Photo by Tennie Tallman
The interaction between prey (clam), host (Evasterias troschelii) and symbiont (Arctonoe fragilis) in the intertidal.

The coastal waters of the North Pacific are home to a variety of marine invertebrates.  The interaction between  many of these obligate symbiont hosts ( sea stars, chitons and several gastropods ) and polyniod polychetes is an important part of the marine ecosystem. The ecology of polychetes is important in understanding the geographical range and host selection of three species of commensal scale worms, Arctonoe spp.. Scaleworms have no known predators except parasites so coloration and location on the host are poorly understood.  In some cases, Arctonoe spp. is found within the protective cover of a snail's mantle cavity but it is also found on the exposed outer surfaces of sea stars. The coloration is also affects perdition.  Arctonoe fragilis is cream colored, matching the arboreal coloration of it's primary host Evasterias troschelii.  In the photo above, A. fragilis is highly camouflaged, can you see it?

Scaleworms have a long larval stage which results in a wide dispersal range. They are broadcast spawners and release their gametes into the sea where they are fertilized. The larva are dispersed in the plankton before they settle on a host. Arctonoe spp. locate new hosts using waterborne chemical signals and are never found free-living except as larvae. The worms will change hosts if contact occurs but they show strong preference for the original host species.

Studies conducted by Wagner and Phillips show that the relationship between Arctonoe spp. and its host is not only commensal, but mutualistic as well.  The association between the host and its worm is ecologically imbricata (common Arctonoe spp. host) show that given the choice between the symbiotic polychetes or prey, the sea star showed preference for its symbiont 80% of the time. The experiment was also conducted between A. fragilis and prey and the results were repeated with the host selecting A. fragilis 75% of the time. The experimental results suggest that a chemical attractant coming from the worm itself may attract the host to its symbiont and vise versa.

The benefits of the interaction are not fully understood but both the host and worm benefit from the relationship.  Arctonoe spp. may feed on mucous secreted by its host and the amphipod epibonts present on the arboreal surface of the sea stars. The polychete is fed; while the host, which often lacks pedicellariae, is kept clean.

Pernet describes the relationship between Arctonoe fragilis and its primary host, Evasterias troschelii, as more commensal than mutualistic because the symbiotic worm feeds on its host's tissue. Arctonoe fragilis is the only species of scaleworm that can survive by living on the sea star Evasterias. The other two species, A. pulchra and A. vittata died when they were paired with Evasterias. A sopaonin isolated from Evasterias is toxic to all scale worms except A. fragilis.  The ability of A. fragilis to survive on Evasterias allowed it to fill a niche left open by other species of scaleworms. A. fragilis feeds on the spines and pedicellariae of its host while the other species feed on annelids and arthropods. The pedicellaria of Evasterias may damage or remove elytrae on A. pulchra and A. vittata . Two adaptations may prevent extensive damage to A. fragilis. The ruffled elytra of A. fragilis may be more resistant to damage by the sea star, and a coating on the body surface of A. fragilis may inhibit the closing of pedicellaria.

Allozyme variation and the differences in host preference, confirm that genetic distinction is present in the Arctonoe spp. The life history and reproduction page contains in-depth details relating to the development, reproductive period and ontogenies of A. fragilis.