In aquatic ecosystems, what role do leeches primarily serve?

Leeches primarily serve as parasites and predators within aquatic ecosystems. This role is significant because leeches feed on the blood of various host organisms, such as fish and amphibians, which categorizes them as ectoparasites. In addition to their parasitic behavior, many species of leeches also contribute to the ecosystem by preying on smaller invertebrates and detritus, thus helping to regulate populations of these organisms and maintaining ecological balance.

The roles played by leeches as both parasites and predators position them as important components of the food web, influencing the dynamics of the ecosystem. Their feeding habits contribute to nutrient cycling by breaking down organic material and redistributing nutrients within the water column.

In contrast, other roles mentioned in the options do not accurately reflect the primary functions leeches serve. For example, leeches do not primarily act as pollutant decomposers; rather, this role is typically filled by microorganisms and macroorganisms like bacteria and certain fungi. Similarly, leeches are not classified as primary producers, which are usually plants or phytoplankton that convert sunlight into energy through photosynthesis. Lastly, while some species of organisms may indicate water quality, leeches are not standard indicators used for assessing water quality, as their presence is