Lythrypnus dalli, commonly known as the blue-banded goby or Catalina goby, is a species of goby. It is native to the eastern Pacific where it is found from Monterey Bay, California to northern Peru, including the Gulf of California. It can be found in coastal waters at depths of from 0 to 76 m with rocky substrates in which there are crevices for concealment. It is also known to hide amongst the spines of sea urchins. It is a bidirectional hermaphrodite and capable of rapidly switching sexes. Having a size advantage and certain behavioral interactions can indicate which individual changes sex. This species can reach a length of 6.4 cm TL. It can also be found in the aquarium trade. The specific name honours the malacologist William Healey Dall (1845-1927), who when trawling for specimens off Catalina Harbour, California, caught one of the type specimens. The blue-banded goby (Lythrypnus dalli) is a small fish found in shallow coastal waters of Southern California, commonly inhabiting eelgrass beds and rocky reefs. Known for its vibrant blue stripes, it exhibits both simultaneous and sequential hermaphroditism, with larger females more likely to transition to male roles. The Blue Band Goby, Lythrypnus dalli, exhibits protandrous and protogynous sex changes, allowing individuals to adapt their sex based on social status within their populations. These fish can switch between male and female reproductive functions depending on reproductive success, allowing for flexible adaptation to environmental conditions.This occurs when a male is eliminated from his territory by predation or when numerous females come together on a territory that is not occupied by a male. Research has shown that males have higher androgen receptor (AR) levels, which are linked to male-specific courtship behaviors and sexual selection, enhancing their display vigor.