Corals are considered the beautiful, exotic, plant-like beings that grace our ocean floor. In the past, not a lot was known about their life cycles, means of sustenance, and behavior. A majority of us still consider them ‘ocean plants that display a range of colors and textures. Not a lot of effort has gone into understanding their role in the ecosystem and their interactions with other creatures in the sea.

We all know the symbiotic relationship clownfish share with anemones or the relationship between gobies and crabs. But a team of researchers have observed a startling and fascinating behavior in corals that might change your opinion about them. 

A team of researchers from the University of Sydney published a recent study in Biology letters on crown of thorns starfish, a ferocious coral predator. They found that juvenile crown of thorns starfish can keep their carnivorous instincts dormant for up to 6  years and wait for a coral population to regenerate before launching a vicious attack. The team described this behavior as ‘hidden army waiting to consume reefs as the reefs recover,’ showing how voracious the starfishes’ appetite for coral is.

As a part of the same study, the team also documented a previously unknown behavior of coral that might give you some perspective of the ferocity of battles between these ocean inhabitants.

The researchers found that corals have evolved their own defense against these predators capable of decimating an entire reef bed in a single go. The team documented proof of life-threatening mortal injuries inflicted on the crown of thorn starfish, pointing to a sophisticated and effective defense mechanism from vulnerable corals. 

When a starfish appendage (small tube feet used for movement) touches a coral, it is met with stinging cells present on the coral’s polyps. These polyps have tentacles that help the coral catch food drifting in the currents. These tentacles, known as ‘sweepers’ or ‘sweeper tentacles’ have a specialized outer tissue called nematocysts, that are also used to capture food. This tissue is capable of inflicting severe damage to any intruder, including the starfish. 

Why is This an Amazing Find?

When the starfish comes in contact with the coral, the tentacles are released and this inflicts enough damage to remove chunks out of the arms of the juvenile crown of thorn starfish. Some injuries were so severe that an entire arm was missing and the team also observed a 10 percent fatality rate among the juvenile crown of thorns starfish that came in contact with coral. They also found coral stings caused injuries that severely reduced the arm length of the starfish by up to 83 percent. Thanks to the starfishes’ incredible ability to regrow damaged parts, most were able to recover, although it caused a significant delay in maturing into adulthood. 

Previous research by the team showed that these starfish remained in a juvenile state for up to 6.5 years and lived completely as vegetarians until nearby reef populations replenished from past damage. Then, the sudden switch triggers a mass push towards attacking nearby coral. This discovery shows that coral are not defenceless and are well-equipped in deterring this vicious predator. 

“This shows that the coral use stinging cells as protection to strike back in an attempt to give itself a fighting chance against attacking coral predators.” said Dione Deaker, a PhD student at the University of Sydney in the media release. 

“Sometimes the juveniles never recovered and died, but in most cases injured juveniles recovered and can regenerate their arms in about 4 months. Despite being prey of crown of thorns starfish, coral can potentially influence the survival of juveniles and the appearance of a population outbreak on a reef by delaying their transition into an adult that can reproduce,” she added. 

Professor Byrne and other colleagues at the national Marine Science Centre studied this behavior using 37 juvenile crown of thorns raised in captivity. These amazing creatures that grow up to 1 meter in diameter, were raised in isolation away from potential predators and reared them on a diet of coral prey for over 3 months.

Significance of Findings

The team were also able to isolate the injured specimens, capture images and video of the aftermath to document how coral defences affect predators. This significant documentation can help reef building efforts across the globe choose predator resilient species in regions overrun with critters looking for a quick coral snack. 

“This Peter Pan effect means that populations of the juvenile crown of thorns starfish can build up on reefs in the absence of coral,” Ms. Deaker said in a previous media release. “They could become a hidden army waiting to consume reefs as the reefs recover.”

In this previous study, the team documented the life-cycle of the amazing crown of thorns starfish. During their juvenile stage, they can fee don algae for up to 6.5 years and then make a sudden change in diet. Juveniles remain on this vegetarian diet for at least four months. In areas where there is plenty of reef cover, the starfish switch to a carnivorous diet after this 4 month period. 

“Another important implication of our findings is the possibility that the current adult starfish killing programs used to manage crown of thorns starfish might, in fact, trigger a feedback mechanism in the starfishes’ transition to coral predator as juveniles are released from adult competition,” added Ms. Deaker. 

Using these findings, population control effects across the Indo-Pacific waters could be controlled and made more efficient. Using the starfish population statistics, scientists can help barren areas repopulate with corals or also study if there is a correlation between the presence of juvenile crown of thorn starfish and corals in a particular area. This could lead to more fascinating insight into the symbiotic lives of our marine neighbours.