How Male Cichlids Trick Females into Fertilizing Eggs

The article How Male Cichlids Trick Females into Fertilizing Their Eggs appeared first on AZ Animals.

Nature always finds a way, but those ways aren’t necessarily pretty. In fact, in regions where competition for resources is high, deception is a surefire way to ensure that your genetics will be passed on to the next generation. Take it from male haplochromine cichlids, a large tribe of cichlid fish best known from the African Great Lakes and other waters in parts of Africa and the Middle East. In many modern haplochromine species, males develop yellow, orange, or reddish “egg spots” on their anal fins that resemble eggs. To make things weirder, females often engage in mouth incubation (holding eggs in their mouth).

In a typical spawning sequence, the female lays eggs and quickly takes them into her mouth. Males then display the egg spots on their anal fins, and females may muzzle or plug these marks when the male simultaneously releases sperm, thereby fertilizing the eggs in his mouth. It’s a sneaky deal, of course. However, research suggests that egg spots may play a more complicated role than simple mate attraction, with evidence pointing to both spawning behavior and male-male signaling. Let’s learn more about this strange, slightly disturbing and evolutionarily advantageous method that male haplochromine cichlids use to pass on their genes.

Meet the haplochromine cichlids

Haplochromine cichlids are a tribe of cichlid fish that includes hundreds of species, with some estimates placing egg-spot-bearing haplochromines at around 1,500 species. This tribe lives in eastern, southern and northern Africa, as well as the Middle East. However, they dominate the African Great Lakes. Their presence in large bodies of water like Lake Malawi and Lake Victoria has led to one of the most remarkable examples of adaptive radiation on Earth. African Great Lakes cichlids exhibit an extraordinary range of behaviors, including territoriality, specialized feeding strategies, complex courtship displays, and mouth brooding. The latter involves incubating the eggs or young in a parent’s mouth for long periods of time. Several types of creatures engage in this behavior, including cichlids and Darwin’s frogs, in which the male broods his young in his mouth or vocal sac.

Smoke and mirrors

Oral incubation has given rise to one of the most deceptive approaches to reproduction in the animal kingdom. Male haplochromine cichlids have spots on their anal fins that resemble female eggs. Since females take the eggs into their mouths during spawning, the egg-shaped markings may stimulate females to mouth or nuzzle the male’s anal fin. In doing so, males surreptitiously fertilize eggs in the mouths of females who come close enough, thus ensuring the transmission of their genes. This may seem unusual, but egg spots are widespread among modern haplochromine cichlids and have long been linked to their courtship and spawning behavior. Research on female preferences is mixed. Previous studies on some species suggested that females preferred males with egg spots, while a 2012 study Astatotilapia burtoni found no preference for males with more egg spots and even a tendency for females to prefer males without them.

In Astatotilapia burtonilater work found no clear female preference for males with more egg spots and reported a tendency for females to prefer males without them. However, the latest research in this area is changing this picture. This shows how integral, yet plastic, this breeding practice is for mouth-brooding cichlid species. Scientists recently discovered the cellular composition of these egg spots on anal fins. They also discovered that the gene responsible for producing these egg spots appears to be suppressed when males are kept in social isolation.

Colors

A 2024 study in Evolution and development examined how egg spots form at the cellular level and how their development may evolve depending on the social environment. The study found that the formation of egg spots in Astatotilapia callipterae involves iridophores and xanthophores, with iridophores playing a key role in initiating pigment cell aggregations.

Previous genetic research identified fhl2b as a strong candidate gene involved in haplochromine egg spot formation. These markings form through the coordinated development of pigment cells, including iridophores and xanthophores, and previous work has linked the gene fhl2b to the morphogenesis of egg spots. Given their realistic appearance, it’s no wonder these egg spots entice female cichlids to approach.

It takes a village

This same study found that the social environment influenced the timing of egg spot formation, with iridophores appearing to be the pigment cell type most sensitive to this change. In the study, individuals isolated earlier began to form aggregations of egg spots earlier. Rather than showing that isolated males lost their egg spots, the research suggests that the development of these spots is developmentally flexible and can change depending on social conditions.

In some ways, this helps reconcile the conflicting research on the importance of egg spots in reproduction. Males use their egg spots to attract females close enough for fertilization. However, females did not necessarily breed more with males spotted with eggs than with those without them. This suggests that egg spots are more akin to peacocking than a foolproof breeding approach. In fact, they can also serve as visual signals in male-to-male communication.

Much like the size of a red deer’s antlers or the complexity of a paper wasp’s facial spots, egg spots are a biological gauge of a male cichlid’s social status and reproductive health.

The article How Male Cichlids Trick Females into Fertilizing Their Eggs appeared first on AZ Animals.