An extremely high biodiversity with thousands of species living together: that’s the common vision of a tropical coral reef. Like in every society we know, there is always somebody ready to take advantage of the weaknesses of others, using his resources and vital energies to grow up and better survive. And the coral reef is no different.
In a relationship between two species, there is a wide range of interactions that are possible. In some cases, animals behave in ways where both receive some advantage by working or even living together. When species cannot survive without the other, this is called “mutualism”, or mutualistic symbiosis. In some cases, when both species receive an advantage from cooperating with the other, but the benefits are not entirely balanced, this is known as “commensalism”. But when one species takes full advantage of the relationship, sometimes leading to the death of the other species, this is what is called “parasitism”. Examples of parasitism on tropical coral reefs are almost uncountable with some parasites reaching very high degrees of specialisation, and evolving very complex life cycles as a result.
First of all, it’s important to note that the death of the host is not the goal of the parasite. Quite the opposite, a long-living host is the dream of every good parasite — resources that can be used by the parasite for a long time. Evolution drives parasitism in this direction. A parasite can live inside their host (endoparasites) or clinging to the outside (ectoparasites). While endoparasites are almost invisible, ectoparasites are quite easily observable while diving in a rich and biodiverse coral reef.
Almost every animal group can be parasitic, and at the same time can be parasitized. While worms are the kings of endoparasites, a large number of marine ectoparasites are crustaceans. A quite common example is the Pennellid copepod, which often infects the Loki’s whip goby, or other small fishes. This family of crustaceans (Pennillidae) contains a large number of species, which can penetrate hosts’ bodies through the gills, probing until reaching the heart to feed on oxygenated blood. The appendages that are observable by divers are the eggladen abdomens of the females. Usually, the poor little infected fish leads a difficult life, swimming in an unbalanced fashion due to the weight of the parasite. The destiny of the infected goby is to die with his parasite, after the release of hundreds of parasite larvae.
Other crustaceans of the Cymothoidae family are external parasites of fish. They are generally less “intrusive” than copepods and they can leave their host and transfer to another in case of necessity. An exception is the “tongue-biter” isopod that infects many species of fish, and has recently achieved some fame in clownfish images. This parasite completely replaces the clownfish’s tongue.
Copepods and isopods don’t spare other animal groups, targeting crustaceans as well as fish. Isopods of the Bopyridae family are ectoparasites of crabs and shrimps, living on the carapaces or inside the gill chambers, and are often externally observable because of the formation of a characteristic “bulge”. The initial stage of a copepod infection shows the ovigeral process, appearing like longs antennas. It is sad to think that some of these cleaners are not themselves cleaned.
Crustaceans can infect many other animal groups, from gastropods to sea anemones, or even gorgonians. A typical example is the infection by huge colonies of caprellids. Recently, it has been discovered that a caprellid infection was the cause of high mortality of some species of gorgonians in the Bunaken National Park, in Indonesia. Caprellids act as predators of gorgonian polyps, and some intensive infections drive the coral colonies to a fast death.
Gorgonians are also subjected to another kind of parasite — one that is less deadly. Egg cowries are gastropods of the Ovulidae family and are found in gorgonians, sponges or soft corals all along the Indo-Pacific (Aclyvolva lanceolata and Phenacovolva birostris). These parasites perform something very similar to a grazing activity, eating the host’s tissues, polyps and mucus, while absorbing pigments to closely match the host’s colour. The host continually regrows the lost tissues, so this grazing activity never leads to the host’s death, and the cowry never risks going short on food. In this case the word “infection” usually associated with dangerous diseases, assumes an altogether lighter meaning.
Another kind of strange parasitism is perpetrated by acoel flatworms against bubble coral colonies. Apparently not harmful, these animals most likely feed on organic detritus deposited on the host’s tissues. These small worms can duplicate themselves by a process called fragmentation, where little pieces of the worms can form other individuals. In this way, this kind of infection can be really huge and cover the host entirely.
The Animal Kingdom is full of parasites. Almost every animal group can be parasitic upon another, even vertebrates. Birds for example can be parasites of other birds’ nests, while fish can inhabit the internal organs of big holothurians. All of these parasites utilise the host resources, leaving it impoverished, tired, and with a decreased ability to resist other dangers. No animal group can be parasite-free, but it’s important to remember that parasitism plays a very important role in controlling populations of wild species, and driving evolution towards the selection of the fittest.
Taken from ScubaDiver Australasia 03/2012