The scaleworm Arctonoe fragilis
, along with other species in the genus, are obligate symbionts of several species of sea stars and sea cucumbers. This means that, for best health and fitness, they are obligated (required) to inhabit a host. However, it is known that most or all Arctonoe
species can live for at least short periods off their hosts. The hosts provide protection for the worms, for example, among its spines or in the ambulacral grooves of its arms, and the worms may also scrounge food scraps from the host while the host is feeding. What the hosts get in return is debatable, although being cleaned of organic debris from its skin, or perhaps even dead skin, is possible. So, whether the scaleworms are strictly parasites, which seems likely, or commensals (one participant benefits while the other is not affected), which is the more common term for them, is a matter of opinion. More on their relationships with host sea stars can be found in A SNAIL'S ODYSSEY: LEARN ABOUT SANDWORMS & RELATIVES: SYMBIOSES
Note Author: Tom Carefoot, Department of Zoology, University of British Columbia.