Based on: Tavares, A.I., Assis, J., Larkin, P.D., Creed, J.C., Magalhães, K., Horta, P., Engelen, A., Cardoso, N., Barbosa, C., Pontes, S. and Regalla, A., 2023. Long range gene flow beyond predictions from oceanographic transport in a tropical marine foundation species. Scientific Reports, 13(1), p.9112.
Habitat-forming marine species, like seagrasses, corals and macroalgae have wide geographical ranges and migrate between regions via passively dispersed propagules. But, the long-distance dispersal of these marine organisms is a relatively understudied phenomenon, despite its profound ecological implications. Ocean currents are often hypothesised as being the main propagule transporters between different areas. However, there might be other contenders…
To test this hypothesis, researchers, some of which belong to RESTORESEAS, examined whether simulated oceanographic transport could predict the genetic differentiation observed in the extensive biogeographic range of the seagrass species, Halodule wrightii.
Halodule wrightii is distributed on both east and west tropical Atlantic coastal shores. Its vast geographical range makes it the perfect candidate to study oceanic dispersal. Seagrasses are crucial components of coastal ecosystems, and understanding their dispersal dynamics is of paramount importance to understanding their future in a changing planet.
Researchers collected genetic data from Halodule wrightii populations distributed along the east and west Atlantic coastlines. Genetic diversity, often used as a proxy for historical ecology, offers insights into the connectivity between populations located in different sites. In other words, it gives us an idea of the extent of exchange of individuals between the populations.
To test the influence of ocean currents on the connectivity between these populations and their genetic diversity, biophysical models, that simulate how Halodule wrightii propagules might disperse passively across the ocean, were employed. If oceanic currents are indeed the main responsible for propagule transport, the model’s predictions should have aligned with the genetic findings. But that’s not what happened. Caribbean-Brazil and Atlantic Africa populations, despite being separated by an ocean, were genetically more similar between each other, than either of them was with the Gulf of Mexico population. This suggests that some level of passive dispersal must have occurred between the two, contradicting the biophysical model.
The biophysical model predicted limited or no connectivity between distant populations through ocean currents. This discrepancy suggests that alternative transport mechanisms might be at play, like for example biotic vectors. Grazers, such as sea turtles and manatees might be instrumental in transporting Halodule wrightii across long distances, and therefore be key players in shaping the fate of this seagrass species. For example, effective seagrass endozoochory, which refers to seed dispersal by animals after passing through their digestive tracts, has been documented in fish, sea turtles, and birds. The substantial dispersal distances covered by these animals, known to frequent seagrass meadows, might enable long-distance dispersal of seeds. However, to substantiate this hypothesis, more research is required.
This study serves as a testament to the intricate web of interactions that govern the dispersal of marine organisms.
Written by Maria Pinto