Arctonoe fragilis (Baird, 1863)

Common name(s): Commensal scaleworm, Ruffled scaleworm, Fragile scaleworm, Frilled commensal scaleworm, Scaleworm

Synonyms:  Polynoe fragilis, formerly part of Arctonoe vittata
Phylum Annelida
 Class Polychaeta
  Order Phyllodocida
   Superfamily Aphroditidae
    Family Polynoidae
Actonoe fragilis on the ray of its host seastar, Evasterias troschelii.
(Photo by: Dave Cowles, July 2005)
Description:  As with all Polynoids, this species is mostly benthic, few if any of the segments are longer than wide when contracted, the dorsal surface has clearly visible elytra, and all of the neurosetae are simple.  Arctonoe fragilis has more than 18 pairs of elytra which have extensively ruffled or folded edges, leave a large gap between them down the center of the body, and are not on every segment posterior to segment 38, but are on every other segment near the posterior end of the body.  The point of attachment of the elytrum is often white.  The species has no prominent nuchal fold.  The prostomium has a pair of palps, a pair of tentacles, and 3 pair of antennae, the lateral pair of which are inserted slightly ventral to the edge of the prostomium, notosetae are present, and there is no dark band across the body behind the head on segments 7 and 8.   Two pairs of black eyes on the upper side of the prostomium.  Usually pale colored, similar to the seastar Evasterias troschelii which is its most common host.   Width to 1 cm and length to 8.5 cm.

How to Distinguish from Similar Species:Arctonoe pulchra and Arctonoe vittata have elytra whose margins are smooth or only slightly ruffled, plus Arctonoe vittata has a dark band across segments 7 and 8 behind the head.

Geographical Range:  Alaska peninsula to San Francisco Bay, CA (uncommon in CA)

Depth Range:  Low intertidal to 275 m

Habitat:  Mostly symbiotic on the seastar Evasterias troschelii

Biology/Natural History:  These worms are most often commensals on painted seastars Evasterias troschelii, but it may sometimes be found on other species of seastar or may be freeliving.  Alternate hosts include Leptasterias aequalisLeptasterias hexactis, Orthasterias koehleri, Stylasterias forreri, Luidia foliata, Pisaster ochraceous, and Solaster dawsoni.  They are attracted by the smell of their host, at least as long as the host is uninjured.  Usually only one individual occurs on a host at a time (they are antagonistic to one another), but up to 4 have been found.  They feed on detritus

Members of Family Polynoidae, unlike most other errant polychaetes, have parapodia specialized for walking rather than as paddles.  Their longitudinal muscles, which caused lateral undulations in other polychaetes, are poorly developed and they don't undulate much.  As a result, although they can walk efficiently they are poor swimmers.

Return to:
Main Page Alphabetic Index Systematic Index Glossary


Dichotomous Keys:
  Flora and Fairbanks, 1966
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Brusca and Brusca, 1978
  Harbo, 1999
  Morris et al., 1980
  O'Clair and O'Clair, 1998
  Ricketts et al., 1985
  Sept, 1999

Scientific Articles:

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:
On Solaster
These two views show Arctonoe fragilis on the ray of Solaster stimpsoni. Photos by Dave Cowles, July 2005

This large 7 cm-long individual was commensal on the seastar Orthasterias koehleri.  Click Here to see the worm on its host seastar.

On Evasterias troschelii Arctonoe fragilis symbiont on Evasterias troschelii.

Authors and Editors of Page:
Dave Cowles (2005):  Created original page and updated it since then