Modelling developmental changes in the carbon and nitrogen budgets of larval brachyuran crabs

Abstract

The uptake and partitioning of nutritional carbon (C) and nitrogen (N) were studied during the complete larval development of a brachyuran crab,Hyas araneus, reared under constant conditions in the laboratory. Biochemical and physiological data were published in a foregoing paper, and complete budgets of C and N were now constructed from these data. Regression equations describing rates of feeding (F), growth (G), respiration (R), and ammonia excretion (U) as functions of time during individual larval moult cycles were inserted in a simulation model, in order to analyse time-dependent (i.e. developmental) patterns of variation in these parameters as well as in bioenergetic efficiencies. Absolute daily feeding rates (F; per individual) as well as carbon and nitrogen-specific rates (F/C, F/N) are in general maximum in early, and minimum in late stages of individual larval moult cycles (postmoult and premoult, respectively). Early crab zoeae may ingest equivalents of up to ca 40% body C and 30% body N per day, respectively, whereas megalopa larvae usually eat less than 10%. Also growth rates (G; G/C, G/N) reveal decreasing tendencies both during individual moult cycles and, on the average, in subsequent instars. Conversion of C and N data to lipid and protein, respectively, suggests that in all larval instars there is initially an increase in the lipid: protein ratio. Protein, however, remains clearly the predominant biochemical constituent in larval biomass. The absolute and specific values of respiration (R; R/C) and excretion (U; U/N) vary only little during the course of individual moult cycles. Thus, their significance in relation toG increases within the C and N budgets, and net growth efficiency (K ₂) decreases concurrently. Also gross growth and assimilation efficiency (K ₂; A/F) are, in general, maximum in early stages of the moult cycle (postmoult). Biochemical data suggest that lipid utilization efficiency is particularly high in early moult cycle stages, whereas protein utilization efficiency is higher in later stages. Only the zoea II appears to accumulate lipid from food constantly with a higher conversion efficiency than protein. The cumulative C and N budgets show in subsequent larval instars conspicuously increasing figures in all of their parameters.F andG increase to a particularly high extent from the first to the second zoeal instar, whereasR, U, exuvia production (G _E), and total assimilation (A) reveal a greater increase from the zoea II to the megalopa. Respiratory, excretory, and exuvial losses increase in subsequent larval instars at higher rates than tissue growth and, hence,K ₂ decreases in the same order. In the C budget,K ₂ values of 0.63 (zoea I). 0.56 (zoea II), and 0.29 (megalopa) were calculated (or: 0.56, 0.46, and 0.16 after subtraction of exuviae). In the N budget, corresponding values of 0.76, 0.66, and 0.45 (or: 0.72, 0.62, 0.38 without exuviae) were obtained. AlsoK ₁ decreases slightly in subsequent instars, whereasA/F reveals rather an increasing tendency, at least from the zoeal instars to the megalopa. Changes in the uptake and partitioning of matter in crab larvae are discussed in relation to developmental events and changes in life style before metamorphosis.