Ricerca ProgettiMECOThe ProjectMethodology and activitiesEcologyBioassay-2Choice experiments

Choice experiments

Live individuals of five key species of Zouaraa (Talitrus saltator, Tylos europaeus, Phaleria acuminata, Eurynebria complanata and Scarites laevigatus) were brought back to Florence. These were maintained alive and used in laboratory experiments. Preferences to temperatures, sand moistures, grain sizes and salinities were tested under controlled laboratory conditions. Phototaxis and scototaxis experiments were conducted to test the responses to an artificial light source and a black shape. At the end of the experiments all species were brought back to their beach of origin.

 

• Temperature preference experiments were made at constant room temperature (20°C). Each species was tested individually in a linear arena divided in ten connecting compartments, where a gradient with a 40 - 10°C range was maintained. Air humidity was 100% and was kept constant in the compartments of the arena throughout the experiments. Controls were made at 25°C. Key species were tested during daytime with artificial illumination. The position of the animals and the temperatures were recorded after 15 minutes. Mean temperatures of the compartments chosen by the individuals were calculated together with 95% confidence limits.
  
  
• Preferences to sand moistures were tested using circular arenas, in which key species could choose alternatively between dry and moist sands. The sand was previously washed and oven dried at 105°C (0%). Sand moistures of 1, 2, 5, 10, 20% of water added to the total weight of sand were prepared and the following combinations tested: 0/1%, 0/2%, 0/5%, 0/10%, 0/20%, 1/2%, !/5%, 1/10%, 1/20%, 2/5%, 2/10%, 2/20%, 5/10%, 5/20% and 10/20%. For each species 50 individuals were used. For Eurynebria complanata and Scarites laevigatus each animal was tested singularly, whereas for Talitrus saltator, Tylos europaeus, Phaleria acuminata tests were made with groups of 10 individuals. Controls were made at 0/0% and 2/2% at 20°C room temperature. For statistical analysis Chi-square test was used.
  
  
• Also for sand salinity preference experiments key species could choose between two alternative substrate salinities. Sand was previously washed with tap water, oven dried and again moistened to 2% with a solution of de-mineralised water containing 0.5, 1, 2, 5, and 10% of NaCl. The following combinations tested: 0/0.5 0/1%, 0/2%, 0/5%, 0/10%, 0.5/1%, 0.5/2%, 0.5/5%, 0.5/10%, 1/2% !/5%, 1/10%, 1/20%, 2/5%, 2/10%, and 5/10%. For each species 50 individuals were tested using the same criteria of the above experiments. Controls were made at 0/0%. For statistical analysis Chi-square test was used.
  
  
• For grain size preference experiments the above experimental design was here used. Sand was sieved and the following grain sizes were selected: 180, 250, 355, 500, 710 µm. The combinations tested were 355/180 µm, 355/250 µm, 355/500 µm, 355/710 µm, 180/250 µm, 180/500 µm, 180/710 µm, 250/500 µm, 250/710 µm, 500/710 µm. Controls were made at 355/355 µm because it was the intermediate grain size. For statistical analysis Chi-square test was used.
  
  
• To test if the animals responded positively or negatively to a light source (400 lux) phototaxis experiments were made using a slide projector. The light source was placed alternatively to the north and to the south of an arena with 72 pitfall traps at the circumpherence.
  
  
• To test if key species could use cues, such as shapes (beach debris, tufts of vegetation), to choose their preferred beach zone, an experiment on their scototaxis abilities was performed. An arena with 72 pitfall traps surrounded by and covered with a white Plexiglas cylindrical screen and lid was used. Fifty individuals were tested on dry substrate in groups of ten. Twelve neon tubes which formed a regular hexagon around the arena diffused light homogeneously. Shapes were made from a rectangular piece of black cardboard (10 cm in height) that took up 45° of the horizon subtending an angle of 25° from the centre of the arena. Alternatively the shape was placed to the north and to the south. Experiments were conducted at 12:00 and 24:00 h in a dark room at a temperature of 20°C. Replicates were made the following day.

 

Circular statistics analysis (Batschelet 1981) was used to evaluate if arthropod responses to light source or black shapes were significant. The Rayleigh-test was employed to check whether the population was significantly concentrated, and the V-test was applied to determine if the population was significantly concentrated towards the light source or the silhouette’s centre (expected direction = 0°). When two equal and opposite sets of points were obtained in the distribution, bimodality was considered. By doubling of angles (Batschelet 1981) the bimodal distribution was transformed into a unimodal distribution, and the second trigonometric moment was calculated.