Jeremy enters the bowels of Lighthouse Cave, San Salvador, Bahamas.
A Symbiotic Lifestyle:
Cassiopeia xamachana and Zooxanthellae
Cassiopeia xamachana, the Upside-Down Jellyfish, is part of the phylum Cnidaria, which contains approximately 9000 living species worldwide  including corals, anemones, and of course jellyfish. They are called Upside-Down Jellyfish because unlike the stereotypical jellies, C. xamachana spend most of their time inverted on the sea floor with their oral cavities exposed to the overlying water. Once their bells are in contact with the sea floor, a rock, or some other relatively flat surface, they use a gentle pulsing action to stay in position. These jellies look more like underwater flowers than animals and come in many colors, usually greenish-brown, because of the zooxanthellae living within them. They are a favorite meal for ocean sunfish and the leatherback sea turtle.
“Zooxanthellae” is a term used to describe a collection of species of algae, photosynthetic dinoflagellates to be exact, that have symbiotic relationships with various invertebrates . That is, the algae and the host organism derive mutual benefits from their prolonged relationship with each other . The zooxanthellae provide nutrients and oxygen for the C. xamachana, and in return, the algae can live in the photic zone, are supplied with abundant carbon dioxide, and are protected from ultraviolet rays .
While we may understand the benefits both organisms derive from the relationship, scientists are still looking for answers to such questions as when did the algae and the jellyfish begin their symbiotic lifestyle and how do they co-exist in one body. As scientists study these and other questions, they are also uncovering ways C. xamachana and zooxanthellae are helping to provide answers to some other problems.
One of the most confounding issues concerning these algae is exactly how many species are there. At one time, it was believed that there was only one species, Symbiodinium microadriaticum . In fact, the identification of the algae within Cassiopeia sp. as S. microadriaticum caused the long-held assumption in the scientific community . Due to their tiny sizes and often very similar physical characteristics, scientists often find it hard to distinguish between different species of zooxanthellae. However, despite these difficulties, today there are as many as five different groups of zooxanthellae (A, B, C, D, and E)  with a possible sixth and seventh (F and G) theorized . Each group is identifiable through restriction fragment length polymorphism (RFLP’s) . Basically, restriction enzymes are added to the algal ribosomal DNA which cut the DNA at specific sites. Based on the different lengths of the resulting fragments, the algae can be categorized . Algae from group A are the most commonly found within the Upside-Down Jellyfish.
Upside-Down Jellies can be found in shallow tropical marine environments such as mangrove bays and back-reef lagoons ranging from along the Gulf of Mexico to the Bahamas and West Indies . As cnidarians, the jellies have radial symmetry and posses the most common type of cnida (the explosive organelles that fire off stinging cells when stimulated): nematocysts . These stinging cells are used to catch food  and for protection. Fortunately, the nematocysts of C. xamachana are relatively weak compared to other species of jellyfish. Usually one can recover from being stung merely by stepping out of the water.
Their life-cycle is typical of most jellyfish. As adults (or medusa), C. xamachana reproduce sexually resulting in larvae (or planula). The planula anchor themselves to a firm object and grow into polyps that can then clone themselves and bud . They can stay in the polyp stage for months, even years. Once the polyps phagocytize S. microadriaticum, they mature  and enter the ephyra stage of development, and from here the C. xamachana grow into adult medusa . Adults can grow upwards of 35cm wide and 51mm tall.
As mentioned earlier, the relationship between the zooxanthellae and C. xamachana benefits both organisms greatly. The steady supply of oxygen and nutrients from the algae allow C. xamachana to live in waters with poor oxygen and lacking in food sources. Meanwhile, the jellyfish protect the sensitive algae from harmful UV rays while inhabiting the photic zone .
Both organisms are quite sensitive to light levels and temperature . Additionally, C. xamachana are sensitive to the salinity of the surrounding water. If any of these factors are disrupted, for example not enough light or too much heat or salinity shock, the zooxanthellae are expelled from the jellyfish in a process called bleaching , named so because the jellyfish become white when the algae are lost. The algae can live outside of the jellyfish for up to a few months. However, C. xamachana rely almost exclusively on the zooxanthellae for their nutrition in the wild and can only survive without the algae for prolonged periods in a lab environment where they can be given brine shrimp every few days.
Research with and on C. xamachana and Zooxanthellae
One recent study has introduced the hypothesis on the early evolution of Hox genes . Hox genes are a particular subgroup of the so-called Homebox genes, which are involved in the formation of bodily segmentation during embryotic development, that function in patterning the body axis . Hox genes determine where limbs and other body segments grow in a developing fetus or larva . Kuhn et al., as well as some other researchers, have presented the idea that perhaps Hox genes evolved earlier than first thought. C. xamachana and other cnidarians hold the potential to reveal the hypothesized intermediate stages in the evolution of Hox clusters and axial complexity .
Other research has shown that the jellyfish can recognize the specific zooxanthellae after it has been phagocytized. Zooxanthellae can be acquired vertically from the mother or horizontally from the environment . It was commonly thought that animal hosts could only harbor specific algal species and that each animal could only host the algae it had acquired as juveniles. However, the past few years have brought forth the theory that cnidarians, mainly coral but studies by Dr. Adrienne Sloan are based on C. xamachana , can in fact not only acquire new zooxanthellae as adults but also that the zooxanthellae need not be the same species as that acquired as juveniles . This has serious implications in studies regarding coral bleaching and reef health and restoration.
Problems Facing C. xamachana
Along with coral reefs around the world, the Upside-Down Jellyfish faces great danger from the effects of global warming. The shallow tropical waters they occupy are subject to great temperature changes, and studies have shown that an increase in temperature of as little as 1°F for prolonged periods is enough to cause jellyfish bleaching .
Loss of their natural habitat also threatens the survival of C. xamachana. While they are still present in their native habitats, longitudinal monitoring has shown that their numbers are beginning to dwindle in areas where they have historically been abundant . The loss of mangrove bays and the ongoing increase of human populations along coastlines only heighten the threat to the jellyfish .
Human activity comes with environmental change and degradation, and the human population growth along coastlines is no different. The change in local ecosystems and the pollution introduced into the surrounding areas bring danger to the C. xamachana . Their hypersensitivity to water temperature, substrate composition, turbidity, and salinity make it difficult for the jellies to adjust to human-introduced changes. The range in which they can be found continues to grow smaller over time .
The relationship between C. xamachana and its zooxanthellae help us to better understand symbiosis. Additionally, the jellyfish and their family members may hold the key to better understanding the processes that cause us to development in the forms that we do. These organisms also serve as a type of measuring tool to gauge the health of an ecosystem and the oceans in general. Bleaching events worldwide are showing us that global warming and pollution do not only affect the land. As the jellyfish and coral continue to disappear, we are learning that we must take action to reduce human contribution to global warming or face losing these beautiful creatures altogether. We can only watch and hope to see whether C. xamachana and its relatives can weather the storm.
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(12) Dictionary: Homebox gene. Answers.com. 1 Jun 2005.
(13) Finnerty, John R. “Cnidarians Reveal Intermediate Stages in the Evolution of Hox Clusters and Axial Complexity1.” American Zoologist 41.3: 608-620.
(14) Sloan, Adrienne. Personal Interview. Jun 2003.
(15) Lewis, Cynthia L. and Mary Alice Coffroth. “The Acquisition of Exogenous Algal
Symbionts by an Octocoral After Bleaching.” Science 304.5676 (4 Jun 2004):
(16) “Upside-down Jelly.” Monterey Bay Aquarium: Living Species List. 1999-2005.
1 Jun 2005.
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