Spengler’s freshwater mussel (Margaritifera auricularia)
|Also known as:||giant European freshwater pearl mussel, Spenglers pearlshell|
|Size||Shell length: up to 20 cm (2)|
Spengler’s freshwater mussel is classified as Critically Endangered (CR) on the IUCN Red List (1).
Thought to be one of the most threatened invertebrates in the world (2) (3), Spengler’s freshwater mussel (Margaritifera auricularia) is a large mollusc with a shell that consists of two pearly valves (4). These two shell parts, which enclose the animal’s soft body (4), are black on the outside and pearly white on the inside (2). The beautiful inner layer of the shell, or nacre, of Spengler’s freshwater mussel was used by craftsmen until the 1920s to make buttons and decorate jackknife hilts (2) (5).
Despite the fact that this species is also known as the giant European freshwater pearl mussel, it is not thought to be a true pearl mussel (2).
The shell of Spengler’s freshwater mussel is exceptionally heavy (5), and is much thicker at the front compared to the back, where the shell is thinner and more fragile (2). Growth lines are present on the shell, and can be counted to make a rough age estimate for the individual (5). However, because the lines near the shell edge are extremely close together, this method cannot be used to accurately estimate the age of Spengler’s freshwater mussel (2).
The foot of Spengler’s freshwater mussel is white and very large, and when it protrudes it can be as big as the entire shell (2).
Once widespread throughout Europe (1) (6), Spengler’s freshwater mussel has suffered an alarming decline since the beginning of the 20th century (7), and is now believed to be extinct throughout most of its range (2).
Until the 1990s, the last records of live specimens dated from 1933. As a result, the species was thought to be extinct until a population of approximately 2,000 Spengler’s freshwater mussels was discovered in 1996 in a channel of the Ebro River Basin in Spain (7).
The current range of Spengler’s freshwater mussel seems to be limited to France and Spain, with subpopulations occurring in just a few rivers including part of the River Ebro in southern Catalonia, Spain (1) (5).
Spengler’s freshwater mussel is found in large, slow-flowing rivers (1) (5) (8), and it is the only species within its genus to live in such a habitat (5). The rivers this species inhabits must have clean, well-oxygenated water (1) (5) (6) (7) (8), with substrate formed from a mixture of gravel, pebbles and sand (1) (5) (7) (8).
Spengler’s freshwater mussel can be found in the main channel of the River Ebro at depths greater than eight metres (5).
Spengler’s freshwater mussel is a very sedentary species, and will partially bury itself in the gravel and pebbles of its habitat, leaving wide, circular traces visible in the substrate. This species tends to place itself in an almost vertical position with the back end exposed, particularly during the breeding season, or in a horizontal position with only the dorsal section visible (7).
It is not known exactly when the eggs of Spengler’s freshwater mussel are fertilised (7), but the thin-shelled larval stage, known as glochidia, are brooded for a short period of time in the four gills of the adult mussel before being released in February or March (3) (7). The ratio of hermaphroditic individuals in the population of Spengler’s freshwater mussel is very high (3).
The microscopic larvae are parasitic, and require a specific fish host in order to progress to the juvenile stage (4) (8). The glochidia use little hooks to attach themselves to the fins or gills of a fish host, where they remain for several weeks before metamorphosing into free-living juveniles (7). The probability of an individual larva reaching maturity is relatively low (5).
The common or European sturgeon (Acipenser sturio), a bottom-feeding fish, is thought to be the main host species for Spengler’s freshwater mussel larvae (1) (5) (8). In captivity, Spengler’s freshwater mussel has been sucessfully bred using the Siberian sturgeon (Acipenser baeri) and the freshwater blenny (Salaria fluviatillis) (2), with the latter being a potential host species in the River Ebro (1).
Spengler’s freshwater mussel faces many threats, most of which are the result of human activities (1).
Historically, overexploitation by pearl and nacre collectors was the major cause for the decline of the highly valued Spengler’s freshwater mussel (5) (6) (7) (8). However, more recently pollution has become a key threat (1) (2) (6) (8), as bivalves are sensitive to both pollutants dissolved in the water and those that accumulate in sediment in the river bed (5). Pesticides leaching into the water from the large irrigated fields along the banks of the River Ebro pose a major threat (1) (2).
Badly planned water use is frequently named as the reason behind all of the problems faced by Spengler’s freshwater mussel (1) (2), and the habitat of this species is altered in many ways through human activities such as water extraction and engineering works (2) (6).
Spengler’s freshwater mussel is extremely dependent upon its fish hosts (4), and therefore any activities which impact upon the fish and their habitat will also negatively affect this Critically Endangered mussel (1) (2) (5). For example, the European sturgeon, the main host fish for Spengler’s freshwater mussel, is thought to be extinct in the River Ebro, and is suffering severe declines in most other western European rivers (1). A lack of host fish due to dams, which block migration routes, and dredging, which may prevent the fish from breeding (1) (2) (7), could be a major cause for the decline in Spengler’s freshwater mussel (2) (3) (8).
Plans to modernise the Canal Imperial in Spain by laying down concrete slabs are expected to potentially kill the entire population of Spengler’s freshwater mussel in the area (2) (7). Very little or no reproduction is occurring in the various subpopulations of this species, and in 2010 it was estimated that nearly all of these smaller, isolated populations will disappear in the next 20 to 50 years (1).
A further potential threat to Spengler’s freshwater mussel is poaching, once the risk to the species becomes well known and its value increases (2).
Spengler’s freshwater mussel is strictly protected in France and Spain (1), as a result of being listed on Annex IV of the EU Habitats Directive (9) and on Annex II of the Bern Convention (10). It is also protected by Spanish national legislation (3), being included on Annex I (Endangered) of Spain’s National Endangered Species List (2), the first invertebrate to be placed on such a list (7).
In 1983, a large part of the Ebro Delta was declared a Natural Park, although this does allow some unspecified exploitation of the natural resources (5). In 1990, dead specimens of Spengler’s freshwater mussel were found near the Ebro estuary, which led to the proposal and acceptance of a set of recommendations aimed at protecting this species and others found within the Natural Park (7). The accepted conservation measures within Ebro Delta Natural Park are outlined in various Action Plans and include the protection of key habitats and restrictions on the collection of Spengler’s freshwater mussel, as well as the establishment of breeding colonies (1) (2) (5) (6) (7).
Further research into Spengler’s freshwater mussel is necessary (2) (5) (6) (7), including studies into its ecology (1) (7), population size and structure (7), and the identification of new populations (2).
Studies into potential fish host species such as the common eel (Anguilla anguilla) and the river blenny are already underway (7), and the protection of such fish hosts is one of the key conservation recommendations for Spengler’s freshwater mussel (2) (5) (6) (7). The restoration of host fish populations is also of utmost importance (1), and reintroduction of stocks of these species could ensure the future of Spengler’s freshwater mussel (1) (2).
Further recommendations include reintroduction of Spengler’s freshwater mussels from captive breeding colonies to the wild, as well as public education programmes, and water quality control within this species’ habitat (5) (6) (7).
Further information on the conservation of freshwater habitats and biodiversity:
IUCN Red List Freshwater Initiative:
IUCN Freshwater Biodiversity Unit:
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- Bivalve: a group of aquatic molluscs in which the soft parts are encased in a shell consisting of two parts, known as valves.
- Dorsal: relating to the back or top side of an animal.
- Fertilisation: the fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.
- Genus: a category used in taxonomy, which is below ‘family’ and above ‘species’. A genus tends to contain species that have characteristics in common. The genus forms the first part of a ‘binomial’ Latin species name; the second part is the specific name.
- Hermaphroditic: possessing both male and female sex organs.
- Invertebrates: animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones) and echinoderms.
- Larva: immature stage in an animal’s lifecycle, after it hatches from an egg and before it changes into the adult form. Larvae are typically very different in appearance to adults; they are able to feed and move around but are usually unable to reproduce.
- Larval: of or relating to the immature stage in an animal’s lifecycle, after it hatches from an egg and before it changes into the adult form. Larvae are typically very different in appearance to adults; they are able to feed and move around but are usually unable to reproduce.
- Metamorphosis: an abrupt physical change from the larval to the adult form.
- Molluscs: a diverse group of invertebrates, mainly marine, that have one or all of the following: a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.
- Parasitic: describes an organism that derives its food from, and lives in or on, another living organism at the host’s expense.
IUCN Red List (December, 2011)
- Araujo, R. and Ramos, M.A. (2001) Action Plans for Margaritifera auricularia and Margaritifera margaritifera in Europe, Issues 18-117. Council of Europe, Strasbourg, France.
- Bauer, G. and Wächtler, K. (2001) Ecology and Evolution of the Freshwater Mussels Unionoida. Springer, USA.
- Oscoz, J., Galicia, D. and Miranda, R. (2011) Identification Guide of Freshwater Macroinvertebrates of Spain. Springer, USA.
- Altaba, C.R. (1990) The last known population of the freshwater mussel Margaritifera auricularia (Bivalvia, Unionoida): A conservation priority. Biological Conservation, 52(4): 271-286.
- Maitland, P.S. and Morgan, N.C. (1997) Conservation Management of Freshwater Habitats: Lakes, Rivers, and Wetlands. Springer, USA.
- Araujo, R. and Ramos, M.A. (2000) Status and conservation of the giant European freshwater pearl mussel (Margaritifera auricularia) (Spengler, 1793) (Bivalvia: Unionoidea). Biological Conservation, 96(2): 233-239.
- Koomen, P. and van Helsdingen, P.J. (1996) Listing of Biotopes in Europe According to their Significance for Invertebrates,Issues 18-77. Council of Europe, Strasbourg, France.
EU Habitats Directive (December, 2011)
Council of Europe: Bern Convention (December, 2011)