Travelling back from Cumbria in early February, I decided to stop off at Bolton-le-Sands at the top of Morecambe Bay to stretch my legs. It was nearly high tide and so offered the prospect of good birdwatching. Whilst pottering about on the marsh by the high water mark, I found a stranded mermaid’s purse (Thornback Ray Raja clavata (L., 1758)) and idly picked it up. On returning to the car I noticed the purse was studded with tiny gastropods and, assuming they were Hydrobia, tucked it away in a bag for identification to species level later. When the time came, the snails soon began to emerge when encouraged with the prospect of some food and water. However, where were the tentacles? Examination under magnification revealed mere stubby appendages instead, with eyes towards the ends, identifying the snails as not Hydrobia at all but Assiminea grayana. However, this was confusing as the text books state this is a species not found on the western coast of the UK (Fretter & Graham 1978; Graham 1988; Poppe & Goto 1991). Yet here I had specimens, self-collected (13 in all, from the one purse) and undoubtedly from the west coast. Somewhat confused, I contacted Jan Light, who kindly confirmed my identification and informed me of a recent discovery of the species present in some numbers in a locality in Cumbria, just the other side of Morecambe Bay. Further to this, records were publicised of apparently well established populations in Merseyside (Charlton & Ruscoe 2007) and earlier 1990s records from the Shannon estuary in western Ireland came to light (Capel 2004).
All in all this represents a considerable and relatively rapid expansion in distribution for the species. The small size of the species (H<5mm) and its durability as an inhabitant of the extreme HWST zone suggest that it would be readily transported by many vectors, either natural (on the feet of migratory birds perhaps) or influenced by the activities of man. However, such vectors are not new phenomena, further suggesting that other factors have allowed the species to establish on western coasts when previously this was thwarted. The obvious conclusion is that this represents further evidence for the influence of climate change. But in what way? Has there been a change in sea level, mean water and/or air temperature, salinity or perhaps the extremes of any or all of these criteria? Capel (2004) suggests salinity is a key factor for the species. The answers are certainly not presented here, but the question is posed for future research.
References:
Capel, J., 2004. Hull Biodiversity Action Plan: Saltmarsh Snails. [Online]. Hull University. Availablefrom:http://www.hull.ac.uk/HBP/ ActionPlan/SSnails.htm [cited 23 May 2007]
Charlton, D., & Ruscoe, C., 2007. Assiminea grayana Fleming, 1828 found on the west coast of England. Pallidula 37 (1) pp. 17
Fretter, V., & Graham, A., 1978. The Prosobranch Molluscs of Britain and Denmark Part 3 – Neritacea, Viviparacea, Valvatacea, Terrestrial and Freshwater Littorinacea and Rissoacea. The Journal of Molluscan Studies Supplement 5
Graham, A., 1988. Molluscs : Prosobranch and Pyramidellid Gastropods. Synopses of the British Fauna (New Series) No 2 (2nd Edition). Leiden: E.J. Brill.
Poppe, G. T., & Goto, Y., 1991. European Shells Volume 1 (Polyplacophora, Caudofoveata, Solenogastra, Gastropoda). Wiesbaden: Hemmen