The response of the terrestrial slug Deroceras laeve to the mucus and air-borne odours of con- and heterospecifics (Pulmonata: Agriolimacidae)

Gastropods produce mucus for a number of reasons. 1 One of these is attracting potential mates through (volatile) metabolites that are released in the mucus. 2,3 Hence, mucus may to a greater or lesser extent be responsible for influencing the behaviour of conspecific and even related heterospecific individuals. Moreover, interference competition through mucus of conspecifics and related heterospecifics may reduce growth rates and reproduction in individuals, thus affecting their fitness.4‐6 We tested the response of the terrestrial land slug Deroceras laeve on the mucus of unstressed and stressed con- and heterospecifics, and on air-borne odours of conspecifics. D. laeve (Muller) individuals were obtained from a laboratory culture. D. reticulatum (Muller) and D. panormitanum (Lessona & Pollonera) were used for trials with heterospecifics. All individuals were kept isolated in plastic containers with a piece of sponge and damp paper towel. Dried cat food and poplar leaves were given as food. Animals were kept in a climate room at a constant temperature of 18°C and a 16 h light/8 h dark rhythm. Only equally sized adult individuals were used. Individuals that were used to obtain stressed mucus were not used in the trials. Individuals were only used once in each experiment. All experiments were carried out in complete darkness. After each trial, all experimental equipment was cleaned with water and detergent to remove all mucus. In a first experiment we tested the response of D. laeve on mucus of unstressed conspecifics using two different set-ups. In the first set-up, we used 12 � 17-cm plastic boxes that were surrounded with copper foil to prevent slugs from escaping. Half the surface of a box was lined with untreated damp paper towel and poplar leaves. The other half of a box was lined with damp paper towel and poplar leaves containing mucus from unstressed conspecifics. Unstressed mucus was obtained by letting three slugs move for 30 min in complete darkness in a 12 � 8.5-cm plastic box surrounded with copper foil. The bottom of the box was covered with damp paper towel and poplar leaves. The paper and leaves with the mucus were transferred to the larger experimental box. At the beginning of each trial one experimental animal was placed at the contact line of the two surfaces. The behavioural response of 30 slugs was filmed for one hour with a Sony DLRTRV 130 E digital handycam attached to a VHS videorecorder in a climate room at 18°C. The ‘super nightshot’ option of the handycam allowed filming in complete darkness. Each time, two experimental boxes were filmed at the same time with a different orientation of the treated surface to avoid any bias from movements towards a certain direction. After 1 h we recorded the position of each slug, calculated the time that was spent on each surface and calculated the time that the individual was active on each surface. Activity was expressed as: (total time an individual was crawling in area/total time spent in area) � 100% and arcsin square root transformed to meet the parametric assumptions of normality. 7 Individuals that showed no activity within 10 min of the start of the experiment were excluded from the analysis. The preference for one of the two areas was tested with a � 2 test. Differences in the time spent on the two areas and differences in activity on the two areas were tested with a paired Student’s t-test. There was a tendency for individuals at the end of an hour to be on areas with mucus of conspecifics rather than on areas without mucus although the difference was not significant (� 2 = 3.33; df = 1;