Medicinal leeches and drought

25th February 2011, Friday

Returning to a familiar theme, the impact of drought on a series of grazing marsh ditches at East Guldeford, my latest post brings me to the medicinal leech, Hirudo medicinalis.  Drought would seem an obvious killer for this animal, however the leech is a resiliant beast.  Last summer I netted the ditches that had dried out in 2009 and found the leech to be distributed in good numbers through most of them, even an extremely isolated and ephemeral ditch that dries out in most years.

Whilst I guess the animals could have been brought in on the legs of waterfowl the 2009 drought was very severe with almost every ditch drying out at East Guldeford.  The numbers of leeches found last summer suggests these animals aestivated in the damp mud until wet conditions returned.  Infact this animal is more terrestrial than you might imagine.  The adults leave the water in search of a suitable location to lay a cocoon containing eggs.  At Dungeness these are laid amongst the roots of plants such as great willowherb Epilobium hirsutum, about 60 cm above the water table, whereas on Walland marsh they have been found in hollows in damp turf.

So once again we have another aquatic species that is superbly adapted to the droughts that frequently afflict this ditch system.

Today, which was mild and spring-like, had medicinal leeches responding to the movement of my net, fooling them into thinking a blood meal was on offer.  This would have been a substantially better fare than the sticklebacks, marsh frog and smooth newts which were the main vertebrates I was able to find.  No doubt the pair of mallard on a neighbouring ditch offered meals on wings.