It almost sounds like a punishment worthy of Hades: most morsels slurped up by tiny hungry fish fry are ripped from their lips at the final instant. ‘For example, when 8 day old seabream fry attempt to capture their prey, they need five or six strikes to capture one prey item’, says Roi Holzman from Tel Aviv University, Israel. Yet, no one knew exactly how the unfortunate chain of events that snatches food from the youngsters’ mouths unfolds. Intrigued by the mystery, Holzman and Krishnamoorthy Krishnan, Asif Nafi and Roi Gurka from Costal Carolina University, USA, knew that there was only one way of finding out: by using a computer simulation of the events occurring within the fry's miniscule mouths. But first, the team needed to understand how the tiny fish larvae move their jaws and flare open their gill covers as they slurp in water through their mouths when attempting to gulp down a titbit.
Turning to Victor China's movies of gilthead seabream (Sparus aurata) larvae – ranging in age from 7 to 37 days post-hatching (dph) – attempting to feed on one of their favourite snacks (0.16 mm long rotifers), Holzman detailed how the fry opened their mouths and gills as they slurped in water and how that changed as they grew. Over a month, the diameter of the fry's mouths doubled to 0.5 mm and their mouths also grew longer (0.7 mm to 2 mm). In addition, the larvae were eventually able to open their gill covers almost 1 mm wide. The team then designed a simulated fish larva mouth based on the measurements of the growing larvae, shaped like an elongated rugby ball, which could open wide at the front as the mouth drew in water. In addition, the team added simulations that took account of how long it took the larva to open its mouth, how the stickiness of the water reduced as each larva grew and the way the water surged forward as the larva's mouth gaped wide, to get a better sense of how water flowed through the minute fry's mouths.
After months of patient computer programming, the team was relieved when the first simulations successfully recreated the water flow through the cyberlarva's mouth and out of the gill slits at the back. Also, as the cyberlarva grew, the speed of the simulated water flow through the mouth increased from 28.3 mm s−1 in the youngest larva up to 136.2 mm s−1 in the oldest larva as it swallowed water at a rate of 5.9 mm3 s−1. However, when the youngest cyberlarva closed its mouth at the end of a gulp, the team was astonished to see approximately one-tenth of the water gush back out of its mouth. This unexpected turn of events perfectly explained why food fragments that should have been swallowed suddenly popped back out of the frustrated fry's mouth. Fortunately, the inconvenient backwash vanished when the larvae grew older (23 dph); however, if the larger fish only took a leisurely gulp, the problem came back and they too could lose their dinner.
Puzzled by the young larvae's misfortune, the team took a closer look at the trajectory of the water surging into the mouths of the tiniest simulated fry and realised the strongest outwash was generated when the mini fish opened their mouths slowly. It seems that opening their mouths fast is essential for famished gilthead seabream larvae to fill their mini bellies.