The team at Mas Mireille, in the Vallée des Baux, recently found an individual of the species Dolomedes plantarius. This species of semi-aquatic spider is rare and threatened in France.
Where does its pretty name come from?
The scientific name Dolomedes plantarius was given by the Swedish arachnologist Carl Alexander Clerck in 1757. The term “Dolomedes” comes from the ancient Greek dolos, meaning “cunning” or “deception”, in reference to its stealthy hunting techniques. The term “plantarius” may refer to its preferred habitat among wetland plants.
There are two species of Dolomedes in Europe: Dolomedes plantarius and Dolomedes fimbriatus. The two species are very similar and need detailed examination to be able to differentiate between them.
How to recognise the great raft spider?
This spider has a chocolate brown body with cream or white lateral stripes. Its legs are long and slender, making it ideal for travelling on water. Females can reach up to 20 mm in body length, with a total wingspan (legs extended) approaching 7 cm.
How does this species live?
The great raft spider is a semi-aquatic spider that hunts by detecting vibrations on the surface of the water. It often stands motionless, its rear legs anchored to the vegetation and its front legs resting on the water, ready to capture prey such as aquatic invertebrates, aquatic beetles or odonates (dragonflies). In the event of danger, it can dive and remain submerged, trapping air with its hydrophobic hairs.
During courtship, the male taps the surface of the water with his two pairs of front legs and slowly approaches the female. By generating vibrations at frequencies not used by their prey, males are able to identify themselves as potential suitors rather than as prey. The female carries her eggs in a cocoon that she keeps with her until they hatch, then she builds a “nursery” where the young spiders stay for a few days under her supervision.
How did the Mas Mireille scientific team find it?
As part of the inventory of spiders that our Mas Mireille team is currently carrying out in the Ilon Regional Nature Reserve, we knew that the Ilon marsh could potentially be home to the reed dolphin. However, it had not been detected during recent surveys of nearby marshes to the east of the Rhône. We also expected to come across the more common Dolomedes fimbriatus. But to our great surprise, the first Dolomedes we saw turned out to be the reed dolphin! A wonderful discovery, which underlines the importance of these natural environments and their preservation.
Did you say "endangered species"?
The great raft spider is classified as “endangered” in France (and “critically endangered” in the Nord-Pas-de-Calais region), mainly due to the destruction and fragmentation of its natural habitats, such as marshes and peat bogs. Drainage of wetlands, urbanisation and climate change are also contributing to the decline in its populations. In addition, its low dispersal capacity makes it particularly vulnerable to environmental disturbance.
(Photo credits : Romain Barthel)