Anything fishy...as long as it swims or lives underwater.

Ichthyology is the scientific study of fish. This includes skeletal fish (Osteichthyes), cartilaginous fish (Chondrichthyes), & jawless fish (Agnatha). An estimated 25,000 species exist, comprising a majority of vertebrates. While a majority of species have probably been discovered & described, ±250 new species are officially described each year.

The practice of ichthyology is associated with marine biology & limnology.