Jellyfish are well known for their ability to multiply en masse and influence coastal ecosystems. But what is less known is that they have very diverse and sometimes surprising life cycles. In many species, the life cycle alternates between two forms:
- A
polyp, a
benthic form that remains attached to the seafloor.
- A
jellyfish, which swims freely in the water.
This is the case, for example, with
Aurelia or
Rhizostoma jellyfish, common along European coasts. But some species have evolved differently: they have completely abandoned the benthic phase to live entirely in the water column. This lifestyle, known as
holoplanktonic, is found in many species, such as jellyfish of the genus
Pelagia or among siphonophores.
Illustration image from Pixabay
The ecological and evolutionary factors behind this diversity of life cycles had never been studied before. This gap has just been filled in a paper published in the journal
Proceedings of the National Academy of Sciences (PNAS).
Leveraging data from the Tara Oceans expedition
For this study, scientists used data from the Tara Oceans expedition (2009–2013), which collected samples from all regions of the globe.
By combining genetic data, environmental measurements, and evolutionary analyses, researchers discovered that the holoplanktonic lifestyle emerged at least eight times in different jellyfish groups, independently, and in some cases, over 100 million years ago. These evolutionary changes took several forms:
- Some species completely lost the polyp form, which is attached to the seafloor.
- Others saw their polyp transform into a floating, drifting form or one that parasitizes other planktonic organisms.
Each time, this change was linked to a shift from coastal to open waters, confirming a hypothesis proposed by Ernst Haeckel in the 19
th century.
The evolutionary success of holoplanktonic species
The study showed that holoplanktonic species are more abundant and widely distributed than those retaining a fixed phase. They are particularly more common in tropical and subtropical oceans in deep, clear waters. Notably, it is often these fully planktonic species that dominate, even near coasts.
To better understand their ecological roles, scientists examined the plankton interactome using databases that map potential interactions between planktonic organisms. They observed that jellyfish with a fixed phase occupy central positions in this network, meaning they interact more specifically with other species. In contrast, holoplanktonic jellyfish appear more flexible: they adapt more easily and participate in a wider variety of interactions.
(top) World map showing the relative abundance proportion of holoplanktonic jellyfish (blue) and those alternating between a benthic and pelagic stage (yellow) in each surface sample from the Tara Oceans expedition. Holoplanktonic jellyfish overwhelmingly dominate these samples, except for a few coastal stations and in the Arctic.
(bottom) Schematic representation of jellyfish life cycles including (yellow) or excluding (blue) a benthic phase, the polyp. A key study to predict future changes
In summary, this work shows that the jellyfish life cycle has a significant impact on their presence in the ocean and their role in the ecosystem. By abandoning their benthic phase, some species have conquered vast areas, becoming dominant players in the marine ecosystem.
This study highlights the need to pay greater attention to holoplanktonic species to assess the impact, abundance, and future evolution of jellyfish in the ocean. In the era of climate change and human pressure on the oceans, it is important to understand how jellyfish adapt to predict which species will thrive in the future and which may struggle.