Fen Raft Spider

Fishing spiders, propel themselves across the water surface using two gaits: they row with four legs at sustained velocities below 0.2ms-1 and they gallop with six legs at sustained velocities above 0.3ms-1. Because, during rowing, most of the horizontal thrust is provided by the drag of the leg and its associated dimple as both move across the water surface. The integrity of the dimple is crucial. (1) The dolomedes plantarius can be found with its entire body on the water anchored to shore with only a silk thread, or resting on the surface with legs anchored on a leaf or rock near the shore. (2)

Ground-water abstraction between 1960 and 1999 dried out the pools on which the spider depended, reducing its range by over 80% and leaving two small, isolated populations. A population index, based on systematic monitoring from 1991 to 1999, showed significant variation between years and between the two populations, one of which showed a slight but significant positive trend. Changes in the density of spiders on the pools and the numbers of occupied pools contributed to variation in the index to differing extents in the two populations. The age structure of the populations was highly variable between years and there was no relationship between the abundance of spiders and the numbers of nursery webs. (1)

Body length (female): 17 - 22 mm Body length (male): 13 -18 mm. (1)

Animals, such as fishing spiders, water striders and basilisk lizards, which run on the surface of water, occupy a locomotor niche that has only begun to be explored empirically. (1)

The fen raft spider, like all pisaurids, exhibits extended maternal care. Adult females carry their egg sacs in their chelicerae for several days, then construct a silken protective web around the egg sac amongst vegetation, guarding from the outside until the hatchlings complete their first molt and disperse. Dolomedes plantarius has only been observed at the fen to construct these nursery webs on great fen sedge, which is disappearing from the fen. (1)