Nicholas Carey2,3, Januar Harianto2, Maria Byrne1,2
1 School of Biological Sciences, The University of Sydney, NSW, 2006, Australia
2 School of Medical Sciences, The University of Sydney, NSW, 2006, Australia
3 Hopkins Marine Station, Stanford University, Pacific Grove, CA, USA
Body size and temperature are the major factors explaining metabolic rate, and the additional factor of pH is also a major driver. These three factors also frequently cause altered feeding rates due to changes in energetic demand. Body size, temperature and pH have frequently been found to interact, with both synergistic and antagonistic relationships common, and this has complicated the formulation of broad models predicting metabolic rates and hence ecological functioning. We measured metabolic rate (R) and feeding rate (F) for the first time across a large size range in the sea urchin Heliocidaris erythrogramma after two months acclimation to increased temperature and lower pH.
A large size range (0.04-9.87g ash-free dry mass) of H. erythrogramma were separately and gradually introduced to warming (+5°C at 1°C every 3-4 days) and low pH (-0.5TOTAL at -0.1 every 3-4 days), and acclimated for 2 months in flow-through seawater. R was determined through intermittent-flow respirometry at 8 and 11 weeks, and these validated that successful acclimation had been reached. F was determined through five-day feeding trials with urchins fed on cubes of powdered algae suspended in agar; each day food was replaced, leftover food weighed and the mass per day ingested averaged over the five days. The separate and combined effects of the three factors (body size, temperature and pH) on both R and F were assessed through Analysis of Covariance.
The three factors, body size, temperature and pH, all strongly influenced R; +5°C and –0.5 pHT coincidentally both increased R by around 20%, and R increased with body-size with a scaling exponent (b) of around 0.7. Remarkably however, all combinations of the three factors acted purely additively; the combined effects of +5°C and –0.5 pHT increased R by 44%, and body size did not modulate the effects of either in any combination; b was essentially identical in all four treatments. By contrast, despite increases in metabolism of up to 44%, F was barely altered by any combination of factors; F increased slightly in higher temperatures, but only in the very smallest specimens, and was not affected by pH at all.
This study demonstrates that these three fundamental drivers of metabolism are highly consistent and partitioned in their effects. In a high CO2 world however, Heliocidaris erythrogramma will be under a substantial additional energetic burden which may not be alleviated through additional food intake. These added costs may come at the expense of other important life processes such as growth and reproduction.