Appendix C. Determination of biomass: methods and discussion.
Individuals were placed in tin cartridges and freeze dried for 48 h (GT2E Finn-Aqua Lyovac vacuum dryer), weighed (Metler M2 microbalance), and processed in an elemental analyser (Carlo Erba EA 1108) to determine carbon (C) and nitrogen (N) content. I used individuals that were in the laboratory as megalopae for a period <48 h in order to reduce laboratory effects on juvenile biomass. Measurements were made in 2-d-old juveniles, instead of recently metamorphosed individuals as a way to reduce errors in the estimation of biomass without increasing the laboratory interference. The errors in biomass estimation arise as the consequence of the high growth rate of individuals during the hours following exuviation. During that period, differences in the time of processing animals for DW, C and N may lead to differences in biomass that are not related with larval history, but to the first feeding and the moult cycle (stages A and B following of Drach 1939, see Anger 1996). Errors should be smaller as individuals are in intermoult (beginning stage C).
This study highlights the importance of careful design of experiments to integrate physiological process to population studies. This study required the determination of the age and size (or biomass) of each individual but these quantities are interrelated as biomass increases and individuals grow in size; Carcinus maenas megalopae accumulate 50% of its initial dry mass in ca. 10 days (Dawirs et al. 1986). However, it was not technically possible to determine both quantities simultaneously. The estimation of age since the last moulting event, with the necessary precision of 1 day, required keeping individuals in the laboratory until metamorphosis, although this may blur the effect of field conditions on the individual biomass and size. However, estimations of biomass of fresh collected megalopae, without knowledge of the age, cannot be used at all. For that reason, the determination of size and biomass were done by keeping and feeding individuals in the laboratory; size was estimated at metamorphosis and biomass was estimated in individuals metamorphosing in <48 h. Although the increase in size at metamorphosis with longer time reflected a laboratory effect, it enabled making conclusions about growth limitation in the field.
Anger, K. 1996. Physiological and biochemical changes during lecithotrophic larval development and early juvenile growth in the northern stone crab, Lithodes maja (Decapoda: Anomura). Marine Biology 126:283296.