The use of metabolic ratios for determining the catabolic substrates of a solitary ascidian

Abstract

In a previous study, Hatcher (1989: Mar. Biol. 102: 445–452) found that variations in CO2 and O2 respiration rates in individual marine invertebrates led to RQ (respiratory quotient) values which were variable and often outside theoretical limits. The present study was designed to examine the variability in several excretionbased metabolic ratios which are often used as alternatives for the RQ in qualitative predictions of catabolic substrates. The experimental organism was a solitary ascidian,Herdmania momus (Savigny), collected near Perth, Western Australia, between January and July 1984. Respiration and excretion rates ofH. momus were examined as a function of a progressive nutritional stress, and covariation was examined. Nutritional stress accounted for more of the variation over time in respiration rates (40 to 50%) than in NH4+ excretion rates (20%). Significant net exchanges of dissolved organic compounds were measured. Qualitative predictions of catabolic substrates were based on a comparison of metabolic ratios with theoretical limits. The O:PO43- ratios were lower than the theoretical limits. The values of the O:NH4+ and NH4+:PO43- ratios were not influenced by nutritional conditions but changed as a function of reproductive condition of the ascidians. Based on the results of this study, it was concluded that metabolic ratios measured on individual ascidians cannot be reliable predictors of catabolic substrates.