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Deep-sea octocorals: An ecosystem of their own

Octocorals dominated the seafloor on Chris Mah's Okeanos mission to Johnston Atoll in the remote Pacific Ocean.
Credit: NOAA Office of Ocean Exploration and Research, 2017 Laulima O Ka Moana

The deep-sea is a vast part of the world’s ocean. Even today, it often surprises people to discover how much life lives in this largely unexplored and inaccessible world of darkness, cold and high pressure. In fact, it was once thought to be lifeless, before early explorers dredged the deep-sea bottom and pulled up creatures never before seen.

This summer, I joined the NOAA research vessel Okeanos Explorer, America’s premier deep-sea exploration vessel, and had a unique opportunity to study habitats of the deep-sea. Among the most striking were deep-sea “coral forests.”

Many of the corals we saw were octocorals, which aren’t the “typical” warm water coral found in shallow waters that most people think of when they hear the word. These octocorals can form very large and very old colonies, some can be thousands of years old. Deep-sea octocoral colonies are only predators (unlike their shallow water relatives that often are also photosynthetic thanks to symbionts in their tissues). They feed on organic material as it flows past them on the water current and is caught by one of the eight feeding tentacles that surround each of the mouths of their many polyps.

Corals (and sponges) are what’s been described as “ecosystem engineers”—their very presence provides habitat for many other animals, making them a habitat or “ecosystem” unto themselves. Several serpent stars, for example, live in close symbiosis with their octocoral host—so close in fact that they are almost never present as free-living individuals. Many of these stars spend their entire lives on the coral, with observations showing that some are present even as very tiny individuals. Museum specimens show us that many will not release from their host even after death. In the case of one species, this type of relationship can even protect the corals from human-induced threats—like an oil spill.

Serpent Star on Octocoral

Many serpent stars spend their entire lives on a coral host.
Credit: NOAA Office of Ocean Exploration and Research, 2017 Laulima O Ka Moana

But, it’s not just friendly relationships in an ecosystem, and octocorals have a very diverse and colorful “rogues gallery” of predators to deal with as well. Among them are a tiny jellyfish (Aegina citrea) that settles on the feeding polyps present on the branch tips of some corals, devouring the tissue.

The Okeanos crew comes across the tiny jellyfish (Aegina citrea) that devours the tissue of octocoral feeding polyps. 

The worm-like mollusk called an aplacophoran can be found coiled around the stalks of corals after devouring the tissue on the stalk.

Aplocophoran on Octocoral

Credit: NOAA Office of Ocean Exploration and Research, 2017 Laulima O Ka Moana

Prominent, often spiny goniasterid sea stars methodically feed on the soft tissues of the corals as they slowly climb up the body. Many of these predatory sea stars are known to remain on the corals for a very long period of time, with some observations suggesting that they have been present for over a year, making them more like parasites.  

Mah says of the spiny goniasterid sea stars, "They're very efficient and we've seen them devour the polyps inside primnoids kind of the way I just had my cookie." 

Giant sea spiders (or pycnogonids) are predators on multiple types of corals and will use their large proboscis to drain fluids out of the corals for food in a vampire-like fashion.

A giant sea spider on octocoral

A pycnogonid (or giant sea spider) on an octocoral. 
Credit: NOAA Office of Ocean Exploration and Research, 2017 Laulima O Ka Moana

Although not strictly speaking a predator, a sea-anemone-like animal called a zoanthid overtakes the surface of some Paragorgia octocoral colonies, crowding over the original coral animal and competing with the coral’s polyps for food.

The Okeanos team comes upon an octocoral colony overgrown by zooanthid cnidarians, sea anemone-like animals, that gives it a beautiful red and orange coloring. 

We tend to think of an ecosystem as being large, like the deep waters off the Johnston Atoll that we explored. But even one single octocoral specimen can have its own collection of predators, prey and friends.