The world of marine ecology isn’t a very nice place right now. The ocean is still as awe-inspiring as ever, but its diversity is waning. Scientists are ringing new, ever-louder alarm bells on a daily basis. Maybe that’s why I feel such a surge of affection every time I think of the coral guard crabs of the genus Trapezia: they represent a tiny speck of light in an otherwise pretty dark outlook.
Let’s forget about coral die-off and deep-sea trawlers for a second, and instead find out how one of the reef’s tiniest inhabitants plays its own part in keeping the fragile reef ecosystem intact. Crown of thorns starfish: meet your smallest, biggest nightmare.
Meet the Trapezia
Coral guard crabs are tiny but mighty. They belong to a group of closely related families (united in the superfamily Trapezoidea) that exclusively inhabit branching corals. The best-known genus is Trapezia, whose members—along with related genera like Tetralia and Domecia—are found on tree corals like Pocillopora and Acropora. The crabs never stray from their hosts if they can help it, being completely dependent on the shelter and meals provided by the corals. That doesn’t mean they’re freeloaders, though: crab and coral maintain a mutualistic relationship, a type of symbiosis whereby both participating species obtain some kind of advantage. Not surprising, then, that up to 90% of these corals are revealed to host crabs when inspected!
The crabs obviously benefit from the protection afforded by the coral’s stinging cells and intricate branching growth pattern. Any predator looking to dine on crab sticks would have to exert considerable effort to pry loose their dinner, which is unlikely to be worth it for such a small and overly crunchy reward. But Trapezia doesn’t just use its host for shelter. The crabs also feed on it, scraping its tissues to obtain fleshy bits and nutritious protective mucus when other foods run low. Although this irritates the coral, it clearly seems to think that’s worth it, probably because the crabs also perform the invaluable duty of keeping it clear of debris.
There are few things corals hate more than being covered in bits. The accumulation of debris affects photosynthesis, increases the risk of disease, and can even suffocate the coral. A 2006 study confirmed the crucial importance of Trapezia for corals in areas with abundant sedimentation. When scientists “seeded” corals with guard crabs before planting them, they would survive, whereas up to 80% of corals placed on the reef without crabby protectors gave up the ghost within a month. Even the corals that did survive would bleach and struggle to grow as quickly as their seeded counterparts. The crabs would dutifully spring into action whenever sediment accumulated, kicking and shoveling it off their hosts. Although they occasionally sampled particles, most of the debris wasn’t consumed. Detritus does make up part of a Trapezia crab’s diet, as does a small amount of coral tissue, but researchers have found they mostly focus on swimming plankton in the form of amphipods, copepods, isopods, and other critters.
The best part for the coral? Cleaning isn’t the only service included in their deal with the crabs. Coral guard crabs are exceedingly vulnerable when they’re not tightly wedged between their host’s branches, which is why you’re unlikely to ever spot one scuttling freely across the reef. They therefore have a vested interest in helping to keep the corals they live in alive, and they’ve taken up this responsibility with zeal.
A starfish “supervillain”
Despite their stinging tentacles, corals are vulnerable to predation by several specialized reef species. Several snails snack on coral tissue, for example, including the appropriately named Coralliophila violacea, a type of murex snail. But echinoderms are the real problem, particularly the unusually pillowy pincushion star (Culcita novaeguineae) and a highly notorious spiky number known as the crown-of-thorns (Acanthaster planci). The latter in particular is considered problematic—it’s one of those species that has accidentally benefited considerably from human activity.
Whereas some sea creatures are quickly wiped out as a result of factors like warming seawater and nutrient pollution from agricultural runoff, crown-of-thorns stars have thrived. Their numbers have exploded in recent years, seemingly mostly due to increased larval survival rates. Although this is obviously not the stars’ fault—they’re just responding to conditions created by humans—it’s certainly considered problematic. Their favorite food, after all, is coral tissue. Branching species like the Acropora and Pocillopora that coral guard crabs inhabit are no exception, although thanks to their tiny defenders, these seemingly easy snacks can end up being more than the starfish bargained for.
Arthropoda vs. Echinodermata: Place your bets
A 2024 overview compares Trapezia crabs to the tiny ants that protect Acacia trees from large herbivores on the African savanna. Although an ant taking on an elephant sounds like the set-up for a bad joke, no one likes getting a venomous stinger plunged into their tongue. When the ants start swarming, the elephants often quickly decide it’s not worth the effort and move on to the next tree, limiting the damage to this particular colony’s home.
The crabs operate in a similar fashion. Dozens can inhabit a single coral, often in monogamous pairs, and they can work together to defend their host when huge (in comparison) COTS starfish move in for a meal. Despite looking like the aquatic equivalent of a cactus, after all, this echinoderm is not untouchable. Like elephants posses sensitive mucus membranes for ants to target, the crown-of-thorns has a soft underside, which inevitably ends up partially exposed as it moves around. Its thousands of tube feet—a miracle of echinoderm locomotion also used by sea urchins—happen to fit perfectly in a small crab claw.
As soon as it comes too close, the crabs swarm the starfish (or other predator) and simply begin pinching it wherever they can. And when they can’t pinch, they shove! The predator doesn’t even have to touch the host coral for the crabs to spring into their defensive positions, because they can “smell” damaged coral tissue, as well as the signature chemicals emitted by the predators themselves. If a COTS or murex is wrecking their neighborhood, they’ll know.
Even the children do their part: a 2014 study showed that baby Trapezia crabs were highly defensive of baby corals. Considering how vulnerable juvenile coral colonies are, this added protection can make a big difference for the reef’s overall health.
Photo by me. See more of my photos →
Every little bit helps
The crabs’ defensive measures aren’t enough to kill predators outright. In fact, they don’t even always cause the crown-of-thorns or carnivorous snails to give up on their meals and leave—but they certainly help prevent it from enjoying a long, leisurely picnic. The larger Trapezia species, like the red-and-purpleT. cymodoce, in particular, can make a real difference to the host’s outcome. As an example, a 2001 study showed that COTS have clear preferences in terms of which corals they like to munch. Flavor has little to do with it; the starfish picked their meals based on how many crabs they hosted, often not bothering with the well-protected species. The energy expense associated with getting pinched while you try to eat is apparently not worth it. I too, would choose the establishment where my feet are left alone.
Coral guard crabs help to support the survival and growth of coral colonies as a whole through this reduction of predator pressure. It’s not just the coral host that benefits, but all the critters that call the reef their home. And luckily, the crabs don’t have to handle guard duties entirely by themselves: some shrimp, like certain snappers of the genus Alpheus, will gladly join any starfish pinching party taking place close to their tree coral hosts. Other coral crabs, like the smallerTetralia, will also join the fray. These weaker crabs are generally better at sediment removal than fighting, but I doubt the corals mind their efforts.
Conclusion
Humans mess up a lot, and that’s bad. But sometimes, help is found in unexpected—and delightful—corners. Tiny crabs whose bright colors and fun patterns (polka dots!) belie their aggressive disposition may not be the first thing you think of when you consider how to save a coral reef. But now you know: if you want to protect the corals, you gotta protect the crabs, too.
Sources & further reading
Glynn, P. W. (1980). Defense by symbiotic crustacea of host corals elicited by chemical cues from predator. Oecologia, 47(3), 287-290.
Pratchett, M. S. (2001). Influence of coral symbionts on feeding preferences of crown-of-thorns starfish Acanthaster planci in the western Pacific. Marine ecology progress series, 214, 111-119.
Rouzé, H., Lecellier, G., Mills, S. C., Planes, S., Berteaux-Lecellier, V., & Stewart, H. (2014). Juvenile Trapezia spp. crabs can increase juvenile host coral survival by protection from predation. Marine Ecology Progress Series, 515, 151-159.
Shmuel, Y., Ziv, Y., & Rinkevich, B. (2022). Coral-inhabiting Trapezia crabs forage on demersal plankton. Frontiers in Marine Science, 9, 964725.
Stewart, H. L., Holbrook, S. J., Schmitt, R. J., & Brooks, A. J. (2006). Symbiotic crabs maintain coral health by clearing sediments. Coral Reefs, 25(4), 609-615.
Stier, A. C., & Osenberg, C. W. (2024). Coral guard crabs. Current Biology, 34(1), R5-R7.