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  Life in the intertidal zone
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Ocean acidification


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

Research study 1

graph showing growth of juvenile sea stars Pisaster ochraceus at different temperatures and carbon-dioxide concentrationsgraph showing feeding rates of juvenile sea stars Pisaster ochraceus at different temperatures and carbon-dioxide concentrationsEcological effects of climate change are increasingly becoming evident, but what about physiological effects related to increasing ocean acidification and temperature?  This is investigated by researchers at the University of British Columbia, Vancouver using growth in sea stars Pisaster ochraceus feeding on mussels as an indicator of physiological response to different combinations of  temperature and carbon-dioxide concentration.  Interestingly, and contrary to results from similar types of studies on other marine invertebrates, not only does feeding and growth increase with increasing temperature, as expected, but both parameters respond positively to higher carbon-dioxide concentration (see graphs).  Content of calcified ossicles actually decreases over the course of the study, but still is accompanied by soft-tissue growth.  The authors think the difference in response to carbon-dioxide concentration between their study and other similar studies on echinoderms and molluscs may relate to a lesser relative dependence upon a calcified endoskeleton in P. ochraceus as compared with, say, in heavily calcified ophiuroids and echinoids.  The study represents a valuable addition to the growing scientific literature on effects of climate change in the ocean environment.  Faster future growth of this keystone predator could have significant consequences on the dynamics of intertidal-mussel communities. Gooding et al. 2009 Proc Nat Acad Sci 106: 9316.

NOTE  a value of 380ppm is used to represent current carbon-dioxide levels in the ocean, and this is approximately doubled to 780ppm in the growth experiments to represent a climatically realistic value based on predictions for the year 2100. The study suffers from a fault common in other such investigations on predicted future climatic events; namely, the tests are done at acute levels, whereas Pisaster in the future would have many generations of past experience with gradually increasing carbon dioxide levels, and consequently there may be acclimatisation/heritable factors not being accounted for 

NOTE  the authors cite a value of 11% for mass of calcareous ossicles relative to total live mass in P. ochraceus, which corresponds to a dry-mass value of about 60-65%.  On the basis of this, one might argue that the species is not “lightly” calcified in comparison with other invertebrates.  The authors’ contention, however, is that unlike in sea urchins, mollusks, and brittle stars that have containment-type skeletons, the comparatively loosely arranged system of endoskeletal ossicles in Pisaster and other asteroids possibly would not be so physically restrictive to soft-tissue growth

Research study 2

graph comparing growth rates in larvae and juveniles of sea stars Crossaster papposus at pHs of 8.1 and 7.7photographs of juvenile sea stars Crossaster papposus at 38d of age grown at pHs of 8.1 and 7.7Results of a concurrent study by Swedish workers on development of the sea-star Crossaster papposus1 also support the idea that ocean acidification of the level predicted for centurary’s end may not be all bad for marine organisms.  The researchers show that when grown at low pH2over a 38d period, larvae and juveniles grow faster than at normal seawater pH, with no observable effects on survival or ossicle formation (see photographs on Right and graph3 on Left).  Ossicle development begins in the rudiment at settlement of the larva (about 18d post-fertilisation at 12oC). Crossaster papposus has non-feeding lecithotrophic larvae and, in the authors’ view, may possibly be less susceptible to environmental changes than asteroid species with planktotrophic larvae.  Dupont et al. 2010 J Exp Zool photographs of ossicle development in juvenile sea stars Crossaster papposus in pH seawater of 7.7 and 8.1(Mol Dev Evol) 314B: 382.

NOTE1 this species is cosmopolitan and is found in Europe, and in the north Atlantic and Pacific regions.  The study is included here to promote interest in the field of climate-change induced acidification in west-coast species.  The authors remark that echinoderms are popular candidates for such studies based on their comparative richness in marine communities and on the fact that their skeletal parts are made up of magnesium calcite, which is 30 times more soluble than normal calcite

NOTE2  a low-pH 7.7 treatment is created by maintaining pCO2 in the air above the culture vessels at 930ppm (normal seawater of pH 8.1 has pCO2  = 372ppm).  A pH of 7.7 is equivalent to that predicted to occur in the year 2100.  Both this study and Research Study 1 above are open to criticism for their failure to consider acclimatisation that might occur from now to Century’s end.  At first glance there would appear to be no solution to this, but other researchers have approached it from the standpoint of heritability, and that is indeeds measurable (see LEARN ABOUT SEA URCHINS/PH & OCEAN ACIDIFICATION/RS4 and RS2)

NOTE3  given that each point in the graph is based on the mean of 20 individuals, the authors appear to have made an error in the fitting of the regression line for pH 7.7 for the larvae