2 minute read
Emulate: Coral Reefs
WRITTEN BY EDITORIAL STAFF IN CONSULTATION WITH JUAN ROVALO OF BIOHABITATS
Coral itself comprises the living substrate upon which reef colonies develop. A single coralpolyp, the result of sexual reproduction, reproduces asexually to create a coral body madeup of thousands to millions identical polyps that share a nervous system and some digestivefunctioning. Each polyp secretes calcium carbonate (the genesis of limestone) and lives withinthe resulting cup shape, or calyx. Periodically, the polyp will secrete a new calyx floor, resultingin slow growth (.3 – 10 centimeters per year).
While they make up less than 1% of the ocean floor, the world’s coral reefs contain upwards of 25% of all marine life, which, through a complex web of relationships thrive in an impressively challenging ecotone, marked by wave energy, massive storm disturbance, temperature changes and UV exposure. There are three main coral reef typologies: fringing, barrier and atoll. While the conditions that give rise to these typologies differ, all reefs share some similarities in the condition in which they grow, including bottom topography, wave and current strength, light, temperature and suspended sediments.
Inside its custom calyx, a polyp remains safely ensconced in the daytime. At nighttime,however, the polyp will emerge from the calyx to feed on the currents. In addition to thesenighttime current feedings, coral and algae have formed an obliged symbiosis; polyp tissuecontains photosynthetic algae that aids in coral’s growth. Coral, in exchange, protects the algaewhile providing it with compounds it needs for photosynthesis. In some species of coral, thissymbiosis is so specialized that the coral body has the ability to filter UV light to maintain idealconditions for the algae’s photosynthesis.
This foundational relationship between coral and algae is just the first example of symbiosis in an ecosystem marked by cooperation. Coral is half of the symbiotic pair in many different relationships, providing habitat and protection to crabs, lobster and shrimp, which in return provide protection against aquatic worms and other predators that would feast on the softbodied polyps housed in the coral. Clown fish and anemones live in close symbiosis; the fish supply the anemone with food, the anemone protect the fish, which have a special immunity to the anemone’s sting. Moray eels and coral trout, two entirely distinct fish species, hunt in pairs and equitably split their catch. A variety of cleaner fish eat parasites off larger fish – an exchange that feeds the former and keeps the latter healthy.
A precisely and dynamically balanced ecosystem, small, herbivorous fish eat seaweed and algae that, if left unchecked, would smother the coral below. Parrot fish eat the coral itself, digesting the algae within and excreting the calcium carbonate that washes to shore as fine, white sand; a single parrot fish can excrete as much as 11,000 pounds of sand per year. Larger predatory fish keep the smaller fish populations in check and crabs and sea cucumbers ingest the detritus that falls to the sea floor.
Tragically, coral reefs’ characteristic resiliency is flagging with sustained stresses; ocean acidification and rising water temperatures as a result of carbon dioxide uptake from the atmosphere, food chain collapse due to over fishing, and structural damage as a result of careless tourism have combined into a lethal force threatening the total collapse of these ecosystems in the next twenty to thirty years. Already in the last thirty years an estimated fifty percent of the world’s coral reefs have died. In 2016, a sustained warming period caused a widespread bleaching event that alone killed as much as 29% of the world’s remaining reefs, hitting the Great Barrier Reef especially hard, where as much as 85% bleached.
Coral bleaching is a stress response in which the coral body expels all its algae, thereby erasing its characteristic color, distinct by species, and leaving behind just the white, calcium carbonate exoskeleton. If water temperatures return to normal before algae smothers the reef, it can revive. Given the warming trend of our oceans, this possibility is increasingly slim. Conservation efforts focused on the protection of herbivorous fish and other organisms that keep algae blooms in check have proven effective but slowing and reversing global climate change is imperative for the future of the reefs.
