Download The thick-shelled river mussel has been given strict legal protection

Notes on the thick-shelled river mussel Unio crassus in Stradomka (southern Poland), a
tributary of the Raba river
Maciej Bonk
Institute of Nature Conservation, Polish Academy of Sciences
Al. A. Mickiewicza 33
31-120 Kraków
Institute of Zoology, Department of Entomology, Jagiellonian University
ul. Gronostajowa 9
30-387 Kraków
[email protected]
Keywords: thick-shelled river mussel, Unio crassus, Stradomka, Raba tributary
Introduction
The thick-shelled river mussel Unio crassus, PHILIPSON, 1788 is a large unionid (Bivalvia:
Unionidae) species (Fig. 1). In general, it is morphologically similar to other unionid mussels
often occurring sympatrically. However, several shell characteristics enable identification of
this species under field conditions. U. crassus is considered to be a primarily riverine species.
It lives both in lowland and highland rivers. As in other Unio species, the life cycle includes
obligatory fish hosts. During this stage, larvae (glochidia) feed mostly on gills. At least
several fish species may host U. crassus, i.e. Leuciscus leuciscus, L. cephalus, Scardinus
erythrophtalamus, Cottus gobio (ZAJĄC 2010 and references therein), Phoxinus phoxinus and
other species (TAEUBERT et al. 2012). Formerly the thick-shelled river mussel was a common
species in Europe including Poland, however water pollution and river drainage have led to a
decrease in population numbers (ZAJĄC 2004, ZAJĄC 2010). River bottom types occupied by
U. crassus include sand, gravel, stone and clay.
The recently confirmed distribution of U. crassus in Poland encompasses localities in Baltic
Pomerania, Mazurian and Suwałki Lake District (i.e. Romincka Forest; MARZEC 2010,
Krutynia river-lake system; LEWANDOWSKI & Jakubik 2014), tributaries of the Warta, Pilica
and Nida rivers, rivers in the Carpathian foothills (i.e. Jasiołka, Cedron; ZAJĄC 2010), and
Podlasie and Mazowia regions (Liwiec river; KORYCIŃSKA 2002). Several unpublished
records of this species include Korzenna (a tributary of Kacanka) and Tuszymka rivers
(southern Poland; M. BONK 2015-2016 unpublished). Several other sites in which only empty
shells were found (Jasieńka – a tributary of Strwiąż river, Wschodnia near Połaniec,
Zgłowiączka near Wieniec and Wisła in Włocławek; BONK 2015, unpublished) need to be
verified in the context of viable population existence.
The thick-shelled river mussel has been given strict legal protection under Polish law. It is
also mentioned in Annex II of the EU Habitats Directive. In Poland it is considered
endangered (ZAJĄC 2004). Recently, it was included in National Nature Monitoringof Annex
II and IV of the EU Habitats Directive species(Zając 2010).
This note contributes new data on the occurrence of the thick-shelled river mussel in
Stradomka river, in which it was not previously recorded.
Methods
Study area
Stradomka is a right side tributary of the Raba river. It starts in Beskid Wyspowy and exhibits
a mostly mountainous or submountainous character. In the lower part of the river, the
substrate tends to contain more sand, gravel and clay. The river channel is mostly regulated by
large stone embankments. However, many natural hydromorphological elements are still
present (Fig. 2). The two largest right-side tributaries of the Stradomka river (Tarnawka and
Sanecki (Trzciański) streams) were also searched for the thick-shelled river mussel.
Mussel detection
This contribution shows anecdotal data collected irregularly in shallow and accessible parts of
the mentioned watercourses. Visual surveys in clear transparent water were conducted from
April to June 2016. The lengths of the controlled sections of the river ranged from 10 to
700m.
Usually, only the first or several mussels at a given site were dug from the substrate and
identified, to avoid unnecessary interruption of mussel behavior. It was assumed that other
mussels buried in the substrate from which only siphons were visible were also thick-shelled
river mussels. This is a reasonable assumption as 1) in Polish mountain and submountainous
rivers other unionids usually do not occur, 2) in all surveyed sites the first mussels found were
always thick-shelled river mussels, 3) no other unionids or empty shells were found in this
river, 4) this species is typically found almost entirely dug into substrate showing only
siphons. However, it cannot be excluded that small numbers of other Unio or Anodonta are
present in the area. Nonetheless, this would have only affected the number of recorded
mussels but not the number of localities because the thick-shelled river mussel was always
present.
Results
I found 13 localities inhabited by U. crassus (Tab.1; Fig 1.). Nine were noted in Stradomka
river, two in Trzciański (Sanecki) stream and one in Tarnawka river. Almost every site
contained at least one living specimen (Fig. 2). However, only empty shell was found in the
most upstream site of Trzciański (Sanecki) stream.
In the Stradomka River this species reaches at least Grabie village (208 m a.s.l.); it was not
recorded upstream from this site. In Trzciański stream the highest locality was Trzciana
village (238m a.s.l.) with no species detection upstream as well. The only site in Tarnawka
river was noted near Boczów (234 m a.s.l.).
In three sites in the Stradomka river one-year specimens were found; one well preserved oneyear empty shell was recorded in Tarnawka river.
The minimum number of observed individuals ranged from one to 30.
Habitats where U. crassus individuals were detected included gravel rapids, mud deposits
within large stone artificial embankments and clay depositions (Fig. 3)
Potential fish hosts of glochidia in these watercourses included L. cephalus and Phoxinus
phoxinus.
Discussion
Recently, the number of known sites of U. crassus has increased in Poland. Most probably
this is a result of growing interest in this species as a “Natura 2000” species and inventory
efforts following design and protection plans for “Natura 2000” sites. However, the quality of
surface waters in Poland has improved in recent decades, which might have also allowed
recolonization of some areas by this clean-water mussel. Additionally, some of the
watercourses have recovered after large scale drainage and river regulation, which also might
have accelerated recolonization by U. crassus.
Records from the Stradomka river show that U. crassus may be widely distributed among
sub-Carpathian or Carpathian watercourses. In the Raba river, U. crassus was recorded only
from one site near Pcim (BERGER 1973). There is no information on whether the species is
still present in Raba. BERGER (1973) did not detect this species below the mouth of the
Stradomka river near Chełm. Some attempts to find U. crassus in Stradomka were made in
the last decade near Łapanów, but no mussels were found (K. & T. ZAJĄC, pers. comm. 2016).
This may suggest recent colonization of the watercourse, however in that part of the river
single specimens are found irregularly and could have gone undetected.
Despite large scale regulation, i.e. construction and maintenance of embankments, the
Stradomka river and its tributaries provide natural hydro-morphological elements appropriate
for U. crassus. Notably, despite this species being present along artificial river elements,
almost only natural or renaturalized parts of the river supported mussel concentrations (3 – 30
specimens in one microhabitat such as a clay patch or natural mud deposition). On the other
hand, the presence of the mussel in various microhabitats suggest that knowledge of habitat
preference and its response to man-made river bed modifications should be better studied.
The presence of U. crassus as well as other rare species (i.e. the noble crayfish Astacus
astacus; BONK et al. 2014) in Stradomka river are strong arguments for avoiding riverbed
modification or other construction works affecting natural river valley dynamics.
Abstract
This contribution presents anecdotal data on the occurrence of the thick-shelled river mussel
Unio crassus in the Stradomka river (southern Poland), a previously unsurveyed habitat. Live
specimens and empty shells were found in 13 localities, mostly in Stradomka river.
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