Rare mussels could be the jewel in a new gem industry for Wales

Oct 2 2009 by Robin Turner, Western Mail

Distribution of SACs with species 1029 Margaritifera margaritifera. Click image for enlarged map.

THERE was a time when Welsh rivers were rich with natural pearls.

But in recent years illegal poaching, pollution and river engineering have made the freshwater gems a rarity.

Now research work by Swansea University aims to safeguard the most endangered and puzzling aquatic species, the freshwater pearl mussel.

And it is possible the research could eventually lead to Wales having its own cultured freshwater pearl industry.

Lead researcher on the project, Gethin Rhys Thomas, 25, of Ammanford, of the university’s school of the environment and society is working with Dr Carlos Garcia de Leaniz, a senior lecturer in the department of pure and applied ecology to look at ways of preserving what is regarded as one of Wales’ natural treasures.

Mr Thomas said: “Surprisingly few people are aware that there are freshwater pearl mussels in Welsh rivers, or that they are now possibly the most endangered aquatic organisms anywhere in the world.”

Freshwater pearl mussels live for up to 200 years, and have unique parasitic lifecycles, relying on fish like brown trout or salmon for their survival.

The juvenile mussels attach to the gills of trout and similar fish where they develop for several months until they eventually fall off to the river bed and begin feeding by themselves.

Freshwater mussels are critically endangered, yet they are considered “flagship” species as they play a key role in the recycling of nutrients in rivers. Their well-being, or otherwise, is seen as a good indicator of the well-being of a river, and their decline can have important knock-on effects on the entire local ecosystem.

Freshwater mussels used to be widespread across Europe, but have suffered a 95% decline over the last few decades, mainly as a result of

habitat loss, pollution, illegal pearl fishing, and river engineering works.

Wales, in particular, has undergone major river engineering works because of historic flooding problems affecting towns and cities.

Studies in some Welsh rivers show that few or no mussels have been able to reproduce over the last 30 years, resulting in ageing populations.

Trout and salmon have also declined, making it even more difficult for mussels to reach maturity and reproduce.

Mr Thomas said: “Once, most salmon rivers would have had freshwater pearl mussels.

“But now, there are only half a dozen mussel populations in Wales, and they’re not reproducing – which has obvious consequences for the survival of the species.”

Mr Thomas has recently received a grant from Cymdeithas Edward Llwyd, the Welsh Naturalist Society, to investigate the decline in the mussels in collaboration with the Environment Agency Wales.

In particular, the work will focus on identifying the traits that make trout and salmon become receptive to hosting freshwater mussels as part of a captive breeding programme.

The work could eventually lead to a cultured freshwater pearl industry of the type which is currently thriving in China.

He said: “Our research is important for the conservation of freshwater mussels, particularly for Welsh rivers, as it is helping us understand critical bottlenecks and to develop better conservation strategies

 

 

The freshwater pearl mussel Margaritifera margaritifera

The freshwater pearl mussel Margaritifera margaritifera grows to 140 mm in length, and burrows into sandy substrates, often between boulders and pebbles, in fast-flowing rivers and streams. It requires cool, well-oxygenated soft water free of pollution or turbidity. The mussel spends its larval, or glochidial, stage attached to the gills of salmonid fishes. The larvae attach themselves during mid to late summer and drop off the following spring to settle in the riverbed gravel where they grow to adulthood.

The freshwater pearl mussel is one of the longest-lived invertebrates known, and individuals can survive for over 100 years (Bauer 1992).The mussels live buried or partly buried in coarse sand and fine gravel in clean, oligotrophic, fast-flowing and unpolluted rivers and streams.They inhale water through their exposed siphons to filter out minute organic particles on which they feed. Where the species was formerly abundant, it is possible that this filtration acted to clarify river water to the benefit of other species, including juvenile Atlantic salmon (Salmo salar) and brown or sea trout (Salmo trutta) (Zuiganov et al. 1994). The freshwater pearl mussel is typically dioecious, in common with other freshwater bivalves. It matures at 10–15 years, when the length generally exceeds 65 mm. In early summer (June to July), the

males shed sperm into the water, and it is inhaled by the females.The fertilised eggs develop in a pouch on the gills for several weeks, and are released from July to September as tiny larvae, measuring  0–70:m (0.6–0.7 mm), known as glochidia.These resemble tiny mussels, but their shells are held apart until they encounter a suitable host, when they snap shut on to the host’s gill filaments (Young & Williams 1984)  Each female ejects between 1 and 4 million glochidia in a sudden, highly synchronised event, usually over one to two days (Hastie 2001b). It is likely that a threshold temperature, or other environmental cue, may trigger glochidial release.The proportion of adults producing glochidia is relatively high,  arying from 30–60%, even in sparse populations (Young & Williams 1983). Almost all the glochidia are

swept away and die, but a few are inhaled by juvenile Atlantic salmon and brown or sea trout. Infective glochidia can remain viable for up to six days (Zuiganov et al. 1994), but most attachments probably occur within a few hours (Young & Williams 1984).

Glochidia attached to the gills of juvenile fish encyst, live and grow in the hyper-oxygenated environment until the following spring.They drop off in May and early June, and must land in clean,sandy or gravelly substrates to settle and start to grow.This association does not appear to harm the
fish, and enables young mussels to colonize new areas upstream. Young mussels are typically yellowish brown, becoming darker with maturity. Freshwater pearl mussels develop very slowly and can live for more than 100 years, reaching 12–15 cms in length.The life span and maximum size reached are highly variable between populations, depending on environmental conditions, particularly hydrochemistry and water temperature (Zuiganov et al. 1994). Freshwater pearl mussel populations tend to be faster growing and shorter lived in the
southern part of their range, with a lower reproductive output (Hastie 2001b) than those in the upper (cooler) reaches of catchments and more northerly latitudes (Bauer 1992, Hruska 1992).The mussels can re-bury themselves if dislodged, and can also move slowly across sandy sediments. Most have about a third of their shells exposed, but some adults and virtually all juveniles burrow completely into the substrate and under loose stones (Hastie et al. 2000a). The huge losses involved in this unusual life cycle make the freshwater pearl mussel particularly  vulnerable to adverse conditions.