Baby, You Light Up My World Like Nobody Else: A Guest Post

One Direction was inspired by the brightly
shining love of the bioluminescent ostracod.
Photo by Fiona McKinlay at Wikimedia.

by Rachel Wang

You might not have guessed that the song lyrics of the band One Direction could apply to the courtship of bioluminescent marine animals, but the female ostracod crustacean (relatives of crabs and shrimp) might want to sing her heart out when she finds a bright guy to light up her world. 

This month's cover of the Journal of
Experimental Biology features a picture
of one of Trevor and Jim's ostracods!

Bioluminescent ostracods, also called “marine firefleas,” are tiny creatures (at most just 2 mm!) that live at the bottom of the ocean and have the awesome ability to light up the water in the western Caribbean with their natural bioluminescence. Males wait until it’s completely dark to put on a light show for the ladies. They secrete molecules that react with the water and produce pulses of bright blue light. To court females, males flash brightly 3-4 times in the same spot before swimming up in a spiral pattern and producing up to 16 flashes. Then they do this over and over for an hour! Talk about a bright time!

Males have three different types of courtship tactics as they compete for a mate. Males who initiate a display are described as “leading,” males who synchronize their flashes with another male are “following,” and males who stay close to another guy without doing any flashing are “sneaking.” Males aren’t just capable of all three tactics – they can also switch between tactics within seconds, even multiple times within a single 12-second-long display!

You can see an example of a light display here:
Video provided by Trevor Rivers

Jim Morin at Cornell University and Trevor Rivers previously at Cornell (now at Bowdoin College) provided an illuminating look at what affects a male’s decision to lead, follow, or sneak. Since the males only show off in the dark, Trevor and Jim used infrared video to shed some light on the situation and track each male’s movement, speed, and distance to other males. They randomly picked five males (a normal group size) and observed their behavior for 30 minutes. They looked at how much time each male spent leading, following, or sneaking, as well as the distance and angle to other males.

Pay close attention to see some followers in this infrared video:
Video provided by Trevor Rivers

The researchers found that once one male decided to be the leader and initiate a display, the other males’ choice between following and sneaking was strongly predicted by their distance to the leader – mainly vertical distance! The figure below illustrates what they found. The leader is represented by the point where the vertical and horizontal lines cross. At 8 cm above the leader, males were equally likely to follow or sneak (green area). Those more than 8 cm above the leader (the blue area) were more likely to follow, while males who were less than 8 cm above (pink area) were more likely to sneak! 

This figure shows that males way above the leader (at the center of the circle) choose to
follow (i.e. "entrain"), whereas males near the leader choose to sneak. The graph on the
left (a) shows the starting position and eventual tactic of each male. The graph on the
right (b) shows where each male was when he started using his chosen tactic. Figure
from Rivers, T.J., & Morin, J.G. (2009). Plasticity of male mating behavior in a marine
bioluminescent ostracod in both time and space. Animal Behavior, 78(3): 723-734.

So why would vertical distance make a difference in picking a tactic? Trevor and Jim offer some possible explanations. Males who are closer to the leader tend choose sneakiness. Rather than wasting energy on a flashy show, they focus all their attention on snatching a female. Males who are farther away from the leader tend to follow. They decide to go for it, and synchronize their flashes with the leader to compete for the Best & Brightest Award, with the winner receiving the prize of a lovely female mate. Followers also swim farther out, which could help them intercept females approaching from the side and increase their chances of scoring a touchdown. The bottom line is that these male bioluminescent ostracods are always checking out their competition and quickly deciding on the best move!

So what can we learn from this enlightening tale? Guys, keep your friends close and your competition closer. Imitating or sticking close to the guy who’s got it all could pay off in the end. Just remember: stalking your competition? That might work for bioluminescent ostracods, but it’s not sexy by human standards.

If you’re interested in more info, check out:

1. University of Wisconsin-Lacrosse’s great, informative website on bioluminescent ostracods

2. Rivers, T.J., & Morin, J.G. (2009). Plasticity of male mating behavior in a marine bioluminescent ostracod in both time and space Animal Behavior, 78 (3), 723-734 DOI: 10.1016/j.anbehav.2009.06.020