Front-flip: parasitic fish embryos must learn it when they are two days old

04. 03. 2024

All animals undergo embryogenesis, the development of the embryo. Until recently, it was thought that the main goal of early embryogenesis was mainly to initiate developmental processes correctly. But an international team of scientists has found that in parasitic bitterling fish, the embryos already develop a set of important adaptations during this early stage that give them major survival advantages.

Bitterling fish are abundant in East Asia, especially in China and Japan, where they parasitize bivalves. Bitterling fish eggs and embryos spend their first weeks of life in the gill apparatus of their hosts. Hatching from the eggs occurs very quickly - only one or two days after spawning.

Crossing and anchors

"And it is during these first hours of development that the embryo performs a unique front-flip that has never been described in any other vertebrate species. "During this movement, the embryo flips around the yolk sac head first and breaks through the egg envelope on the other side, allowing it to be safely trapped in the gill apparatus of the bivalve. In addition, after hatching, anchors will grow on the yolk sac, which will further enhance embryo capture."

This ensures that the embryo, which is shaped like an arrow thanks to the yolk sac, is correctly positioned to resist the water flow that flushes the bivalve through its gills. "This is a remarkable example of how natural selection shapes very early vertebrate development," says Martin Reichard.

Mussels resist parasitism. If the embryo did not make a front-flip aroonund the yolk, it would hatch on the wrong side of the egg, where it would not have time to attach itself, and the stream of water with which the mussel wash their gills would carry it away from its safe hiding place. And if the anchors had not formed, the embryo might not attach properly and as a result would be washed away from the bivalve gills again.

Unique fish explored by the EXPRO project

 "We've known for 20 years that the early development of these fish is unique. But it is only thanks to the EXPRO project that we have finally been able to tackle this question comprehensively, and this study is perhaps just a gentle start of what we have in the pipeline," adds Martin Reichard.

The five-year project, entitled The importance of coevolution for ecological speciation, answers the questions of why and how some parasites become specialists, while others remain generalists and can therefore parasitize a wide range of host species. "The whole aim of the EXPRO project is to find out how the evolution of life strategies and adaptations to parasitic reproduction affect diversification into new species and the emergence of whole sets of adaptations to new environments," the scientist adds.

An international team of scientists has been able to describe this unique process in detail from different perspectives. "Our research shows and details the adaptations of early embryos - from morphology, histology, time-lapse photodocumentation, microtomography and gene expression," says Wenjing Yi, a postdoctoral researcher at the Institute of Vertebrate Biology of the CAS and lead author of the study.

This groundbreaking study published in the prestigious journal PNAS not only expands our understanding of embryogenesis, but also highlights the dynamic interactions between hosts and their parasitic counterparts.



Martin Reichard
Institute of Vertebrate Biology of the CAS

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