The bacterial symbiosis of deep-sea shrimp is the result of gradual evolution
An international study reveals how shrimp living near deep-sea hydrothermal vents have repeatedly and independently developed symbiotic associations with chemosynthetic bacteria. This research reveals an evolutionary gradient ranging from complete absence of symbiosis to complete dependence, challenging our understanding of evolution in extreme environments.
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n the depths of the ocean, where sunlight never penetrates, a fascinating ecosystem thrives around hydrothermal vents. These underwater chimneys spew fluids rich in chemical compounds that serve as a source of energy for unique microbial communities. In the heart of this hostile environment, shrimp from the Alvinocarididae family have found a remarkable solution for survival by establishing symbiotic partnerships with bacteria capable of exploiting this chemical energy through the process of chemosynthesis.
An international team of scientists has undertaken an unprecedented global study of 22 species of shrimp belonging to this family, from 32 hydrothermal sites spread across the world's oceans. Samples were collected between 2004 and 2023 during 31 oceanographic expeditions. The expertise of researchers from the FOCUS research unit at ULiège played a crucial role in this discovery, through the analysis of stable isotope ratios, which made it possible to accurately characterise the feeding strategies of the different shrimp species. "The stable isotope ratios of carbon, nitrogen and sulphur are essential for understanding trophic relationships in these ecosystems," explains Loïc Michel, a marine biologist at ULiège. "By comparing the isotopic compositions of the shrimp's muscle tissue and mouthparts, we were able to determine the extent to which each species depends on its bacterial symbionts for food."
The results revealed dramatic differences between species cohabiting in the same hydrothermal site. For example, at the Puy des Folles hydrothermal field on the Mid-Atlantic Ridge, at a depth of nearly 2,000 metres, the blind shrimp Rimicaris exoculata shows an isotopic composition characteristic of an almost exclusively symbiotic diet (the bacteria hosted in its head and cephalothorax). Conversely, its cousin Alvinocaris markensis has values indicating a traditional non-symbiotic diet. Between these two extremes, Rimicaris chacei has intermediate compositions, suggesting a mixed diet combining symbiosis and other food sources.
An evolutionary continuum rather than two distinct categories
"Thanks to observations under a scanning electron microscope, we were able to establish a rating system for bacterial colonisation (symbiosis) ranging from 0 – representing a total absence – to 24 – representing very dense colonisation on several parts of the head and mouthparts," explains Loïc Michel. The results reveal a continuous gradient rather than a clear division between symbiotic and non-symbiotic species. This discovery suggests that chemosynthetic symbiosis (which transforms chemical elements into nutrients) in these shrimp is a gradual and dynamic evolutionary process."
Beyond its purely scientific interest, this study has important implications for the conservation of hydrothermal ecosystems. The diversity of feeding strategies within the same family challenges the simplistic categorisations used in some conservation studies. This functional complexity reinforces the idea that hydrothermal vent ecosystems are particularly vulnerable, with little functional redundancy between species. This makes this discovery particularly critical in the context of talks on deep-sea commercial exploitation, on which the scientific community has called for a moratorium.
"This study overturns what we know about symbiosis," says Pierre Methou, a marine biologist at IFREMER. "It has enabled us to highlight that its evolution in deep-sea shrimp is a dynamic process influenced by complex factors, which develops gradually. The independent emergence of exclusive reliance on symbiosis in different shrimp species, alongside a range of different feeding modes, illustrates the immense amount we still have to discover about these species that inhabit hydrothermal vents and extreme deep-sea environments."
Understanding these intermediate cases of symbiosis is as essential as studying complete symbiosis in emblematic species. It provides a better understanding of the evolutionary trajectories that lead to the establishment of symbiotic relationships, a dynamic process that is still ongoing in certain lineages.
This study was led by IFREMER (French Research Institute for Exploitation of the Sea) in partnership with the University of Liège, JAMSTEC (Japan Agency for Marine-Earth Science and Technology), Sorbonne University, Temple University (USA), the University of Southampton (UK) and the University of Rhode Island (USA).
Scientific reference
Methou P., Mathieu-Resuge M., Michel L.N., Cueff-Gauchard V., Kayama Watanabe H., Cowell E.J., Copley J.T., Beinart R., Zbinden M., Pradillon F., Cambon M.-A., Chen C., Evolutionary convergence and trophic diversity in hot vent and cold seep shrimps showcase a continuum of symbiosis, Proceedings of the Royal Society B: Biological Sciences, 2026.
