Maybe its the HEAT and not the acidity? A study of how Climate Change Affects Echinoderms!

(from SaSpotato on Flickr!)

So, there was big climate change news on the BBC website with a headline of "Sea urchins tolerate acid water" yesterday. An interesting article that showed the effects of carbon dioxide-rich water Atlantic shallow water sea urchin, Psammechinus miliaris to quote:

Whilst the scientists found no adverse effects on larval development or soft tissue production in the present study, they did observe a significant decrease in the amount of calcium carbonate that the organisms produced, resulting in smaller and thinner skeletons.

I should note that I haven't seen or had a chance to read the BBC paper as yet.. but a discussion on this topic with my colleague Dr. Allison Gong of UCSC (and who provided some Pycnopodia pix) reminded me of a paper I had seen awhile back.

Dr. Maria Byrne and her colleagues in Southern Australia published this 2009 paper with a telling title " Temperature, but not PH, comrpomises sea urchin fertilization and early development under near-future climate change scenarios".

I have previously written about how climate change may affect intertidal invertebrates here (effects on Pisaster and such)..

Byrne and colleagues studied the ecologically important sea urchin Heliocidaris erythrogramma which occurs widely throughout temperate-water Australia.

(from SaSpotato on Flickr!)

So, climate change's biggest impacts will most likely be in terms of sea-surface warming and ocean acidification. Good introductory articles on Wikipedia to global warming are here and for ocean acidification, here (short version: increased carbon dioxide creates more acidic ocean water) .

(this image from the BBC)

Echinoderms are composed of an internal calcium carbonate skeleton that is made up of millions of tiny pieces that are all infused with tissue.

As a consequence, echinoderms are one of the organisms likely to be affected by climate change because their skeletons are most likely be affected by the animal's uptake of calcium carbonate. Acidic (or warmer) water can dissolve/change/alter the chemical nature of calcium carbonate (aka chalk, limestone, etc.),

Byrne and her colleagues ran experiments that simulated future conditions of increased sea surface temperature AND an increasingly acidic ocean (i.e., a higher pH).

But rather than focus on adults, their efforts were directed at different life stages of Heliocidaris.
Namely, their larvae and the fertilized eggs.
This is actually one of the MOST important aspects of understanding the biology and evolution of marine animals. Like almost ANY animal, their "baby" stages are influenced and/or changed by the environment.

Larvae are also often WIDESPREAD. The little floating eggs and "babies" are found all throughout the water column and can be changed/affected by environment in ways that the adults can't be...
So, Byrne and her colleagues ran experiments where they changed the temperature and acidity during rearing conditions of fertilized/developing eggs of Heliocidaris.

They compared experiments versus a "control" standard was set at 20 degrees C (which is 68 degrees F) (with a PH of 8.2)

They varied environmental conditions up to 24 degrees C and 26 degrees C.

There were two experimental conditions outlining +4 and +6 degrees over the "control" temperature.

Their results are summarized in their Figure 1 below:
The top graph represents % of eggs that underwent fertilization
The middle graph represents % of normal cleaving embryos as temps increase from left to right.
The bottom represents % of normal gastrulation as temps increase from left to right.
Bar color represents the pH: black-8.2, dark grey-7.9, light grey-7.8, white-7.6 (so becoming MORE acidic from left to right)

What's Happening?
-The % of normal cell cleavage AND gastrulation (these are critical stages of cell development) were significantly LOWER. (dropping from the 65% that developed normally to LESS than 20% that developed normally!) at the warmest temperatures!
Basically, higher temperature created a BIG Developmental FAIL!!! Regardless of the pH (i.e., the acidity). Temperatures may affect various physiological processes that affect the development of the egg resulting in developmental failure!

How does this compare with other urchins?
Byrne et al. compared these results with the temperature and pH tolerances of other species and found that for five other species most of the known tolerances are only affected by a pH of LESS than 7.4 (the lowest they reached in the experiment was a pH of 7.6).

(thanks to Wikipedia for the pH scale!)

Normal seawater has a pH of about 8 (8.2 was measured for the sea urchins), and distilled water is about a pH of 7. Hydrochloric acid, what we use to digest food, is a pH of about 1.

Only the WORST CASE of ocean acidity would begin to seriously and fatally affect fertilization and development of baby sea urchins.

But it turns out that acidity on its own is a mixed bag for sea urchins (and probably other echinoderms)...

-on one hand, many sea urchins are quite tolerant to low pH (i.e., acidic conditions) owing to the relative acidity associated with fertilization. Sea urchin sperm actually has a pH of about 7.6!!

Indeed..some studies (such as this one by Wood et al. ) have shown that some echinoderms, such as brittle stars will actually show increased growth under acidic conditons!

(image from EPOCA Arctic Campaign 2009)

-BUT on the other hand, decreased pH may have a negative effect on larval calcification i.e, the ability to use calcium carbonate to develop their endoskeletons after sea urchins undergo early development. Animals could be weaker or have weaker "bones" as adults.

In the here and now- temperature is an important consideration!
Projections for eastern Australia indicate a surface sea temperature warming of UP TO 2 to 4 degrees C in the summer off the coast of New South Wales. Resulting in water temperatures of UP to and above 26 degrees C-Close to the temps indicated in their experiments!

And all this IN ADDITION to whatever possible stress may be caused by acidification!

So, while reports such as the one reported by the BBC are important, perhaps temperature will be the more important consideration in future studies???

Increased temperature appears to be one of the most important factors impacting many facets of the biology of adult echinoderms (and indeed-many other marine invertebrates) , including distribution, feeding, behavior, reproduction...but most importantly, the development and healthy survival of fertilized eggs and juveniles.