Northern Ireland's Priority Species

Aplexa hypnorum – moss bladder snail

Aplexa hypnorum

Aplexa hypnorum (Linnaeus)
Family: Physidae

The moss bladder snail has a dark red-brown to yellow-brown, highly polished shell with aoperture on the left side (sinistral) and a tall, sharp spire with nearly flat sutures. The animal is a beautiful dark grey to almost black and it is found seasonally in ponds and ditches.
A widespread but declining species in Ireland due to loss of habitat, such as infilling of farm ponds and ditches (Kerney, 1999), and land drainage in general. The species prefers ditches and temporary ponds and pools which it shares with other species adapted to seasonal habitats such as the button ramshorn Anisus leucostoma.

In brief

  • This is a Northern Ireland Priority species and has been red-listed as Vulnerable in Ireland (Byrne et al. 2009)
  • It has suffered a substantial decline in records since 1980
  • Favoured habitat: floodplain pools and swamps on river and lake margins and ditches and temporary ponds

Species description
There are four species of bladder snail in Britain and Ireland. Two of these are alien introductions form N. America, and two are native. Aplexa differs from the other three in having a tall spire with flattened sutures, while the others have squat, globular shells with very low spires and impressed (distinct) sutures between the whorls. The shell is medium-sized (9-12 mm), highly polished and slippery, a feature which it shares with the other native, though smaller and more globular common bladder snail Physa fontinalis, and the animal is dark, nearly black. It lacks the mantle digitations (projections like fingers) found in the mantles of the other three bladder snails.

Life cycle
Adults may be present at all times of year but are more abundant and obvious at the end of the winter period when flood levels are usually at their highest. Conversely, they will be rare in dry summers when habitat water levels are at their lowest or absent. Breeding may take place whenever conditions are suitable but are probably concentrated in the winter months.

Similar species
See Species Description

How to see this species
Areas with good populations of this species include the low-lying shores of Lough Neagh where seasonal flooding of back-marshes and alder scrub still occurs. River floodplains which have not been disrupted by main drainage schemes are also good. An example is the River Lagan between Waringstown and Lisburn. Select an area of shallowly flooded riverbank or lakeshore, or ditches connected to the main water course and sweep with a water net in winter or early spring. This will usually provide an abundance of Aplexa, Anisus leucostoma and other temporary water species.

Current status
Widespread in the main watersheds where main drainage has not destroyed natural riverbank architecture. Also along lakeshores such as those of Loughs Neagh and Erne. Probably not as threatened in Northern Ireland as in southern counties where declines have been severe in recent decades.

Why is this species a priority in Northern Ireland?

  • This species is in decline because of land drainage across its range in Europe
  • It is a Priority Species in Northern Ireland and listed as Vulnerable in a recent red list for Ireland
  • It is in decline across Ireland although this may not be as severe in northern counties

Threats/Causes of decline
The main causes of decline are land drainage, major arterial drainage schemes on rivers and the intensification of agriculture generally.

Conservation of this species

Current action
Declaration as a Priority Species.

Lough Neagh is designated as an ASSI.

Proposed objectives/actions
Most of the damage caused to its habitats in N. Ireland is historical. However, intensification of agriculture continues to nibble away at its habitats. Support for agri-environmental schemes is needed to encourage responsible, long-term husbandry in beef and sheep rearing areas where most remaining habitat resides.

What you can do
This is not a difficult species to identify, but it is seasonal in its occurrence. Examine small, temporary ponds, pools and marshes near you to see if it is present. If you encounter something which suggests this species please note the locality from an OS map and report the details, with a specimen or specimens to CEDaR (Record Centre Manager, CEDaR, National Museums Northern Ireland, Cultra, Holywood, Co. Down, BT18 0EU;

Further information


iNaturalist: Species account : iNaturalist World Species Observations database