Category Archives: The Fen Raft Spider

Mother and child

The Fen Raft Spider displays a remarkable degree of maternal protective behaviour. the fact that many higher animals (for example reptiles) show no protective behaviour for their young as soon as egg laying is over makes this all the more remarkable.

The following pictures were taken whilst on a routine survey of the Carlton dykes in mid July 2019.

The next three show two females carrying egg sacs. The first two pics are of the same lady, the third is a different individual.






She has wrapped her eggs in a silk cocoon which has turned a brown colour due to oxidation. She will carry them like this until they are ready to hatch. Then she will build a nursery web for them and help them out of the sac by picking a hole in the silk.



Here’s a ball of hatched spiderlings in a (fairly thin) nursery web



Mum guarding the nursery. You can see two balls of spiderlings in the web above her.



Another mum with her nursery. She has a thin abdomen and could be mistaken for a male, but the non-clubbed palps give away her gender. She has eaten very little for weeks, being too busy carrying the sac, and guarding the nursery to hunt. This is on top of the massive energy expenditure in creating her young, the egg sac, and the web. Slimming World could learn a thing or two from her. It has just rained. Spider surveyors are a dedicated bunch!



Here are a few spiderlings on the move within the web.



This is a male. I’ve included the pic only because it displays so beautifully how the fen raft spider is perfectly adapted to aquatic life -walking on water. His first left and right legs are resting on the water, feeling for vibrations in the meniscus, possibly signalling the arrival of the next meal.



Another male -see his clubbed palps in front of him -the “boxing gloves”. These are full of sperm transferred from his primary genitalia and stored ready for mating. The shot is slightly fuzzy as I’ve enlarged it considerably to show his eyes and the palps. He has uncommonly bright bands and you can see his two rows of eyes at the front of the cephalothorax (combined head and chest).

Males don’t have anything to do with the nurseries or young -they are basically sperm donors in their world. I have included this shot not because it’s particularly good, but because it took me ages to get the view and exposure I wanted, He was in a relatively dark recess at a distance. I sweated over it!

Save the Spiders!

During a couple of very cold days in early December 2017 I trotted off to Castle Marshes in North Cove along with our Assistant Warden and my fellow volunteers and pals J and R. Our mission: to look for, and rehome in the dykes, fen raft spiders (FRS) that had been displaced by the dredging of the dykes.

Every year, the dykes need to be dredged to recreate a channel of water. A digger moves along the bank and lifts out the mud, sunken water soldiers (that the FRS lives on), and grass and sedge that has begun to encroach on the water. Without the dredging, the dyke would gradually fill, eventually becoming a continuous part of the surrounding land. The aquatic environment would be lost, along with all its denizens.

If the spiders are left in the debris, they risk being crushed by the digger, which flattens the debris the day after the dredging; they also risk being eaten by herons and other hungry predators. Our efforts would have mitigated that risk, even if only a little. Every spider saved means potentially hundreds to hatch the following year.

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The digger starts its work

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Picking through the debris


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You can see the massive encroachment onto the dyke beyond the clear water, where the digger has stopped.

R was very clever and found 11 FRSs over the two days. I found not a single one, despite the the cold numbness in my fingers and the developing sciatica in my left leg. Neither did J. Harrumph!I managed to find only minute specimens of probably every other arachnid species in the dyke. I have no idea how R managed it -better eyesight and keener to throw herself into the mud than me I expect. We were not helped by the ambient temperature of 5 Celsius, causing all the insects and spiders to be very torpid -not moving around meant they were harder to spot.

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Finding a FRS in the weeds and mud. You can see its brown legs poking out a little below the pointing index finger.

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One for the pot. But not for cooking -he was put back into the water nearby to live on and, with luck, breed next year.

However I was delighted to find a couple of large and very handsome great diving beetles. Click here for more information on this beautiful insect, Dytiscus marginalis.


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Hello! I think I must be having an out-of-water experience. I can see a bright light….

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Beautiful green iridescence on his head

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A formidable and well-equipped hunter

While we were out on Castle Marshes, a Chinese water deer was startled and ran away from us.



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Big round Mickey Mouse ears and a pair of tusks

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Bounding away

One of the last things we saw that day was the spectacular rise of a flock of Greylag Geese, disturbed by something or someone, wheeling across the sky and cackling en masse. I’ve tried to discover if there is another collective noun for a number of geese in flight, and can’t find agreement amongst different sources. Here is a selection -take your pick: flock, plump, skein, team, wedge, gaggle.

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This video is a little shaky -no time to get my Monopod which was 20 feet away in the mud. Still, I’m pleased with my amateur efforts on a handheld bridge camera.


Spider surveys

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.

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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.

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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


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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.


Resting up….



…. 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.

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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.