It's a calm, moonlit summer's evening, and the perfect scene for a romantic encounter; well, it's perfect if you're a marine ragworm (Nereis succinea) in search of a mate. Emerging from their estuary mud homes,males and females swarm to the surface, embarking on brief close-range courtship dances, which culminate in duos circling around each other `as they release clouds of sperm and eggs into the water', says Jeffrey Ram from Wayne State University. How the 2–4 cm-long worms orchestrate their moonlit encounters in vast estuaries puzzled Ram's long-time collaborators Jörg Hardege and Thomas Breithaupt. Could trails of the metabolically costly female pheromone, which stimulates males to spawn, lead a courting male to his girl?In the summer of 2006, Ram travelled to Hardege's Hull laboratory (UK) to test out whether the males follow pheromone trails to find a mate(p. 757).
Collecting immature animals from Cardiff Bay in the Severn Estuary, the team reared the youngsters back in the lab before testing the worms' reactions to artificial pheromone trails. Unfortunately, the worms persistently swam along the tank's sides, often getting lodged in corners instead of swimming through the pheromone trail. That was until Hardege suggested introducing an internal apex into a circular tank to make a heart-shaped arena. Ram built the surprisingly shaped tank and, this time, the worms happily swam towards the pheromone trail, speeding up and veering to either the left or right as they crossed it.
Teaming up with student Michelle Danaher, Ram filmed successive pheromone trail encounters and realised that the animals predominantly circled over concentrated trails, while they weaved back and forth along dilute pheromone trails. And, when the trail was dilute enough, Danaher and Ram recorded nine worms swimming along the trail; one followed it for 18 cm.
But were they turning in the right direction to catch a mate? After weeks of digitally capturing and annotating the worms' trajectories in Hull, Danaher sent the results to Ram, who realised that the worms veered towards the left and right in roughly equal numbers. The worms' choice of direction was random. Given that the worms' responses to pheromone trails were governed by probability, Ram wondered whether he could simulate the worms' trail-following behaviour to find out more.
Working with Xubo Fei and Shiyong Lu from the Computer Science Department at Wayne State University, Ram designed a computational male worm based on previous observations of their behaviour to see if he could reproduce the animals' responses to pheromone trails. According to Ram `the results were surprisingly life like'. Testing whether the simulated male could locate a female by following her pheromone trail, the team found that the simulation's chances of finding a mate improved 3-fold compared to worms that had left it to chance. More significantly, the calculations showed that the males could track females at pheromone concentrations well below the levels needed to stimulate spawning. Ram adds `despite the high cost to the female of releasing pheromone all the time, this dual use of helping the male to find her and then turning him on makes it all worth while'.
