GYRKPPFNGSIFamide (Gly-SIFamide) modulates aggression in the freshwater prawn Macrobrachium rosenbergii

Abstract

The freshwater prawn Macrobrachium rosenbergii is a tropical crustacean with characteristics similar to those of lobsters and crayfish. Adult males develop through three morphological types-small (SC), yellow (YC), and blue claws (BC)-with each representing a level in the dominance hierarchy of a group, BC males being the most dominant. We are interested in understanding the role played by neuropeptides in the mechanisms underlying aggressive behavior and the establishment of dominance hierarchies in this type of prawn. SIFamides are a family of arthropod peptides recently identified in the central nervous system of insects and crustaceans, where it has been linked to olfaction, sexual behavior, and gut endocrine functions. One of the six SIFamide isoforms, GYRKPPFNGSIFamide (Gly-SIFamide), is highly conserved among decapod crustaceans such as crabs and crayfish. We wanted to determine whether Gly-SIFamide plays a role in modulating aggression and dominant behavior in the prawn. To do this, we performed behavioral experiments in which interactions between BC/YC pairs were recorded and quantified before and after injecting Gly-SIFamide directly into the circulating hemolymph of the living animal. Behavioral data showed that aggression among interacting BC/YC prawns was enhanced by injection of Gly-SIFamide, suggesting that this neuropeptide does have a modulatory role for this type of behavior in the prawn.

Figure 1

Gly-SIFamide injection into YCs. Effect on mean dominance index (DI) before (pre) and after (post) injection of the YCs with 1 × 10⁻³ mol l⁻¹ Gly-SIFamide (n = 5). Before the injection, BCs established themselves as the dominant animals and YCs as the subordinates (one-way ANOVA, Tukey's test, P ≤ 0.001). When the YCs were injected with Gly-SIFamide, they behaved very much as do BCs under control conditions. The behavior of BCs confronted with Gly-SIFamide-injected YCs was also modified, showing a significant decrease in the levels of aggression compared to control conditions. The asterisk (*) over the YC post-injection group represents a significant difference from the YCs in control conditions (before the injection). The cross (†) over the BC post-injection group represents a significant difference from the BCs in control conditions. BC: blue clawed prawns; YC: yellow clawed prawns; filled bar indicates the mean DI of the injected (YC) prawns; *P ≤ 0.001; **P ≤ 0.025; † P ≤ 0.050.

Figure 2

Gly-SIFamide injection into BCs. Effect on the mean dominance index (DI) before (pre) and after (post) injection of the BCs with 1 × 10⁻³ mol l⁻¹ Gly-SIFamide (n = 5). When the BCs were injected, they further increased their aggression toward YCs when compared to control conditions (asterisk [*] over the BC post-injection group, P ≤ 0.001). BC: blue clawed prawns; YC: yellow clawed prawns; filled bar indicates the mean DI of the injected (BC) prawns; *P ≤ 0.001.

Figure 3

Vehicle injection into YCs. Effect on the mean dominance index (DI) before (pre) and after (post) injection of the YC with prawn saline (n = 5). Injection of vehicle solution alone did not change the DI for YCs. BC: blue clawed prawns; YC: yellow clawed prawns; filled bar indicates the mean DI of the injected (YC) prawns; #P ≤ 0.002; ##P ≤ 0.040.

Figure 4

Vehicle injection into BCs. Effect on the mean dominance index (DI) before (pre) and after (post) injection of the BCs with prawn saline (n = 5). Injection of vehicle solution alone did not change the DI for BCs. BC: blue clawed prawns; YC: yellow clawed prawns; filled bar indicates the mean DI of the injected (BC) prawns; *P ≤ 0.001; #P ≤ 0.002.

Figure 5

Effects of vehicle and SIFamide injections on number and duration of attacks between prawns. No statistically significant effects were observed on the number of attacks and fight duration for any of the injection experiments. Effect on the mean number of attacks (A) and attack duration (B) before (pre) and after (post) injection of the BC (n = 5) and YC (n = 5) with prawn saline, respectively. Effect on the mean number of attacks (C) and attack duration (D) before (pre) and after (post) injection of the BC (n = 5) and YC (n = 5) with Gly-SIFamide, respectively. BC: blue clawed prawns; YC: yellow clawed prawns; Pre: pre-injection behavior; Post: post-injection behavior; filled bar indicates the injected prawns in each set of experiments.

Figure 6

Effects of vehicle and SIFamide injections on the prawn's locomotion. No statistically significant effects were observed on locomotor activity for any of the injection experiments. (A) and (B) Effect on the mean number of lines crossed in 30-s intervals during a 30-min recording period, before (pre) and after (post) injection of the BCs (n = 5) and YCs (n = 4) with prawn saline, respectively. (C) and (D) Effect on the mean number of lines crossed in 30-s intervals during a 30-min recording period, pre- and post-injection of the BCs (n = 5) and YCs (n = 5) with Gly-SIFamide, respectively. BC: blue clawed prawns; YC: yellow clawed prawns; Pre: pre-injection behavior; Post: post-injection behavior; filled bar indicates the mean number of lines crossed in a 30-s interval by the injected (BC or YC) prawns.