Due to their sensitivity and dramatic declines, freshwater mussels are prime targets for conservation and environmental
monitoring. For this, however, information is needed on life history and ecological traits, which is
lacking in many taxa, including threatened species. Species recently described or recognized as valid are of
particular concern, due to the shortage of even basic knowledge. A case in point is the recently recognized and
Near Threatened dolphin freshwater mussel Unio delphinus Spengler, 1793, which is endemic to the western
Iberian Peninsula and has suffered marked population declines. To overcome information gaps for U. delphinus,
we carried out a holistic biological study across the species range, aiming to: i) estimate the area of occupancy
(AOO) and extent of occurrence (EOO) based on updated distribution data taken from the literature and recent
surveys; ii) estimate growth patterns from biometrical (shell dimensions and growth annuli) measurements taken
on specimens from seven populations; iii) estimate sex ratios from gonad tissue biopsies collected on specimens
from eight populations; iv) estimate gametogenesis and sex ratio through histological examination of gonad and
gill tissues collected monthly for a year, from a single population; and v) determine host species from infestation
trials of glochidia with co-occurring fish species. We estimated an EOO of 706 km2 and an AOO of 61 km2, which
together with data on declines assigns the species to the Endangered category using IUCN criteria. Unio delphinus
was found to grow faster and to be shorter-lived (up to 11 years, maturity at around 2 years old) than other
European freshwater mussels. Growth and life span are similar across the range in lotic habitats, but different
from that in lentic habitats. The larvae of U. delphinus may attach to most co-occurring fish species, but only
native species were effective hosts. Native cyprinids, especially those from the genus Squalius, seem to be the
primary hosts. Overall, the information provided contributes to a better conservation status assessment, selection
of conservation and rehabilitation areas, guidance for the establishment of propagation programs and better
timing for specimens’ manipulation including monitoring and possible translocations. The framework presented
here highlights the importance of basic biological studies to define good ecological and physiological status.
This work was supported by Portuguese FCT – Foundation for
Science and Technology, Projects FRESHCO: Multiple implications of
invasive species on Freshwater Mussel co-extinction processes (PTDC/
AGR-FOR/1627/2014-04/SAICT/2015), MUSSELFLOW: Host-dependent
evolution, ecology and conservation of freshwater mussels under
varying hydrological conditions: consequences of climate change
(PTDC/BIA-EVL/29199/2017), and ConBiomics: The missing approach
for the Conservation of freshwater Bivalves Project N° NORTE-01-0145-
FEDER-030286, co-financed by COMPETE 2020, Portugal 2020 and the
European Union through the ERDF. FCT also supported MLL under
grant (SFRH/BD/115728/2016), CM under grant (SFRH/BD/111133/
2015) and also through Strategic Funding UID/Multi/04423/2019. PB
was supported by EDP Biodiversity Chair.
