Interdependence of NPS corals with brittle stars

At a great depth below the surface of the Gulf of Mexico, there are colonies of non-photosynthetic corals (NPS). This dark, seemingly barren area reveals a rich biodiversity upon closer inspection. Among the NPS corals, crabs, fish, brittle stars, and anemones can be found, with some organisms appearing to use the coral structures for protection and as a food source. It was previously believed that this relationship was one-sided, where marine fauna benefited from the corals without providing any advantage in return. However, recent observations suggest that the corals might also gain benefits from these interactions.

The Deepwater Horizon drilling disaster provided scientists with a unique opportunity to better understand the relationships between non-photosynthetic corals and the organisms living near them. Oil slicks from the spill eventually settled on the ocean floor. In the months following the spill, researchers from the Ecosystem Impacts of Oil and Gas Inputs to the Gulf (ECOGIG) consortium, part of the Gulf of Mexico Research Initiative (GoMRI), began documenting the state of NPS corals using remotely operated vehicles (ROVs), capturing high-resolution images of the coral colonies. The findings were alarming – more than half of the corals were covered in a suffocating, brown substance containing traces of oil from the spill site. Additionally, many corals produced excessive amounts of mucus and over time lost tissue along with polyps, revealing skeletal fragments – all symptoms of stress and disease. These conditions increased the likelihood of coral coverage by hydroids, small anemone-like animals that compete with corals for space and resources.

After more than four years of monitoring the state of NPS corals, scientists noticed an intriguing trend. Brittle stars of the species Asteroschema clavigerum were consistently found on healthy sections of the coral Paramuricea biscaya, which was most affected by the spill. This observation led researchers to wonder whether the brittle stars were simply avoiding sick parts of the coral or actively protecting the corals from the harmful effects of the oil spill.

Researchers analyzed images of corals, some of which had brittle stars wrapped around their branches, while others did not. They found that corals with brittle stars were healthier than those without. Branches of corals located closer to the brittle stars were in better condition and, when damaged, had a greater chance of recovery compared to more distant branches that lacked contact with brittle stars. Even years after the oil spill, brittle stars were still observed with selected corals, suggesting that their relationship might be based on mutual benefit rather than one-sided exploitation.

How might brittle stars help corals? It’s difficult to say for certain. Information about deep-sea creatures is limited by the challenges of reaching such depths, requiring the use of submersibles or robotic vehicles. However, scientists have some theories. Brittle stars feed by extending their long arms to catch nutritious particles falling through the water column. Previous studies of corals in New Zealand fjords showed that symbiotic brittle stars could clear potentially suffocating sediments after catastrophic landslides simply through their feeding movements. It is believed that the same applies to oil contaminants. Brittle stars might also deter hydroid larvae from settling on the coral branches.

NPS corals can live for thousands of years. Damage to a single branch is a costly loss, considering that an individual branch may take hundreds of years to grow. This study of corals coping with an oil spill shows that a seemingly insignificant brittle star clinging to their branches could be the first line of defense for NPS corals.