Current weather for Marathon, FL
Click here for a Local Weather Forecast
84
84
Dive Time with Tim Grollimund
Let’s wake up an eel and go hunting
E-Mail this story
Print this story
tool name
close
tool goes here
Posted - Monday, October 01, 2012 01:58 PM EDT
Green Moray eels are easily approached during the day. (photo by Tim Grollimund)
On any dive, at any time, progressing along the reef can come to a halt quickly. I’m not talking about accidents or incidents, but animal sightings.
I don’t think I have ever not stopped when I’ve seen an eel. And now and then, if they are out and about, I get to see them at their full length. To me one of the most graceful motions in the sea is an eel undulating effortlessly between coral heads. They can also disappear with seemingly no effort into in a crevice that looks way too small for them to fit.
Relative to some other places I’ve been, we have more than our fair share of green, spotted and goldentail eels. When we were out on Molasses Reef waiting for the coral to spawn in August, I saw a purplemouth eel, with its bright yellow eyes, but it receded into the reef before I could capture an image. My split-second “what-the-heck-is-that?” hesitation cost me the image.
My REEF fish identification book cites 13 species of morays, five conger and five snake eels.
While the vast majority of observations during the day are while eels are resting or on a cleaning station, a few times I have seen an eel coupled with a grouper. The first time I saw this was on the Winch Hole. A graysby came to a stop on a pile of rubble, and a goldentail came out from its lair. At first they just sort of looked at each other, then came cheek to cheek, touched, and took off together.
I have seen it a couple of other times with a green moray and black grouper.
This behavior has not been greatly documented according to the research I did this week. I could only find a few scientific references based on cooperation and communication between different fish species.
Other cooperative hunting behaviors among single species are well documented — from dolphins to chimpanzees — but inter-species communication coupled with cooperative behavior studies among fish are rare.
There are many relationships of joint foraging, such as wrasses hanging out with trunkfish. These involve a nuclear species (the main dude) and a follower (the one cleaning up the scraps).
What differentiates the grouper/eel behavior is the grouper seeks the eel and communicates its intention to hunt with a specific signal.
The one study all the other scientists referenced was done in the Red Sea. Google “Plos Biology grouper eel” and the paper should pop right up for you. It’s called “Interspecific Communicative and Coordinated Hunting between Groupers and Giant Moray Eels in the Red Sea,” with principal investigator Redouan Bshary.
Their observations begin with the grouper initiating the behavior by approaching an eel and shaking its head rapidly back and forth. If the eel chooses — remember, it’s nap time for the eel — it leaves its parking spot and motors along the reef with the grouper. The head shake by the grouper is the message to the eel that it’s time for a cruise. But morays don’t always come out to play.
This cooperation benefits both species quite well. Since morays hunt in crevices in the reef, and groupers hunt in open water, the combined hunting techniques cut off the avenues of escape for the prey. The cooperative efforts of the grouper and the eel leave the prey with nowhere to go.
By their measurements, the scientists showed that combined hunting efforts provided significantly more hunting success than the grouper normally achieved on its own. Morays also were successful on the daytime hunts, but the researchers could not compare an eel’s daytime success with their normal nocturnal hunting results.
They also demonstrated, by feeding a grouper a fish, that the behavior is dependent on the grouper’s appetite. Satiated groupers did not seek help hunting. They could not make any observations on the incidence of cooperation by the eel based on whether or not it was hungry.
Groupers made different head shakes when they found a prey item in the reef, including taking a vertical posture, which signaled the eel to investigate for prey in that spot. Another unique feature is that the bounty is not shared. The grouper and the eel both swallow their prey whole — and the other does not complain. There was no mention in this study whether or not he who eats first stops the hunting first. The video clips used in the study are here: http://youtu.be/2A7qqfJYXOc
The communication and cooperation among groupers and eels is fascinating. So is the structure of the eel’s jaws. Groupers, and most other bony species, suck in their prey with the rapid opening of their mouths. Many species of fish have a second, internal jaw, called pharyngeal jaws, that function more for crushing and grinding.
But the eels’ pharyngeal jaws are unique. At the University of California/Davis, Rita Metha has established the ELVER Lab to study eels and their unique characteristics.
She calls this prey ingestion mechanism pharyngeal transport. Since morays have a reduced capacity for suction feeding, they protract the pharyngeal jaws to bite down on prey items to transport the catch into the esophagus. You can see her explain it all here: http://youtu.be/Rv2DkzOPBXw My next eel encounter will be different. I’ll be trying to look as far down its gullet as I can to see if I can see that second set of chompers.
Tim Grollimund is a freelance photographer and PADI divemaster based in Key Largo. He can be reached at tim@timgimages.com or through his web site at www.timgimages.com. Tim is actively involved with the Coral Restoration Foundation and the Aquarius Foundation.