Growth characteristics of the endangered thick‐shelled river mussel (Unio crassus) near the northern limit of its natural range

Abstract

1. The thick-shelled river mussel, Unio crassus (Bivalvia: Unionoida), is one of Europe's most-threatened mussels. Finnish populations of U. crassus lie close to the northern limit of its natural distribution. Extirpation of these populations will reduce the range of this endangered species.2. Growth characteristics of U. crassus were measured in a river running through the Helsinki metropolitan area. Shell dimensions (size-at-age data) and annual shell growth increments were used to reconstruct growth rate and its variation during the lifespan of individual mussels and to investigate the relationship between growth rate and longevity (age-at-death).3. Reconstructed growth rates compared well with size-at-age data conventionally used to study individual growth in natural populations, and fitted von Bertalanffy growth functions very well. Based on the same function, reconstruction and size-at-age methods resulted in similar estimates of growth rate. 4. Shell weight explained the body size more reliably than age, suggesting that growth rate varied significantly among individuals. Comparison of individual growth histories revealed a negative correlation between age-atdeath and growth rate, i.e. slow-growing mussels lived longer, and vice versa.5. In comparison with populations from central and southern Europe, U. crassus populations in the north of its range grew more slowly but lived longer, a phenomenon most likely explained by latitudinal changes in ambient temperature.6. Although northern populations are expected to benefit from a relatively high number of reproductive periods and lower juvenile mortality, the individuals studied here died earlier and suffered higher mortality than expected and an elevated conservative status is identified. Any management programme should take into account the lifehistory traits essential to the recognition of management units of U. crassus.

Local adaptation studies and conservation: Parasite–host interactions between the endangered freshwater mussel Unio crassus and its host fish

“…The unionoid thick‐shelled river mussel Unio crassus occurs in soft substratum and sand‐ and gravel‐rich middle watercourses of varying hydrology (Björk, ; Lopes‐Lima et al, ; Stoeckl & Geist, ) where it averages about 8 to 50 years and a maximum life‐span up to 90 years over its distribution area (Helama, Valovirta, & Nielsen, ; Hochwald, ; Lundberg & von Proschwitz, ). The mussel spawns in late spring, and parasitizes on fish species such as minnow Phoxinus phoxinus , bullhead Cottus gobio and chub Squalius cephalus for about 1 month from late spring to early summer (Hochwald, ; Lundberg, Bergengren, & von Proschwitz, ; Maaß, ; Taeubert, Gum, & Geist, ; Taeubert, Martinez, Gum, & Geist, ).…”

Determining Bioecological and Biometric Properties of Freshwater Mussels (Unio crassus Philipsson, 1788)

“…The shell measurements and population dynamics of U. crassus are related to the water conditions and the region [10,[29][30]. In studies on U. crassus, the maximum shell length was reported as 66.9 mm in the Wkra River (Poland) [31]), 67.4 mm in the Palakaria River (Bulgaria) [32], 81 mm in Bohemia (Czech Republic) [30], 83 mm in Estonia [9], 85.4 mm in the Vantaa River (Finland) [33], and 95 mm in the Susa River (Denmark) [34]. During this study, which was done in Çine Creek, the largest length was measured as 92.50 mm, while another study conducted in Sapanca Lake on the west coast of Turkey reported that U. crassus were found with lengths of 74 mm [35].…”

“…When all these studies are taken into consideration, it is seen that the mussel measured in Çine Creek has the longest length except for the Susa River. Helama et al [33] reported that the average age of death for U. crassus individuals in the Vantana River (Finland) was 12.5 years and the longest living individual has a lifespan of 38 years, and that these individuals weighed 58.7 mm on average and weighed 14.9 g on average. The largest individual measured 85.4 mm and weighed 44.4 g and this individual was 15 years old.…”

“…The largest individual measured 85.4 mm and weighed 44.4 g and this individual was 15 years old. It is estimated that the major part of the population in Çine Creek is over 15 years old, as most individuals distributed in Çine Creek are larger than the individuals in the Vantana River [33]. This shows that the U. crassus species in this region can live for a long time.…”

Effects of mussel and host fish density on reproduction potential of a threatened unionoid mussel: prioritization of conservation locations in management trade-offs

“…A mussel with flat and transparent gills was categorized as non-gravid, including male individuals. The individual shell length (nearest mm, Helama et al 2017) was noted. The ratio of gravid/non-gravid mussels was calculated for every sampling occasion.…”