CORALS MIGRATING POLEWARD? NOT SO FAST….

By Rebecca Gibbel, MS, DVM

BACKGROUND

As the ocean’s temperature increases under the effects of climate change, corals across the globe are in hot water. With a projected rise in global warming of 1.8–5.6 °C (35.24-42.08 °F) and atmospheric CO₂ levels reaching even higher levels (1100 ppm) by the end of this century [1], marine conditions are expected to be unsuitable for coral life in virtually all locations. Corals are especially sensitive to temperature and can only survive within a narrow thermal range. A large proportion of the world’s corals are already living at the top of what they can tolerate for survival. As corals become stressed by marine heat, they dissociate from the symbiotic algae that feed them and become bleached, which often leads to death by starvation. Increasing atmospheric CO₂ is absorbed by the ocean, which renders it more acidic and inhospitable to organisms like corals that grow skeletons and mollusks that grow shells. Coral diseases and infections are becoming prevalent, and the overall loss of living coral reefs due to myriad anthropogenic factors is increasing at an alarming rate worldwide.

Orbicella annularis coral bleaching in a marine heat wave

Numerous studies have documented both terrestrial and marine species moving past their home range in response to warming regional conditions. Species that are living on the edge of their thermotolerance may shift poleward, from the tropical zone to the subtropical. Poleward means away from the equator, or south in the southern hemisphere and north in the northern hemisphere. It is an appealing idea that corals might survive the heat by migrating, though new locations are unlikely to host the networks of inter-dependent species of the original range. And species that do manage to relocate (or invade, depending on your viewpoint) can also gravely upset the pre-existing environmental balance in their new location. 

HOW CAN SESSILE ANIMALS RELOCATE?

1. Range Expansion/Contraction:  

It’s easy to understand how fish or birds can migrate, but stationary animals like corals have a harder time. The individual corals don’t move, but a population of them can slowly alter its distribution area by surviving better on one side of a range than another. This can happen quickly for species with rapid reproduction and few predators, which is the case for many invasive species, but for stony corals, their gradual growth rate means that this type of organic movement toward better ocean conditions is expected to be extremely slow. 

Corals do have the capacity to switch the ratios of their symbiotic microalgae (Symbiodiniaceae, or zooxanthellae), and that can happen quickly, potentially with an exchange of the original type of microalgae for a more thermally tolerant one. There is hope that corals harboring Durusdinium (a species of microalgae known for its heat tolerance) and other heat tolerant microalgae might survive better in a warming ocean and be able to grow better, at least on the cooler side of a range.  

An Acropora cervicornis coral starting to lose symbiotic algae and bleach in elevated sea temperatures

2. Larval migration:  

Another way that corals can move is via their water borne larvae that can disperse long distances and survive up to four months in their pelagic state [2]. In this way, coral species can potentially move to new areas that are more hospitable.  This is similar to plant seeds that can travel to new locations by air currents or hitchhiking on passing birds and animals. But coral larvae are not passive drifters and they actively swim in the open ocean. Despite having no eyes, ears or noses, they respond to complex cues including light, sound, and water chemistry to help them select a new location to settle in and live.  In many cases the new settlement location may be very close to their parent colony, but they do have an impressive ability to potentially relocate. 

A Tubastrea coccinea coral larva setting out for new horizons

ARE CORALS ACTUALLY MIGRATING? YES? NO? SOMETIMES?

Yamano et al. (2011) is a frequently cited study that examined historical records of coral distribution in Japan over a period of 80 years, during which detailed sea surface temperature records were maintained. The authors concluded that Acropora hyacinthus expanded its range during this warming period, with a rapid poleward migration reaching 14 km/year. 

But when Fifer et al. (2022) used molecular tools to examine the corals in Yamano et al.’s study, they found that rather than being one type of coral that migrated, the corals were actually visually similar but genetically distinct lineages. Each of these subspecies occupy slightly different environmental niches, with local ranges that may have only had limited expansions and contractions over the research period.

There are other examples of corals appearing in new poleward locations that are harder to discount. In Sydney, Australia, at least four species of branching corals including Pocillopora aliciae have been observed in new locations 120 km to the south of their former range. These corals are accompanied by other migrants, such as trapezia crabs, tropical gobies, and multiple species of damselfish [5].

To evaluate whether corals really migrate or not, Fuchs et al. (2024) analyzed large Australian datasets of 662 reef species. The study period spanned a decade that included significant temperature extremes, so migration was expected. In fact, minimal net movement was found, with range retreat often balancing expansion. They concluded that “previous studies based on extreme species observations, rather than tracking all species through time, may have overestimated the prevalence, magnitude and longevity of range shifts amongst marine taxa.”

KEEPING UP?

Another important question to consider is whether corals can make changes quickly enough to match the increasing ocean heat and acidification.  It’s important to note that the rate of current environmental alteration is faster now than in the past tens of millions of years [7].  To keep up, corals either need to adapt or to move. The rate of latitudinal coral migration needed to offset the effects of climate change and avoid extinction has been estimated at 15 km/yr, which corals are unlikely to meet [8]. Tropical corals cannot just move down to cooler deeper water because if they move too deep, they have insufficient sunlight to fuel their algal photosynthesis.  The corals that inhabit deep waters, like the red and black corals, are generally without Symbiodiniaceae, and are very slow growing.  They feed exclusively by capturing food from the water column, which is a different life strategy from that of tropical corals.

Discouragingly, Matz et al. (2017) analyzed the natural rate of genetic mutations in Great Barrier Reef Acropora corals and determined that if adaptations to climate change are based on the rate of genetic mutations, they will not be sufficient for corals to survive under the new climate conditions. 

WHAT ABOUT THOSE TEMPERATE CORALS?

Astrangia poculata, with and without symbiotic Symbiodiniaceae

Corals have a particularly daunting number of barriers to moving outside their native region to which they are acclimated.  If a migrating coral species manages to travel far from home, its planktonic food may not be present in the new location. The grazing fish that keep algae overgrowth at bay will not necessarily have migrated with the coral either. Most importantly, the sunlight in higher latitudes is not intense enough to sustain the vital symbiotic Symbiodiniaceae that fuel the coral hosts, particularly in winter when both temperature and light are significantly diminished. 

But there are a few hardy cnidarians that manage to survive into high latitudes of temperate zones of the Mediterranean, the Red Sea and North America. Examples include the Northern Star coral, Astrangia poculata, which is a scleractinian coral with a range up to Massachusetts and the soft coral Alcyonium digitatum which lives as far north as Iceland.  These corals are not numerous enough to form reef structures but are astonishing in their resilience. The cold-water corals have evolved special survival strategies, presumably over eons, that distinguish them from their tropical cousins. Astrangia poculata is a versatile coral that lives in an incredible temperature range of -1℃ to 25℃ [10]. It has an optional relationship with symbiotic Symbiodiniaceae which is one of the keys to its success. These corals have a metabolically part time existence, saving energy in the winter by entering into a dormant state called quiescence. During this period, the polyps, or soft structures of the coral, pull inward and dissolve, jettisoning the Symbiodiniaceae.  In warmer temperatures, Astrangia poculata grow new polyps, which is analogous to a person growing hundreds of new hands every spring. In warm weather they host symbiotic algae again if there is enough daylight to make the symbiosis advantageous, though they generally derive most of their nutrition from the zooplankton that the polyps capture. 

THE BIG PICTURE

A poleward migration may be occurring for some marine and terrestrial species, but since corals are so sensitive to environmental changes, their prognosis for successful migration is very uncertain. Some fortunate corals have previously developed adaptations that will allow them to survive in challenging conditions, and others may be able to expand their ranges in a limited way. Overall, corals’ “Goldilocks zone” is shrinking and they are being squeezed out of the tropics by ocean warming, and out of cooler waters by ocean acidification and harsher conditions.  At the rate that climate change is progressing, it sadly seems likely that the heat will defeat them first, and in increasingly acidic seas eventually even the corals’ skeletons will be dissolved. 

Skeleton of a Northern Star Coral, Astrangia poculata

REFERENCES

1. O’Neill, B. C., Tebaldi, C., Van Vuuren, D. P., Eyring, V., Friedlingstein, P., Hurtt, G., Knutti, R., Kriegler, E., Lamarque, J.F., Lower, J., Meehl, G. A., Moss, R., Riahi, K.,  Sanderson, B. M. (2016). The scenario model intercomparison project (ScenarioMIP) for CMIP6. Geoscientific Model Development, 9(9), 3461-3482.
2. Connolly, S. R. & Baird, A. H. Estimating dispersal potential for marine larvae: dynamic models applied to scleractinian corals. Ecology 91, 3572–3583 (2010).
3. Yamano, H., Sugihara, K., & Nomura, K. (2011). Rapid poleward range expansion of tropical reef corals in response to rising sea surface temperatures. Geophysical Research Letters, 38(4).
4. Fifer, J. E., Yasuda, N., Yamakita, T., Bove, C. B., & Davies, S. W. (2022). Genetic divergence and range expansion in a western North Pacific coral. Science of the Total Environment, 813, 152423.
5. Jones, N. (2011). Coral marches to the poles. Nature.
6. Fuchs, Y.H., Edgar, G.J., Bates, A.E. et al. Limited net poleward movement of reef species over a decade of climate extremes. Nat. Clim. Chang. 14, 1087–1092 (2024). https://doi.org/10.1038/s41558-024-02116-w
7. Hoegh-Guldberg, O. (2014). Coral reef sustainability through adaptation: glimmer of hope or persistent mirage?. Current Opinion in Environmental Sustainability, 7, 127-133.
8. Lucas, M. (2013). Will our coral reefs survive climate change?. Quest, 9(4), 6-9
9. Matz, M. V., Treml, E. A., Aglyamova, G. V., van Oppen, M. J., & Bay, L. K. (2017). Potential for rapid genetic adaptation to warming in a Great Barrier Reef coral. PLoS Genet, 19, e1007220.
10. Dimond, J., and E. Carrington.  2007. Temporal variation in the symbiosis and growth of the temperate scleractinian coral Astrangia poculata. Marine Ecology Progress Series 341:161–172.