Figure 1.

Immersion in serotonin (5-HT) increased swimming but not crawling in Melibe leonina. Animals were placed individually in a small, clear plastic container containing artificial seawater (ASW). The container had a 1-cm grid pattern drawn on the bottom and sides to track the distance that animals crawled. Each animal was allowed to acclimate to the container for at least 30 min before the experimental solution was added. One hundred milliliters of 5-HT solution (Sigma-Aldrich, St. Louis, MO), or ASW, was added to the container such that the final concentration of 5-HT in the container was 10⁻³ mol l⁻¹, 10⁻⁵ mol l⁻¹, or 10⁻⁷ mol l⁻¹. Animals were then visually monitored for at least 45 min. (A) Increasing concentrations of 5-HT resulted in a trend toward decreased latency to locomotion (i.e., the amount of time before the animal exhibited locomotor behavior). However, this trend was not significant (P = 0.29, F_3,44 = 1.30). (B) Serotonin had no significant effect (P = 0.34, F_3,44 = 1.14) on the distance crawled, which was calculated as the number of centimeters crawled in 45 min. (C) In contrast, the highest concentration of 5-HT significantly increased the average number of swim cycles observed during the experiment (P < 0.001, F_3,44 = 22.25). A swim cycle was determined as a bend to one side, followed by a bend to the other side, and then a return to the central longitudinal axis. (D) In the highest concentration of 5-HT, some animals exhibited some irregular swim behavior, which typically consisted of incomplete swim cycles. However, the frequency of these irregular swim cycles was not significantly different between bath solutions (P = 0.23, F_3,44 = 1.50). Forty-eight animals were used, with 12 in each treatment group. Error bars indicate standard deviation. Statistical comparisons of means here and elsewhere were made in InStat ver. 3 (GraphPad Software, San Diego, CA) with ANOVAs and Tukey post hoc analysis. For all statistical tests, a P value less than 0.05 was considered significant.