A 200-kilogram Amazon river fish that produces ‘milk’: Scientists explain the arapaima’s surprising parenting secret

Deep in the vast rivers and flooded forests of the Amazon Basin, there lives a fish that appears to be prehistoric in both size and appearance. The arapaima, also called the “pirarucu,” is one of the biggest freshwater fish on the planet. This massive fish may grow nearly three meters in length and weigh 200 kilograms. It is a giant fish that rules the waters of the Amazon Basin.The Amazon River is home to an extraordinary range of wildlife, many of which still surprise scientists with unusual behaviours and adaptations. Among these remarkable species, the arapaima stands out not only for its enormous size but also for its unique biology.But it is not the size of the arapaima alone that has caught the attention of both scientists and fishermen. There is something even more surprising about the way the arapaima takes care of its baby fish. It appears that the arapaima may actually secrete a milk-like substance that may be used to feed its baby fish. This is a rare trait in fish.The arapaima is an ancient species of bony-tongued fish that has been around for millions of years. Its body is covered with large scales that resemble armour. These scales are so hard that they even withstand the attacks of piranhas. The arapaima’s long and streamlined body and powerful tail also make it an effective hunter in the slow waters of the Amazon River. But despite its enormous size, the arapaima also has another interesting characteristic. The arapaima comes up to the surface of the water to breathe. This is because its swim bladder is similar to the lungs of the fish. This characteristic allows the arapaima to survive in the low-oxygen waters of the Amazon River. All of these characteristics of the arapaima make it one of the most interesting species of freshwater fish.The most astonishing thing about arapaima is how it feeds its young. After the eggs hatch, the young are always in proximity to the head of the parent, usually the male.Scientists have found that the adult arapaima has glands located on its head that secrete a nutrient-rich fluid, commonly known as “arapaima milk,” which is used to feed its young. This is not milk in any way similar to that produced by female mammals. However, it is believed to be used to feed the young. This has fascinated scientists because it is a unique occurrence in the world of fish. This is because such behaviour is only common in mammals and birds.In their natural habitat, newly hatched arapaima stick together in a tight group close to their parent. The dark colouration of arapaima helps them blend in while they swim close to the head of their parent.The parent arapaima releases pheromones that help stick its young together and lead them to areas that are safe and have plenty of plankton and other food sources.This kind of parental care is unusually long for most fish. The young arapaima gradually begin to eat from the food sources found in the water while still under the protective care of their parent.This behaviour greatly enhances the survival chances of arapaima young in the Amazon waters that are full of predators.The scientists who have been studying the arapaima have looked at the content of the secretion and found that it contains hundreds of biological compounds, proteins, and peptides that are associated with growth and immune processes.A study titled “Comparative proteome and peptidome analysis of the cephalic fluid secreted by Arapaima gigas during and outside parental care,” published in PLOS ONE and listed in the U.S. National Library of Medicine database, analysed the fish’s cephalic fluid and identified compounds that may support the development of its young.The secretion may assist the young fish in growing quickly and fighting off diseases during their early stages of development.The secretion may be a communication medium that keeps the young fish close to the parent and ensures group coordination in the murky river waters.Although the exact purpose of the secretion is still a subject of study, it is evident that the behaviour is a complex parental care that is rare in fish.The discovery of this unusual behaviour highlights the extent to which scientists still have much to learn about the lives of the world’s rivers.The arapaima is already known for its massive size and ancient lineage. However, the production of a milk-like secretion by the arapaima highlights a new dimension of biological complexity. In the Amazon ecosystem, the key to survival is often the development of unusual adaptations. Even a massive fish species may develop unusual means of protecting its offspring.To the scientific community, the arapaima is more than a massive fish species. It is a symbol of the fact that evolution often provides us with unusual means of protecting our offspring.