Slugs and snails are parasitized by a range of organisms including nematodes, bacteria, microsporidia, mites and flies (Barker, 2004). Of these, the nematodes are the most numerous and diverse as 108 species have evolved to parasitize molluscs (Grewal et al., 2003). One of these nematodes (Phasmarhabditis hermaphrodita) is a lethal parasite of a range of pestiferous slugs and snails, including Deroceras reticulatum, Arion ater and Cornu aspersum (Wilson, Glen & George, 1993). Phasmarhabditis hermaphrodita has been formulated into a biocontrol agent available for farmers and gardeners (Nemaslug®) by Becker Underwood-BASF (Rae et al., 2007). Nematodes are mixed with water and applied using spraying equipment to soil where they then search for potential gastropod hosts. They are attracted to slug mucus and faeces (Rae, Robertson & Wilson, 2006, 2009) and on contact they enter through the slugs' mantle and kill it between 4 and 21 d later (Wilson et al., 1993; Tan & Grewal, 2001).

Some slug and many snail species are resistant to P. hermaphrodita (Coupland, 1995; Wilson et al., 2000; Williams & Rae, 2015). The mode of resistance has remained unknown in most cases, but it has been shown recently that Lissachatina fulica could trap, encase and kill invading P. hermaphrodita on the inner surface of its shell (Williams & Rae, 2015). This encapsulation of nematodes has also been shown in slugs; Rae, Robertson & Wilson (2008) showed that P. hermaphrodita were trapped in the shell of Limax maculatus underneath the mantle. A characteristic sign of heavy P. hermaphrodita infection of slugs is a swollen mantle (Wilson et al., 1993) due to the shell adding more calcareous tissue upon nematode contact.

Wilson et al. (2000) found that the snail Cepaea hortensis (Müller, 1774) was susceptible, but Rae et al. (2009) found that it was resistant. Both this species and its congener Cepaea nemoralis (Linnaeus, 1758) are polymorphic for colour and pattern of the shell (Jones, Leith & Rawlings, 1977). This polymorphism was not considered in the studies mentioned above, but it might influence susceptibility. Studies on this polymorphism have identified many selective agents that might influence variation among populations. While most of these have related directly to the consequences of different colours and patterns (Silvertown et al., 2011), there is evidence that behavioural and physiological differences might also be significant (O'Donald, 1968). As these might also be involved in resistance to nematode infestation, we carried out an experiment exposing adult C. nemoralis to infection by P. hermaphrodita, to see if susceptibility towards P. hermaphrodita would differ among specific colour and banding morphs of C. nemoralis and to examine whether the this species of snail could trap P. hermaphrodita in their shells.

Adult C. nemoralis were collected from Festival Gardens, Liverpool, and were stored in nonairtight boxes and fed ad libitum with cabbage and cucumber. The snails were split into groups of pink or yellow shells and within each into those with no bands, one band and three to five bands. Phasmarhabditis hermaphrodita was supplied by Becker Underwood-BASF, UK and stored at 10 °C until use.

Thirty-six plastic nonairtight boxes (10 × 10 cm) were fitted with copper tape around the sides (to prevent snails from reaching the lid of box) and half filled with moist soil (c. 25 g). Eighteen boxes had nematodes applied and 18 had only water and no nematodes applied and acted as the control. Phasmarhabditis hermaphrodita was applied at a density of 30 nematodes per cm2 of soil, which is the recommended rate of nematodes applied in the field (Wilson et al., 1993). Groups of five snails of each colour morph (pink or yellow with 0, 1 or 3–5 bands) were added to three replicate boxes with either nematodes or water added (n = 15) and survival was monitored every 3–4 d for 72 d. Snails were fed with cucumber every 3–4 d. Survival was compared using log rank test in OASIS (Yang et al., 2011). At the end of the experiment the snails were killed and the shells inspected for encapsulated nematodes (n = 8). Numbers of nematodes present in the shell were compared using a one-way ANOVA. We also checked whether the box the snails were placed in had any effect on survival using a two-way ANOVA.

Phasmarhabditis hermaphrodita had no effect on the survival of any class of C. nemoralis after 72 d exposure (P > 0.05) (Table 1). There was also no significant difference between the survival of snails in each replicate box of the nematode-treated or untreated snails (P > 0.05). We found that P. hermaphrodita was trapped and killed in the shells of each morph, but there was no significant difference between the numbers of nematodes encapsulated between the different morphs (P > 0.05) (Table 1; Fig. 1).

Table 1.

Survival of different colour and banding morphs of Cepaea nemoralis (n = 15 for each morph in each treatment) exposed to Phasmarhabditis hermaphrodita for 72 d and mean number of nematodes found encased in their shells (n = 8).

Colour Number of bands Mean percentage alive ± SE (n = 15) Mean number of nematodes found in shell (range) (n = 8) 
Yellow 86.67 ± 6.7 
100 ± 0 
93.33 ± 6.7 
Pink 100 ± 0 
100 ± 0 
100 ± 0 
Yellow 100 ± 0 7.38 (0–19) 
100 ± 0 8.63 (2–31) 
93.33 ± 6.7 12.88 (4–21) 
Pink 100 ± 0 6.5 (0–16) 
86.67 ± 13.3 14.5 (2–23) 
93.33 ± 6.7 15.13 (3–28) 
Colour Number of bands Mean percentage alive ± SE (n = 15) Mean number of nematodes found in shell (range) (n = 8) 
Yellow 86.67 ± 6.7 
100 ± 0 
93.33 ± 6.7 
Pink 100 ± 0 
100 ± 0 
100 ± 0 
Yellow 100 ± 0 7.38 (0–19) 
100 ± 0 8.63 (2–31) 
93.33 ± 6.7 12.88 (4–21) 
Pink 100 ± 0 6.5 (0–16) 
86.67 ± 13.3 14.5 (2–23) 
93.33 ± 6.7 15.13 (3–28) 
Figure 1.

A. Numerous Phasmarhabditis hermaphrodita encased and killed in the shell of a yellow Cepaea nemoralis. B. Close-up of individual P. hermaphrodita trapped in pink C. nemoralis. Scale bars: A = 1 mm; B = 100 µm.

Figure 1.

A. Numerous Phasmarhabditis hermaphrodita encased and killed in the shell of a yellow Cepaea nemoralis. B. Close-up of individual P. hermaphrodita trapped in pink C. nemoralis. Scale bars: A = 1 mm; B = 100 µm.

The experiment showed that C. nemoralis was able to defend itself from P. hermaphrodita by producing shell tissue that traps and encases invading nematodes. This ability is not affected by colour of the shell or by the number of shell bands. The nematodes appear as if perfectly preserved in amber and are completely covered by unknown cells. We do not know if the nematodes degrade over time or remain indefinitely in the shell. This defence mechanism is now known from several families of stylommatophorans; it might be a very ancient characteristic. As it leaves traces of its operation, it will be possible to detect its presence in museum or other collections of specimens from a wide range of families. Traces of a similar reaction to other parasites should also be explored.

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