Pond bivalves 
 
Bivalves are molluscs which specialise in filter-feeding and live in the sea or freshwater, not on land.  They are an ancient group, first found in in the early Cambrian period c 500 million years ago, and there are about 10,000 species alive today.  As aquatic animals, they have had little attention from garden ecologists, and are not included in any of the garden wildlife guides. This is a group where some careful work by keen gardeners could bring in a lot of useful data.
 
Species in Britain and Ireland
There are about 31 freshwater bivalve species in Britain and Ireland from 5 families1. Some, like the swan mussel Anodonta cygnea  are large, up to 15cm long, as is the threatened pearl mussel Margaritifera margaritifera, but these larger species don't occur in ponds unless introduced deliberately.  The majority of species are within the family Sphaeriidae, and they are small - all less than 15mm length, or very small indeed, down to 2mm.  We have three genera, Musculium with two species found in muddy habitats, Sphaerium which has four species  up to about 12mm found in lakes, rivers and ponds, and the pea mussels Pisidium with 16 species, which range from 2 to 12mm and occur in all freshwater habitats.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Left: Pea mussel Pisidium amnicum c 12mm       Right: 3-4mm Pisidium spp from Steve Head's pond. The one on the right has its foot extended - there is also an ostracod at bottom left.
 
Finding abundant Pisidium in my own pond - where they have survived for 20 years - means that we can state with confidence that this genus can be found in garden ponds. How often of course we don't know, but I suspect they are pretty common in established ponds.
 
Biology of small pond bivalves
These clams feed mainly on bacteria living interstitially in the sediment at the bottom of ponds, and probably utilise phytoplankton and tiny detritus particles too.  They are unusually mobile, with a large foot which they use to move over the bottom, and in the case of Sphaerium to climb plant stems.
 
These sphaerids are hermaphrodite, and brood their eggs internally, releasing the offspring as miniature adults.  It is interesting to speculate how they come into isolated ponds, probably with translocated vegetation, or perhaps on the feet of birds.
 
References
 
1.   Anderson, R. 2005, An Annotated List of the Non-Marine Mollusca of Britain and Ireland, Journal of Conchology,
      London, 38: 607-638.
2.   Webpage on Pisidium casertanum on AnimalBase
 
Other sources of information
 
Book
Ian Killeen, David Aldridge and Graham Oliver (2004)  Freshwater Bivalves of Britain and Ireland  Field Studies Council AIDGAP Guides Volume: OP82
 
Website
Website of the Conchological Society of Great Britain and Ireland
 
 
Page written by Steve Head
Left: European fingernail clam Sphaerium corneum, shell about 10mm across.  This is probably the most common Sphaerium, but we don't know if it occurs in any garden ponds. Note the extended foot.
 
 
While writing this page, I remembered having seen what looked like Pisidium 20 years ago, shortly after I created my garden pond, so I took some samples on the sloping "beach area" from the sparse gritty sediment adjacent to and under overhanging grass. Examination under a stereomicroscope showed Pisidium was still present in large numbers. They have been kindly identified by Dr Ben Rowson of the National Museum of Wales as probably Pisidium casertanum, considered one of the world's most widely distributed non-marine mollusc2.
Pond bivalves
 
Bivalves are molluscs which specialise in filter-feeding and live in the sea or freshwater, not on land.  They are an ancient group, first found in in the early Cambrian period c 500 million years ago, and there are about 10,000 species alive today.  As aquatic animals, they have had little attention from garden ecologists, and are not included in any of the garden wildlife guides. This is a group where some careful work by keen gardeners could bring in a lot of useful data.
 
Species in Britain and Ireland
There are about 31 freshwater bivalve species in Britain and Ireland from 5 families1. Some, like the swan mussel Anodonta cygnea  are large, up to 15cm long, as is the threatened pearl mussel Margaritifera margaritifera, but these larger species don't occur in ponds unless introduced deliberately.  The majority of species are within the family Sphaeriidae, and they are small - all less than 15mm length, or very small indeed, down to 2mm.  We have three genera, Musculium with two species found in muddy habitats, Sphaerium which has four species  up to about 12mm found in lakes, rivers and ponds, and the pea mussels Pisidium with 16 species, which range from 2 to 12mm and occur in all freshwater habitats.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
European fingernail clam Sphaerium corneum, shell about 10mm across.  This is probably the most common Sphaerium, but we don't know if it occurs in any garden ponds. Note the extended foot.
 
While writing this page, I remembered having seen what looked like Pisidium 20 years ago shortly after I created my garden pond, so I took some samples on the sloping "beach area" from the sparse gritty sediment adjacent to and under overhanging grass. Examination under a stereomicroscope showed Pisidium was still present in large numbers.  They have been kindly identified by Dr Ben Rowson of the National Museum of Wales as probably Pisidium casertanum, considered one of the world's most widely distributed non-marine mollusc2.
Left: Pea mussel Pisidium amnicum c 12mm   Right: 3-4mm Pisidium spp from Steve Head's pond The one on the right has its foot extended - there is also an ostracod at bottom left.
 
Finding abundant Pisidium in my own pond - where they have survived for 20 years - means that we can state with confidence that this genus can be found in garden ponds. How often of course we don't know, but I suspect they are pretty common in established ponds.
 
Biology of small pond bivalves
These clams feed mainly on bacteria living interstitially in the sediment at the bottom of ponds, and probably utilise phytoplankton and tiny detritus particles too.  They are unusually mobile, with a large foot which they use to move over the bottom, and in the case of Sphaerium to climb plant stems.
 
These sphaerids are hermaphrodite, and brood their eggs internally, releasing the offspring as miniature adults.  It is interesting to speculate how they come into isolated ponds, probably with translocated vegetation, or perhaps on the feet of birds.
 
References
 
1.   Anderson, R. 2005, An Annotated List of the Non-Marine Mollusca of Britain
      and Ireland, Journal of Conchology,  London, 38:607-638.
2.   Webpage on Pisidium casertanum on AnimalBase
 
Other sources of information
 
Book
Ian Killeen, David Aldridge and Graham Oliver (2004)  Freshwater Bivalves of Britain and Ireland  Field Studies Council AIDGAP Guides Volume: OP82
 
Website
Website of the Conchological Society of Great Britain and Ireland
 
 
Page written by Steve Head
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