Coral reef ecosystems have a vast diversity of creatures, which means many more niches than you might expect so species aren’t directly competing with each other. The many different strategies for finding food make that possible. As I swim around the reef most of the activity I see is food-related, with predator avoidance built into that. And breeding has its seasonal behavior added into the mix. Reef creatures are busy all the time working at basic survival, which is one reason why zoos and aquariums can’t realistically represent natural environments. There’s really no "down-time” in nature. When animals are inactive they might be sleeping, but it could also be part of a feeding strategy.
Surprisingly few animals are fast-swimming predators chasing down smaller fish, like the high-profile sharks and barracudas you may envision as “hunting”. Invertebrates in the ocean are generally very slow-moving animals, and their predators don’t have to chase them the way we think of predation on land, cheetahs running down an antelope or swallows snapping up gnats. The Triggerfish above is a carnivorous predator but it swims very slowly when hunting because its invertebrate prey is almost motionless. That’s a nice thing for divers; we get to watch wild creatures fairly close up in their wild environment, sometimes for a long time. Alert to their own predators, they can move fast when they want, but they know human divers are slow.
As on land, big predators like sharks are few, being at the top of the trophic (eating) “energy pyramid”. With each transition in a food chain, most of the potential energy in food that’s consumed is lost since the consumer uses it to carry on its own life processes (metabolism, movement, etc). Sharks eat big fish or marine mammals which eat smaller fish which eat even smaller fish or invertebrates...so not much of the energy packaged by photosynthetic producers in the reef ecosystem, at the base of the pyramid, gets into the shark.
In general, the higher up the trophic pyramid an animal is, the fewer of them there are.
Plankton means “drifters”. In temperate marine ecosystems plankton are primarily drifting unicellular algae which can get so thick the water is murky. In a coral reef ecosystem the unicellular algae are mostly packaged inside the bodies of the individual coral animals, each inside its coralline home. That’s why tropical waters are so clear compared to temperate waters. But there are drifters on the reef….tiny animals floating in the currents. Many reef fish are plankton-feeders and hunt in schools. The edge of reef drop-offs, where water currents swirl, are prime spots for that. These Creole Wrasses swim in huge schools, “pouring” along the edge up and over formations, snapping up small crustaceans, invertebrate larvae and tiny jellyfish.
The Daily Bucket is a regular feature of the Backyard Science group. It is a place to note any observations you have made of the world around you. Rain, sun, wind...insects, birds, flowers...meteorites, rocks...seasonal changes...all are worthy additions to the bucket. Please let us know what is going on around you in a comment. Include, as close as is comfortable for you, where you are located. Each note is a record that we can refer to in the future as we try to understand the patterns that are quietly unwinding around us.
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(All these photos were taken by me on the reefs surrounding Lttle Cayman Island in April 2016. I used a GoPro camera on a stick.)
Hunting down your prey is one kind of species interaction. www.globalchange.umich.edu/...
Mutualism, when both species benefit, is another. A critical one on the reef is the relationship between the photosynthetic algae living inside corals: the algae get nutrients from the animal’s waste, and the coral animal gets food and oxygen from the algae. It’s the microscopic endosymbionts that give healthy corals their colors.
Commensalism is when one animal benefits and the other is neither helped nor harmed, like the stingray and fish below:
Hunting vertically:
Grazing….
“Cleaning stations” are a unique reef niche, spots on the reef known to all the fish as a place to get parasites removed by particular tiny wrasses and shrimp. Large fish which would otherwise eat those small creatures do not, opening their mouths and gills to be cleaned. It’s a zone of peace.
Many fish are stealthy predators.
Invertebrates feed too, mostly in slow motion.
Fun Fact! That pretty white sand on tropical beaches....where does it come from? Watch this video to find out!
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All nature observations are welcome in the comments below. Tell us what you’re seeing in your own natural neighborhood, in your part of the world.
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