The jellies that loved sunlight
Jellyfish have had a bad press. In some respects, they deserve it. For many people they are a slimy nuisance at the beach. They can also be problematic for fishermen, aquaculture operators, and even power plants. Some tropical species can—although rarely—cause fatalities. However, specialists recognize the roles of jellyfish are complex and much more diverse than previously thought.
To illustrate this, and to try to improve the public image of jellyfish, I would like to introduce a little-studied, yet fascinating, group: zooxanthellate jellyfish. These jellies are peculiar in that they host in their tissues tiny algal symbionts (zooxanthellae). While the jellyfish itself hunts for planktonic prey, the zooxanthellae use sunlight and nutrients to perform photosynthesis. The two partners then exchange resources, complementing one another’s needs (Fig. 1). The zooxanthellate jellyfish, as a whole, is therefore mixotrophic (i.e. simultaneously predator and photosynthetic).
One of my first surprises when I started working on these jellyfish was their stunning diversity. From the data currently available, it can be estimated that the symbiosis between jellyfish and zooxanthellae originated independently at least seven times during their evolution, and it is likely that there are still more originations to be discovered. When looking at the large and most-studied jellyfish (called scyphozoans), it appears that being zooxanthellate is quite normal, as 20–25% of scyphozoan species can have zooxanthellae.
Figure 1. The symbiotic relationship between jellyfish and zooxanthellae. Organic matter is provided by both photosynthesis of zooxanthellae (using sunlight, CO2 and inorganic nutrients) and predation. This organic matter is then used by both partners which, through respiration, regenerate CO2 and inorganic matter. Through this symbiosis, the jellyfish and the zooxanthellae complement each other’s needs.
The fact that zooxanthellate jellies are mixotrophic has many fascinating implications for their adaptations, life cycles, and roles in ecosystems. For instance, the adoption of mixotrophy implies that one resource might compensate for the other if one is lacking. This may allow populations of zooxanthellate jellyfish to be generally more stable and less 'bloomy' than populations of comparable, non-zooxanthellate, jellyfish. Another example of the peculiarities of these jellyfish is in their behaviour. Zooxanthellate jellyfish indeed tend to favour exposure to sunlight by swimming near the surface during the day, actively avoiding shadows, and even, in the very peculiar upside-down jellyfish Cassiopea spp., spend all their time lying on shallow seafloors exposed to the sun. Such adaptations may, however, imply trade-offs such as reduced ability to hunt for prey. These trade-offs and their low propensity to form blooms imply that zooxanthellate jellyfish have differing ecologies compared to non-zooxanthellate jellyfish. Although few studies are available, future comparison of zooxanthellate and non-zooxanthellate jellyfish could allow a better understanding of the roles of mixotrophic organisms in ecosystems, making them a novel ecological model.
Jellyfish can be a tourist attraction
This is the case most notably in the Palau archipelago (Western Pacific Ocean) where Ongeim’l Tketau, the 'jellyfish lake', attracts tourists from all over the world and represents an important source of revenue for the local economy. This marine lake hosts millions of zooxanthellate jellyfish of the species Mastigias papua. No stings are to be feared, as the stinging cells of these jellyfish are not large enough to pierce human skin, and I can testify first hand that snorkelling in the lake is an amazing experience.
Population of the zooxanthellate jellyfish Mastigias papua in one of Palau’s marine lakes. The large population in this lake is present year-round and is part of normal ecosystem functioning
Jellyfish in general have a complex life cycle, beginning their life as a polyp living on the seafloor. The polyp then forms young jellyfish through a process called strobilation. Interestingly in zooxanthellate jellyfish, the polyp does not have zooxanthellae at the beginning of its life and needs to acquire them from its environment. However, most of the time zooxanthellae are needed for strobilation. It is not known how and why the polyp appears to 'wait' to acquire zooxanthellae before strobilating, but it may be to ensure that the young jellyfish, which can rely heavily on them, have zooxanthellae. This particular aspect is of tremendous interest for developmental and symbiosis biologists.
To conclude, I hope I have contributed to a more balanced view of jellyfish. The zooxanthellate jellyfish, in particular, may be of enormous, but largely unexplored, scientific interest. Jellyfish can also inspire new research routes, and perhaps more importantly, some sense of wonder.
Dr Nicolas Djeghri (nicolas.djeghri@gmail.com)
Further reading
Djeghri, N., Pondaven, P., Stibor, H. & Dawson, M.N. (2019). Review of the diversity, traits, and ecology of zooxanthellate jellyfishes. Marine Biology, 166: 147.
Ohdera, A.H., Abrams, M.J., Ames, C.L., et al. Upside-down but headed in the right direction: review of the highly versatile Cassiopea xamachana system. Frontiers in Ecology and Evolution, 6: 35.
