Off of our coast are other masses of land which still are grouped under the one name of Bermuda; we are surrounded by an archipelago. The hundreds of smaller islands provide, from a marine biologist’s point of view, coastline in shallower water for snorkeling, and beaches for turtles to nest. From an explorer’s point of view, the islands are shrouded with seclusion, mystery, and legend.

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West Indian Top Shell

Many of the smaller islands are now occupied by summer camps, and whether the counselors make up the ghost stories or not, they are still widely believed. It’s no surprise that ghost stories pop up around these islands, since many a ship has been wrecked on their small, but still rock solid, shores surrounded by reefs. Forts from military times are decrepit but present on the islands, providing a ghostly dwelling. The scarcity of human contact on the island means the stories live well there, but living there too are many unique and diverse species.

A few years ago, when the population of West Indian Top Shells was still dwindling in Bermuda, Paget Island was still home to an astonishing number of accessible specimens. They were amazing to interact with, and being kids, we picked them up and they browsed around on our hands. It was the first time one has ever come out of it’s shell for me to see. Now, the West Indian Top Shell population is a lot healthier, but I returned to Paget Island just yesterday.

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A Kayaker Enters Paget Island Lagoon

There is a man made cut in the island, granting passage through to a small lagoon, the perimeter of which is overgrown with both red and black mangroves. As I kayaked through the cut, eager to see the lagoon again, I noticed that the corals were exceptionally bright.  Mexican Pepper trees growing overhead made the cut dark in comfortable shadow, but it seemed that did nothing to stop the corals from flourishing. Small crabs darted into their nooks as the kayak moved smoothly through the water and all around the boat were bright corals. The cut ended, along with the shade, and brilliant sunlight which allowed the lagoon to flourish so amiably shined brightly. Once the lagoon came into view it was much shallower than I remembered it, but still occupied by vast populations of Upside-Down Jellyfish (Cassiopea Xamachana).

Paget Island is home to the Outward Bound summer camp. Basically, you can overnight or sign up for day camps that increase your team work skills and build character. They do this via extreme sports. Built on the island is an extensive high ropes course, designed to make you conquer your fears and have fun doing it. Among the high ropes is a zip line. By the end of the camp, the zip line is easily everyone’s favourite thing. You begin by climbing up a very high telephone pole, easily the highest point of the island. There you wait on a small platform for the instructor to bring back the zip line rope.

I will always remember the amazing view. From so high up the whole island is visible, including the lagoon. I marveled at the sight, but will never forget the sheer numbers of small pale circles in the lagoon, the sight of the Upside-Down Jellyfish haven from a distance.

Up close, Upside-Down Jellyfish look a lot like cauliflower. Their creme, textured bottoms, the same colour as a tourist without a sunburn, dominate the shallow water. They are most numerous with mangrove trees around them, and it seems like mangroves are their environment of choice. Also with us are mangrove periwinkles, lizards, and small fish, all happily residing in the island paradise. The sludge at the bottom of the saltwater pool is only disrupted by the jellies and the movements of fish.

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Upside-Down Jellyfish aerial view

Upside-Down Jellyfish are able to swim upright, but they rarely choose to. To most people they may look dead, lying motionless on the bottom of ponds, but in actual fact they are feeding. Most jellyfish navigate the seas in search of food for capture, but the Upside-Down Jellyfish uses a much more relaxing technique, a symbiotic relationship with microscopic zooxanthellae. Zooxanthellae are a form of microscopic algae, a single-celled phytoplankton which live in the tissues of animals. The dinoflagellates are plants, and so they must use photosynthesis to get their nutrients. For photosynthesis to work, the algae must be in the presence of sunlight, which is significantly more difficult in water than on land, and this is why they take advantage of symbiotic relationships with animals like the Upside-Down Jellyfish.

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A mobile Upside-Down Jellyfish among Red Mangroves

Laying bell-down on muddy bottoms of inshore bays with their small tentacles reaching for the sunlight through the water, these jellyfish provide their zooxanthellae with access to the sunlight they need. In return, the algae, who live in the jellyfish’s mesoglea, provide their host with carbon to sustain the jellyfish. However, the carbon which the zooxanthellae provide is not enough to satisfy the metabolic needs of the jellyfish, so they are forced to absorb some of their food from the water, or catch prey with their nematocysts. When they feel prey around, the jellyfish can actually fire stinging cells from its tentacles to stun small fish (and make human skin tingly!). Due to the jellyfish’s largely sessile lifestyle, the symbiosis with zooxanthellae is very convenient for easy nutritional access. However, the fact that the algae cannot provide all the food the Upside-Down Jellyfish needs probably is the reason for the jellyfish to live in nutrient rich water filled with detritus. Some studies have even suggested that Upside-Down Jellyfish are attracted to fallen Red Mangrove leaves.

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A coral polyp with zooxanthellae attached.

All this I knew while observing the jellyfish that day, but what I didn’t know was the more widely known fact, that zooxanthellae and corals also have a symbiosis. It was not until getting home and researching more about this that I realized the lagoon is not just an island paradise for Upside-Down Jellyfish, it’s a haven for zooxanthellae! It seems that zooxanthellae provide carbon for the animals they are in a mutualistic relationship with, allowing corals to flourish  with their calcium carbonate skeletons, with the help of their symbiosis. The corals provide the algae with a safe, protected environment and sunlight which allows them to flourish. In return, the zooxanthellae give off oxygen, the waste product of photosynthesis, which helps the corals to remove wastes. Most essentially, the algae provide the coral with glucose, glycerol, and amino acids which the coral converts into calcium carbonate.

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Coral bleaching

In addition to satisfying the coral’s metabolic needs, the zooxanthellae are also responsible for coral’s vibrant and amazing colour. This also makes zooxanthellae responsible for coral bleaching. When the coral is physically stressed from chemical changes in the water surrounding it, the algal cells of zooxanthellae are expelled and the symbiosis ends. The corals can no longer grow without the relationship and they take on a white shade without the zooxanthellae providing colour. If this goes on for too long, the coral can expire for good; a problem we face today with the ocean’s decreasing pH and increasing acidity.

Interestingly, zooxanthellae seem to flourish in a number of conditions. Coral reefs thrive in clear, nutrient poor water because the algae infused in their tissues do, since it allows for easier access to sunlight. However, the water that Upside-Down Jellyfish occupy is largely cloudy. The bottom line is that the hosts of zooxanthellae will do anything for them to stay, and this means seeking out sunny areas. The only reason Upside-Down Jellyfish can keep the algae is because they live in very shallow water and avoid shade, it also explains why they seek out a new dwelling on the sea floor if the bottom mass is disturbed around them.

Fascinating!

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