When you're not as big as the rest of the guys, sometimes you have to come up with different strategies to thrive. That seems to be what's happened with a rare fish found in Cuatro Ciénegas, Mexico. While big males of the species horde the females, the smaller guys have figured out a sneaky way to make sure their DNA gets passed on.

The fish is known as a Minckley's cichlid, marking it as a member of the cichlid group, which are tropical fish including angelfish, oscars and and tilapia. It is endemic to the spring-fed pools in Cuatro Ciénegas in northern Mexico. Most members of the cichlid group are monagamous, but not the Minckley. Instead, for this species, one large male lords over several females, meaning that for smaller fish, there is a lack of potential mates.

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To overcome the shortage, the mini males hide out nearby when the dominant fish is breeding with a female. During this process, the female lays her eggs on a rock and then the male comes by and fertilizes the eggs with an invisible cloud of sperm. Instead of waiting for the large male to shoot his sperm, however, Ron Oldfield, a senior biology instructor at Case Western Reserve University (CWRU), figured out that the smaller male darts in and beats him to it.

He even caught the act on video as it took place in the 300-gallon aquarium in his office, in what is believed to be one of the few videos showing sneaking behavior – which is exhibited by just a few other species in the wild.

"A large, macho male – the largest fish in the tank – was mating with the largest female in the tank," Oldfield said. "Another, smaller male was hovering obliquely in the water, hiding next a large piece of wood near the water surface, and was pointed straight at the mating pair. As the female finished a pass, in a sudden burst, the small male dove to the rock, tilted his underside toward the eggs, and then leisurely swam away and went back to his hiding place."

Oldfield said that he noticed that the sneaking fish had an enlarged genital papilla, the external pouch that carries the sperm, which further lends evidence to the fact that the fish was there to deposit his DNA onto the eggs. He also observed the sneaking fish hanging around on the periphery of the mating pair, fighting off other males who might have had the same designs, and making several dashes to fertilize the eggs.

The enlarged genital papilla (Credit: Ronald G. Oldfield)

The fish from Oldfield's office will soon be relocated to Odysea, a new public aquarium in Scottsdale, Arizona.

The find was originally published in the journal Hydrobiologia, and the video below along with two others and several photos were recently put out last month by CWRU on its "DigitalCase" archive, an initiative to make free information on research available to the public.

Source: Case Western Reserve University

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