The Pisaster Post! Posterchild or Portent ??

(from MarineBio.net!)

This week, we're talking about the Pacific Northwest Intertidal!!

And what animal is more iconic to this area then Pisaster ochraceus (family Asteriidae)-the familiar intertidal Ochre Star found on rocky, mussel-laden substrates on from Alaska to California to Mexico. Some more basic information is here.Interest on this species has shifted over the years and seems to change based on some of the "big science" of the day. Yet another sign of how the humble starfish has incorporated itself into the fabric of the BIG scientific picture!

So, today some highlights of the importance of the ever-humble intertidal Pisaster ochraceus and how its study has varied over the years....

1. The Keystone Species Concept-Ecology's Posterchild. Probably one of the most lasting ideas from the 1960s and 1970s was the hypothesis developed by ecologist Robert T. Paine who identified Pisaster ochraceus as a keystone species (keystone shown below in grey).To quote Wikipedia:

A keystone species is a species that plays a critical role in maintaining the structure of an ecological community and whose impact on the community is greater than would be expected based on its relative abundance or total biomass

This keystone is a crucial block in an arch that keeps it from collapsing. This is analogous for Pisaster's influence on the mussels and the other invertebrates that exist in a rocky intertidal ecosystem.
The loss of the "keystone species" results in a drastic shift among these species....The idea has endured and while not embraced by everyone- remains a mainstay in basic ecology books. More details on this notion can be found here.

Pisaster along with its prey, the mussel Mytilus are almost ALWAYS the featured example... the POSTER child for the keysone concept-and for this reason, is probably even better known then the Atlantic Asterias!
This was (and continues to be) an important ecological notion during a time when the ecosystem and ecologists were in ascendance and ecology was a huge primary mainstay of biological research!

2. Pisaster as a Portent of Change?? The Canary in the Cage of Climate Change??
Probably one of the biggest, new research directions these days?

Understanding Climate Change and in turn...increased ocean temperature, which has a HUGE impact!

It affects ocean water chemistry. Water chemistry in turn can change everything from mineral absorption, feeding behavior, physiological systems to larval settlement. These in turn can have influence on MILLIONS of tiny larvae in the water. As well as the MANY adults those starfish grow into.

This translates into many people interested in the effects of increased temperature and heat relative to the ability of common species to adjust. Will Pisaster ochraceus take on a new status as a possible indicator species (i.e., canary in a coal mine) for climate change effects in marine systems?

Here is a survey of three recent studies (2008-2009) that have looked at how Pisaster holds up!

Elevated water temperature and carbon dioxide concentration increase P. ochraceus growth!

(Diagrammatic graph by Echinoblog Art Department!)

Rebecca Gooding, Christopher Harley and Emily Tang at the University of British Columbia published this study in the Proceedings of the National Academy of Sciences wherein they found that increases in temperature from 5 to 21 degrees C led to increases in feeding AND overall growth.

This bucked the predictions that the decreased carbon dioxide resulting from increased temperature would prevent animals that use calcium carbonate to form their skeletons (such as coral)!!

Solar radiation plays a role in P. ochraceus habitat selection

(Diagrammatic graph by Echinoblog Art Department!)

A 2008 paper by Jennifer Burnaford and Melissa Vasquez at the University of Puget Sound studied where P. ochraceus occupied habitat and their tolerance of Ultraviolet radiation.
The short version of this-the authors found that in artificial lab experimetns, P. ochraceus avoided ultraviolet and "photosynethetically active radiation" and observations of Pisaster in the intertidal found that 85% of them occurred in shaded habitat underwater where they were shown to preferentially avoid direct exposure to sunlight (see diagrammatic graph above!).

P. ochraceus avoids extreme body temperature by pumping its body full with cold sea water!!

(Diagrammatic graph by Echinoblog Art Department!)

Sylvain Pincebourde, Eric Sanford, and Brian Helmuth recently published this paper (2009). A popular account can be found here (and for shame to Live Science for misspelling "ocher").

Their paper details how Pisaster ochraceus was observed to increase the amount of colder water in their body cavity lowering their body temperature during the subsequent low tide in response to the temperature.

Sense it getting warm? don't like it? Just PUMP IT UP! with cold water! (see diagram above)

But climate change has a huge potential impact on animals that do this...to quote in their words:

When placed in a global change context, these results suggest that a continued increase in ocean temperature may compromise the ability of sea stars to avoid thermal stress during aerial exposure at low tide.

Has the humble Pisaster ochraceus gone from ecological poster child to a possible portent of climate change to come??? Time will tell....