Featured Image Caption: Crown-of-thorn starfish have experienced a disproportionate growth in population in the past decade, putting a considerable strain on their ecosystem. (Image Source: “_MG_1466” by Chris Wilson, licenses under CC BY-NC 2.0).
Reference: Hue, T., Chateau, O., Lecellier, G., Marin, C., Coulombier, N., & Le Dean, L. et al. (2022). Impact of near-future ocean warming and acidification on the larval development of coral-eating starfish Acanthaster cf. solarisafter parental exposure. Journal of Experimental Marine Biology and Ecology, 548, 151685. https://doi.org/10.1016/j.jembe.2021.151685
Crown-of-Thorn starfish are coral-eating echinoderms that inhabit the Indo-Pacific region. Preying on the polyps of stony coral, these starfish can be a real threat to coral reef ecosystems. When looking at them, it isn’t hard to tell where they got their name from: they most literally look like a crown of thorns. Their thorns are venomous and make one of the largest starfish in the world look even more menacing than it would already look due to its sheer size.
Crown-of-Thorn starfish can be found in the Indo-Pacific region, ranging from the west coast of Central America to the East African coast—but it is most prolific through Oceania, a continental region located in the South Pacific Ocean that includes Australia, New Zealand, Polynesia, and various more countries. In Australia, this starfish has come to be known as a “coral reef killer” due to its noxious effect on the Great Barrier Reef. In the past three decades, the Great Barrier Reef has lost half its coral life, and it is estimated that Crown-of-Thorn starfish are responsible for 42% of the loss reported.
Ocean Temperature and Acidity as Rising Threats
Predation hasn’t been the only factor affecting the Great Barrier Reef and the Indo-Pacific region: stressors like ocean warming and acidification have been linked with higher mortality rates, not only for coral but for other animal species as well. Ocean warming is the gradual mean increase in temperature that is being seen in our oceans, while acidification is the increase in acidity in our oceans for long periods of time, usually due to an increase in the uptake of CO2 by the oceans. So, is this changing environment helping or hurting the population of Crown-of-Thorn starfish?
Prior research has looked at the effect of ocean warming and acidification on the larval state of Crown-of-Thorn starfish, yet these have come to conflicting results. For instance, some researchers predicted negative effects on their fertilization success under acidic conditions, while others found no significant effect on fertilization success under these conditions. In a similar fashion, some researchers found that larval mortality increased at 30° Celsius (86° Fahrenheit), while others found larval mortality decreased at high temperatures if food availability was high.
A group of researchers with lead author Thomas Hue noticed that all of these studies focused only on the larval stages—not taking into consideration parental exposure. That is to say, prior research focused only on the larval stage, without looking at the life cycle as a whole. Taking into consideration the whole life cycle is extremely important if we’re trying to replicate a real-world situation. For instance, in the real world, Crown-of-Thorn starfish would be experiencing increasing temperatures and acidity in their environment before even conceiving their offspring, and so we should take into consideration how that will affect these offspring in their larval stage later on. That’s why Hue et al. set to study the effects of ocean warming and acidification by first exposing adult Crown-of-Thorns starfish to the experimental conditions and then studying their offspring and the effect of these conditions on their larval stages.
Staying Alive: Bad News for Coral?
The researchers tested the acclimation capabilities of Crown-of-Thorn starfish by observing them under four conditions: normal, acidification (with a pH of 7.75, considering normal ocean pH is around 8.1), warming (with a + 2° Celsius above normal temperature), and acidification and warming conditions (with a pH of 7.75 and a + 2° Celsius above normal temperature). The Crown-of-Thorn starfish were exposed to these conditions for 20 weeks and then were allowed to mate. The researchers observed that the stressful conditions resulted in developmental delays but they did not affect larval mortality. That is to say, under ocean warming and acidification, Crown-of-Thorn starfish populations will still survive and reproduce.
With ocean warming and acidification also comes weakened coral reefs and loss of biodiversity, including the loss of natural predators of these starfish. Outbreaks have been observed in localities where triton sea snails, damselfish, triggerfish, and pufferfish— which are some of the Crown-of-Thorn’s natural predators—have been removed, usually through disorganized fishing practices. These results may give some hope that outbreaks could be mitigated, since the researchers found that ocean warming and acidification seem to cause developmental delays, leaving Crown-of-Thorn larvae exposed to predation for longer periods of time.
So, the answer is yes: Crown-of-Thorn starfish populations will survive ocean warming and acidification. But hopefully, these almost inevitable conditions will also make it more plausible for their natural predators to control future outbreaks.