Orthasterias koehleri (de Loriol, 1897)

Common name(s):  Rainbow star, long-armed sea star, longrayed starfish, red banded sea star, painted star, fragile star

Synonyms: Asterias koehleri, Orthasterias columbiana Orthasterias koehleri
Phylum Echinodermata
Class Asteroidea
Order Forcipulatida
Suborder Asteriadina
Family Asteriidae
Orthasterias koehleri from 70 m depth, San Juan Channel.  This individual recently lost two of its rays.  Ray length is 20 cm.
(Photo by: Dave Cowles, July 2008)
Description:   This large, brightly colored seastar ideally has 5 long arms (rays), though since it often suffers loss of one or more rays and grows them back only slowly it often is seen with less than 5 rays.  Although there are several different kinds of spines and plates along the rays, its marginal plates are not greatly enlarged.  The aboral row of plates on each ray contains very long spines, often 4-5 mm long, with each spine surrounded with a fleshy cushion of small crossed pedicellariae (photo).  This row of spines is fairly well-defined along the aboral surface and is called the carinal spines (photo).  There are several ragged rows of similar spines along each ray lateral to the carinal row (photo).  Bordering the ambulacrum on the oral side are mouth plates which are usually so sunken that they cannot be seen, then a row of prominent adambulacral plates, each of which contains two diverging spines (photo), then a row of small "oral intermediate" spines.  Most large spines and many areas between the spines contain abundant pedicellariae, many of them prominent, and of many different sizes and shapes (photo).  The papulae, coelomic pouches used for respiration and excretion, are also large and numerous, especially near the oral side next to the adambulacral plates (photo).  The rays can be up to 25 cm long each, and the central disk diameter is only about the same as the diameter of the rays, or about 1/6 to 1/10 the ray length.  The color is blotchy, generally with various colors of pinkish red with white or tan or cream but some individuals may be straw colored or blue.  The large spines are usually white but may be purple.

How to Distinguish from Similar Species: Stylasterias forreri is similar in shape and spines but is brown, olive, or gray without red and has differences in its spines and pedicellariaeEvasterias troschelii has a generally similar body shape and variable coloration but its rays are thickest a short distance from the central disk and it does not have the pronounced series of large spines surrounded by fleshy cushions of pedicellariae.

Geographical Range: Eastern Aleutian Islands to northern Mexico.

Depth Range:  Intertidal or (usually) subtidal to 283 m or more

Habitat:  Rock, pebbles, sand, and mud

Biology/Natural History:  Studies have indicated that this species may eat a variety of prey such as snails, limpets, bivalves, chitons, squid, brachiopods, barnacles, crabs, fish, and tunicates but it seems to especially eat bivalves.  It can use its large tube feet to pull clams up out of the sediment, then chips away some of the margin of the shell until it can insert its stomach and digest the clam.  Breeding occurs from June to August.  Spawning individuals elevate themselves from the bottom on their arm tips.  Lifespan is at least 9 years.  The polychaete worm Arctonoe fragilis is commensal on this seastar (photo).



 

References:

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

General References:
  Furlong and Pill, 1972
  Gotshall and Laurent, 1979
  Harbo, 1999
  Kozloff, 1993
  Johnson and Snook, 1955 (As Orthasterias columbiana)
  Lambert, 2000
  Lamb and Hanby, 2005
  Morris et al., 1980
  Niesen, 1997
  Sept, 1999

Scientific Articles:
Mauzey, K.P., C. Birkeland, and P.K. Dayton, 1968.  Feeding behavior of asteroids and escape responses of their prey in the Puget Sound region.  Ecology 49: 603-619

Web sites:


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

Older references such as Johnson and Snook (1955) state that this species is common intertidally.  However, my experience and several more recent references confirm that now this species is much less common intertidally than subtidally.

Aboral view of ray
This aboral view (in other words, view from the top) of the ray shows the line of large carinal spines down the aboral surface and the ragged rows of spines on each side.  Notice the cushions of small pedicellariae which surround each spine.  Also notice the large individual pincer-like pedicellariae and the abundant papulae between the spines.  Photo by Dave Cowles, July 2008


Ray lateral view
This lateral view of the rays shows the scattered rows of spines, the abundant large pedicellariae and papulae among the spines, and the rows of adambulacral spines along the bottom.
 


Views of the oral (ambulacral) region of the rays.  Photos by Dave Cowles, July 2008
Lateral view of ambulacrum Oral (ambulacral) view of ambulacrum
This lateral view of the ambulacral area of the rays shows the scattered rows of ray spines surrounded by cushions of small pedicellariae along the sides of the ray.  Below that is a section of prominent papulae which are largely obscuring the row of small oral intermediate spines.  At the bottom of the ray can be seen the forked adambulacral spines which also have pedicellariae around them, and the tube feet are at the very bottom. This oral (ambulacral) view of a ray shows the 4 rows of long tube feet.  The forked adambulacral spines are visible along the margins of the ambulacral groove, though the tissue around the spines is so dense that the two forks look like two separate rows.  The mouth plates, which are between the ambulacral groove and the adambulacral spines are apparently sunken and not visible.


Spines and pedicellariae
This closeup view shows the abundant pedicellariae and papulae among the large spines.  The small crossed pedicellariae in the cushions around the spines are mostly retracted in this view but a number of the individual pedicellariae between the spines are open.


Arctonoe fragilis symbiont
The polychaete worm Arctonoe fragilis is often found as a commensal on this seastar.  I cannot distinguish the dorsal plates (elytra) on this worm but perhaps they have broken off and simply left the white bases.  The worm seems to spend the large majority of the time on the aboral surface of the rays, even though that is where all the large pedicellariae are.  I wonder what kind of relationship it and the seastar have that prevents the worm from being severely pinched?



Authors and Editors of Page:
Dave Cowles (2008):  Created original page
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