One of the highlights of my Sea of Cortez cruising was an experience with a school of manta rays I encountered in San Evaristo, a well-protected bay adjacent to the coastal village of that name. Tucked into a cozy little cove at the north end of the bay, I was getting ready to toast the sunset when the rays caught my attention. It wasn't just that they were jumping several feet into the air and then slapping back down into the water - you see that often in these waters - it was more attention getting because it was going on so frequently and so continuously. In fact, as I watched what appeared to be a large group of these rays, the group seemed to be slowly proceeding around the cove right toward the boat. As they got closer, it was apparent that the main group of 40 or 50 rays were pretty closely grouped into a tight circle right at the surface of the water. Of course, this isn't all that remarkable, because they are surface feeders who feed on krill they find there. What was remarkable, was the behavior of the jumping rays. As they approached, it was easy to see that the jumps were nearly all performed outside of the closely grouped school, and the jumpers were consistently swimming and jumping in a clockwise circle around the main group! It also appeared that the jumps were concluded with a loud, belly flopping, slap when the rays landed back in the water.
Now, considering that manta rays are closely related to the shark family, some of the most primitive fish in the ocean, this sure looked like the kind of organized group behavior we don't usually attribute to this kind of critter. But a little online research also reveals that krill are stunned and tend to bunch up when they sense an impact nearby. Is it possible that these rays were engaging in some kind of co-operative feeding behavior - the outriding jumpers herding the krill into the circle of their feeding buddies ?! Gotta confess, I'm not enough of a biologist to answer that one, but it sure is an interesting question...
For more interesting information about manta rays, check out the website: http://www.freedive.net/mantas/mantas.htm
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