Experiment 3: Detection of life stages
The presence of C. sivickisi medusa in waters surrounding Magnetic Island is highly seasonal (Schlaefer et al. 2020), restricted to the spring-summer period. This species, therefore, provided a good model to determine if eDNA could detect the presence of cubozoan polyps when adults were absent. Water for eDNA analysis was collected from nearshore waters surrounding Magnetic Island, Queensland, at three sites separated by 300 - 400 m during the Austral winter (June 2019). Depth stratified water samples were collected into 1L General Oceanics Niskin near the surface and just above the substratum; the total water column was 3 – 6 m deep. Sampling was targeted over coral reef largely covered by the macro alga Sargassum and dead coral matrix, as adult medusae are abundant over this habitat in the Austral spring and generally stay close to this preferred habitat (Schlaeferet al. 2020). Accordingly, this habitat it is where embryo bundles would likely be released following copulation (García‐Rodríguezet al. 2018). Water samples (2 L) were taken from ‘jellyfish hot spots’ at Geoffrey Bay (Site 1, 19.15.332o S & 146.86479 o E; Site 2 - 19.15.486 oS, 146.86.183o E) and Nelly Bay (Site 3, -19.17.102o S, 146.84.831o E), Magnetic Island. Samples collected during the winter were also taken after dark (1830-1930 h as they were during the medusa season (Sept-Nov). Water was taken in sterile 2 L containers and placed on ice which were then filtered within 12 h of collection. Filter holders were changed for each replicate to minimize the chances of contamination among sites and replicates. Individual filter holders were loaded with 5 μm nylon filters. After filtration of each sample and equipment blanks, the nylon filters were placed in 1 mL of Longmire solution and stored at 4 oC for DNA extraction.
It was possible that stratification of the water column would influence level of detection of eDNA. Accordingly, conductivity (PSU), temperature and depth (CTD), measurements were taken at each site during the jellyfish season. These data were used to identify the presence/absence of thermoclines and haloclines in the water column, which could potentially inhibit the vertical dispersal of DNA through the water column.