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  Life in the intertidal zone
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  Colour morphs of Pisaster
 

The topic of life in the intertidal zone includes a section on other physiological stresses considered here, and sections on TEMPERATURE & DESICCATION, SALINITY, OTHER PHYSIOLOGICAL STRESSES, and SYMBIONTS presented elsewhere.

 
Research study 1
 

For several decades scientists have been interested in coloration of Pisaster ochraceus.  The polymorphism is striking, with individuals on some beaches within a few meters of one another being brilliant purple, orange, or a range of intermediate colours.  The greatest admixture of colours photograph of colour morphs of ochre stars Pisaster ochraceusoccurs in wave-exposed open-coast habitats, while the greatest conservancy of colours (mainly purple hues) occurs in quiet-water habitats.  Two main carotenoid pigments have been isolated and identified: mytiloxanthin, which is present in mussels Mytilus spp. and presumably accumulated in the sea stars through feeding, and astaxanthin, which is an end-product of specific metabolic pathways.  Both observations suggest that the coloration is environmentally driven, but solid evidence has been hard to come by.  A major reason for this is that juveniles may be quite uncommon, especially in protected waterways and, because it may require 4-6 years to rear an individual to full expression of colour, long-term diet studies have not been done, nor have the appropriate genetic crosses been undertaken.  Researchers, however, have not lain idle, as the next entries show.  Fox & Scheer 1941 Biol Bull 80: 441. Photograph courtesy Rafe Sagarin, Duke University, Durham, NC.

NOTE animals generally do not produce carotenoids

Colour morphs of ochre stars
Pisaster ochraceus 0.3X

 
Research study 2
 

photograph of colour morphs of ochre stars Pisaster ochraceus in Barkley Sound, British Columbiacluster diagram of sea stars genetically analysed for colour-morph study of ochre stars Pisaster ochraceusAspects of coloration in Pisaster ochraceus1 is investigated in a large-scale study involving collections from Alaska to southern California (see map).  In addition to recording colours of individuals collected, the researchers tally the prey being eaten and prey types available in each site, and body size map of collecting sites for study  of coloration in ochre stars Pisaster ochraceusof the sea stars, extent of injuries, and various characteristics of the habitats.  They then analysed the genetic makeups of the populations using sequence variation in mitochondrial cytochrome oxidase I gene in preparations of tube feet. 

Cluster analysis of colour, diet, size, and injury discloses 3 strong geographic groupings: 1) wave-exposed shores of California and Oregon, 2) wave-exposed areas of northwest Washington (Olympic coast), and 3) sheltered areas of Georgia Strait and Puget Sound (see diagram above Right).

histograms showing colour-morph frequencies of ochre stars Pisaster ochraceus in different geographic areas of the west coast of North AmericaColours in the first 2 sites2 are 6-28% orange and 68-90% brown, respectively, while the third site is 95% purple (see colour histograms on Right).

Diet seems to be implicated in the colour distributions, with a few exceptions (see histograms lower Left). Thus, the sea stars in the first 2 sites are eating 15-78% mussels histograms showing diets of ochre stars Pisaster ochraceus in geographic areas of the west coast a propo their involvement in colour differencesMytilus californianus, while in the third site they are eating 33-70% acorn barnacles and 9-39% gastropods. The authors also suggest that salinity may be involved as a causative factor, as the Georgia Strait/Puget Sound region is characterised by more estuarine conditions in comparison with the 2 higher
salinity open-coast regions.

sizes of ochre stars Pisaster ochraceus in different geographical areas of the west coastOchre stars are smaller3 in the first region (California and Oregon) than in the other 2 regions (see histogram lower Right).

The genetics part of the study reveals a low population-genetics structure, suggesting that gene flow is high, expected based on the presence of a long-lived, widely distributing planktotrophic larva in the life cycle.  High gene flow suggests that the geographic pattern in colours is not a leftover from a Pleistocene glacial refugium4

The authors conclude that the colours are largely ecologically controlled, likely with a genetic component, and with diet and salinity featuring strongly as causative factors.  Because of the general absence of Mytilus californianus in the third region (Georgia Strait/Puget Sound) and thus the absence of the orange pigments it contains, the authors suggest that brilliant purple may be a default coloration in Pisaster ochraceus.  Harley et al. 2006 Biol Bull 211: 248.

NOTE1 lit. “pale yellow” G., referring generally to yellow to orange (also yellow-brown, reddish-brown) coloration

NOTE2 there is remarkable historical consistency in colours at some sites.  For example, in Pacific Grove, for which data go back to 1947, orange colour-morphs appear in the following percentages: 1947 (25%), 1951 (25%), 1997 (28%), and 2004 (27%). In the Pacific Grove area, P. ochraceus is eating 30-60% M. californianus.  Data from Harley et al. 2006 Biol Bull 211: 248

NOTE3 "size" in the graph is based on the radius of the longest arm

NOTE4 such refugia are considered elsewhere in the ODYSSEY: LEARN ABOUT WHELKS & RELATIVES: REPRODUCTION: DISPERSAL, GENETIC HETEROZYGOSITY, & GLACIAL REFUGIA

 
Research study 3
 

map showing collecting sites along the west coast of North America used in a study of colour polymorphism in ochre stars Pisaster ochraceushistogram showing percentage orange morphs in populations of ochre stars Pisaster ochraceus ranging from southern California to northern OregonAnother perspective on causes of colour morphs in Pisaster ochraceus is provided in a more recent study.  Here, researchers assess colour frequencies in 14,720 specimens of P. ochraceus from 26 coastal locations from northern Oregon to southern California, and find that orange graph showing percentage orange morphs with increasing size of ochre stars Pisaster ochraceus at sites along the west coastmorphs comprise 20% of all samples.  There is relatively little variation (for all sites: 13-27%) across the latitudinal range of 1850km encompassed in the survey (see histogram above Right). The remarkable aspect of the results is that the frequency of orange morphs increases with body size, a feature apparently never noticed in other studies (see graph lower Right).  This proportion does not change seasonally, nor do the orange morphs segregate into different intertidal habitats or aggregate based on colour.  After considering several alternate hypotheses, the authors conclude that colour in P. ochraceus is a selectively neutral genetic trait that is expressed ontogenetically. The authors raise many interesting questions in their paper and call for research into the genetic basis for colour in Pisaster, mating experiments to determine inheritance patterns, and studies on dietary and other environmental effects on colour, in order to solve the continued mystery of this striking colour polymorphism.  Raimondi et al. 2007 Pac Sci 61: 201.

NOTE  for convenience, the sea stars are scored as either “orange” or “purple” (all brown and darker shades are scored as “purple” because evaluators in the field have difficulty in differentiating these colours consistently)

NOTE  the authors note that regenerating arms in orange morphs are often purple in colour, possibly representing reversion to an early ontogenetic state

 

These photos show several orangeish-coloured Pisaster ochraceus with varying degrees of (residual?) purple pigmentation.. Some individuals appear to have purple arm tips, while others have a purple cast on an otherwise orange-colored body. Based on the conclusions from the foregoing Research Study, these individuals are becoming more orange as they grow. But what is the adaptive value of colour to the sea star? Photos courtesy Rafe Sagarin, Duke University, Durham, NC.

photograph of 2 Pisaster ochraceus, one typical showing purple coloration; the other, purple arm tips photograph of Pisaster ochraceus showing purple arm tips on an otherwise orange body
photograph of Pisaster ochraceus showing purple cast an otherwise orange body photograph of Pisaster ochraceus showing purple arm tips on an otherwise orange body photograph of Pisaster ochraceus showing purple cast on an otherwise orange body
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Here are a few hypotheses regarding the adaptive value of colour morphs in ochre stars Pisaster ochraceus. One or two of these hypotheses might be worth pursuing, but others are "iffy" and/or make little sense. Think about them, then CLICK HERE for explanations.

Pisaster ochraceus use orange colour as an indicator of size and, thus, as an indicator of value of a mate. 

Orange acts to camouflage at depth. 

Orange pigment is metabolically cheaper than purple so, if you’re going to be coloured, go for the more economical version. 

Orange individuals are less palatable than purple and thus the orange colour acts as a warning signal. 

Orange morphs suffer less heat stress in summer through their greater reflectivity. 

 
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