Fen Raft Spider
Latin name: Dolomedes plantarius
Notable feature: This is a large spider with a rich brown or black body and has white or cream stripes along the side of the abdomen and cephalothorax.
Conservation Status: Vulnerable
Where in the UK: This species is now found at 7 sites in the UK. There are three natural populations found at Pevensey Levels in East Sussex, Redgrave and Lopham Fen in Suffolk, and Pant-y-Sais Fen and Crymlyn Bog near Swansea. Between 2010 and 2015 four new populations were established in the Norfolk Broads through a translocation programme, namely Castle Marshes, Carlton Marshes, The mid-Yare Marshes and Ludham Marshes.
Fen Raft Spider (Dolomedes plantarius) © Steven Falk
The Fen Raft Spider (Dolomedes plantarius) is a large, native, semi aquatic spider with a rich brown or black body and has white or cream stripes along the side of the abdomen (rear part of the body) and cephalothorax (front part of the body).
Fen Raft Spiders live in fens and other wetlands in southern England and Wales. They are able to sit on the surface of the water and move across it thanks to their hairy legs; these hairs are also very sensitive and are used to detect the movement of their prey.
- Size: The body length of female Fen Raft Spiders can be up to 23mm and have a span of about 7cm including the legs.
- Life span: The lifespan of the Fen Raft Spider can be quite variable from one to three years. They emerge as adults in the Spring (late April or May) and find a mate almost immediately. Females can be seen with egg sacs in June and then the characteristic nursery webs can be seen from the end of June. Most nursery webs are seen in July and August and females can survive until the autumn.
- Diet: Fen Raft Spider diet is mostly made up of other invertebrates such as small spiders, dragonfly larvae and pond skaters, however the spiders have been known to catch small vertebrates such as fish and tadpoles!
- Reproduction: Female Fen Raft Spiders are very caring mothers. After mating, the females build an egg sac which they carry for approximately three weeks, dipping the egg sac in the water every few hours to keep the eggs moist. When it is time for the eggs to hatch the female builds a nursery web – a silken tent of up to 25cm across which is suspended above the water attached to plants. The mother spider will guard her nursery for the first week of the young spiders’ lives after which they will disperse into the surrounding vegetation.
- When to see: Can be seen in the summer months, although adults can be difficult to find. It is much easier to look for the conspicuous nursery webs.
- Population Trend: Fen Raft Spiders populations are historically declining across Europe. In the UK they appear stable after a historical decline. The recent translocation work has increased the number of populations in the UK though it remains vulnerable on both the global and GB IUCN Red List and is listed as a Priority Species in England under Section 41 of the NERC Act 2006 in England, and under Section 7 of the Environment Act 2016 in Wales.
- Threats: This is a very rare species in the UK and across much of its European range due to the destruction and degradation of the lowland wetland habitats on which it depends. A reduction in water levels, open habitats and water quality have all played their part. Climate change and sea level rises have the potential to further impact low lying populations.
- Fun Fact: The Fen Raft Spider is an ambush predator. It doesn’t build webs to capture its prey or actively pursue its prey until it is within easy reach. They are able to sit on the surface of the water and move across it thanks to their hairy legs. These hairs are also very sensitive and are used to detect the movement of their prey.
How you can help:
Buglife is working to increase awareness of invertebrates and the Fen Raft Spider through specific projects, such as Natur am Byth!, and campaigns, but we need your help!
Join a recording scheme and log your finds – volunteers are needed to help monitor known populations – contact via the Fen Raft Spider website to be put in touch with reserve staff. Or if you think you have seen a Fen Raft Spider, submit your record to the National Spider Recording Scheme
For more information you can also visit the Fen Raft Spider project website
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