Chilean Mussel Could Reveal Clues About the Evolution of Animal Reproductive Systems

What can a small mussel found along the Chilean coast reveal about the origin of the sexes and the evolution of animal reproduction? This is one of the questions that the Fondecyt project «The role of mitochondrial DNA in sex determination of Mytilidae: a unique case in Animalia that reveals the evolution of trioecy and hermaphroditism» seeks to answer. The project is led by Pablo Oyarzún, Director of the Marine Biology Program at Universidad Andrés Bello (UNAB), together with Dr. Juan Antonio Valdés, also at UNAB, and Dr. Sophie Breton of Université de Montréal. By studying the Chilean mussel, the researchers seek to understand how reproductive strategies emerge in the ocean and the role that DNA plays in these processes.

The focus of the study is the mussel Semimytilus patagonicus, also known as the «chorito negro,» a species that has inhabited the Chilean coast from Arica to Puerto Montt for millions of years. The species is highly abundant in spring and summer but virtually disappears from the shoreline during winter. Today, it is considered one of the most extraordinary species in the animal kingdom. It is the only known species in which hermaphroditism has evolved within a group of mollusks whose other members all have separate sexes.

«In this project, we are studying the reproduction of mollusks that are highly important to Chile’s coastal ecosystems. One of them, the mussel Semimytilus patagonicus, possesses an extraordinary characteristic: its populations include males, females, and hermaphrodites, meaning individuals that possess both male and female reproductive functions,» Oyarzún explained.

This phenomenon, known as trioecy, is extremely rare among animals and has been documented in only seven species worldwide.

According to the researcher, this mussel also possesses a mechanism for mitochondrial DNA inheritance that challenges one of biology’s most firmly established principles. While this genetic material is inherited exclusively through the maternal line in nearly all animals, this species is an exception. As a result, it now represents a unique model for studying the evolution of sex-determination mechanisms.

A Unique Species for Understanding Evolution

Scientific interest in Semimytilus patagonicus is not new. In a study led by Oyarzún and published in the journal Frontiers in Marine Science, the team confirmed for the first time the existence of trioecy in this marine mussel, making it the first documented case among mollusks.

The study analyzed more than 4,100 specimens collected along approximately 2,500 kilometers of the Chilean coast and found that the proportions of males, females, and hermaphrodites remain stable across different populations. This finding established the species as an exceptional model for investigating how reproductive systems evolve in animals.

«We have discovered that reproduction in marine animals is far more complex than we had imagined and is strongly influenced by the environment. Animals have existed on Earth far longer than humans and have therefore developed different reproductive strategies that have allowed them to survive for millions of years. These are the strategies that we study and seek to understand.»

The results obtained to date suggest that hermaphroditism and self-fertilization, the ability to produce offspring without a mate, may constitute evolutionary strategies rather than anomalies.

«Hermaphroditism is a reproductive strategy widely used by animals, and it has enabled them to survive under adverse conditions. Even self-fertilization, which has traditionally been regarded as detrimental to populations, can in some cases be the reproductive strategy necessary for survival. That strategy is retained in their DNA and passed on to future generations,» he said.

In a species whose populations simultaneously include males, females, and hermaphrodites, the likelihood of finding a reproductive partner increases. «Hermaphrodites can perform both male and female reproductive roles, increasing their mating opportunities,» Oyarzún explained.

«Therefore, in the face of events that cause a severe population decline, a species with this type of reproductive system may have a greater capacity to recover,» he added.

Next Steps

The team is currently conducting fieldwork and controlled experiments to study fertilization and early development in marine invertebrates. The next steps will focus on examining the genetic and molecular mechanisms that control sex determination and understanding how these processes respond to environmental changes.

According to Oyarzún, this knowledge could also provide tools for addressing future challenges related to marine conservation and climate change:

«Understanding how marine organisms reproduce is essential to protecting biodiversity and addressing the environmental changes affecting the oceans.

«This knowledge could have future applications in aquaculture and in the conservation and sustainable management of marine resources. In addition, understanding the mechanisms that regulate reproduction could allow us to anticipate how certain species may respond to climate change or marine pollution,» he concluded.