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 metres (0 to 249 ft) with rocky substrates in which there are crevices for concealment. It is also known to hide amongst the spines of sea urchins. This species can reach a length of 6.4 centimetres (2.5 in) 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 species exhibits both simultaneous and sequential hermaphroditism, with larger females more likely to transition to male roles. This allows 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. Research has discovered that there is an increase in length of the female’s dorsal fin when they become dominant. However, it is unknown whether the fin elongates due to the sex-change process or by dominance status. Research has shown that the neuropeptide arginine vasotocin (AVT) plays a crucial role in the sex change of L. dalli, with AVT-immunoreactive forebrain cells increasing to male-typical sizes during transition. Having a size advantage and certain behavioral interactions can indicate which individual changes sex.

In a peer-reviewed study performed by Kuk-Jeong Chin, Matthew S. Grober, and Stacy Pirro, DNA was extracted from a single goby, and 17.9 G bases were obtained through experimentation of the goby tissues. This is significant, as gobies demonstrate sex changes, and this data highlights the biological reasoning behind it. <ref> Chin, K.-J., Grober, M. S., & Pirro, S. (2022, February 27). The complete genome sequence of Lythrypnus Dalli, the bluebanded goby: Published in biodiversity genomes. Biodiversity Genomes. https://biodiversitygenomes.scholasticahq.com/article/33575-the-complete-genome-sequence-of-lythrypnus-dalli-the-bluebanded-goby

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