The relative influence of temperature and food on the metabolism of a marine invertebrate

Many benthic marine invertebrates exhibit a seasonal cycle in activities su ch as feeding, growth and reproduction. In temperate regions, this seasonal ity is typically correlated with coincident cycles in photoperiod, temperat ure and food availability and it can be difficult to determine which of the se environmental factors is the key driver. Polar regions are characterised by greatly reduced seasonal variation in temperature, and an enhanced seas onality of food availability; they therefore form a natural laboratory for distinguishing the ecological effects of food from those of temperature. He re, we report a study of the common shallow water urchin Sterchinus neumaye ri from Rothera Point, Antarctica. This species exhibits a marked seasonal variation in metabolic rate and feeding activity (which ceases completely i n winter). In this study the metabolic rate of urchins collected in late wi nter and held in the laboratory without food was compared with that of wild urchins undertaking the transition to summer feeding and growth. Starved u rchins showed a small rise in metabolic rate in summer which could be expla ined entirely by the small increase in temperature (Q(10) = 2.5). At the sa me time, the wild population showed a much larger increase in metabolic rat e related largely to the costs of feeding and growth. Rates of nitrogen exc retion were also much larger in wild urchins, and the O:N atomic ratio indi cated that starved urchins were depending to a greater extent on lipid and carbohydrate. Gut mass and test organic content showed no change in starved urchins, indicating that metabolic substrate was being provided by the gon ad. The data suggest that in wild S. neumayeri only 15-20% of the summer in crease in metabolism is caused directly by the temperature rise whereas 80- 85% is caused by increased physiological activity associated with feeding, growth and spawning. (C) 2001 Elsevier Science B.V. All rights reserved.