A bluegill sunfish uses a cunning combination of ram and suction to close the gap between itself and its dinner; it propels itself forwards while rapidly expanding its jaws. But is a sunfish more likely to catch a juicy treat if it lunges faster? Timothy Higham, Steven Day and Peter Wainwright have now worked out how a sunfish's attack speed affects the speed, area and shape of the fluid that it swallows().
To visualise sunfish sweeping food into their mouths, the team scattered microscopic silver beads in a tank and shone a laser light sheet onto them to see water flow. They dangled a tasty morsel 0, 30 and 50 cm from a fish's mouth to entice it to lunge forwards at different speeds, and recorded the prey-engulfing process on high-speed video. The team measured ram speeds of up to 25 cm s–1. They saw that the speed of water rushing into the fish's mouth wasn't affected by the animal's swim speed. But faster sunfish did suck up water from a narrower region right in front of their mouths, which is where the prey is likely to be. Faster fish also gulped down water from further away, which could limit the escape chances of prey struggling against the current. But there's a potential drawback when sunfish boost their attack sprint; sucking up a narrower volume of fluid means that the fish need to attack pretty accurately. The team concludes that a moderate ram speed might improve a sunfish's chances of seizing a hapless victim.
