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.