Populations of Microcondylaea bonellii ( Férussac 1827 ) , Unionidae – an european freshwater mussel at rapid decline-and Unio mancus in Istria , Croatia

Microcondylaea bonelli (Férussac 1827) inhabits rivers in the northern and eastern region of the adriatic sea. Actual IUCN conservation status is VU (vulnerable), it is listed in Annex V of the Flora-Fauna-Habitat directive of the EU. Due to construction works, frequent severe flooding and habitat changes the species is in rapid decline in its whole area. Conservation status should therefore be altered to CR (Critically Endangered) and the habitats of the remaining populations especially in Croatia managed to stop the decline.


Introduction
Microcondylaea bonelli (Férussac 1827; syn.Microcondylaea compressa Menke 1830) lives in rivers that flow into the adriatic sea, mainly in northern Italy and adjacent Slovenia, in river Mirna and some of its tributaries in Croatia (Fischer 1999, Bößneck 2002), in Montenegro and Albania in Lake Shkodra and its tributaries, in river Drin (Wohlberedt 1909) and in lake Ohrid (Dhora & Welter-Schultes 1996, Korniushin 2004, Reischütz & Reischütz 2002, Reischütz 2008).In 2012 Reischütz found one corroded shell in river Shkumbin near Cerme in Albania (Reischütz 2013).In Italy it was considered to be ecxtinct in the 1980s, in 1984 few living individuals were found in river Versa (Fechter & Falkner 1989).Surveys in accordance with NATURA 2000 monitoring confirmed living populations also in tributaries of river Po in the 1990s and 2000s (Bogliani et al. 2007, Fondazione Lombardia dell´Ambiente 2008, WWF Italia 2013).Older records from Greece in Lake Trichonis ("Vrachori"; Jaeckel et al. 1958) have not been confirmed recently.There is one doubtful record from the district of Brcko in northeastern Bosnia (Redzic 1996).There are no records from Dalmatia (Wohlberedt 1909).The actual IUCN status is VU (vulnerable).In the most recent paper on distribution and vulnerability of european freshwater mussels (Lopes-Lima et al. 2015) the albanian populations are not mentioned.
Microcondylaea is listed in Annex V of the Flora-Fauna-Habitat directive of the EU, that means that "Member States must ensure that their exploitation and taking in the wild is compatible with maintaining them in a favourable conservation status".This classification seems quite strange, because there is no known commercial or other exploitation of the species itself.Microcondylaea should better have been listed in Annex IV -"a strict protection regime must be applied across their entire natural range within the EU, both within and outside Natura 2000 sites".
In the Species assessments at EU biogeographical level in Art. 17 report 2012 the species is reported from the Continental and Mediterranean biogeographical regions.
In the Continental region the Conservation Status has changed from U2 -Infavourable Bad before 2007 to U1 -Infavourable Inadequate due to use of different Methods.Here the vast majority of the range and distribution is found in Italy with 23 populations (Estimated range and number of populations is based on expert opinion with no or minimal systematic survey or sampling).Population assessment here: U1 inadequatedeclining.The croatian populations are not mentioned in the report 2012, because Croatia was no EU-member state in 2012.Due to construction works, severe flooding and habitat changes the species is in rapid decline also in Croatia (own observations 2009 -2016) and Albania (Dhora in litt.).
In the Mediterranean region the species occured in Greece in Lake Trichonis ("Vrachori"; Jaeckel et al. 1958), where the actual status and threats are unknown.
High importance threats and pressures mentioned in literature are diffusive pollution to surface waters, dredging, canalization and water deviation.

Material and Methods
To confirm the partly vague informations on populations and habitats all specimens of Microcondylaea in the Collection of the Natural history Museum in Vienna were examined.Accessible photos with labels from the internet, mainly specimen-photos at www.animalbase.uni-goettingen.dewere also examined.
To estimate the status and number of individuals of the population in river Mirna (Istria/Croatia) the area was visited yearly from 2009 to 2016.In 2011, 2013 and 2015 two tributaries of Mirna, the brooks of Bračana and Butonigla were also surveyed.After severe flooding in winter/spring 2015 numerous empty shells and ± freshly dead specimens could be found along the riverbed of Mirna.The material was collected either from the dry interspaces between the big stones of regulated parts of the riverbank or from the rough vegetation on the riverbanks on an area of 30 m 2 at each location.With this material the occurence and ratio of Microcondylaea and Unio mancus in different parts of the river could be cleared and the population sizes roughly estimated by extrapolation of numbers of living individuals in suitable habitats in the surveyed parts of river Mirna and brook Butoniga.

Results
The localities of the examined older specimens in the collections of the Museum of Natural History Vienna and animalbase.uni-goettingen.degive a quite clear picture of the (historical) distribution that correlates well with the area represented by actual data.Quite remarkable ist the absence of specimens from river Mirna (Istria/Croatia) in the older collections.According to literature (Fischer 1999) the population in Mirna was not discovered before 1997, although Istria was visited by many biologists in the 19th and beginning 20th century.The population in river Mirna is also not mentioned in Welter-Schultes 2012 and the standard data form for the NATURA 2000 reserve Mirna (EUNIS), although it had been published long before by Fischer (1999).Surveys in other rivers in Croatia e.g.Zrmanja, Jadro, Zrnovnica, Cetina and Neretva by Reischütz (2002) and river Raša (Istria) by me in 2017 gave negative results.Fig. 2 and 3 give the results of my surveys in river Mirna in 2009 to 2016.In the upper part of the river from the city of Buzet to the confluence with Butoniga the artificially straightened riverbed is dominated by coarse gravel and shows ± rapid current.Although Microcondylaea was recorded from this part of the river near Istarske Teplice (Fischer 1999) no shells or living specimens were found actually.Populations of Unio mancus were mainly found in tributaries like Bračana and drenches, less frequently in the riverbed of upper Mirna.
Between 2009 and 2016 the Mirna-riverbed was reconstructed in several places, eroded banks with coves and fine sand substrate were replaced by blocks of stone and the riverbed straightened again.Thus many suitable habitats and eventually existing mussels-populations were destroyed, so much the worse as heavy machines were driving in the riverbed over many weeks for the construction works and mobilized the substrate, which led to high accumulations of fine sediment in the lower part of the river, especially in parts with low current and coves which were inhabited by Microcondylaea.Between the confluence with Butonigla and the road bridge near Ponte Porton situated 10 km to the west, the numbers of living individuals and empty shells of Microcondylaea increased, adult and young living individuals were observed in high densities of up to 15 individuals per square meter in shallow waters of eroded banks with coves and sandy substrate (Fig. 4) or accumulated fine substrate but not in the straight parts oft he riverbed with higer current and substrat of gravel/stones.These suitable sandy habitats are not frequent, as the riverbed is artificially straightened in most parts.In this part of the river construction works have been carried out only in a few places until now.The heavy influence of muskrat on the mussel populations mentioned by Reischütz (2002), who was worried about total extinction because of the thousands of mussels killed by muskrat could not be noted in my recent surveys.Most of the empty shells showed no traces of muskrat.Possible other habitats of Microcondylaea in the Mirna area are the drenches between the village Livade and the plain near the mouth of river Mirna into the adriatic sea (where Reischütz found thousands of dead Unio mancus elongatulus und some Microcondylaea bonellii in 2002 after the drenches were dredged) and the lowest part of river Mirna with deep water and no shallow banks.This part cannot be surveyed without diving equipment.
The highest density of living adult and young Microcondylaea was found in the brook Butoniga (Fig. 5, 6) which emerges from the dam of the drinking water reservoir Jezera Butoniga and flows into river Mirna about 4 km northwest.The substrate of the brook is dominated by stones, fine sand and clay and protected from frequent heavy floodings by the reservoir.By extrapolation of the numbers of individuals in populated habitats, e.g. 4 km of brook Butoniga and 10 km of river Mirna, and the occurence of suitable habitats in the surveyed river-parts the population size in the Mirna river system was roughly estimated between minimum 5.000 and maximum some 10.000 living individuals of Microcondylaea, with the uncertainty of possible additional populations in drenches or the lower part of river mirna near the mouth.Due to construction works, unusually frequent severe floodings in the last 15 years and habitat changes the population of Microcondylaea is rapidly declining in Croatia (Reischütz 2002(Reischütz , own observations 2009(Reischütz -2016)).
Also in Albania the populations are at rapid decline: "Today there is a very different ecological situation due to the transformation of landscapes, deviation of river Gjadri, agricultural and hydrotechnical transformations etc. Very much of the populations of this species […] was destroyed in these decades."(Dhora in litt.).
The IUCN status of Microcondylaea bonellii should thereforeas already stated by Reischütz 2002be altered to CR (Critically Endangered) and a wise management of the remaining populations and habitatsespecially while construction works in the riverbe established to stop the decline.

Figure 3 .
Figure 3. Numbers of shells of Unio mancus (blue) and Microcondylaea bonelli at the different sites (red).