One of the things I love about the ocean is that every separate surface is its own ecosystem. If you look at the shell of a crab you're likely to find plants and animals growing there. And that's just the ones that don't deliberately decorate themselves. Many times the organisms living on and around each other form some kind of symbiotic relationship. As you probably remember from school, symbiosis is when two organisms live together and can affect each other's behavior. Often times we say that both benefit one another, but that's only one of three types. Mutually beneficial symbiosis is conveniently called mutualism. You've probably already heard of the symbiosis where one organism benefits and the other is harmed; it's called parasitism. Finally when one organism benefits and the other doesn't get anything but also isn't harmed it's called commensalism.
All three types are frequently observed in this environment
Courtesy Canyon 289 via Flickr
Chris Mah, who is an awesome scientist and blogger over at the Echinoblog did a post in 2010 about a team of scientists who took a closer look at a cool symbiosis. You can read it here. In his post Dr. Mah talks about how we thought that the banded scale worm (Arctonoe vittata), which lives on a number of invertebrates in the Pacific northwest, probably didn't benefit its host organisms. Consequently they are generally considered commensals. However a team of scientists decided to see if one of the worms' hosts, the leather star (Dermasterias imbricata), preferred to have a worm over not. If they did prefer to have the worms it would suggest the worm benefited the star and would actually be a case of mutualism. Well it turns out the stars chose worms over nothing, other stars, and even their favorite foods! The scientists who wrote the paper offer suggestions for how the worm benefits the star, but also recommend looking into it more. I'd like to offer the video below as evidence of the worm's benefit to the star.
That is the same banded scale worm biting the hell out of a sunflower star's (Pycnopodia helianthoides) rays. Seriously the worm looks like the love child of a graboid and a xenomorph when it chomps down. In this case the worm is protecting a keyhole limpet (Diodora aspera). As far as I'm aware sunflower stars don't eat leather stars, but the morning sun star (Solaster dawsoni) consumes pretty much everything; especially other stars. Solaster dawsoni is such an invertebrate killer that it's also commonly called the vampire or death star.
Oof! The leather star is probably not gonna regenerate from that one.
Courtesy Brooke Reiswig at:
Clearly the leather star could potentially benefit from the predator warding prowess of the scale worm. Wouldn't it be cool if the scale worm also occurred on morning sun stars so they could be protected from one another, because they're totally cannibalistic!? Oh wait they do!
Courtesy same as above
That specific worm could be jumping ship from the star being eaten, but banded scale worms have been documented living on morning sun stars. Obviously no scientific rigor has been applied to this hypothesis yet, so it may turn out to be a load of hooey, but it's exciting to look at evidence and begin to form questions. I hope we'll see a study or some video evidence of stars also being protected in the future.