Assessment of endangered freshwater pearl mussel populations in the Northern Iberian Plateau in relation to non–native species: xenodiversity as a threat

Abstract

In the last two decades, small populations of freshwater pearl mussels, Margaritifera margaritifera, have been recorded in Mediterranean rivers of the Iberian Northern Plateau. A survey was carried out in Castilla and León in 2018 to assess the development of populations of this species in all the rivers of known distribution and to update the threat classification. Thirty sections in the rivers Negro, Tera, Alberche and Águeda were positive for its presence, and another 50 stretches of seven rivers were negative. The species is currently distributed over about 22.5 km. Águeda and Tera populations have decreased dramatically in the last 14 years and are on the threshold of extinction. The Negro river supports the largest population, although the species has now disappeared in at least 61 % of the stretches that were inhabited in 2004. All populations showed very low densities and an ageing population structure, with no recruitment for decades. The presence of non-native invasive alien species (NIS) was higher than in a previous regional survey, with the signal crayfish representing the greatest threat. We observed changes in benthic microhabitats and direct predation of adults and glochidia conglutinates. In the Alberche River, in strict syntopy with M. margaritifera and two other mussel species, 10 NIS were detected. The current hydrological and ecological conditions in the Duero watershed support the settlement of exotic species to the disadvantage of native mollusks, which are more demanding in terms of microhabitats.

The signal crayfish Pacifastacus leniusculus (Dana, 1852) (Crustacea, Decapoda) is threatening the near future of Margaritifera margaritifera Linnaeus, 1758 (Bivalvia, Unionoida) in the Negro River (NW Zamora, Spain)

“…In addition, selective predation on glochidia (larvae) conglutinates has been observed to occur in the Negro River, thus directly affecting FPM reproduction. This is a particularly critical condition in this mussel population, which is known to have not recruited for decades (Morales et al 2004, Morales 2022, and could impede the population's recovery by the eventual release of individuals derived from captive breeding.…”

“…Waters were oligotrophic and lowly mineralized, with the FPM population distributed in low-density patches. Rainfall in the basin area varies between 800 and 1,000 mm/ year, and in the mountain, ranges can frequently reach up to 1,500 mm/year (Morales andLizana 2014, Morales 2022). The precise location of sampling station N2/7, located at 834 m a.s.l.…”

“…The freshwater pearl mussel (FPM) Margaritifera margaritifera Linnaeus, 1758 is a species catalogued as endangered 'EN' in Spain and occurs exclusively in a limited number of rivers of the northwest Iberian Peninsula. In the most recent FPM regional survey, carried out in 2018 (Morales 2022), the first data on the negative pressure exerted by the signal crayfish on the mussels -and on the Negro River ecology-, were assessed, one decade after the release of this invasive species. In this context, the predation exerted by the crayfish on the mussels in nearby basins of northern Portugal was assessed elsewhere (Meira et al 2019 based on direct observations of clamping on siphons, presence of cutting marks at the edge of the shells, and the bullying of mussels by crayfish.…”