I became involved in a survey of the Fen Raft Spider (FRS) starting in July 2017 and running till October 2017. Dolomedes plantarius is the UK’s largest and least common species of spider. It is Red Listed by the IUCN and classified as ‘Vulnerable’. It is also Red Listed by several European countries, including the UK where it is currently classified as “vulnerable” -an “under threat of extinction” category.
Our foremost UK scientific authority on the FRS, Helen Smith, came to talk to us about it at Carlton, and I was immediately drawn to this strikingly beautiful complicated and elegant predator. The more I learnt about it, the more I wanted to know. So my hand went up when the call for survey volunteers went out. As a former mild arachnophobe I surprised myself.
Helen had bred around 6,000 spiderlings in a laboratory, and her own kitchen (!) and populated a section of ditches at Carlton marshes with them, as part of a conservation translocation programme run by Natural England. The purpose of our survey was to measure the extent (or not) of their spread and thus the success (or not) of the translocation. Translocation because the FRS is not naturally resident at Carlton. There are only three other populations in the UK -Redgrave and Lopham Fen in Suffolk, Pevensey Levels in East Sussex, and and area outside Swansea in Wales.
Our job was to amble along the dykes, covering both sides of the ditches in a mapped area of wetland, noting the location of FRS webs, and the presence of any spiderlings in the webs, or adults nearby, using the naked eye and close-focussing binoculars. On our first survey I couldn’t see any at all, until our guide had, with great patience, pointed out the location. We all found it hard at first, but, like any skill, we improved with practice.
The technique was a bit like looking at a “Magic Eye” picture. You have to de-focus your eyes and relax. You begin to notice hazy areas, then looking at them through binoculars in clear focus, you can see the webs and the detail. Spotting the adults was also tricky, as they camouflage well into the melee of reed, sedge and water-soldier leaves.
My first sighting of a ball of spiderlings.
To the left of centre, my first sighting of an adult Fen Raft Spider
A ball of spiderlings tight together
Are these flowerheads, or spiderlings spread out and sunbathing? It can be very hard to tell sometimes. Here the plant material (blunt-flowered rush) is in the middle and left of the web, and a ball of spiderlings is on the right.
Spiderlings spreading out on a sedge leaf, warming up in the sun in the safety of their nursery web
We had to learn to spot both new and old webs as both needed to be logged. Here’s an illustration.
I have been expertly advised that she’s a post-partum female. In a male, the two palps at the front of his head are used to store sperm in to inseminate the females. In this picture, because the palps curl under, it’s very easy to think they are clubbed at the tip when in fact they’re not; this would mark a spider out as male. In general, adult males have a wider cephalothorax (the head/chest segment with eyes and legs attached), a very narrow abdomen, and proportionately longer legs than the females.
Here she is again, closer in
In the centre you can see the shed skin of an adult FRS. It was dangling in the breeze.
and again -slightly different after being moved by the breeze. It looked as light as gossamer.
The funny thing about the photo above is that we only spotted the adult after I’d taken the pic -I was interested in the widely scattered spiderlings.
The pics below show a particularly dark, handsome individual, with striking stripes, enjoying a snack. The wide abdomen marks her out as a female.
On one of the last days I wished for a perfect pose from a FRS. Within five minutes my wish had been granted and this gorgeous creature came out from his rest and posed for me in a classic waiting-for-prey pose. This is a girl.
…. the spider climbed onto a water soldier and is resting her back legs on the meniscus of the water, waiting to sense vibrations signalling nearby prey
Below are two shots, far and near, of a lovely example of a labyrinth spider web.
A ditch on our last survey day of the year, a cloudless beautiful October morning. The water soldiers are scarce, most have sunk to the bottom of the ditch where they will stay till next spring. There were no spiders to see at all. No one is absolutely sure where they go, but it is thought they might go down into the mud at the bottom of the ditch with the water soldiers.
Goodbye till next year
Doing the survey yielded many positive things for me -the thrill of learning biology and botany again was invigorating -always my two most favourite subjects at school and in life. This alone made me feel 30 years younger! Then I have made new friends (you know who you are) and discovered the layout of Carlton Marshes. I’m also learning how to use my camera and a pair of binoculars properly -I thought I knew but thinking that is often a sign of ignorance.
It’s great to be directly involved in helping the recovery of an endangered species, no matter how small it is. It has its part to play in our world, and its absence is a red flag highlighting degradation of the environment, for the FRS will only live where the water quality is good. Conversely its recovery will signal a return to health of the local water and land. Head over to the definitive resource on the FRS here if you’d like to learn more about this fascinating creature.