Marbled reef eel (Anarchias seychellensis)

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What at first appears to be a blanket of fine snow on the sea bed is actually a patch of bleached coral off the northern shore of Papua New Guinea. As sea temperatures rise, this scene is becoming more and more common in coral reefs around the globe. Rising temperatures make corals expel their zooxanthellae (algae with which corals have a symbiotic relationship), leading to starvation.

This is particularly bad news because once a reef dies, it turns to rubble, making it impossible for new coral to grow on top of it. This essentially turns what was once a flourishing biodiversity mini-hotspot into a sort of ecological deadzone.

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The blacktip reef shark (Carcharhinus melanopterus) is a common shark found in the tropical waters of the Indo-Pacific. Their bodies are streamlined, making them powerful swimmers, and—as the name suggests—the tips of their fins are black. They usually inhabit shallow waters and are very curious as a species. They will often approach divers to investigate, which has led to numerous attacks, giving the species a bad reputation. However, as with all sharks, the odds of being attacked by one are still extremely low.

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“Reefs in the Caribbean are experiencing a budget crisis: Corals’ production of calcium carbonate — their bony material that creates reefs — is way down, a 16-month-long investigation finds. Shallow-water reefs are in especially bad shape, with growth rates that are 30 to 40 percent of historical values. Many of these shallow sites also lack Acropora species, which are key reef-building corals that typically produce a lot of carbonate. These degraded reefs also have a lot of smothering seaweed and few critters to graze upon it, the study of 19 sites found.

The new analysis, published January 29 in Nature Communications, suggests that when the amount of live coral in a reef drops below about 10 percent, erosion begins to outpace growth of new reef structures. Many Caribbean coral reefs are approaching this tipping point, the team led by Chris Perry of the University of Exeter in England found. Ongoing assaults such as warming waters and ocean acidification may further hinder reefs’ efforts to get their budgets back in the black.”

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Southern reef squid, Sepioteuthis australis

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Caribbean reef squid, Sepoiteuthis sepoidea

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A nudibranch Pteraeolidia ianthina, common name Blue Dragon, glides across the coral.

Photo © Christopher Bartlett

Dendrophyna soft branching coral and a feather star, a marine animal of the class Crinoidea of the echidnoderms.

Photo © Christopher Bartlett

Sea whips, cabbage coral and juvenile damselfish on Vanessa’s Reef, Kimbe Bay.

Photo © Christopher Bartlett

Deep-sea corals were first discovered in 1869, but it took the advent of sonar and deep-sea submersibles to reveal the size and abundance of the reefs that they build. Although less well studied than their tropical counterparts, these cold-water reefs are just as rich in life. The stony corals that form deep-water reefs flourish in water temperatures of 39–55°F (4–13°C). Unlike tropical corals, they can live in total darkness because they do not rely on zooxanthellae living inside them to produce nourishment by photosynthesis in sunlight. Instead, they survive by filtering food from the water.

One of the biggest reefs—covering 38 square miles (100 square km)—was discovered during an oil-related survey of the Atlantic Frontier, norwhest of Scotland, in 1998. Lophelia pertusa is the main reef-forming coral at these reefs, called the Darwin Mounds, which lie at a depth of 3,300 ft (1,000 m). Lophelia reefs occur at similar depths on many seamounts in the Atlantic, and also in shallow cold water such as in Norway’s fjords.

Pictured is Lophelia pertusa.