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  Life in the intertidal zone
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Waves & currents


The topic of life in the intertidal zone includes a section on waves & currents considered here, and sections on TEMPERATURE & DESICCATION, SALINITY & OSMOTIC REGULATION, SEASTAR WASTING DISEASE, OCEAN ACIDIFICATION, OTHER PHYSIOLOGICAL STRESSES, COLOUR MORPHS OF PISASTER, and SYMBIONTS presented elsewhere.

Research study 1

graph showing relationship of arm shape in ochre stars Pisaster ochraceus with degree of wave-exposure in habitatAlthough the idea in retrospect seems obvious, it has taken a long time for someone to investigate whether intertidal sea stars alter their shape in response to wave action.  Researchers at Bamfield Marine Sciences Centre, British Columbia compare arm dimensions of ochre stars Pisaster ochraceus  at 4 sites differing in degree of wave exposure.  They find that individuals in wave-exposed habitats have arms that are narrower and lighter in mass than ones in more sheltered habitats (see graph).  Reciprocal translocations of individuals between wave-exposed and wave-protected habitats leads to significant arm-slimming or arm-fattening depending upon direction of transfer, within 3mo, indicating that the change is a phenotypic plastic response to environmental stimulus.  The authors suggest that the advantage of being slim-armed in wave-exposed conditions is most likely in reduction of lift and drag forces (the authors’ calculations of theoretical coefficients of lift and drag support this).  Why not be thin-armed in all areas?  Fatter arms provide more space for digestive- and energy-processing pyloric ceca and for gonads.  The work is a nice example of adaptive morphology, and the authors are to be complimented for their interesting story.  Hayne & Palmer 2013 J Exp Biol 216: 1717.

NOTE  degree of water motion is compared among sites using a type of device that measure corrosion rates of dissimilar metals

NOTE  individuals are marked for later recovery with coded bandings of neutral red dye applied to the lighter-coloured aboral side of an arm

photo schematic comparing relative arm sizes in ochre stars Pisaster ochraceus from habitats differing in degree of wave exposure Arm form is defined by aspect ratio, or L/W. 
A smaller aspect ratio, then, indicates a fatter
arm; a bigger ratio, a slimmer arm.  Arms of
individuals from the most wave-exposed site
are 12% slimmer than arms of individuals from the
most wave-sheltered site for a given arm length