Written by: Catalina R-P Edited by: Erika Gress
Picture the detective Sherlock Holmes solving a tough case while captaining Odysseus' ship in his seemingly never-ending journey. The coral taxonomic puzzle does not look distant from that quest. With a history stretching over more than two hundred years, delineating coral species boundaries has been an epic voyage. But why should we endeavor that quest?
Why does it matter?
Beyond names in biodiversity counts, numbers in conservation records, or service providers for the ecosystems, species are groups of organisms that share particular properties and function as units. As such, species histories are singularly impacted by different factors, which will determine their rise or demise over time. In turn, these trajectories will influence the status of biodiversity as species are its main currency. These alterations in biodiversity shape our world, and that is why we need to first understand how diverse our planet is and how our actions impact key organisms, such as corals.
It is a fact that distinct coral species respond differently to stress. For instance, differences in temperature tolerance —even between closely related species— can compromise their survival in increasingly warming ocean conditions such as the ones we are currently experiencing. This year's sea surface temperature has surpassed the average value reported since the 1980s, and last March was the warmest in the 175-year record of NOAA (US - National Oceanic and Atmospheric Administration). It is no coincidence that coral reefs around the world are experiencing the fourth mass bleaching event on record and the second in less than a decade.
Nailing down where one species ends and another begins may help us understand their specific differences in susceptibility to climate change and human activity impacts. Contrastingly, failing to define species boundaries could throw off our conservation strategies (either by downplaying the issue or leading us in the wrong direction). Thus, creating and testing methods for accurately distinguishing between coral species is crucial to enhance our capacity to protect and manage the ecosystems they are part of. In short, why should we care about telling coral species apart? Because we cannot effectively protect what we do not know.
The coral species puzzle: much more than the looks 🪸
Outlining species boundaries is as challenging as it is crucial for getting a genuine picture of biodiversity on Earth. For more than a hundred years, coral shapes and structures were traditionally used to delineate species boundaries. However, the complexity of morphological features and how these are shaped by the environment led to potentially erroneous coral species boundaries. Species delimitation has advanced thanks to multiple lines of evidence more recently. DNA molecular analyses combined with ecological factors, such as reproduction times and strategies, and geographic and depth ranges, are currently being used to solve the puzzle and help navigation in a sea of suspects (Figure 1).
Past technological hurdles impeding our grasp of coral species have largely been surmounted (although navigating coral taxonomy can still feel like an ongoing journey). We are transitioning into a phase where species boundaries are being reevaluated through the lens of type specimens (i.e., preserved representative samples designated as reference for a species or taxon), thereby improving initial descriptions with diverse datasets. Therefore, distinguishing between coral species relies on much more than what they look like, necessitating collaborative efforts from fieldwork, laboratories, museum collections, and data repositories to enhance and fine-tune coral taxonomy.
Figure 1 | More than two hundred years advancing coral taxonomy
Diagram of the “brief” history of coral taxonomy, highlighting the main approaches used at different stages. While rough time frames are provided for each stage, it is worth noting that some methods emerged gradually and span multiple periods. Additionally, many advancements occurred simultaneously leading to overlapping developments (Ramírez-Portilla, 2023).
Delineating Acropora species: a task worthy of Sherlock 🕵🏼
If you are familiar with the famous detective of 221B Baker Street, you will know that his most challenging cases involve his arch-nemesis, Moriarty. If Sherlock’s antagonist had been responsible for obscuring the species boundaries of some coral genera, Acropora would surely be among them. Acropora is the largest reef-building coral genus, with hundreds of species distributed globally on shallow tropical and subtropical reefs. However, species in this genus are also some of the most affected by climate change and human activities. Thus, the Acropora exemplifies an extraordinary case study reflecting the challenges of coral taxonomy.
The diversity in morphology among corals, especially the Acropora species, presents a challenge for traditional identification methods: many share similar shapes, live together, reproduce simultaneously, and resist classification through standard DNA-based techniques. However, recent studies demonstrate that previously unclear species boundaries can be resolved with the right tools and incorporating knowledge about species ecology, reproduction, biogeography, DNA, and morphology. These findings provide valuable insights for taxonomical detectives to navigate species distinctions odyssey effectively, laying a solid groundwork for conservation and management strategies for coral reefs.
Conclusion 🔑
We have not even figured out what a third of the species are in the ocean. Yet, breaking down what makes each species unique is crucial to figuring out how marine ecosystems deal with challenges like climate change. Therefore, if we want to manage and protect coral reefs from extinction, we need to step up our game in sorting out the organisms that integrate them. That is why it is necessary to use genera such as the Acropora to test and validate new methods to come up with ways to tell corals apart.
References
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Furukawa, M., Kitanobo, S., Ohki, S., Teramoto, M. M., Hanahara, N., and Morita, M. (2024). Integrative taxonomic analyses reveal that rapid genetic divergence drives Acropora speciation. Mol. Phylogenet. Evol. 195, 108063. doi:10.1016/j.ympev.2024.108063.
Ramírez-Portilla, C., Baird, A. H., Cowman, P. F., Quattrini, A. M., Harii, S., Sinniger, F., et al. (2022). Solving the coral species delimitation conundrum. Syst. Biol. 71, 461–475. doi:10.1093/sysbio/syab077.
Ramírez-Portilla, C. (2023). Treading on thin lines: delineating species boundaries in corals. Department of Biology of Organisms, Free University of Brussels (ULB) & Department of Animal Ecology and Systematics, Justus Liebig University Giessen (JLU). doi:10.22029/jlupub-17838.
Ramírez-Portilla, C., and Quattrini, A. M. (2023). Perspectives on the grey zone of species delimitation with notes on invertebrates in the marine environment. Bull. Soc. Syst. Biol. 2, 1–23. doi:10.18061/bssb.v2i2.9255.
Veron, J. E. N. (2013). Overview of the taxonomy of zooxanthellate Scleractinia. Zool. J. Linn. Soc. 169, 485–508. doi:10.1111/zoj.12076.
Wallace, C. C., and Willis, B. L. (1994). Systematics of the coral genus Acropora: implications of new biological findings for species concepts. Annu. Rev. Ecol. Syst. 25, 237–262. doi:10.1146/annurev.es.25.110194.001321.