Solar Power

The act of eating is, without a doubt, the most enjoyable way of obtaining energy. As I sink my teeth into a juicy cheeseburger or try the first bite of an unusual new dish, I am blissfully unconcerned with the fact that I am meeting my metabolic obligation as a heterotrophic organism. My focus is exclusively on pleasing my senses of smell and taste while achieving that completely satisfied, comfortably-full feeling, not meeting my body’s caloric demand to ensure continued growth, maintenance and response. It is so easy to forget that we eat to live, that savoring a grilled cheese sandwich is merely a delicious way of staying alive.

While humans have made seeking, preparing, and consuming food a delightful means of survival, it is arguably an inefficient way to obtain energy. Heterotrophs like us must devise some scheme of capturing the appropriate food, the energy expenditure of which can be enormous. Autotrophic organisms such as plants and algae utilize solar (and sometimes chemical) energy to fix atmospheric carbon through photosynthesis and build the complex organic compounds needed for metabolism. In doing so they create their own food, skipping the expensive steps that we so enjoy. Autotrophs are the primary producers in the food web, creating the organic compounds upon which all other organisms rely for energy. Heterotrophs are the consumers, filling distant trophic levels which correspond to the number of links separating them from the sun’s energy. Humans typically eat as primary and secondary consumers, although consumption of some foods, such as large prize fish, can make us quaternary or even more distant consumers.

Over the course of evolution, organisms have spread out among different trophic levels. While humans enjoy the role of apex predator, occupying the end of the food web, some animals have moved in the opposite direction. Outdoing other primary consumers, some marine invertebrates have managed to acquire most of the energy they need without hunting or even eating. The ultimate convenience in predation, this unique trophic status is made possible by a special partnership between select invertebrate species and zooxanthellae algae. This partnership is considered an endosymbiotic relationship because the zooxanthellae live inside the body of the host, allowing both organisms to enjoy mutual benefits. (Our intestinal micro-flora and fauna are endosymbionts as well). Zooxanthellae are single-celled organisms that line the superficial tissues of the host like tiny solar panels, performing photosynthesis which provides the host with the organic compounds it requires for metabolism. In exchange the zooxanthellae receive shelter, nutrients and a constant supply of carbon dioxide – metabolic waste from the host. To maximize the benefits of this relationship, the invertebrate host seeks environments with conditions that are favorable for photosynthesis, as if it were an autotroph itself. The nudibranchs, sea anemones, jellies and reef-building corals that host zooxanthellae all live in shallow water for maximum sunlight.

Behind the scenes at the New England Aquarium, Upside-Down Jellies, Cassiopea andromeda, are raised under high-powered sun lamps in shallow tanks that re-create their natural sandy-bottom habitat. The Cassiopea rests on its flat bell, its reduced tentacles extended up towards to the light. At some point in the jelly’s lifecycle, zooxanthellae are incorporated into its tentacles, causing them to appear many different shades of blue-green. While they rest comfortably on the bottom, Cassiopea are by no means sessile. If you gently agitate the sand below their bell, they will casually swim off, still upside down, to another sunny spot. Small Cassiopea, about the size of a quarter, are held in a separate tank under the sun lamps. With a few exceptions, these young organisms do not appear to have zooxanthellae in their tentacles, and they must be provided with a normal diet. When the jellies are large enough, they are moved to the main tank where their tentacles eventually develop that typical blue-green color. It is likely that the adult jellies regularly expel excess zooxanthellae into the water in order to maintain a balanced number. The free zooxanthellae can then be absorbed by other jellies and will proliferate to colonize their new hosts. Even though they actively produce their own food, we would still feed adult Cassiopea small amounts of zooplankton to supplement their photosynthetic diet.

Similarly, reef-building corals (which, like the jellies, are members of the Phylum Cnidaria) acquire about 90% of the energy they need from zooxanthellae. Corals can capture and consume other plankton, cnidarians and sometimes even small fish, but they are completely dependent on their endosymbiotic relationship with zooxanthellae for survival. Research has shown that host-zooxanthellae associations in corals are extremely specific. This means that only one species of zooxanthellae will populate a colony of coral. As the zooxanthellae multiply, the host coral must regulate the number of individuals it holds within its cells in order to maintain its finite capacity. One way that the coral achieves this balance is by digesting excess zooxanthellae or expelling individuals into the water column. Zooxanthellae expulsion is responsive to environmental changes and often occurs when the corals are under stress caused by rising temperatures, changes in water chemistry, reduced light and disease. The expulsion of zooxanthellae is called coral bleaching, because the loss of the pigmented algae leaves the corals bone-white. If the corals survive the period of stress, zooxanthellae may be able to repopulate the colony within a few months. At first the species of zooxanthellae that repopulates the colony may be different from the species that was expelled, but over time the species will be replaced much like forest succession, eventually leading to that original inhabitant.

An endosymbiotic relationship with zooxanthellae may seem like the most efficient way for an animal to acquire energetic organic compounds, but one unique group of sea slug has managed to get even closer to autotrophic status. Sacoglossans, or solar-powered sea slugs, are small marine gastropods - the herbivorous cousin of the predatory nudibranch. These organisms are absolutely animals, yet amazingly they can perform photosynthesis on their own without the help of functional zooxanthellae. As a sacoglossan feeds on algae, it can isolate intact chloroplasts (the cellular machinery that performs photosynthesis) from its food and retain the chloroplasts within specialized cells in its body. While the other components of the algae are digested as food, the chloroplasts are incorporated into the body of the sacoglossan and perform photosynthesis the same way they did in the algae. This symbiotic phenomenon, aptly known as kleptoplasty, enables the sacoglossan to reap the energetic benefits of photosynthesis without playing host to an independent organism. While the functionality of the chloroplasts within the sacoglossan’s body is finite, the species Elysia chlorotica can maintain its chloroplast associations for up to ten months.

Sacoglossans thrive with the convenience of independently producing food within their own tissues, and even though they cannot glean as much pleasure from meeting their caloric requirements as we do, they don’t have a sense of taste either. Kleptoplasty and zooxanthellae endosymbiosis have allowed some animals to enjoy the ultimate reliable food source by acting more like plants. So the next time you are hungry enough to eat a horse, imagine yourself as a solar-powered sea slug, bathed in sunlight – warm, energized, and comfortably full.

To learn more about sarcoglossans and nudibranchs that host zooxanthellae, visit The Sea Slug Forum.



Dimond J. and E. Carrington. 2008. Sybiosis regulation in a facultatively symbiotic temperate coral: zooxanthellae division and expulsion. Coral Reefs. 27: 601-604

Toller, W. W., R. Rowan, and N. Knowlton. 2001. Repopulation of Zooxanthellae in the Caribbean Corals Montastraea annularis and M. faveolata following Experimental and Disease-Associated Bleaching. Bio.Bull. 201: 360-373

Photo: The sarcoglossan Placida dendritica. Photo by Bill Rudman