4.2.3 LARVAL ASSAYS
Fertilised eggs were first cultured in a 4-L glass jar at a density of 30 embryos/ml for three days, before being transferred to 2-L glass jars at a density of 1 larva/ml. Details
of the experimental set-up are provided in section 3.2.3. The temperature was 16.5°C ±1 throughout the whole experiment. Larvae were fed after each water change with D. tertiolecta from a clean culture or a copper enriched culture, depending on treatment, at a concentration of 8000 cells/ml. Algal stocks were rinsed thoroughly by three successive centrifugations (1201 g for 5 min) and re-suspensions in clean FSW, before being fed to the larvae to remove traces of metal unabsorbed by algae.Jars in the Diet and Diet + Water treatments, including both chronic and pulse exposure, were sampled at 15 and 24 d post-fertilisation, and jars in the Water treatment were sampled 16, 23 and 29 d post-fertilisation. Controls were sampled at 15 and 23 d post-fertilisation. During sampling, the water level in each jar was lowered to 750 ml to increase larval density and 3 x 15 ml aliquots were taken per jar.
Jars were randomly numbered and all measurements were conducted blind with regard to the treatment classification. The number of live larvae was counted in each aliquot using a dissecting microscope (40x magnification). Each live larva was assigned to one of nine developmental categories. The categories were the same as in Table 3.2, except that the 8-armed pluteus category was further divided in three categories: 8-armed pluteus with no visible rudiment, pluteus with small rudiment and competent pluteus (Table 4.3). Plutei with a rudiment and 8-armed plutei were always considered as normal. While 6-armed plutei were considered as normal until day 16 (inclusive). The first ten normal larvae encountered were preserved in 40% ethanol to be photographed under a compound microscope (50x magnification).
Measurements of larval body and arm lengths (average of R and L arms where possible; Table 4.3), as well as size of the juvenile rudiment in the late larval stage,were performed using the software ImageJ.