Ponderal growth of Asellus Aquaticus L. under laboratory conditions and in experimental mesoeosms, consequences of lindane contamination
We estimated the ponderal growth (instantaneous growth rate) of a fresh-water invertebrate Asellus aquaticus L. (Crustacea, isopoda), bred under laboratory conditions and in experimental mesocosms. Contamination by the insecticide lindane (Pepro 99 % purity) was studied. To estimate the instantaneous growth rate we used the formula Wt = Wo exp Mt) in what Wo is the median weight class of Asellus at the beginning of experiment. Wt is the median weight after 20 days, dj is the days number multiplied by median temperature above 3 °C of considered period (3 °C is considered as minimal temperature below that no development is possible). Seven classe were constituted : class 1 (2 mg to 6.99 mg), 2 (7 to 11.99), 3 (12 to 16.99), 4 (17 to 21.99), 5 (22 to 26.99), 6 (27 to 31.99) and 7 (32 to 36.99). The instantaneous growth rate is calculated for this seven class weight and for each replicate, next median and standard error were calculated for each class. The number of replicate vary among class weight. It was : 22, 33, 29, 22, 16, 15 and 13 respectively for class 1, 2, 3, 4, 5, 6 and 7. Asellus were preleved in natural ponds, acclimated in laboratory conditions during a minimal period of 15 days before that they are used for experiment. (glass containers of 30 litres, filled with hall tap-water hall pond-water, feed with maple leaves). In laboratory conditions Asellus were kept in glass aquarium (15 x 20 x 18 cm) filled with 2 litres of water. Physico-chemical parameters of water were : pH = 8.2; total hardness =155 mg 1-1 measured as Ca C03; nitrites = 0.055 mg 1-1; nitrates = 3 mg 1-1; chloride = 73 mg l-1 as Na C1. Laboratory breeding conditions were a tempera-tare of 15 °C and a 12/12 hours photoperiod. Contamination tasted 20 days, lindane concentration was 4,5 μg. l-1 (near the median lethal concentration, 48 hours : 5.14 μg.1-1) at the beginning of the experiment and approximatively 2 μg.1-1 at the end. Twenty Asellus of the same weight class were deposed in each glass. The diet was constitued with maple leaves. For contamination study only Asellus of class 7 to 11.99 was study. Ten replicates were realised. The mesocosms were constituted by two rectangular basins 10 meters in length, 2.5 m in width and 50 to 60 cm in deep. To secure a good water-thightness, bottom and sides were covered with black polyane 150 μm in thick, sediment and sand were deposed on the bottom on 5 to 10 cm in thick. Next basins were gradually (July 1987 to January 1988) filled up with tapwater. Natural colonization by phytoplankton and insects were observed, whereas vegetable (Ranunculus aquatilis, Typha angustifolia, Scirpus palustris and Ceratophyllum submersum) and invertebrates (Asellus, Planaria and Leech) were introduced by us. One mesocosm was contaminated by surface spraying with lindane acetonic solution. Thirty of a same weight class were placed in 25 x 12 x 7 cm plastic box, with a total of 20 lateral openings on either side (1.2 cm in diameter) disposed in two rows and covered with fine mesh net (150 μm in opening). The center part of the lid was cut out and also covered with the same fine mesh net. A bed of maple leaves was placed on the bottom of the box. This containers were then distributed throughout bath the control and lindane contaminated basins. Between 15 to 20 days later, this containers were collected and brought back to the laboratory. All the specimens were weighted and once divided into size classes and returned to their experimental basins. The experiment started in June 1988 and lasted till February 1989. The lindane concentration was 4.5 μg.l-1 at the beginning and near zero at the end. For the control, in laboratory condition or in mesocosm, we observed that the instantaneous growth rate decrease when the median weight of the class increase. It vary to 0.1131 mg.mg- 1.dj-1 to 0.0183 mg.mg-1.dj-1 and to 0.2704 mg.mg-1.dj-1 to 0.0879 mg.mg-1.dj-1 respectively in laboratory condition and mesocosm. Significant correlation (level 0.001) was observed between the logarithm of instantaneous growth rate and logarithm of the weight. Slope of regression lines does not vary, only position differ. Growth rate was higher in the mesocosm than in laboratory. In laboratory conditions lindane contamination induce a variation of instantaneous growth rate. An significant increase of 2.6 was observed between contaminated and control for Asellus of weight class 7 to 11.99 mg. In the contaminated mesocosm, a correlation is noted between instantaneous growth rate and weight, it is significant only for 3 collections dates (December 1988, January 1989 and February 1989). No variation in the slope of linear regression is noted, position differ significatively (level 0.01) only in February. For this collection date instantaneous growth rate is higher in contaminated mesocosm than in control. It appear that other factors that lindane contamination may also explain this increase. Among these, food quality has been envisaged by different authors. In conclusion is noted that in laboratory condition lindane induce an increase of instantaneous growth rate, in mesocosm we can't affirm that lindane was the responsible for the increase of instantaneous growth rate. Other experiments are necessary to confirm this observation.