Simon actually did some serious thinking the other day after my blog on the rarity of sexually selected characters in marine systems. He came up with a couple more examples of sexual selection, one of which was the difference between the sexes in mandarin fish. Mandarin fish males are much larger than females and have very elongated fin rays. Males behave differently from females, arriving at the mating grounds well before females do, securing and defending small territories in the broken Acropora rubble mandarin fish mate above.
Males compete fiercely over the ownership to especially attractive sites. Conflicts are most often resolved by a side by side measuring of each others strengths, where obviously weaker fish fold before fighting, as fighting would not do them any good anyway. However, at times the male pair will be more or less evenly matched and a vicious fight can be initiated, where males push, flick and bite each other.
The more equal the males are in strength, the longer this fight will last, and it can in certain circumstances last even longer than the whole mating session! This kind of mating behaviour, where males gather at certain spots for the sole purpose of mating is called lek, and is best known and studied in a number of birds, such as grouse and peacock.
When the mandarin fish females arrive to the lek, they check out the available group of suitors. Females prefer large males, so the largest males get the largest number of matings, with small males getting few or no matings at all. When a female has compared available males, the female approaches the chosen one, which then initiates a small “dance” around the female. If the female still wishes to proceed with the mating, the pair rises slowly to about half a meter above the rubble patch. The female can, and will often, break of the event at any stage of the rise. Sometimes the pair rises several times before the actual mating, while on other occasions the female regrets her choice, leaves the male and start searching for another male. If the female chooses to proceed, at the peak of the rise the pair will release a small squirt of eggs and sperm.
Preferred males can in lek systems attain a highly disproportionate number of matings. Successful males can mate with several females during one night, and be back in full force the next night. Despite successful males performing with several mates during a night and females generally only mating with one male at each mating event, males can mate every night while females have to refresh their energy reserves between matings. This is obviously because eggs are much more energy rich and costly to produce than the miniscule amounts of sperm needed to fertilize a batch of eggs from a female. Such a mating system, with males potentially fathering high amounts of offspring or very few to none at all, will be very conducive to high levels of sexual selection.
Mandarin fish are one of the most popular fish in the tropical fish tank hobby. Fish collectors target big males, as big males achieve higher prices. Sadly, mandarinfish have very specific dietary requirements, eating only small live crustaceans such as amphipods and copepods. A large proportion of the wild caught mandarinfish never adapt to aquarium life, and die after quite a short time. This fuels the demand for even more mandarinfish being caught, leading to either a decrease in male sizes at mating sites, or, in extreme cases, a serious deficit of males. It is without doubt a testament to our ability to penetrate into the most hidden corners of nature that certain populations of mandarin fish, where one male is enough to fertilize the eggs from quite a number of females, now have low reproductive success due to a lack of males! Mandarin fish do breed in captivity, a feature that with time might solve the problem, but for now better control over wild caught fish numbers and sex distribution would be needed.