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Nudibranchs and the Effects of Pollution on Coral Life
It usually takes something to be placed out of its everyday setting for me to really start thinking about it like a scientist. Say I walk by a certain tree everyday and there have always been ants, constantly marching up and down, carrying bits of food to their hill. I wouldn’t normally pay much attention or question what or how they are accomplishing such a feat because that’s how it’s always been. I just accept that they are finding food and taking it back to the colony and that’s that. If, however, I saw that same line of ants marching across my kitchen counter, toward a piece of rotting fruit far in the corner, I would immediately question how they knew that fruit was there, how they signaled the others to follow and how they intended to carry it off piece by piece all the way back home.
I think this is perhaps why I started being interested in Nudibranchs or the more commonly known “sea slug”. Although land slugs and sea slugs are actually quite different, I first became intrigued by them simply because they were slugs living under water. I see slugs in my garden so often that they are almost just another part of the flora for me. But slugs in the sea? Craziness!
As soon as I started learning about sea slugs in the 6th grade I was awestruck. How could these slippery, little devils survive under water and how closely related to the garden slug actually were they? Well, years have gone by and while some of my questions about sea slugs have been answered I still have a lot to learn and continue to see them as amazing creatures, both physically and characteristically. With coral reefs suffering all over the world, sea slugs, which rely heavily on the reefs for food and protection, are no exception to the deadly effects. Hopefully, through educating others about the mystery and beauty present in our oceans, protecting sea slugs and all other organisms that rely on the coral reefs will not seem so hopeless.
Known for their extraordinary colors and abnormal body shapes, sea slugs or nudibranchs, meaning “naked gills“, are among the most audacious of all marine life. With hundreds of different species from all over the world they exist in an incredibly broad range of colors, sizes and shapes. Although they share many characteristics with land slugs, they also have many traits which make them extraordinarily unique.
One of the boldest characteristics of a nudibranch is its bright and flaunty coloring. A nudibranch’s fascinating colors aid the sea slugs to warn off predators that they are poisonous and therefore dangerous to eat. The danger comes from tiny glands which contain toxins, located all over the nudibranch’s skin which automatically secrete their contents when disturbed. Another defense capability of some sea slugs (including the Cerberilla nudibranch) is the ability to “recycle” at least some of the stinging cells (nematocysts) from the cnidarians they eat. When the sea slugs digest their prey (sea anemones, corals, hydroids, jellyfish, etc.) the nematocysts are able to pass through the digestive system intact and are then placed on the cerata of the nudibranch. The cerata, an organ on the back of the slug, has the ability to contain the stinging cells in a sort of sac to keep them alive and act as a helpful defense for the nudibranch.
A nudibranch’s shape varies from species to species and works to aid the specific sea slug with traits unique to it. The size of a sea slug is a similar idea because those slugs that are smaller (1-2 inches) have the ability to fit into smaller nooks and take on small organism lifestyles while those that are much larger (up to 19 inches!) dominate their surrounding areas while scraping large chunks of coral and other food up with their powerful radula.
Nudibranches eat in a manner that is quite similar to their land relatives. A sea slugs mouth is located at the anterior end of the animal and is connected to a muscle called the buccal bulb. The buccal bulb contains the radula, or tongue of the slug, which is used to scrape food off of surfaces. The radula acts in a similar way to teeth because of its sharp structure but is structurally more similar to a muscular tongue. Secreted from the buccal bulb are digestive juices contained in the salivary glands which help to expedite the breakdown of food particles for easy digestion.
In a similar manner to many other animals, the next location for a sea slug’s food to travel is through the esophagus to the stomach where more digestive juices are added and then to the digestive ducts. The digestive ducts are an extremely important element of a mollusk’s anatomy because they are the organs which secrete most of the digestive enzymes, absorbs most of the nutrients and excretes the waste products. This single organ on the nudibranch is comparable to the stomach, intestine and liver combined in humans. When a sufficient amount of nutrients has been absorbed from the food the waste product is compacted into small pellets and sent out through the intestine and anus.
Another interesting organ of the nudibranch is the rhinophore. Rhinophore, meaning “nose carrier” gives the sea slug the ability to smell. The organ comes in a variety of different shapes, each designed to provide as much surface area as possible for the chemoreceptors. The chemoreceptors of the sea slug work in much the same way as those in any other animal’s nose do and aid the sea slug to find food. Rhinophores are located on the mid to posterior end of the slug’s body and look similar to a blossoming feathery flower. As the sea slug moves along it picks up stronger signals from different directions and then travels toward the area promising the most food or a mate searching another mate. A nudibranch also has a set of tentacles located on the head of its body which also help in locating food and mates.
Although sea slugs can be highly poisonous and dangerous to predators, they rarely use their defensive toxins to capture their own prey. Sea slugs, which for the most part are carnivores, use their radula to scrape off bits of coral and munch on anemones and sponges as they travel along. Because they are slow and relatively small they are usually limited to stationary prey which they can inch right on up to without it swimming off and hiding.
Reproduction in nudibranchs is particularly interesting because they are all hermaphrodites. To mate, which usually occurs in pairs (sometimes more are involved), they align themselves side by side in opposite directions so that their genitalia line up properly. The slugs then exchange sperm and part their separate ways to each lay their own egg sack which often happens on or around their specific prey.
Once the sacs are laid they are present in numbers ranging from hundreds, thousands to even millions of individual eggs. When the nudibranchs hatch they appear as small larva with tiny shells that are then lost during adulthood. They tend to hatch when they are near their particular prey so that their first meal is within easy reach.
Such huge amounts of eggs are released because, like many small creatures, the young have a low survival rate. The survival of these small, delicate eggs relies on many factors including variations in water temperature, numbers of predators, locations and the increasing presence of pollution from human activity.
Sea organisms of all types are drastically effected more each year as human populations grow exponentially. As the number of humans soars we require more resources and continually attain them in environmentally harmful ways. Sea slugs are definitely not the only organisms suffering from our mistakes but they are definitely an easy target. Because coral reefs are put under a great deal of stress from almost any sort of pollution, toxins, eutrophication as a result of sewage dumping, sediment from construction and harmful fishing practices, nudibranchs, which rely on the coral for food, must endure many lethal changes. Not only are their own bodies harmed from the toxins put in the water and bombs from fishermen but when coral is covered up from algal blooms or sediment the sea slugs starve.
Through researching for this paper I have learned much about sea slugs and their precarious state in their coral habitat. I think it’s easy for people to ignore the effects of pollution in marine waters because the oceans seem extremely vast and powerful. But many creatures, like sea slugs who live in these vast oceans, are so incredibly delicate that even the slightest changes, let alone big ones, alter their habitats in ways their bodies cannot handle. It’s going to take people to see the oceans in this light rather than as all-powerful before we can really start to clean up the coral reefs and protect all of the unique and beautiful creatures that live in them.
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