A murder mystery on a catchment scale: Death by flesh eating bacteria or invasive piranha?

We know that an organism's struggle for survival in the wild can be pretty gruesome. As Richard Dawkin wrote in his book ‘River Out of Eden: A Darwinian View of Life’: ‘Nature is not cruel, only pitilessly indifferent… indifferent to all suffering’ (Dawkins, 1996). Indeed, the fate of some organisms can be so dire that they resemble the torture devices of a supervillain. In this issue, we learn of an investigation into the health of Guri sea catfish (Genidens genidens) in the Doce River estuary, southeastern Brazil (Andrades et al., 2026), which provides a perfect example of this. The catfish population was experiencing an increased incidence of fin lesions, the cause of which was narrowed down to one of two possibilities: flesh eating bacteria or invasive piranha. While nature may be indifferent to suffering, the same cannot be said for humanity, and empathy for a creature's suffering is elevated when suffering is exacerbated by human activity. Rapid environmental change driven by humans and the resulting maladaptation of organisms to new environmental states, and premature extinction, is another burden that hangs heavy on the conscience. This is relevant for the Guri sea catfish, as they are experiencing an increase in fin lesions in the wake of one of the world's largest mining disasters. Specifically, the collapse of an iron ore tailings dam storing waste by-products. Predictably, this had a devastating impact on the water quality in the adjoining river. Poor water quality is known to increase the incidence of fin rot (Douglas Minchew & Yarbrough, 1977), and at first glance, the mining disaster looks to be the smoking gun in the case of the fish lesions. However, piranha can also cause fin lesions through fin nipping, and like in many other locations in South America (Bertaco et al., 2022), in the Doce River estuary, humans have introduced invasive piranha from other drainages. A second smoking gun. However, thanks to the forensic work by Andrades et al. (2026), this mystery has now been solved. By culturing bacterial communities from damaged catfish fins, it was shown that key bacterial taxa associated with the establishment of bacterial fin rot were missing. If that were not enough, fin remains were found in the stomachs of piranha caught in the area, fin lesions were largely found on the caudal fin (a hot spot for fin-hungry piranha), and no lesions were found in neighbouring estuaries where piranha were not present. Case closed? Not quite! Despite the piranha delivering the first blow, the polluted waters of the Doce River estuary allowed bacterial infections to take hold in the wound. After all, catfish in the polluted estuary had severe kidney disorders, high pathogen prevalence, high bacterial diversity, and bacterial coinfection of the fin and liver. This work shows that anthropogenic pressures are compounded and is a cautionary tale for highly degraded systems, while also demonstrating that, like in any good murder mystery, the main suspect rarely works alone.