What is a "gill"?

A gill is a respiratory organ found in many aquatic organisms, including fish and some invertebrates, that enables them to extract oxygen from water. Gills are typically composed of thin, filamentous structures arranged in rows or clusters. These filaments are rich in blood vessels and covered by a thin layer of tissue, creating a large surface area for gas exchange. As water flows over the gills, oxygen is absorbed from the water into the bloodstream, while carbon dioxide is released. Gills are well-adapted for efficient gas exchange in aquatic environments, allowing organisms to extract the oxygen they need to survive. In addition to their respiratory function, gills may also play a role in filter-feeding or sensing the surrounding water for chemical cues.