Variation in the Behavior of the Amphipod Gammarus pulex

Variation in the locomotion behavior of Gammarus pulex was studied using the Multispecies Freshwater Biomonitor (R). Behavior was recorded individually under a 12: 12 h light: dark cycle for a period of 7 d during which the amphipods were not fed. Gammarids behaved differently during the first 7 h in the laboratory chambers. Most specimens were active and decreased their activity but some were relatively inactive after the start of the experiment and increased their locomotion. At least 2 h are needed by G. pulex to accustom to the conditions. No significant relationships existed between type of behavior during the acclimation period or acclimation time and gender or length of the gammarids. There was a large variation in time spent on locomotion during the first night. In general, males were significantly more active than females. Within males, a distinction can be made between very active and less active specimens. The 7-d observation period showed that only 70% of the specimens demonstrated a clear day-night rhythm with higher activity during the night. Unexpectedly, approximately 10% of the specimens were more active during daytime. Furthermore, in this study locomotion became relatively stable only after 4 d. Inter-individual variation in behavior must be taken into account when using behavior as an endpoint in ecotoxicological bioassays.