What's going on with Enoplognatha (mordax?)?

Posted by Graeme Lyons , Tuesday 29 June 2021 10:29

In 2018, I found immatures of what looked like Enoplognatha mordax, in the Brede Valley. Soon after this, they were independently found on the Pevensey Levels (both sites not that far from each other in East Sussex either side of Hastings - just where you would expect a colonisation event from the continent to come from). This is quite different behaviour to Enoplognatha mordax, which is low to the ground on the upper limits of saltmarshes. A very restricted spider. I could not find an adult and that was that for the year. In 2019, it started turning up all over the place as immatures and not just in wet grassland. By this point, I started believing it was something different and Peter Harvey said we should get hold of an adult, preferably a male, as it might be something different. It's a complex genus, so this could be something totally different from Europe or further afield or something genetically distinct. I only found my first adult, a female, in 2020 (below). 

But get this, it was nothing to do with levels/grazing marsh. It was from an arable margin on the Norfolk Estate on the top of the Downs. I sent it to Peter Harvey but what we really need was a male. The other recorder collected a male in 2019 from Pevensey but let it go, believing it to be mordax after photographing it. This shows the importance of keeping specimens, especially with such a bizarre occurrence like this as it's took me another two years to get a male. So finally, in 2021 I have started finding lots of adults. I had two males on a farm in East Sussex yesterday along the Cuckmere, with one running across a farm track in front of me (photo at the top of the page). 

In June alone, I have recorded four males from three sites, all on lush arable margins on the Downs in both East and West Sussex. I also found a number of adult females along the stream restoration at Knepp about ten days ago, again swept from rank vegetation along ditches and streams. It's clearly well distributed in both counties. Enoplognatha mordax is usually found in litter, deep in rush tussocks, by suction or by lifting matts of algae on saltmarshes etc. I would never expect to find it where you find this thing and I don't think I have ever found it sweeping. My money is on a new species, but what species? Here is the habitat of yesterday's males. Not especially specialised. It seems to prefer this stuff than the original sites of grazing marsh/levels. But that's where I have been recording a lot this year, so there could be an observer bias here.

Not the best video, but here is the male moving north across a farm track. This spider is clearly able to disperse well. Quite different to mordax. When I surveyed East Head and Rye Harbour, move a few metres from the saltmarsh edge and that's the end of mordax records. Why would a spider behave exactly the same for over a 100 years, then suddenly behave really differently, but only in one specific part of the country? It must be a colonisation event, surely? 

Here is an actual Enoplognatha mordax from Rye in 2013. Certainly looks pretty similar but this new thing appears a little more robust according to Peter Harvey and I would agree with that.

I am sending more of my specimens to Peter Harvey who is going to pass them on to Peter Merrett. Peter believes there are subtle but significant differences in the male's palps (I struggled with this to be fair but did notice one area on the palps that did look a little different - I am going to see if what I collected yesterday matches up with this now) and that it could be either a new species to the UK (my belief) or a speciation in action. I predict it is going to turn up in neighbouring counties before the end of the year. Whatever it is, it's going to be a very common spider soon. Keep an eye out if you live in Kent, Surrey or Hampshire as this is spreading fast. Do any global spider recorders recognise this or its behaviour as a different species? Any help greatly appreciated. If you could pass this blog on to any spider specialists you know, especially in Europe or America, that would be greatly appreciated. I am wondering if there has been a similar occurrence in northern France an this thing has just hopped the Channel? Does anyone have a contact for a north France spider specialist?

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