Members of several polychaete families enter into symbiotic relationships with other invertebrates. The families include Aphroditidae, Amphinomidae, Hesionidae, Pilargidae, Nereidae, and Myzostomidae, but the ones seen most commonly are scaleworms in the family Polynoidae. Many of the worm species inhabit more than one host. Researchers differ in their interpretation of the types of symbioses being exhibited by the various worms, that is, whether they are commensals, mutuals, or parasites, with the first designation being most popular. Parasitologists would probably consider all such relationships to be parasitic, and perhaps they are correct. At worst, the worm may be eating parts of the host or stealing food from it, but additional costs to the host may be general wear-and-tear as the worm moves about and extra energy required to transport it. The only obvious mutualistic relationship seems to be that of Arctonoe vittata living in the mantle cavity of keyhole limpets Diodora aspera, where even though the worm may steal food from its host and likely be a nuisance when it moves about in the restricted space of its host's mantle cavity, the worm more than returns the favour by coming out to bite at tube feet of attacking sea stars. Better definition of the status of various polychaete symbionts would be a useful research endeavour.

Here are some photographs of west-coast polynoid-host symbioses:

Polynoid polychaetes such as Arctonoe spp. are thought to be obligate symbionts of several sea-star and molluscan hosts, including Evasterias troschelii, Leptasterias spp., Orthasterias koehleri, Solaster dawsoni, Diodora aspera and others.  Experiments at Friday Harbor Laboratories, Washington on the relationship of Arctonoe spp. with its host sea star Evasterias troschelii and sea cucumber Parastichopus californica using a Y-tube apparatus reveal strong host-specificity. Thus, Arctonoe is attracted 96% of the time to its regular host sea star E. troschelii but only 4% of the time to a non-host sea-star Pisaster ochraceus.  Similarly, Arctonoe is attracted to its host sea-cucumber P. californica but not to non-host Cucumaria sp.  The author concludes that the echinoderm host produces a specific diffusible substance that acts as an attractant for its symbionts.  Davenport 1950 Biol Bull 98: 81; drawing of head of Arctonoe courtesy Pettibone 1953 Some scale-bearing polychaetes of Puget Sound and adjacent waters U Washington Press, Seattle.

NOTE  the author does not distinguish between the 2 species of Arctonoe used in the study

NOTE  a dye, borax carmine, is used to ensure that the worm’s tentacles are stimulated asymmetrically at the point-of-choice in the Y-tube

Follow-up studies by the same research group at Friday Harbor Laboratories, Washington address the origin and chemical stability of the attractants involved in asteroid-polychaete partnerships.  Y-tube experiments with a host sea star Evasterias troschelii and its symbiont Arctonoe fragilis show that host-conditioned water retains its attractiveness to the worm for no longer than about 5h. Interestingly, water from an aquarium from which a host sea star has been removed forcibly, such that tube feet are torn off, has no attractiveness and may even repel the worms.  Tests with washings of skin and viscera to ascertain the source of the attractive factor yield negative results, and sometimes produce aversive responses in the worms.  Davenport & Hickok 1951 Biol Bull 100: 71. Photograph of E. troschelii with at least one scaleworm Arctonoe fragilis courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.

NOTE  the authors refer to the worm as a commensal, that is, with benefit for one partner and no effect on the other partner but, as noted above, it might alternatively be thought of as a parasite.  This is based on observations of A. fragilis frequently consuming tissues of its hosts.  Gut analyses of A. pulchra and A. vittata indicate that they feed primarily on small crustaceans and polychaetes, and not on host tissues, but there may be other negative effects of all scaleworm species on host fitness. Pernet 1999 Evolution 53: 435.

A later study at Friday Harbor Laboratories, Washington by the same researchers suggests that commensal Arctonoe vittata taken from specific hosts such as Evasterias troschelii, Orthasterias koehleri, and Solaster dawsoni may in some way become conditioned to their particular host species, and in subsequent choice tests will select their original hosts over others.  This is an interesting observation and would justify further research.  Hickok & Davenport 1957 Biol Bull 113: 397.

NOTE  an alternative idea is that Arctonoe spp. may, in fact, be mutualistic.  This is considered elsewhere in the ODYSSEY: LEARN ABOUT SEA STARS: SYMBIONTS

Juvenile sun star Solaster dawsoni with symbiotic
scaleworm Arctonoe vittata crawling on it 1.5X

The most important behavioral mechanism involved in host-finding in the hesionid worm Ophiodromus (Podarke) pugettensis is a positive chemotropotaxis.  Hosts of the worm are most commonly sea stars Patiria miniata and Luidia foliata.  The researchers seem primarily to be testing a different design of “olfactometer” in this paper, one in which a main flow of seawater can be augmented by streams of seawater introduced from any of 3 drip-tubes coming from 3 large beakers, each containing a different potential host (see drawing on Left).  A removable sheet of ground plexiglass attached beneath the test chamber allows a worm’s movements to be recorded from below, with the sheet being later removed for recording and analysis. 

Typical recordings of responses to different treatments are shown in the figure on the Right.  Note that a free-living worm, that is, one not associated with a host sea star, moves more or less randomly in the presence of “host-factor” (see Left depiction), as does a "commensal" worm Ophiodromus in the absence of host-factor (see Middle depiction).  However, a "commensal" worm Ophiodromus in the presence of host-factor becomes “behaviorally trapped” in the central stream (Right depiction). Davenport et al. 1960 Anim Behav 8: 209.

NOTE this word, perhaps coined by the authors, appears to be a combination of “tropism” (“turning” G.) that is more often applied to growth of plants, and “taxis”, applied to movement of animals.  The movement of the worm to the host-factor could also be defined in the more familiar terms of orthokinesis or change in linear velocity and klinokinesis or frequency of change of direction

NOTE  the chemical given off by a host sea star that acts as a directional cue for the worm.  In these trials the water coming from a beaker containing a host also carries fluoroscein dye

The scaleworm Arctonoe pulchra is symbiotic with several different echinoderms and molluscs.  In Santa Barbara, California and Friday Harbor, Washington these hosts include sea cucumbers Stichopus parvimensis and Parastichopus californicus, limpets Megathura crenulata, and sea stars Dermasterias imbricata and Luidia foliata.  Researchers at Friday Harbor Laboratories undertake experiments to assess the ability of A. pulchra to detect chemical signals emanating from their hosts using a simple Y-maze choice apparatus.  Interestingly, the worms consistently choose their original host when presented with effluents from a number of different, but sometimes closely related, organisms.  The responses of the worms appear then to be affected by previous experience. Indeed, in some cases a worm can be conditioned to respond positively to an organism that previously was unattractive.  The authors suggest that close physical contact may be necessary to effect such a change in olfactory response.  In circumstances where only a single large worm occupies a single host, it is because the worm is aggressive to other conspecifics and keeps them away.  Dimock & Davenport 1971 Biol Bull 141: 472. Photograph courtesy Dave Cowles, Walla Walla University, Washington. wallawalla.edu.

NOTE >20mm length