Halosydna brevisetosa Kinberg, 1855 Common name(s): Eighteen-scaled worm
Synonyms: Halosydna johnsoni, Polynoe brevisetosa
Phylum Annelida  Class Polychaeta Subclass Errantia  Order Phyllodocida  Family Polynoidae
	Halosydna brevisetosa, about 2.7 cm long
	(Photo by:  Dave Cowles, August 2018)

Description:  As with other members of Family Polynoidae, the dorsal side of this species is covered with a series of platelike elytra scales. Halosydna brevisetosa has 18 pairs of shield-shaped elytra (photo). The elytra are often light brown with white or black spots at the point of attachment. The dorsal  chetae are much shorter than the ventral ones, leading to the brevisetosa part of the name. Typically brown or grayish. Often has white or black spots on each elytrum where its stalk attaches to the animal. May have transverse color bands. Up to 11 cm long, but usually 2 cm or shorter if not symbiotic.

How to Distinguish from Similar Species:  No other local scaleworms have 18 pairs of elytra.

Geographical Range:  Alaska to northern Mexico. Very common in California intertidal.

Depth Range:  Intertidal to 545 m.

Habitat: In mussel beds, on floats, seaweed holdfasts, or symbiotic in the tubes of other polychaetes.

Biology/Natural History:  This very common predatory species is often found living symbiotically in the tubes of other polychaetes (especially Terebellids), on the nudibranch Melibe leonina where it feeds on the nudibranch's fecal pellets, or under clumps of Mytilus californianus mussels. It often eats other worms, scavenges, or eats detritus. When disturbed, it may shed some of its elytra scales but it can grow them back within a few days. Sexes are separate, and gonads are found in segments 12-34. The gametes can be clearly seen through the body wall. The sperm are white and the eggs are pale green. Fertilization is external, with the gametes released through the nephridium (kidney duct). Larvae generally settle when they are about 0.9 mm long and have 11 segments and 4 pairs of elytra. 
There is a space between the dorsal surface of Halosydna brevisetosa and the dorsal elytra. Cilia on the body surface pump water posteriorly through this space, while the coelomic fluid below the body wall moves anteriorly in a countercurrent manner. This likely enhances gas exchange and oxygenation of the coelomic fluid. The anus is unusual in that it exits dorsally, anterior to the pygidium. The nervous system can generally be seen as a reddish color because of the hemoglobin associated with it.

References:

General References:
  Harbo, 2011
  Kozloff, 1993
  Lamb and Hanby, 2005
  Morris et al., 1980
  Niesen, 1994
  Niesen, 1997
  Ricketts et al., 1985
 

Scientific Articles:
 

Web sites:

General Notes and Observations: Locations, abundances, unusual behaviors:
 

A dorsal closeup of Halosydna brevisetosa, showing the elytra.  Stacked photo by Dave Cowles, August 2018
 

A ventral view of the same animal, showing the parapodia. Stacked photo by Dave Cowles, August 2018
 
 

A dorsal view of the head. Photo by Dave Cowles, August 2018
 

A ventral view of the anterior end. Note the mouth on the peristomial segment and the red ventral streak denoting the central nervous system.
 

Another dorsal view of the whole animal. Stacked photo by Dave Cowles, August 2018
 

Authors and Editors of Page:
Dave Cowles (2018):  Created original page
CSS coding for page developed by Jonathan Cowles

Salish Sea Invertebrates web site provided courtesy of Walla Walla University