The bull shark (Carcharhinus leucas), a tropical apex predator has long been recognized for its resilience and adaptability. Known for their remarkable ability to tolerate both fresh and saltwater, a trait referred to as “euryhaline,” they can inhabit a variety of environments, including estuaries, rivers, and coastal waters.
Currently under the spotlight due to its vulnerable status on the IUCN Red List, they have been found to often use freshwater environments as nursery grounds for their young. Pregnant females may move into rivers to give birth, providing a relatively safe and food-rich environment for the vulnerable pups. In Fiji, the story is no different – these sharks find refuge in the river systems, and the Rewa River has emerged as a crucial habitat for them, serving as a nursery ground. Originating from the central highlands of the island Viti Levu, the Rewa River meanders its way through lush tropical rainforests, agricultural lands, and urban areas before reaching the coastal plains. The river’s flow patterns, water quality, and vegetation along its banks contribute to the rich biological diversity found here.
Yet the waters here may be the reason for the bewitching red hue juvenile bull shark eyes here are displaying. Out of 75 bull sharks surveyed during the daytime, a staggering 36% exhibited the red eyes; astonishingly, this distinctive eye coloration persisted throughout the pupping season, spanning from December to March. This startling discovery, noted during daylight surveys between December 2017 and April 2018, has unleashed a wave of curiosity among marine researchers globally. To probe deeper, water quality parameters were meticulously examined by researcher Kerstin Glaus of the University of the South Pacific, revealing a compelling narrative. Elevated levels of faecal indicator bacteria and ammonia, exceeding recommended parameters, hinted at potential sources of eye irritation. Could this uncommon phenomenon be a consequence of the river being metal-rich? Surprisingly, no contamination from heavy metals was detected.
This unexpected finding prompted collaboration among researchers globally. “Red-eyed bull shark photos from the Rewa River were shared with 11 researchers from Fiji and globally studying shark sensory systems, bull shark nurseries, and shark behaviour to find similar observations elsewhere,” Glaus explained. This sharing led to the discovery of a similar instance in the nearby Navua River back in 2014. The red-eyed mystery, it seems, is not confined to a singular river system.
As scientists delve into potential causes, various theories emerge. Could it be an external infection, a consequence of high organic matter in the water? Or perhaps, an inherent ocular process is at play, a manifestation of the bull shark’s unique eye structure. One captivating hypothesis suggests that the red eyes could be a result of eyeshine – the reflection of light out of the eye. Sharks, equipped with a tapetum lucidum (a reflective layer behind the retina) made of guanine crystals, possess a unique ocular mechanism that could manifest as red eyeshine, especially in the dimly lit conditions of murky waters. And while the prevailing red-eye coloration is distinct from albinism, it does prompt a closer look at the genetic makeup of Fiji’s bull sharks. Are these red-eyed individuals a product of genetic mutations, or is there an underlying environmental trigger?
This revelation, Glaus believes, sparks a call to action. If water pollution is indeed responsible for this eye-catching phenomenon, mitigation strategies must be devised swiftly. Not only to ‘solve’ the mystery of the red eyes but also to ensure the well-being of these juvenile bull sharks in the face of environmental challenges.
