Scientists have discovered that wrasse females can favor sperm from an individual male even when multiple males try to fertilize her eggs. The findings are in a paper that was just published in the journal Nature Communications.
The ocellated wrasse is a small, colorful tropical fish found throughout the Atlantic and the Mediterranean Sea. Like most fish, wrasses reproduce via external fertilization. The female lays eggs and the male releases sperm over them. This results in an unusual dynamic between males and females. Large, bright colored “nester” males build nests, court females, and then tend to the eggs. There is a second type of male called a “sneaker” male. Sneakers are small and resemble females. They dart in and release their sperm, fertilizing as many eggs as they can. These sneaker males don’t build nests or help with parental care. The female was previously thought to have little control over this process.
A team of researchers conducted experiments to determine whether or not the female’s ovarian fluid made a difference. Female wrasses release ovarian fluid during mating. This encourages fertilization but the exact mechanisms were unknown. The researchers experimented by adding sperm from both nester and sneaker males to a female’s eggs. They did this with and without the added ovarian fluid. The team found that the ovarian fluid enhanced the mobility and speed of sperm, while also improving chemoattraction to eggs.
Previous research has already established that sneaker males produce more sperm than nesters. Sneaker sperm is lower quality, however, and not as mobile as nesters’ sperm. When a female releases ovarian fluid, it further boosts the speed and mobility of the nester male’s sperm. This allows the nester to compete, even with the large amounts of sperm produced by sneakers. The researchers found that ovarian fluid production strongly favored nesters and helped them fertilize more eggs.
By releasing ovarian fluid, female wrasses even the playing field by favoring nester males. Nester males take care of the eggs and are generally larger and healthier. Sneaker males take advantage of external fertilization by rushing in to release large clouds of sperm. This new research shows that female wrasses have evolved a defense against this in the form of their ovarian fluid, which improves the mobility of nester sperm. Sneaker sperm is lower quality and can’t compete, resulting in few fertilized eggs. This is a unique example of females having control over fertilization even when the process is done externally.
Suzanne H. Alonzo, Kelly A. Stiver & Susan E. Marsh-Rollo. Ovarian fluid allows directional cryptic female choice despite external fertilization. Nature Communications (2016).