Image credit: Yellowfin Goby, Acanthogobius flavimanus, in Jawbone Marine Sanctuary, Williamstown, Port Phillip, Victoria, February 2018. Source: Wayne Martin / iNaturalist.org. License: CC By Attribution-NonCommercial.
Edit (6/23/21): The book excerpt uses “gender” when it talks about morphology (body plan) in the Barlow study. Thanks to reader Breanna H. for asking about a previous draft of this post that repeated the book’s wording. I learned recently how much K-6 material leans on gender norms to explain biological things. Models are always imperfect, and Breanna refers to a teaching approach that avoids applying the human concept of gender to animals, and a discussion about the risks in conflating sex and gender in animal model studies. (-RXS)
Edit (6/8/23): Thanks to Wayne, adding this: Editor’s note: The term "hermaphrodite" is appropriate for referring to non-human animals with sex characteristics that do not fit typical binary notions of male or female bodies. For humans, “intersex” is the appropriate term—learn more here!(-RXS)
A species of goby from Lizard Island on Australia’s Great Barrier Reef [is the Australian goby].(EN12) In the Australian goby, all the juveniles mature into females, with some later becoming males. The males, however, can change back into females. In fact, the meaning of male is ambiguous here. The investigators defined a male to be any fish with at least some sperm production. All males, however, contain early-stage oocytes—cells that develop into eggs—in their gonads. So all the males remain part female. The species therefore consists of two genders at any one time: all-female fish and part-male-part-female fish.
(EN12 G. Barlow (2000) The Cichlid Fishes: Nature’s Grand Experiment in Evolution. Perseus.)
Citation: Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. pp. 137-38.