Imagine for a moment that you’re a copepod—a pinhead-sized, torpedo-shaped crustacean drifting along in the warm currents of the Indian Ocean. You’re a favored menu item for many other ocean dwellers, so you need to be on high alert. You spread your two long antennae wide, ready to detect the pressure wave of an approaching predator. At the first sign of danger, you can rapidly propel yourself an astonishing distance with a single flap of your 8–10 jumping legs, a feat that has made you and your relatives some of the most successful creatures in the ocean. Still, there are dangers in these waters you’re unlikely to escape. If you could sense more than just light with your one compound eye, few things would alarm you more than the sight of this mouthy scene captured by photographer Alex Tattersall.
Indian mackerel (Rastrelliger kanagurta) have a widely varied planktonic diet, but a 2009 study revealed that copepods are their most frequent prey. While a single mackerel would be hard pressed to catch many copepods on its own, a large school of these streamlined fish working together can dramatically increase their chances of success. Whenever a group of mackerel encounter a dense aggregation of copepods, they begin pursuing their prey using a strategy called ram feeding. Aligning themselves in a tight-knit grid in which the gap between each fish is equivalent to the copepods’ average escape distance, the mackerel swim forward with their mouths agape. Long gill rakers—stiff projections that catch food while allowing large volumes of water to pass through—help the fish approach prey with as little disturbance to the water ahead of them as possible.
Copepods, which are normally sensitive enough to detect the mackerel coming despite this adaptation, can make multiple jumps before they exhaust themselves. But it takes a moment for them to reset their antennae after executing each jump, a pause that can be deadly. And if they don’t get swallowed during this downtime, it’s often just a matter of time before their escape jump lands them squarely in front of the waiting mouth of another fish. The mackerel close their mouths every few feet during ram feeding to swallow any copepods and other prey they’ve caught, then open wide in tandem once more to continue hunting. As they do, they reinforce a key link in their marine food web, amassing nutrients that can then be accessed by other species. They themselves are a favored food source for many larger animals—including humans.
Marsa Nakari Bay, Red Sea, Egypt
Alex Tattersall is an award-winning UK-based marine photographer with interests in all aspects of ocean life. His boundless enthusiasm for creatively capturing the underwater world and sharing these ideas and techniques with others has become the signature of the photography workshop he leads.