Jellyfish are well known for their ability to multiply rapidly and impact 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 stage, which is benthic and remains attached to the sea floor.
- A medusa stage, which swims freely in the water.
This is the case for jellyfish like Aurelia or Rhizostoma, commonly found along European coasts. However, some species have evolved differently: they have completely abandoned the benthic phase to live entirely in the water column. This lifestyle, called holoplanktonic, is found in many species such as jellyfish of the genus Pelagia or siphonophores
Ecological and evolutionary factors behind this diversity of life cycles had never been studied before. This gap has just been filled by an article published in the journal Proceedings of the National Academy of Sciences (PNAS).
Leveraging data from the Tara Oceans expedition
For this study, scientists utilized 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 appeared independently at least eight times in different jellyfish groups, in some cases over 100 million years ago. These evolutionary shifts took several forms:
- Some species completely lost the polyp form, which is attached to the sea floor.
- Others saw their polyp transform into a floating form, drifting or parasitizing other planktonic organisms.
Each change was linked to a shift from coastal to open ocean habitats, confirming a hypothesis proposed by Ernst Haeckel in the 19th century.
The evolutionary success of holoplanktonic species
The study showed that holoplanktonic species are more abundant and more widely distributed than those that retain a benthic phase. They are particularly common in tropical and subtropical oceans, in deep and clear waters. Remarkably, these fully planktonic species often dominate even near the coasts.
To better understand their ecological roles, scientists examined the plankton interactome using databases that map potential interactions among planktonic organisms. They observed that jellyfish with a benthic phase occupy central positions in this network, meaning they interact more specifically with other species. Conversely, 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 benthic and pelagic stages (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 polyp phase.
A key study for predicting future changes
In summary, this research shows that jellyfish life cycles significantly affect their distribution in the ocean and their ecological roles. 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 better assess the impact, abundance, and future evolution of jellyfish in the ocean. In the era of climate change and human pressures on marine environments, understanding how jellyfish adapt is crucial to predict which species will thrive and which may face challenges in the future.
Source: techno-science
