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  Life in the intertidal zone
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  Symbionts
 

Three species of polynoid scaleworms live symbiotically with west-coast asteroids. About three-quarters of the commonly seen shallow-water sea stars in British Columbia are hosts to these worms, with Arctonoe vittata being the most common worm involved, A. fragilis the next most common, and A. pulchra the least. Only a few sea-star species, e.g., Luidia foliolata, Solaster dawsoni, and Solaster stimpsoni, appear to host more than one scaleworm species. As confirmed in Research Study 2 below, A. vittata tends to associate with sea stars that lack pedicellariae, while A. fragilis, in contrast, tends to be more common on sea-star hosts that have pedicellariae. Lambert 1981 The sea stars of British Columbia BC Prov Mus Handbook No. 39 Province of BC.

NOTE more on scaleworms and sea stars can be found at LEARN ABOUT SANDWORMS & RELATIVES: SYMBIOSES: SCALEWORMS

   
 

The topic of life in the intertidal zone includes a section on symbionts considered here, and sections on TEMPERATURE & DESICCATION, SALINITY, OCEAN ACIDIFICATION, OTHER PHYSIOLOGICAL STRESSES, and COLOUR MORPHS OF PISASTER, presented elsewhere.

 
Research study 1
 

graph showing relationship between seawater temperature and degree of infestation of worms Ophiodromus pugettensis on bat stars Patiria miniata in Long Beach, CaliforniaBat stars Pateria miniata in southern California commonly host a commensal hesionid polychaete Ophiodromus pugettensisStudies on frequency of occurrence of worms and their hosts in the Long Beach area using tagged sea stars show that incidence of infestation peaks in winter (64-92%) and wanes in summer (19-61%, see graph).  Individual sea stars have 1-2 worms in winter and 0-1 worms  in summer.  Single and multiple recapture of tagged sea stars show that the worms move about, presumably from host to host, with the extent of change in number of worms being dependent upon season.  The authors provide data that loosely correlate incidence of infestation of Patiria with water temperature and, thus, possibly with reproductive period of the photograph of leather star Patiria miniata infested with commensal polychaetesworms. Lande & Reish 1968 Bull Southern Cal Acad Sci 67: 104.

NOTE  this species is found on various sea stars and sea cucumbers and, as well, is free-living

NOTE  plastic tags are attached with stainless steel wire twisted through an arm.  The author notes that dyes (indulen, fast green, silver nitrate), water-insoluble inks, and knotted nylon threads are unsuccessful


Several symbiotic worms, possibly Ophiodromus pugettensis,
inhabit the oral surface of a bat star Patiria miniata 1X

 
Research study 2
 

photograph of leather star Dermasterias imbricata courtesy Ron Long, Simon Fraser University, Burnabyphotograph of head end of scaleworm Arctonoe vittata courtesy Dave Cowles, Walla Walla University, WashingtonNine species of west-coast asteroids, including the leather star Dermasterias imbricata, are known to be hosts to the polynoid scaleworm Arctonoe vittata.  The worm lives on another dozen or so hosts, including keyhole limpets Diodora aspera, gumboot chitons Cryptochiton stelleri, and on other molluscs, and on several species of polychaetes.  Because the worms presumably gain something from their hosts, and the hosts appear not to be benefitted or harmed, the relationship is usually thought to be a commensalism.  Another point of view, that the relationship is actually a mutualism, is proposed by researchers at the Bodega Marine Laboratory, California. They find that in Y-tube experiments Dermasterias will preferentially seek out its symbiont over food, other Dermasterias, and other species of scaleworms. 

Typical results are as follows.  When Dermasterias is tested with one arm of the Y-tube empty and the other arm containing Arctonoe vittata, it chooses the worm side in 34/40 trials.  Are the sea stars just looking for food?  A test with one arm of the Y-tube containing a sea anemone Anthopleura elegantissima and and the other arm containing Arctonoe leads to Dermasterias selecting the worms 16/18 times.  A choice between Dermasterias and Arctonoe leads to selection of the worms 14/18 times.  Although Dermasterias seems attracted to other scaleworm species (for example, Arctonoe fragilis: 15/20 trials with the other arm of the Y-tube being empty), when the choice is between A. fragilis and A. vittata there is no doubt of the outcome (16/20 in favour of A. vittata). 

The results suggest that the host may be deriving benefit from its symbiont, but what could this could be is not clear.  The authors suggest cleaning, as they have observed Arctonoe catching and eating amphipods crawling over the surface of Dermasterias, and the worms may also eat detritus. This is interesting because D. imbricata lacks pedicellariae and, in fact, all of the other common sea-star hosts of A. vittata lack pedicellariae.  However, since A. vittata is an obligate symbiont, the authors query how the attraction could have evolved (i.e., Dermasterias would be unlikely to have attracted symbionts from a pool of free-living worms).  Instead, the authors suggest that the "seeking-out" behaviour may work to attract symbionts from other hosts, or to replace missing symbionts.  An interesting research question might be to see whether Dermasterias is attracted to a worm on another host and, if so, whether the worm would ever change hosts, especially from a non-Dermasterias host.  Wagner et al. 1979 J Exp Mar Biol Ecol 39: 205.Photograph of Dermasterias imbricata courtesy Ron Long, Simon Fraser University, Burnaby; photograph of Arctonoe vittata courtesy Dave Cowles, Walla Walla University, Washington rosario.wallawalla.edu.

NOTE  or simply parasitism if you are more of a “lumper” than a “splitter”.  The major difficulty with the concept of commensalism is that it is almost impossible to know what costs might be incurred by the host from the presence of the symbiont

NOTE  another echinoderm host of the worm is Cryptochiton stelleri, which also lacks pedicellariae or other cleaning devices. This brings up several possible research questions: if placed on a forcipulate (pedicellaria-bearing) asteroid does A. vittata get bitten? if so, does it not like getting bitten? does A. fragilis, which commonly inhabits forcipulate asteroids, not get bitten? or do they get bitten and not mind it? what about A. pulchra's response to pedicellariae, this species being the least common symbiont on west-coast asteroids?

   
  Photographs of west-coast asteroids with symbionts:
 

photograph of a sea star Orthasterias koehleri with a polynoid worm Arcotonoe fragilis crawling on it
Orthasterias koehleri with Arctonoe sp.

Polynoid scaleworm Arctonoe sp. crawling on it courtesy Dave Cowles, Walla Walla University, Washington
Scaleworm Arctonoe sp. on sea star Evasterias troschelii
photograph of scaleworm Arctonoe fragilis on a sea star Orthasterias koehleri
Arctonoe fragilis on the arm of Orthasterias koehleri
photograph of a scaleworm Arctonoe sp. in the ambulacral groove of a sea star
Scaleworm Arctonoe fragilis in ambulacral groove
photograph of blood star Henricia sanguinolenta with commensal scaleworm
Henricia sanguinolenta with scaleworm Arctonoe sp.
photograph of scaleworm Arctonoe vittata in ambulacral groove of sea star Dermasterias imbricata
Leather star Dermasterias imbricata with A. vittata
 
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