Sunblock for Plankton

Reference: Bashevkin, S.M., Christy, J.H., and Morgan, S.G. (2019). Photoprotective benefits of pigmentation in the transparent plankton community: a comparative species experimental test. Ecology. 100:5, e02680. https://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1002/ecy.2680

Another sort of sunscreen

Skin damage from ultraviolet radiation (UVR) is a hot topic during the summer, and nearly all of us know at least one person who has been affected by skin cancer. Hopefully, you take steps to protect yourself from UVR produced by the sun; most likely you’ve worn sunscreen countless times throughout your life. Have you ever wondered how other organisms protect themselves from UVR, though? What about those that live underwater?

Zooplankton are small animals that spend their lives in the water column of ponds, oceans, and everything in between. Many are prey to an assortment of fish species. Being eaten by a predator is a serious risk, and many exhibit adaptations that help protect them from being easy snacks. One such adaptation is transparency – a clear body makes them harder for fish to see. Fish are not the only threat to zooplankton, though. UVR damage is also a big threat, as in open water there is little to shield them from the sun’s rays. In fact, zooplankton from lakes without fish have been found to be pigmented, and their coloration helps protect them from sun damage.

In the oceans, an abundance of predatory fish has contributed to most zooplankton appearing transparent. At night, plankton move to shallower waters to feed. Then, they move deeper to avoid being eaten during the day, which helps limit their exposure to UVR. Some larval plankton, however, must travel in shallow water during the day.

Previous studies had found that pigmented larval zooplankton are more susceptible to predation, but Bashevkin and colleagues recognized that pressures from UVR were likely influencing plankton coloration, as well. Prior studies, however, found contrasting results of pigmentation on UVR damage for crab larvae, and no studies conducted in aquatic environments had linked UVR protection to pigmentation. Additionally, some animals use defenses other than pigmentation to protect themselves from UVR damage.  In this study, the authors examined the correlation between the amount of visible pigmentation of crab larvae and their susceptibility to UVR to determine what was driving the variability in transparency between plankton species.

Testing Plankton Sunburn

To determine whether pigmentation is important in protecting larvae from UVR damage, the authors examined survival during exposure to UVR, as well as pigment cover, for 12 different crab species. In the survival experiments, larvae were exposed to typical surface levels of full sunlight (which includes visible light, UVA, and UVB radiation), visible + UVA only, and visible light without any UVR. To do this, freshly hatched larvae of each species were placed in small tray compartments covered with 15 milliliters of seawater and floated in coolers outside. Each cooler was then covered with filters to expose the larvae to the desired type of light. These experiments ran for 2-3 days, and the larvae were examined under a microscope each day to determine survivorship.

Pigment color was also investigated for each species. To measure this, larvae of each species were photographed through a microscope with white LED lights against a white background. From the photographs, the authors could determine the amount of the larvae’s bodies that was pigmented using a specialized software.

Pigment and Protection

            Overall, larvae of more pigmented species were found to be better protected against UVR than their less pigmented counterparts. This was evidenced by their higher survival rates under the different exposure combinations. The strongest benefits of pigmentation in UVR protection were found to be when it was sunny after 2 days of exposure. Additionally, different species of crab were found to vary in their sensitivity to different combinations of UVR exposure. Some species, for example, were found to be sensitive to UVA, while others were not. Additionally, certain species were found to be more sensitive to UVB than others. Interestingly, the groups of species that varied in their response to UVA were similar in their amount of pigmentation, and there were both lightly and heavily pigmented species in each. Within species, differences in survival related to pigment were strongest after 2 exposure days, and more pigmented individuals of one species were found to survive better than lighter individuals across all the treatments.

What We Know Now

This study is the first to provide strong evidence for the protective influence of pigmentation on zooplankton exposed to UVR. This is important, because it can help us better understand how pigmentation influences behavior and ecology of these tiny organisms. It also teaches us about the susceptibility of these organisms to UVR, and the relative impact of UVA and UVB. Under climate change, UVR exposure is mounting, and understanding how organisms respond to such stressors is increasingly important.