The chapter "Invertebrates" by Sónia R.Q. Serra et al. discusses the importance and diversity of aquatic invertebrates in Portuguese rivers. These organisms, which include various groups such as flatworms, nematodes, annelids, mollusks, and arthropods, play crucial roles in maintaining the ecological balance of these ecosystems. They serve as energy links between lower and higher trophic levels, consuming algae, organic matter, and detritus, and serving as food for predators. The chapter highlights how invertebrates adapt to different environmental conditions, such as water flow and dissolved oxygen levels, and how their communities serve as bioindicators of ecosystem health. It also addresses the threats posed by invasive species and the conservation status of endemic species, emphasizing the need for conservation efforts to protect these vulnerable populations. The chapter provides a comprehensive overview of the ecological significance and challenges faced by aquatic invertebrates in Portuguese rivers.The chapter "Invertebrates" by Sónia R.Q. Serra et al. discusses the importance and diversity of aquatic invertebrates in Portuguese rivers. These organisms, which include various groups such as flatworms, nematodes, annelids, mollusks, and arthropods, play crucial roles in maintaining the ecological balance of these ecosystems. They serve as energy links between lower and higher trophic levels, consuming algae, organic matter, and detritus, and serving as food for predators. The chapter highlights how invertebrates adapt to different environmental conditions, such as water flow and dissolved oxygen levels, and how their communities serve as bioindicators of ecosystem health. It also addresses the threats posed by invasive species and the conservation status of endemic species, emphasizing the need for conservation efforts to protect these vulnerable populations. The chapter provides a comprehensive overview of the ecological significance and challenges faced by aquatic invertebrates in Portuguese rivers.