World Aquacluture Magazine - September 2020

60 SEP TEMBER 2020 • WORLD AQUACULTURE • WWW.WA S.ORG three most successful metamorphosis-inducing treatments (NIT, NIT+A and NBIO), each with three replicates. Post-larvae were transferred to nine 130-mm diameter Pyrex Corning crystallizing dishes with 300 mL of aerated and filtered seawater at 20 C. They were fed 0.50 g of Palmeria palmata (Fig. 7). Survival rate was calculated at 10, 30 and 44 days post- metamorphosis (DPM) as the final number of living juveniles in the dish / initial number of post-larvae in the dish × 100. At 44 DPM, the test diameter (μm) of nine randomly selected juveniles, one from each dish, was observed at 40× magnification and measured using the public domain image processing program ImageJ. Juveniles grown on the NIT biofilm (432 µm) were 1.22 times larger than juveniles grown on NIT + A (354 µm) but were not different than juveniles grown on NBIO (430 µm) (Fig. 8). At 44 DPM, the survival rate of juveniles in the NIT+A treatment was greatest at 93 percent. Survival of juveniles in the NBIO treatment decreased rapidly from days 30 to 44, from 22 percent to 9 percent respectively (Fig. 9). At days 30 and 44, significant differences were seen between the treatments NIT + A and NBIO and between NIT and NBIO only at day 44. Discussion The results of this study reinforce previous findings that cues are necessary for settlement and metamorphosis. Purple sea urchin larvae successfully settled and metamorphosed on treatments of NIT, NIT+A and NBIO. Although there was a 13 percent metamorphosis rate on the control of FSW, there was a significant difference between this treatment and that of NIT, NIT+A and NBIO. Little to no metamorphosis occurs in the FSW treatment as there is no biofilm to induce settlement and metamorphosis. The properties and age of the biofilm treatments appeared to play a role in the induction of settlement and metamorphosis. Nitzschia spp. secrete relatively high amounts of mucus that forms the matrix for the biofilm, allowing tight attachment of diatoms to the substrate and leading to high induction of settling (Kawamura, 1994, Xing et al . 2007). Biofilms used in this study were seven days old, which has a greater effect on the induction of settlement because they have a higher biofilm and bacteria density (Rahim et al . 2004). N. laevis + antibiotics unexpectedly had a high settlement and metamorphosis rate, as antibiotics are known to suppress bacterial growth (Jauffrais et al . 2017) and have low induction and metamorphic rate (Rahim et al. 2004). Bacteria are a known cue associated with the induction of metamorphosis and enhance the growth of diatoms (Jauffrais et al . 2017, Johnson et al. 1997). Natural biofilm is an excellent settlement media as it has an abundant supply of bacteria and diatoms that are required by urchin larvae (Agatsuma 2001). However, the composition of natural biofilms varies throughout the year, influencing rates of larval metamorphosis, growth and survival (Rahim et al. 2004). Oyster shells have a rich habitat of epibionts and a variety of diatoms (Barillé et al . 2017) that are likely to be similar to the profile found in sediments of the oyster pond and are a good inducer of settlement and metamorphosis. However, in this study, oyster shells were broken into pieces that were too small and larvae were not able to be seen, resulting in no apparent metamorphosis. Once settled, post-larvae survival must be at least 80 percent to be considered a profitable biofilm or substrate for aquaculture production (Cellario and Fenaux 1990, Grosjean et al . 1998, Rial et al . 2018). In this study, at 30 and 44 DPM, survival was 66 percent for NIT and 93 percent for NIT + A. Survival on the natural biofilm FIGURE 7. Selected treatments and replicates for survival and growth analysis, from top to bottom NIT + A, NIT, and NBIO with replicates. FIGURE 9. Survival rate (mean %± SD) of purple sea urchin juveniles on treatments of NIT, NIT + A, and NBIO at 10, 30 and 44 days post-metamorphosis (dpm). FIGURE 8. Largest juveniles based on test diameter (μm) at 44 days post-metamorphosis. From left to right: NIT (704 μm), NIT + A (511 μm), and NBIO (407 μm). Scale bar = 100 μm.