Volume 4 Number 7
Horse-Eye Jacks slice through the Open Ocean
at Thumbnail Reef near Bimini, The Bahamas
It took a long time to decide that the fish in this photo were Horse-Eye Jacks. These fish were obviously Jacks. They were about two feet long, silver in color, and were fast swimmers. Like other fast-swimming fish, these Jacks have swept-back fins and long and powerful forked tails. Certainly these were Jacks, but what kind?
Horse-Eye Jacks have big eyes, giving these fish their name. They also often have bright yellow tails. But the fish in this picture have medium-sized eyes and appear to have black tails. What makes us think these are Horse-Eye Jacks? Look closely at their eyes and notice that these Jacks' mouths are only about twice as long as their eyes are wide. That makes those eyes pretty big. It is only the large size of these fish that makes their eyes appear small. And although Horse-Eye Jacks usually have yellow tails, sometimes their tails are dark.
Two other details make us think these fish are Horse-Eye Jacks. These fish have an arched line down their sides, called a lateral line. This lateral line is a row of "sound sensors" that work like a second set of ears to help these fish hear underwater. The length of this arch is less than the length of the pectoral fins. This is also a characteristic of Horse-Eye Jacks. Finally, there is a tiny black spot on the edge of their gill plates, just above and behind their eyes and near the front end of their lateral lines. That also is a characteristic of Horse-Eye Jacks. I never would have noticed that if I didn't have a picture to study closely!
Jacks are fish hunters that prowl through open water. They use their speed to catch smaller fish.
The day we saw these Jacks the water was thick with plankton. You can see the light of the camera strobe reflecting from those microscopic plants and animals, lighting up the water and making the water look "spotted."
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