Rare trophic eggs behind the success of the snake


image: Male snakehead guarding his fry.
to see Continued

Credit: Valter Weijola

Fish rarely feed their young. An exception are some snakehead species whose offspring feed on trophic eggs provided by their parent. A new study shows that this exceptional adaptation developed in snakehead fish around 12 million years ago. This behavior may have allowed the fish to conquer new environments and led to the high species diversity seen in a particular group of snakeheads.

Although 20-25% of all fish care for their offspring, this parental care is usually limited to protecting or watching over the eggs and fry. Only in rare cases do parents provide food directly to their offspring. A handful of fish species can produce nutrient-rich skin mucus that is grazed by fry as a dietary supplement during their first few weeks. So far, the kampango catfish living in Lake Malawi appears to be the only species among the approximately 30,000 species of bony fishes known to science that produce unfertilized trophic eggs to feed their offspring.

A new study conducted at the University of Turku, Finland, has found that in addition to kampango catfish, some snakehead species belonging to the family Channidae also feed their offspring with trophic eggs. The family includes 46 species that live in Africa and Asia. All species of the family are predators and some of them can measure up to 120 cm.

“The use of trophic eggs as a form of parental nutrient supply is more common in insects, spiders, and frogs than in fish. In frogs, for example, the use of trophic eggs has evolved independently many times, usually in environments where there is a lack of proper nutrition for the offspring,” explains the postdoctoral researcher and study author. Valter Weijola from the Zoological Museum of the University of Turku, Finland.

Trophic eggs could have created new ecological opportunities

Weijola wanted to study when the use of trophic eggs evolved during the evolution of snakeheads and how widespread it is among extant species.

The study used aquarium experiments to determine in which snakehead species the trophic egg supply exists. A time-calibrated recreation of the snakehead family tree already existed, so it was just a matter of mapping behavior on that tree. The results showed that the behavior evolved in a specific lineage of snakeheads around 12 million years ago, and that the adaptation is most likely present in all of the more than 20 currently living species.

“Although the use of trophic eggs is a relatively recent feature in the history of snakeheads, the group of species where this behavior exists is by far the most diverse of the seven different groups of snakeheads living today. Although it is difficult to prove causation, it may well be that the ability to feed the fry with trophic eggs has encouraged this great species diversification,” says Weijola.

Being able to feed their young could, for example, have allowed these fish to spread into such environments that otherwise lacked suitable food for fry, allowing them to expand their distribution into new areas.

“However, since we still know so little about how these fish live in their natural habitats, this is only an educated guess and we need more research on the subject,” says Weijola.

Why, then, is the food supply so scarce in fish when it seems to be so beneficial under certain circumstances?

According to Weijola, a limiting factor could be that the food supply in fish seems to have evolved only in species where both parents are involved in the care of the offspring. This is relatively rare in fish in general, but common, if not universal, in snakeheads.

“Production of trophic eggs requires a lot of energy, and if the male did not assist the female in guarding the fry, she would probably not be able to feed and cover this extra energy expenditure for an extended period of time. Another limiting factor could be that the offspring need openings large enough to swallow the trophic eggs whole – which are as big as the heads of the newly hatched fry.With this in mind, snakeheads clearly possessed the right combination of qualities. for such behavior to evolve,” Weijola envisions.

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