The ancient lakes of the Indonesian island Sulawesi stand out for their rich species diversity and high levels of endemism, especially of fish, snails, shrimps and crabs. The invertebrates in particular face serious threats, one of which is invasive fish, specifically the flowerhorn, an artificial hybrid of various Neotropical cichlids. This fish has been spreading rapidly in the Malili lake system in South Sulawesi since 2010, when the first occurrence was recorded in Lake Matano, the uppermost lake of the system, probably the result of a release by a local aquarist or pet store owner. Many local endemic shrimp and snail species are categorized as Critically Endangered on the IUCN Red List, and all five crab species in the Malili lakes are categorized as Endangered.

During monitoring in Lake Matano in September 2022 and May 2023, we witnessed flowerhorns attacking crabs. In the first case, it was an adult Parathelphusa pantherina, which was deprived of several limbs in a few minutes and finally pulled out from under a rock and killed. In the second case, an adult Syntripsa matannensis was attacked and lost several limbs. The fact that we have witnessed this behaviour on several occasions indicates it is a common occurrence. Each time, multiple cichlids attacked together.

Crustaceans are not the main prey of flowerhorns; research suggests they primarily eat snails. In Lake Matano, seven species of Sulawesidrobia and six of Tylomelania are endemic. Nearly all are Critically Endangered. The smallest species of the genus Sulawesidrobia may already be extinct. They disappeared as the flowerhorns spread, and so did the smaller species of Tylomelania and the juveniles of the larger species (Haase et al., 2023, European Journal of Taxonomy, 864, 77–103).

This predation of snails also has an impact on the crabs. Two species of Syntripsa are molluscivorous: S. matannensis, endemic to Lake Matano, and S. flavichela, which also inhabits two neighbouring lakes. Flowerhorns are thus also competing for food with the endemic crabs, which are now threatened both by the depletion of their mollusc prey and by predation.

Our observations of interactions between flowerhorns and crabs underscore the urgency of the situation. Invasive cichlids exert great pressure on this lake ecosystem, and flowerhorns have already made their way into neighbouring lakes that have a similarly unique invertebrate diversity.