**4. Discussion**

Oxybenzone has been found to damage and deform planulae larvae, which could explain why the motile skills of planulae larvae of *C. xamachana* and *C. frondosa* were partially inhibited (Figure 2) [4,14]. Larvae in higher concentrations of oxybenzone (228, 22.8 μg/L) were swimming in irregular patterns, some spun in circles in the same spot, and many of them died (Figure 3). The disorientation of the surviving larvae would most likely inhibit their ability to settle and metamorphose (Figure 4). Although the response of the two species is very similar, planulae of *C. frondosa* appear to be more sensitive to high concentrations of oxybenzone than *C. xamachana* (Figures 3 and 4). Since there was no death in the control group of larvae, nor those maintained in 0.0228–2.28 μg/L oxybenzone, it appears that the low amount of DMSO used in the experiments did not detrimentally affect the planulae.

Planulae and newly metamorphosed scyphistomae (= polyps) are aposymbiotic in *Cassiopea* sp., unlike the normally symbiotic planulae of the coral *Stylophora pistillata* [4,14]. Many of the larval responses seen by Downs [4,14] were thought to be partially due to the effects on the symbiotic algae. *Cassiopea* sp. can acquire Symbiodiniaceae, soon after the new polyps develop a mouth. Therefore, Symbiodiniaceae were not involved in the responses of planulae larvae of *Cassiopea* sp. to oxybenzone. The symbionts enable the polyps of *Cassiopea* sp. to strobilate, turning into a medusa (Figure 1), although the mechanism of the symbiotic interaction is not known [17].

Sunscreens washed or flushed into the ocean during tourist season are probably having a negative effect on corals and jellyfish [18]. Downs et al. [14] found that the upper concentration varied in Hawaii (0.8–19.2 μg/L) and the US Virgin Islands (75–1400 μg/L), up to 6 times higher than the top concentration of oxybenzone used in the current experiments. The lethal concentration of oxybenzone that kills half of planulae of *S. pistillata* (LC50) in the light for an 8- and 24-h exposure was 3100 μg/L and 139 μg/L, respectively [14].

Planulae normally use their swimming ability to investigate substrates to settle on. *Cassiopea xamachana* and *C. frondosa* normally settle on specific substrates in their environment [19]. Oxybenzone can be attributed to the decreased motility and settlement/metamorphosis of *C. xamachana* and *C. frondosa* larvae, and even death at the higher concentrations used, posing a threat to the survival of these species.

**Author Contributions:** For Methodology, experimental set-up, original draft preparation W.K.F., methodology, experimental set-up, reading and editing D.K.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors acknowledge the student research of Alex Amalfitano, Anna Schramski, Sarah Gardner, Ethan Turner, and Rebecca Farley. The Key Largo Marine Research Laboratory (contribution #167) provided housing and access to the jellyfish *Cassiopea* sp.

**Conflicts of Interest:** The authors declare no conflict of interest.
