Now even larger than the existing dominant male it had previously mated with when female, the new male defeats the aged alpha in a violent battle for dominance. In the clownfish Clark's anemonefish Amphiprion clarkii for example, females are larger than males the opposite of the situation for the kobudai. Being able to switch sex maximises the chances of passing on genes if environmental or social circumstances should change. Though less common than protogyny, male-to-female sex changes are found in a wide variety of fish, including the Australian barramundi Lates calcarifer , gilthead seabream Sparus aurata and the black porgy Acanthopagrus schlegeli. Tony Wu The footage is remarkable — but the transformation is actually not terribly unusual. This includes the kobudai, other wrasses, many species of parrotfish, and a wide variety of reef fish. In freshwater fish, it is well established that chemical pollutants — such as the pesticide atrazine, fertilizer runoff from livestock operations, ethinyl estradiol the active ingredient in the birth control pill — have all skewed the sex populations of wild populations of fish. In most protogynous fish, some fish will start out lives as male, some will switch from female to male at some point, and some will remain as females for the full duration of their lives. A study published in the journal PLoS ONE , surveying 59 species, found that an increase in water temperature of just one to two degrees Celsius can alter the sex ratio from 1: What happens when you combine them? Amphibians, reptiles and fish employ a variety of methods to determine the sex of the offspring. Thus effect of man-made chemicals on fish sex been little studied. BBC The habit of switching sex appears to have developed during fish evolution multiple times in separate families, and though in all of them a suite of genetic and hormonal triggers come into play, the enzyme aromatase appears to be the key. About two per cent of fish species display some kind of hermaphroditism: If any two fish who happen to find each other can form a male-female mating pair — no matter what sexes they are when they meet — this would be advantageous. They live in small groups within protective sea anemones, with one breeding male and female pair and a number of subordinate non-breeding fish.