Invasive molluscs in Umbrian inland waters: ecological and

Studi Trent. Sci. Nat., 86 (2009): 99-104
ISSN 2035-7699
99
© Museo Tridentino di Scienze Naturali, Trento 2009
Atti XVIII Convegno Gadio 2008: Un mondo che cambia: successioni ecologiche, invasioni biologiche ed alterazioni antropiche
Sessione 5 - Comunicazione orale: Invasioni biologiche
Invasive molluscs in Umbrian inland waters: ecological and reproductive aspects
Elda Gaino*, Francesca Scoccia, Tisza Lancioni & Alessandro Ludovisi
Department of Cellular and and Enviromental Biology, Università degli Studi di Perugia, Via Elce di Sotto, 06123
Perugia, Italy
*
Corresponding author e-mail: [email protected]
SUMMARY - Invasive molluscs in Umbrian inland waters: ecological and reproductive aspects - The increasing colonisation by aquatic
invasive molluscs has only recently received scientific attention in Umbria, notwithstanding the possible great impact on the functionality
and biodiversity of the freshwater ecosystems. The present contribution highlights the results of investigations carried out on Dreissena
polymorpha (Bivalvia) in Lake Trasimeno and on Potamopyrgus antipodarum (Gastropoda) in water courses of the Tiber River basin. This
in view to evaluate the impact of habitat type on colonisation and reproduction. Dreissena polymorpha showed a marked selectivity for
natural and artificial hard substrates. This preference gives inception to competition with the sponge Ephydatia fluviatilis (Porifera), which
overgrow the mollusc, thereby encapsulating its valves. The study of the life cycle showed a tight link between gonadal development and
water temperature, the high values of which limit larval development and species dispersal. Analyses on parthenogenetic populations belonging to P. antipodarum showed a different fecundity in relation with habitat type, and the marked selectivity towards various biological
substrates (macrophytes, decaying organic materials, periphyton, etc.). The scarce availability of these components together with high
current speed, limit the colonisation ability of this species.
RIASSUNTO - Molluschi invasivi nelle acque interne dell’Umbria - La crescente colonizzazione da parte di molluschi acquatici
invasivi ha ricevuto solo recentemente attenzione scientifica in ambito umbro, nonostante il loro elevato impatto potenziale sul funzionamento e la biodiversità degli ecosistemi d’acqua dolce. Il presente contributo illustra i risultati delle ricerche effettuate su Dreissena
polymorpha (Bivalvia) nel Lago Trasimeno e su Potamopyrgus antipodarum (Gastropoda) in corsi d’acqua del bacino del Fiume Tevere. Ciò al fine di valutare l’incidenza delle caratteristiche dell’habitat sulla colonizzazione e riproduzione di queste specie. Dreissena
polymorpha mostra una elevata selettività per substrati duri naturali e artificiali. Questa preferenza innesca la competizione con la
spugna Ephydatia fluviatilis (Porifera), la quale, potendosi sviluppare sul mollusco, ne determina l’occlusione delle valve. Dall’analisi
del ciclo biologico è emersa la stretta dipendenza dello sviluppo delle gonadi dalla temperatura dell’acqua, i cui valori elevati limitano
lo sviluppo larvale e la dispersione della specie. L’indagine su popolazioni partenogenetiche di P. antipodarum ha messo in luce la
diversa fecondità in relazione al tipo di habitat e la spiccata selettività verso substrati biologici di varia natura (macrofite, residui vegetali deiscenti, periphyton, etc.). La scarsa disponibilità di queste componenti, unitamente ad elevate velocità di corrente, limitano le
capacità di colonizzazione della specie.
Key words: Dreissena polymorpha, Potamopyrgus antipodarum, habitat selection, reproduction, Lake Trasimeno, Tiber River basin
Parole chiave: Dreissena polymorpha, Potamopyrgus antipodarum, selezione di habitat, riproduzione, Lago Trasimeno, bacino Fiume Tevere
1.
IntroduCTION
There is compelling evidence that, in the past few
decades, the human-mediated introduction of non-indigenous species (NIS) is the main driver of biodiversity change
in freshwater and terrestrial ecosystems (Sala et al. 2000).
The effects on global biodiversity are expected to increase
quickly over time in both extent and intensity, in concert
with anthropogenic alterations to the environment, such as
climate and land use changes.
Biological invasions are favoured in disturbed ecosystems and communities, because disturbance opens new
niches for the most adaptable invaders (Ross et al. 2001),
whose success depends on a close match between their
physiological requirements and the environmental characteristics of the system being invaded (Gherardi 2007).
A recent list of NIS occurring in Italian inland waters and their spread during the past century is reported
by Gherardi et al. (2008). Among the 112 animal species
identified, molluscs are represented by 11 species (7 Gastropoda and 4 Bivalvia), two of which - Corbicula fluminea
(Müller, 1774) and Dreissena polymorpha (Pallas, 1771)
- are also included among the 100 worst invasive species
of Europe (DAISIE Project: http://www.europe-aliens.org/
speciesTheWorst.do). Most non-indigenous mollusc species were introduced into Italy in the second half of the 20th
century, as a result of the development of commercial traffic (Cianfanelli et al. 2007).
Nevertheless, in spite of the wide literature produced,
a limited number of researches have focused on the ecology
of molluscs in the “alien” environment being invaded. The
success of a freshwater invader or the prediction of the in-
100
Gaino et al.
Invasive aquatic molluscs in Umbria
Dreissena polymorpha
1. lower Mussino Stream
2. upper Tiber River
3. upper Topino River
Lake
Trasimeno
soft sediments
hard substrata
Potamopyrgus antipodarum
Fig. 1 - Map showing the location of sampling sites of invasive molluscs in Lake Trasimeno and in the Tiber River basin in Umbria.
Fig. 1 - Siti di campionamento dei molluschi invasivi nel Lago Trasimeno e nel bacino del Fiume Tevere.
vasibility of an aquatic system depend on the knowledge
of the characteristics of the invader and the specific system
being invaded (Moyle & Light 1996). In agreement with
Simberloff (2004), in order to acquire the necessary confidence with the invasion process, we need a large catalogue
of case studies at local scale. In particular, the knowledge of
habitat selectivity, diffusion strategies and specific demographic parameters is essential for making prediction on the
spreading and establishment of an invasive species.
In this regard, we investigated the reproduction and
ecology in populations of the Ponto-Caspian Dreissena polymorpha in a lentic environment (Lake Trasimeno, Italy)
and of the New Zealand mudsnail Potamopyrgus antipodarum (Gray, 1843) in running-water systems, belonging
to the Tiber river basin.
2.
STUDY SITES AND SAMPLING
The first record of D. polymorpha in Lake Trasimeno (Spilinga et al. 2000) reports sporadic findings along
northern and eastern shores and also around the main is-
lands of the lake. This record was followed by an extensive
investigation performed by Lancioni and Gaino (2006) on
twelve sampling sites located along the shore of the lake
(Fig. 1). In each sampling site, a 5m-long transect along the
shore was inspected monthly from June 2003 to May 2004
and the abundance of D. polymorpha was recorded together
with the type of substrate. On soft sediments, sampling of the
mollusc was carried out by means of a Surber net. In addition, presence/absence data of the freshwater sponge Ephydatia fluviatilis (Linnaeus, 1758), often found to coexist with
the bivalve, was annotated. In each sampling, 10 bivalves >
8-mm-long (underwater measurement) were randomly collected, measured and then transferred in aerated tanks to the
laboratory. The substrate selectivity was evaluated by Pearson’s Chi-squared test of independence by comparing the
aggregated data of presence on the various substrates, with
the overall frequency of substrates encountered during sampling (Lancioni & Gaino 2007). The similarity of substrate
selection between D. polymorpha and E. fluviatilis was also
statistically tested by a Chi-squared test.
Although the presence of P. antipodarum is known
in Italy since 1961 (Berner 1963) and it is fairly widespread
Studi Trent. Sci. Nat., 86 (2009): 99-104
in Italian inland waters, only one case was reported for the
Umbrian region up to 2007 (Bodon et al. 2005), when it was
repeatedly observed within the Tiber River basin, during
water quality monitoring surveys. An investigation performed in the sites of recording, evaluated the state of the
populations and their potentiality of spreading in the basin
in relation with the type of habitat (Gaino et al. 2008).
Observations were carried out in three sites showing different environmental features (Fig. 1): 1) lower Mussino
Stream (depth: 15 cm; width: 2 m; current speed: ~ 0.1 m
s-1; October 2006); 2) upper Tiber River (depth: 25 cm;
width: 14 m; current speed: ~ 0.3 m s-1; May 2007); 3)
upper Topino River (depth: 20 cm; width: 9 m; current
speed: ~ 0.6 m s-1; June 2007). At each sampling site, a
25m-long section of the water course was inspected and
the abundance of P. antipodarum was recorded, together
with the type of substrate. In order to estimate the density,
sampling was carried out by a Surber net (surface area:
100 cm2) and the counting was performed in situ. The collected specimens were then transferred to laboratory in
aerated containers for further investigations. The selectivity of the species with respect to the type of substrate was
evaluated by Pearson’s Chi-squared test of independence,
by comparing the total abundance observed on the various
substrates, with the total area of the different substrates
encountered during sampling.
3.
LABORATORY ANALYSES
The gonadal maturation in D. polymorpha was assessed on the basis of the shape of the gonadal cells according to the method suggested by Gist et al. (1997).
Morphological observations and measurements were carried out by means of an optical microscope (Leica DMLB),
and a photographic archive was created by means of a
Leica DC 300F digital camera.
Sex attribution in P. antipodarum was performed
under Leica MZ6 stereomicroscope (Leica LMS Holdings
GmbH, Wetzlar, Germany), equipped with optic fibres.
Specimens were anesthetised by adding some drops of chloral hydrate to prevent animal suffering, and then measured
from the apex to the inferior rim of the shell by dividing the
whole size range into 6 (1mm-distanced) classes. Females
were dissected and the total embryos brooded inside each
female were counted.
4.
RESULTS
In Lake Trasimeno, D. polymorpha’s shells are
highly polymorphic for their colour, pattern and shape.
Shell length varies from less than 1 mm to a maximum of
38 mm. While in the southern areas of the lake D. polymorpha was only sporadic, huge numbers were found in
the western sites 9 and 10, in the eastern sites 1 and 2 and
in the northern site 5 (Fig. 1). In Lake Trasimeno, D. polymorpha was found to be attached to various hard substrates,
varying from concrete landing stages (maximum density ~
200000 ind. m-2) to rocks (from 114 to 140000 ind. m-2)
and artificial substrata (tires, tiles, plastic buoys, pumps and
boat keels). The submerged portion of Phragmites australis
reed stalks are also colonised (maximum density 2036 ind.
101
m-2), whereas no individuals were observed on silt and only
gathered in clumps on sandy substrates. Clumps consist of
mixed mussel populations, where the individuals adhere to
one another or attach to sand-grain sediments and filamentous algae by means of their byssal threads. The preference
of D. polymorpha for hard substrates, with particular reference to concrete, pebbles and rocks, has been confirmed by
Chi-squared test to be significant (p= 0.002).
Gonadal development of females and males coincide temporally, and is linked to a rapid increase in water
temperature (from 4.3°C to 13.4°C in spring). In the population, 82% of the zebra mussels are sexually mature with a
female:males ratio of 1:1.16 and there is only one period of
successful reproduction per year (from April to June).
In the areas where sponges and molluscs coexist, the
former tend to overgrow the latter up to the shell encapsulation. The growth and size of sponges vary according to the
season, reaching the maximum (300 x 240 mm in length; 60
mm in thickness) in warmer months, while during autumn
and winter sponges survive as thin encrusting specimens.
The comparison between the mollusc and the sponge
distribution showed that, although different in presence, they
have statistically equivalent preferences (p= 0.66) with respect to the type of substrate. In most cases (93%), D. polymorpha was found in the same portions of the transects investigated where E. fluviatilis was present (sites 2, 5, 10 and
11). The time progression of E. fluviatilis/D. polymorpha
interaction was followed in two sampling sites: here, it was
observed that the sponge outcompetes the mussels and gradually envelops the shell, thus leading to the mollusc death.
In the three investigated populations of P. antipodarum, no males were found so far. The analysis of abundance data showed that the highest global average density
is found in Mussino Stream (1600 ind. m-2), the most diversified environment in terms of substrate offer. By contrast, in Topino River, very low densities were found (33
ind. m-2). The results of the Chi-squared statistics showed
that the snail is not evenly distributed on the different
substrates, but it selects positively biological substrates,
although bare rocks and silt are not avoided. In Mussino
Stream, beds of the macrophytes Chara sp. and Vaucheria
sp. are positively selected, whereas encrusting periphyton
(mainly represented by diatoms) is almost disregarded. It
is worth stressing that the highest snail density was found
on floating leaves and wood. The comparative analysis of the populations showed that they slightly differ in
size composition and fecundity, whereas the number of
brooded embryos/female remarkably varies among sites
(Fig. 2). The size distribution of the investigated populations showed that all the size classes (1-6) are represented,
with the classes 4 and 5 being the most abundant. Females
become fecund at a size > 2 mm (Mussino Stream and
Topino River) or > 3 mm (Tiber River) and the fecundity
comes close to 100% for specimens > 4 mm (Fig. 2). Accordingly, the number of carried embryos increases with
size, although it is largely variable within and between
populations (Fig. 2). In particular, a markedly reduced
number of embryos/female is observed in Topino River,
where the population density is also very low. It is worth
stressing that Topino River is characterised by the absence
of aquatic vegetation and by a water speed that slightly
overcomes the upper limit reported in literature for the
snail (Doby et al. 1966; Roth 1987).
102
Gaino et al.
Invasive aquatic molluscs in Umbria
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Fig. 2 - Population structure and size-class fecundity in Potamopyrgus antipodarum from the three sites investigated in the Tiber River basin.
a, size distribution; b, percentage of embryos-carrying females; c, number of brooded embryos/female (±SD). Star indicates that the size class
6 includes 1 (Topino River) or 2 (Mussino Stream) individuals.
Fig. 2 - Struttura per taglia e fecondità per classe di taglia in Potamopyrgus antipodarum nei tre siti investigati nel bacino del fiume Tevere:
a, distribuzione degli individui per taglia; b, percentuale di femmine con embrione; c, numero di embrioni per femmina (±SD). L’asterisco
indica che la classe di taglia 6 è occupata da 1 (Torrente Topino) o 2 (Torrente Mussino) individui.
5.
DISCUSSION
Species spreading, substitution and extinction are
commonplace in nature, and are among the forces which can
trigger species and ecosystem evolution. They occur over the
course of geological times, even in association with climate
change (Graumlich & Davis 1993). Therefore, changes in
the natural distribution of species should not be viewed as
an abnormal event (Lodge 1993). However, human actions
have greatly increased the temporal rate of species dispersal,
producing, in few decades, a cascade of events that could
never be accomplished by means of natural events alone
(Lodge 1993). This is particularly true for inland freshwater environments, which are circumscribed systems. Based
on the number of documented and potential extinctions, it
has been evaluated that freshwater fauna can experience an
extinction rate that is five times that of the terrestrial environment (Ricciardi & Rasmussen 1998).
Humans have contributed to the spread of organ-
isms by the increasing mobility of people and good trade
among nations and continents. However, it seems acceptable that the increasing traffic by itself could not have
caused a so large effect if NIS were introduced in unaltered environments. In fact, ecosystem modification is often coupled with an increased susceptibility to invasion
(Ross et al. 2001). As for our study sites, Lake Trasimeno
is a fragile shallow-water system, whose ecological state
is strictly linked to the hydrological regime. Water regulation practices, together with the meteorological modifications recorded in Central Italy during the last century
(Dragoni 1998; Burzigotti et al. 2003), caused a lowering
of the water level of about 2 m, corresponding to a reduction of about 30% in volume. Such a change, which is still
in progress, has been accompanied by a modification in
the water quality, with particular reference to the salt content and water transparency (unpublished data). Also, significant depletion of the Phragmitetum can be envisaged
during the last two decades (M.A.R.U., 1994, Cecchetti et
Studi Trent. Sci. Nat., 86 (2009): 99-104
al., 2005). Such environmental changes have certainly affected the riparian zoocoenosis of the lake, with particular
reference to the sponge population of E. fluviatilis, which
is now found just as small patches on rocks and pebbles
(Lancioni & Gaino 2007), whereas museum specimens
dated 1963 were collected on reed stalks in the riparian
belt of the lake. In such a perturbed environment, spatial
and trophic niches are subjected to marked changes, thereby allowing adaptable species, such as D. polymorpha, to
invade and spread.
Ephydatia fluviatilis, whose spatial and trophic
niche is similar to that of the bivalve, may represent a “biological barrier” limiting the diffusion of the species in the
lake, provided that the sponge population is not occasionally present.
Paradoxically, in this changing scenario, the main
constraint to the spreading of the bivalve seems to be the
water temperature. In fact, it is well known that the larvae
of D. polymorpha cannot survive, settle and metamorphose
above 29°C, a temperature which is usually reached along
the shore of the lake during the summer period.
As for P. antipodarum, it has been often detected in
disturbed biotopes in Australia and Europe (Léger & Léger
1974; Ponder 1988; Schreiber et al. 2003), even though
high polluted areas are generally avoided (Ghetti 1986).
The snail tolerates wide ranges of temperature (Berner
1971; Quinn et al. 1994), salinity and trophic conditions
(Proctor et al. 2007), and the current speed seems to represent the main limiting factor for its diffusion (Doby et al.
1966; Roth 1987).
Though the first record of this species in Italy dates
back to 1961, no investigation have been carried out on the
ecology and on the effect of ovoviviparous parthenogenesis
on the dispersal of the snail in the Italian peninsula.
The comparative analysis performed by Gaino et
al. (2008) suggested that fecundity, expressed as number
of brooded embryos/female, increases, in parallel with the
mean density of the population, in habitats rich in aquatic
vegetation, where the current speed is low. These results support the hypothesis that run habitats limit the colonisation by
P. antipodarum (Richards et al. 2001), and suggest a direct
effect of the habitat on the reproduction rate of the snail.
In consideration of the fact that Tiber River collects the most part of the Umbrian water courses, the
whole region represents a potential area of expansion for
P. antipodarum. However, owing to the lack of punctual
information about species distribution and ecology at a
regional scale, it is difficult to make prediction about the
snail’s spreading in Umbria.
6.
CONCLUSIONS
The spreading of species out from their native region has an impact on autoctonous biocoenosis. However,
before taking a stand against non-indigenous species, we
need to be aware about the extent and the effects of the invasion at a regional scale, which is the basic level at which
the phenomenon should be faced, from both scientific and
political perspectives.
In this regard, a significant effort has been devoted
during the last decades in drawing maps of distribution and
pathways of spreading in Italy of the most invasive spe-
103
cies. However, in order to acquire a proper confidence with
the invasion phenomenon, an additional effort is needed to
gather data on the biology and ecology of the species in
the areas where they are recorded. This step is also necessary in order to make predictions on the spreading of nonindigenous species, and to plan conservation policies for
safeguarding indigenous biocoenosis.
ACKNOWLEDGEMENTS
The research was partially financed by FISR
M.I.C.E.N.A.
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