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Intertidal Seastars' Responses to Ocean Warming and Acidification
Reference
McElroy, D.J., Nguyen, H.D. and Byrne, M. 2012. Respiratory response of the intertidal seastar Parvulastra exigua to contemporary and near-future pulses of warming and hypercapnia. Journal of Experimental Marine Biology and Ecology 416-417: 1-7.

Background
The authors write that "Parvulastra exigua is a conspicuous member of the intertidal fauna of southeast Australia with a distribution spanning 7000 km of shoreline and is an important grazer on microalgae," citing Branch and Branch (1980), Arrontes and Underwood (1991), Stevenson (1992) and Jackson et al. (2009). In addition, they say that it is "also found in South Africa and has the broadest distribution recorded for the Asteroidea," citing Hart et al. (2006).

What was done
Working with adult P. exigua specimens collected from Little Bay, Sydney (Australia), McElroy et al. measured the metabolic rates of the seastars at conditions characteristic of high tide (ca. 18°C and pH 8.2), as well as at 3 and 6°C warmer conditions and at additional pH values of 7.8 and 7.6 "in all combinations," as they put it.

What was learned
The three Australian researchers report that "the metabolic response of P. exigua to increased temperature (+3°C and +6°C) at control pH [8.2] indicates that this species is resilient to periods of warming as probably often currently experienced by this species in the field." And they also report that they "did not observe a negative effect of acidification on rate of oxygen consumption at control temperature, a combination of stressors that this species currently experiences at night time low tide."

What it means
Although the metabolic response of P. exigua is resilient to current levels of extreme temperature and pH stress - which are equivalent to mean conditions predicted for the end of the 21st century - it is possible that the extreme seawater temperatures and pH levels at that future time (if IPCC predictions prove true) will be greater than the extreme levels of today, which could prove to be a real challenge for the seastars. However, McElroy et al. write, in the concluding paragraph of their research report, that "species such as P. exigua with a broad distribution from warm to cold temperate latitudes may possess scope for adaptation (evolutionary change) and/or acclimation via phenotypic plasticity (Visser, 2008), as suggested for sympatric echinoid and ophiuroid species (Byrne et al., 2011; Christensen et al., 2011)." And based on the information archived under the heading of Evolution (Aquatic Life) in our Subject Index, that possibility is beginning to look like a real likelihood.

References
Arrontes, J. and Underwood, A.J. 1991. Experimental studies on some aspects of the feeding ecology of the intertidal starfish Patiriella exigua. Journal of Experimental Marine Biology and Ecology 148: 255-269.

Branch, G.M. and Branch, M.L. 1980. Competition between Cellana tramoserica (Sowerby) (Gastropoda) and Patiriella exigua (Lamarck) (Asteroidea), and their influence on algal standing stocks. Journal of Experimental Marine Biology and Ecology 48: 35-49.

Byrne, M., Selvakumaraswamy, P., Ho, M.A., Woolsey, E. and Nguyen, H.D. 2011. Sea urchin development in a global change hotspot, potential for southerly migration of thermotolerant propagules. Deep-Sea Research II 58: 712-719.

Christensen, A.B., Nguyen, H.D. and Byrne, M. 2011. Thermotolerance and the effects of hypercapnia on the metabolic rate of the ophiuroid Ophionereis schayeri: inferences for survivorship in a changing ocean. Journal of Experimental Marine Biology and Ecology 403: 31-38.

Hart, M.W., Carson, C.K., Dartnall, A.J. and Byrne, M. 2006. Morphological and genetic variation indicate cryptic species within Lamarck's Little Sea Star, Parvulastra (= Patiriella) exigua. Biological Bulletin 210: 158-167.

Jackson, A.C., Murphy, R.J. and Underwood, A.J. 2009. Patiriella exigua: grazing by a starfish in an overgrazed intertidal system. Marine Ecology Progress Series 376: 153-163.

Stevenson, J.P. 1992. A possible modification of the distribution of the intertidal seastar Patiriella exigua (Lamarck) (Echinodoermata: Asteroidea) by Patiriella calcar (Lamarck). Journal of Experimental Marine Biology and Ecology 155: 41-54.

Visser, M.E. 2008. Keeping up with a warming world; assessing the rate of adaptation to climate change. Proceedings of the Royal Society B 275: 649-659.

Reviewed 21 November 2012