In this underwater video clip, we can see a Southern Stingray (Dasyatis americana) being shadowed by a Bar Jack (Caranx ruber) on a sandy flat in Saba, Netherland Antilles. This unique fish behaviour is called shadowing or shadow-feeding. Shadowing is the act of one fish following another fish of a different species to profit from the hunting success of this fish. This opportunistic hunting practice is used by a number of different fish species. Trumpetfish (Fistularia chinensis) are one of the species that are often spotted shadowing other species like angelfish (Pomacanthidae).
In this short underwater video, a Bar Jack is taking advantage of the Southern Stingray’s hunting skills to catch any fleeing prey or by picking the scraps from the elasmobranch’s meal. Stingrays usually catch prey that is often hidden under the substrate. With their ampullae of Lorenzini, mucus-filled electroreceptors embedded in the pores of their skin, they are able to detect prey that would be impossible to find for most fish species. The jack doesn’t possess this electric detection organ and must thus rely on the stingray’s ability to flush out hidden prey.
Fish shadow for a multitude of reasons, but it is always a feeding behaviour that involves solitary species. However, not all shadow-feeding involves species that are equipped with these ampullae of Lorenzini. Sometimes it is just a hiding technique where the hunter hides behind a non-threatening fish herbivorous species to use the bigger shadowed fish as a means of camouflage. By doing so, the trumpetfish is able to sneak upon unsuspecting little fish and small crustaceans. The Bar Jack in this underwater video clip is probably hoping to catch fleeing little prey that the Southern Stingray flushed out of the substrate.
Bar Jacks are silvery fish with two horizontal stripes running down their backs. The higher one is a dark line almost black, and the lower one is almost electric blue. These fast and solitary carnivores often turn black and sometimes even bronze when hunting near the bottom. This happens when nerve cells trigger the chromatophores in a fish’s skin. The chromatophores expand or retract, causing the fish to rapidly change colour. In a study conducted in 2009 on the island of Bonaire, it is suggested that Bar Jacks are more successful in catching prey when they are dark/black coloured. The darkened jacks had double the feeding rate of that of the silver-coloured jacks. How this increased feeding success is related to the colour change of the jack is not yet fully understood.
This was filmed without any video lights, which resulted in a lot of noise in the image. Image noise is a random variation of brightness or colour information in images. It resembles little colour specks that lower the quality of the overall picture or video shot. It is most visible in footage that has been recorded or filmed in low-light conditions and without the necessary underwater video lights to illuminate the scene properly. To remove this noise, I applied a specialised generator during the editing process. This generator works by smoothing out the most noise-exposed areas in the footage.
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