Approximately how many different fish species are estimated to exist worldwide?

The estimate that more than 21,000 different fish species exist worldwide is based on extensive research and taxonomic studies conducted by ichthyologists. Fish represent the most diverse group of vertebrates, and advancements in genetic analysis and exploration of diverse aquatic habitats have revealed many species previously unknown to science. Recent estimates suggest that the actual number could be significantly higher than 21,000, as many species are still being discovered, especially in less-explored areas such as deep oceans and certain freshwater ecosystems. This ongoing research highlights the complexity and richness of aquatic biodiversity, emphasizing the importance of conservation efforts to preserve these species and their habitats.

The other choices reflect lower estimates that do not capture the full scope of fish diversity recognized by current scientific understanding and research trends.