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Correspondence
ISSN 2336-9744 (online) | ISSN 2337-0173 (print)
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Confusion in the pond: new item on the menu for the naive Grass
snake (Natrix natrix Linnaeus) versus an unexpected Great ramshorn
(Planorbarius corneus Linnaeus)
ADNAN ZIMIĆ* & TAJNA KLISURA
Herpetological Association in Bosnia and Herzegovina – ATRA, Sarajevo, Bosnia and Herzegovina
*Corresponding author: E-mail: [email protected]
Received 5 March 2016 │ Accepted 15 October 2016 │ Published online 21 October 2016.
The Great Ramshorn – Planorbarius corneus (Linnaeus, 1758) is a large aquatic planorbid gastropod, with
shell typically measuring about 3.5 cm in diameter. It inhabits central European wetlands and floodplains
(Jopp 2006). It is a highly opportunistic species, but prefers still- or slow moving waters, with high density of
(semi)aquatic plants. As a species found in the aquarium trade, it can be found in small ponds outside the
main range. (Seddon & Van Damme 2011). Although it is capable of self-fertilization and, hypothetically, a
single released individual could establish a population (Welter-Schultes 2009), its potential for occupying
new habitats is unknown (Wittenberg 2006). Great ramshorn is native for the Balkan Peninsula (Seddon &
Van Damme 2011) and in Bosnia and Herzegovina (Karaman 2006), but its precise distribution is unknown.
To our knowledge, it is present in the entire Posavina region of B&H (Zimić A. personal observation). It
does not inhabit highlands – above 800 m a.s.l. (Welter-Schultes 2009).
Figure 1. Natrix natrix struggling to ingest Planorbarius corneus. Photo: Zimić A.
Ecol. Mont., 9, 2016, 10-12
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ZIMIĆ & KLISURA
On the other hand, Grass snake – Natrix natrix (Linnaeus 1758) is a semiaquatic species that is widely
distributed in Europe, central Asia and north-western Africa (European Reptile & Amphibian Specialist
Group 1996), from lowland plains (e.g. Ćurić et al. 2015) to high-altitude lakes (e.g. Filippi & Luiselli 2002;
Šunje et al. 2014). It feeds primarily on amphibians (e.g. Gregory & Isaac 2004), and is among the top
predators in aquatic ecosystems (Šukalo et al. 2014).
Close encounter of the two abovementioned species was witnessed on 15th of July, 2015 on the
territory of the Ugorsko village (43°53′25.98″N, 18°21′46.59″E; 680 m a.s.l) near the city of Sarajevo
(central Bosnia and Hezegovina). Sub-adult N. natrix was trying to ingest large P. corneus in small artificial
pond were the snail has been accidentally introduced the same year by the locals (Zimić A. & Šunje, E.
personal observation), but no stable population was established. The struggle was being observed for few
minutes in water and for additional 10–15 minutes on dry land. The snake was found floating, with a snail in
its jaws, while it was trying to get out from water. The snake inserted its snout in the snail’s shell trying to
swallow it, while its lower jaw was still free (Fig. 1) Therefore, the snake was not able or notice our
presence: its main sensory organs were covered by the snail’s shell. On land, the snake was not trying to
escape, but was, sporadically and slowly, rolling over. When we separated the animals, the snail had no
injuries. As a result of long struggle, the snake was exhausted and became, supposedly, highly vulnerable to
predator species because (1) its sense organs were covered (blocked) and (2) the individual was easily
noticeable, for exposing its contrast (black and white patterns) coloration of the belly.
In general, maximum prey sizes increase with snake size, but large snakes nonetheless continue to
eat small prey. Adult Grass snakes prey mainly on amphibians (Gregory & Isaac 2004) and occasionally take
fish and small mammals in nature (Luiselli et al. 2005). Very little is known about the juveniles’ food
preferences. New-borns probably eat tadpoles and invertebrates, while newts and tadpoles account for a
major portion of the dietary spectrum of juvenile grass snakes (Eckstein 1993; Consul et al. 2009; Philips
2012). Captive snakes were observed taking earthworms (Brown 1992). To the best of our knowledge, what
we presented here is only the second record of a grass snake trying to eat a snail (Consul et al. 2009). Four
groups of snakes are snail-eating specialists (e.g. Dipsas indica, Pareas iwasakii) which can extract snails’
bodies from their shells, using their slender asymmetric jaws (Sazima 1989; Hoso & Hori 2006; Consul et al.
2009), but others are rarely observed trying to ingest this unusual prey. Such attempts are usually not
successful.
Being tolerant of a wide spectrum of environmental conditions (Seddon & Van Damme 2011) –
which is reflected through its wide distribution range – Planorbius appears highly capable of occupying
novel habitats. But, it is probable that P. corneus causes no concern in terms of invasions, especially in
places where climatic conditions are unsuitable (Wittenberg 2006). However, this case is an example how an
anthropogenically aided can affect an ecosystem, making confusion. These “interspecific confusion” (in this
case: illusion of predation) are common in nature in close encounters of species which are rarely seen
together (Zimić & Jelić 2015). It is also important to say that P. corneus is the first and second intermediate
host for several parasite species whose final hosts are water birds (e.g. Bilharziella polonica), mammals
(including humans; e.g. Echinostoma echinatum), and amphibians (e.g. Astiotrema trituri – described in
Lissotriton vulgaris) (Toledo et al. 1995; Grabda, 1959; Dvořak et al., 1999). Therefore, the impacts of its
possible introduction into previously unoccupied regions could be greater than simple digestive problems in
its predators.
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