from other colleagues that despite the Water Spider being comparatively well-known, not a lot is actually known about it, which came as quite a surprise to me. So I kept my pair of spiders long enough to breed from them and found that mating takes place within the bell, although the bell they used had barely begun construction when I first observed mating behaviour. At first I wasn’t sure if they were merely fighting, but it became apparent that successful mating had indeed taken place for within a few days, a fertile egg batch was deposited inside the completed bell and was positioned close to the top. All prey was taken inside the bell and consumed there. The male was liberated back in his home pond at Trowell as he had survived the encounter. I thought afterwards that I should have kept him to determine how long male Water Spiders survive after mating, for the males of many spider species succumb shortly afterwards, either killed by the female and eaten, or simply die due to having served their purpose.

In due course, all the 4 eggs hatched and when they began to leave the bell as spiderlings, I returned them all to Trowell to continue perpetuating the species. Many years later, on a return visit to the canal, I was horrified to observe that not a drop of standing water could be found within some considerable distance of the original pond and sapling trees had sprung up all around the periphery, no doubt contributing to the lowering of the water table. I intend to continue monitoring the site in the hope that this very local species has indeed survived. I certainly hope so. In the meantime, I have a rather nice selection of images to remind me of them.

(C) Images: David Glynne Fox.