Denizens of the Deep: Are Brittle Stars the Best House Guests?

Featured Pic: A colorful Ruby Brittle Star (Ophioderma rubicundum) in the Flower Garden Banks National Marine Sanctuary. Photo courtesy of NOAA.

By Samuel Georgian, Marine Biogeographer at Marine Conservation Institute

If you have ever seen a brittle star, you may have assumed that you were looking at the closely related starfish instead. Starfish and brittle stars are both members of the Echinodermata phylum, characterized by their radial symmetry – meaning that their bodies are organized into five mirrored parts around a central point (however some exceptions occur). Other commonly known echinoderms include sea cucumberssea urchins, and sand dollars. Brittle stars are members of the class Ophiuroidea along with the closely related basket stars, both known for their thin, long, and highly flexible arms used for mobility and feeding. In fact, their class name originally derives from the Greek words for ‘serpent’ (ophis) and ‘tail’ (oura). Today, we commonly refer to them as ‘brittle stars’ due to their ability to quickly lose an arm to a predator – allowing them to survive and later regenerate the lost limb!

Brittle stars are found throughout the world’s oceans (including a large number of species in the Antarctic and Artic), with more than half of known species occurring in the deep sea. They are generally scavengers or detritivores, feeding on dead organic material on the seafloor. However, some species are predatory (check out this incredible footage of a brittle star catching a squid!), and many species filter feed by perching high in the water column and catching food from the currents flowing through their arms. They come in all shapes, sizes, and colors – some use bioluminescence to deter predators, some are less than an inch across while others are a whopping 24 inches, some have six arms instead of five.

Brittle stars are fairly ubiquitous organisms throughout the deep sea. In fact, the brittle star Gorgonocephalus caputmedusae was among the first ever animals recovered from the deep sea, accidentally snagged in sounding equipment during an 1818 attempt to find a Northwest Passage. In the deep sea brittle stars are found throughout a wide diversity of habitats, including the rocky flanks of seamounts, muddy abyssal plains, deep-sea coral and sponge reefs, and even hydrothermal vents. Where they do occur, they are often present in extremely high abundances, such as the ‘brittle star city’ discovered on a seamount in the Antarctic that houses an astonishing estimate of tens of millions individuals. Brittle stars play important ecological roles in these deep-sea habitats. Many species act as ecosystem engineers by digging through sediments on the seafloor, while others recycle nutrients in their roles as scavengers and detritivores, providing key sources of nutrition to other organisms including fish, starfish, and invertebrate predators such as crabs.

 

A Suenson’s Brittle Star (Ophiothrix suensoni) hides out in a sponge, extending its long arms into the water column to feed. Image courtesy of NOAA.

In addition to these roles, brittle stars have long been known to have a close association with many cold-water coral species. In some cases, this association is incredibly tight. For example, scientists believe that Ophiocreas brittle stars only associate with one species of coral (Metallogorgia), and never relocate to even another colony of the same species after their larval stage. Until recently, it was generally thought that this was a commensal relationship, a one-sided relationship where one organism benefits from an association with another without hurting – or helping – it. In this case, brittle stars use the corals as three-dimensional habitat to elevate themselves in the water column to feed, while ostensibly not harming the corals. While some invertebrate species do like to munch on cold-water coral polyps (for example, corallivorous snails), scientists have not observed brittle stars predating on coral tissue, and the lack of a defensive response from the corals themselves suggest that no harm is being done incidentally either. Perching high in the water column and gaining access to faster, more nutrient-rich currents can be critical for the success of brittle stars, but does this relationship really have no effect – positive or negative – on the corals they inhabit?

 

Brittle star symbionts wrap themselves around the branches of the soft coral Callogorgia in the deep waters of the Gulf of Mexico. Image courtesy of NOAA-OER and BOEM.

Oddly enough, it took the largest marine oil spill in history to unravel the true nature of the relationship between brittle stars and cold-water corals. In 2010, the Deepwater Horizon oil spill released an astonishing 4.9 million barrels of oil directly into the deep sea. Several cold-water coral communities in the vicinity of the spill were subsequently found to be covered in oil and dispersants (chemicals used in the cleanup effort to help disperse oil particles) from the spill. These corals exhibited many common signs of stress including tissue loss, excess mucus production, and colonization by opportunistic hydroids. By imaging these habitats for years after the spill, scientists were able to track the recovery of the damaged corals. While it was suspected that corals that were only partially damaged might be able to recover to some extent, they surprisingly found that brittle stars appeared to play an important role. Damaged coral colonies that had at least one brittle star (Asteroschema clavigerum) recovered significantly faster after the oil spill, and brittle stars were most commonly observed on healthy parts of the corals. Scientists speculate that the brittle stars remove oil contaminants and other sediments from coral branches, and may even prevent the hydroid colonization that often precludes even long-term recovery of damaged tissues. While more research on this association remains to be done, it appears that brittle stars are doing far more than getting free rent (and a free lunch) – they appear to be excellent house guests in return.

For more information visit our website –www.CaliforniaSeamounts.org

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