Sternaspis affinis  Common name(s): Ground-digger dumb-bell worm, mud-owl, gooseberry worm
Synonyms:
Phylum Annelida  Class Polychaeta Order Terebellida  Family Sternaspidae
	Sternaspis affinis, 1 cm long, ventral view. Anterior is to the left and the posterior ventral shield, chetae, and gills are to the right. The introvert is slightly extended on the left, exposing the genital papillae which are between thoracic segments 7 and 8. Collected from mud at 9m depth in Burrows Bay.
	(Photo by:  Dave Cowles, July 2019)

Description:  (from Sendall and Salazar-Vallejo, 2013):

Description of Family Sternaspidae: Sternaspidae is a worldwide polychaete family with just a few species. They are typically subtidal, marine, non-selective infaunal deposit feeders. They may live in rocky or coarse sand, shell hash, soft mud, or deep-sea clay. They have been collected down to depths of >4000 m but are more abundant < 200 m. They bury head-first in the sediment, with only the posterior gills exposed at the sediment surface. They can invaginate or evert the first several sections of their body (called an introvert, consisting of the head and 6 segments of thorax). Behind the introvert is a (slightly?) constricted region composed of thoracic segments 7 and 8. Behind this is the abdomen, which may be all that is visible when the animal has invaginated its introvert. The abdomen may be missing chetae on most of the anterior segments in adults, but it has a distinctive, relatively hardened, caudal shield on the posterior ventral surface. Abundant chetae surround the shield. The gills are on the posterior end, dorsal to the anus. Diagnosis (from Sendall and Salazar-Vallejo, 2013): Body peanut-shaped. Introvert with falcate, tapered or subdistally expanded hooks. Segments 7–8 constricted, with genital papillae protruding ventrally from between them. Pre-shield region of the abdomen with 7 or 8 segments. Ventro-caudal shield usually stiff, often provided with radiating ribs and concentric lines, rarely flexible. Marginal shield chaetal fascicles include lateral and posterior chaetae, sometimes peg chaetae or additional delicate chaetae present. Branchiae coiled, abundant filaments, emerging from two lateral dorsal plates, near the anus, or directly from the body wall. Additional, thinner coiled interbranchial papillae present.

Description of Genus Sternaspis (from Sendall and Salazar-Vallejo, 2013): The hooks on the introvert are tapered. The shield ornamentation incluses ribs, concentric lines, or both. The anterior abdomen has 7 segments. (The name Sternaspis means "breast shield", due to an early investigator "Otto" not realizing that the shield was on the back of the body, not on the front.)

Sternaspis affinis (rom Sendall and Salazar-Vallejo, 2013): Shield features: The shield has both shallow ribs and concentric lines. The anterior margins are rounded and the anterior depression is deep. The lateral margins are rounded, and the fan (the mid-posterior ribbed portion) is crenulated. The shield has 10 lateral shield chetae in an oval pattern and 5 posterior shield chetae in a line. Peg chetae (short chetae on the margin of the shield between the lateral and posterior chetae) are also present.  As with other members of the family, the animal is peanut-shaped when the introvert is retracted and only the abdomen is visible.

How to Distinguish from Similar Species:  Currently, this is considered to be the only species from this family present in the Northwest Pacific. the S. scutata mentioned in the Kozloff key is from the Mediterranean Sea and S. fossor mentioned in Lamb and Hanby (2005) and Carlton (2007) is from the northwestern Atlantic.

Geographical Range:  Southern Alaska to southern California.

Depth Range:  10-at least 409 meters

Habitat:  Infauna in soft bottoms. The individuals pictured here were found in mud.

Biology/Natural History:  These worms are infaunal direct deposit feeders, which feed with their head down and their entire body except for their posterior gills buried in the mud. Some have speculated that the shield serves as a defense for the posterior part of the body which is closest to the surface, but since the shield itself is buried this seems less likely. A series of internal muscles attaches along the outer perimeter of the shield (which may be flexed to a certain extent) so maybe the shield simply serves as an anchor point for those muscles, or anchors for the chetae for defense. Although they look chitinous, neither the shield nor the chetae are made of chitin. Lowenstam (1972) concluded that the shields are composed of 33% ferrous oxide, 22.4% phosphorus pentoxide, 3.4% calcium oxide, 2.8% magnesium oxide, 0.1% barium oxide, and 0.04% manganese oxide; and the chetae may contain calcium phosphate.

Sternapsis species have separate sexes. Their larvae spend only about 2 days as plankton before settling. 

Much more information about the organism can be found in Sendall and Salazar-Vallejo (2013)

References:

General References:
  Lamb and Hanby, 2005 (as Sternaspis fossor)

Scientific Articles:
Lowenstam HA., 1972. Phosphatic hard tissues of marine invertebrates: Their nature and mechanical function, and some fossil implications. Chemical Geology 9: 153-166 doi: 10.1016/0009-2541(72)90053-8

Sendall, Kelly, and Sergio I. Salazar-Vallejo, 2013. Revision of Sternaspis Otto, 1821 (Polychaeta, Sternaspidae) Zookeys, 2013; (286) pp 1-74

Web sites:

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

This dorsal view shows the posterior gills to the right and the invaginated introvert (anterior) on the left. Note the red, hemoglobin-filled blood in the gills and the whitish-colored filaments that are interspersed with them. 
In life, the gills move in a writhing motion, likely to facilitate blood movement and gas exchange. For videos of this movement click here and here.

A close-up view of the ventral shield, chetae, and gills at the posterior end

This ventral view of the anterior end shows the genital papillae. These papillae project from between the 7th and 8th thoracic segments. When the animal is fully retracted, only the abdominal segments are visible so the papillae cannot be seen. Since they are visible, the anterior introvert (head and thoracic sections) is slightly protruded but the anterior 6 thoracic segments and the head are still invaginated inside.

In this photo the introvert (head and thoracic segments) is mostly extended, doubling the animal's length to 2 cm. Note the 3 sets of parapodia with chetae on some of the thoracic segments. Click here for a movie of the animal extending its introvert.

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