Unbalanced relationships: a world of parasites

Unbalanced relationships: a world of parasites

High biodiversity means an incredible amount of interactions between species, included (unfortunately for the host) parasites

An extremely high biodiversity, thousand of species living together: that’s the common vision of a tropical coral reef. Like in every society we know, even in   coral reefs there is always somebody ready to take advantage from someone else’s weaknesses, using his resources and vital energies to grow up and better survive.

In a relationship between two (or more) species, there is a wide range of  interaction possibilities: both can receive some advantage when living together, and they cannot survive without the other species assistance, this is called “mutualism”, or mutualistic symbiosis. In some other cases both species receive a good advantage, even sometimes in different amount, and this is the “commensalism”. In other situations, only one species takes advantage of the relationship, sometimes leading the other species to the death: this is what is called “parasitism”.

The examples of parasitism in tropical coral reefs are almost uncountable and some parasites reach very high degrees of specialization and a very complex life cycle.

A Loki’s whip goby infected by several copepod parasites. The parasite sucks the small fish’s blood making him weaker, and even his swimming ability is compromised. 

First of all, let’s say that the death of the host is not the purpose of the parasite. A long-living host is the dream of every good parasite: he can use the host resources for a longtime and reproduce himself without apparent effort. Evolution drives parasitism in this direction: those relationships where the parasite kills the host quickly are generally supposed to be relatively “young” in terms of evolution. An exception of this rule is the maximally virulent strategy, in which the host is entirely transformed into parasites and killed quickly, and this is a story apart, related mainly to viruses.

Parasite can live inside their host (endoparasites) or outside (ectoparasites). While endoparasites are almost invisible, ectoparasites are quite easily observable while diving in a rich and biodiverse coral reef.

Crustaceans: Parasite or Parasitized?

Almost every animal group can be parasite, and at the same time can be parasitized. While usually worms are the kings of endoparasites, a large number of marine ectoparasites are crustaceans.

A quite common example is the Pennellid copepod often infecting the Loki’s Whip Goby or other small fishes. This family of crustaceans (Fam. Pennillidae) includes a large number of species, which can penetrate the host’s body trough the gills arriving until the hearth to suck oxygenated blood. The appendices normally observed are the abdomens of the females carrying eggs. Usually the poor little fish infected has a difficult life, the swim is unbalanced due to the parasite weight, and his destiny is to die together with his parasite, after the release of hundred of parasite larvae.

An Isopod parasite of a Split-Banded Cardinalfish (Apogon compressus). Less intrusive that other isopods, this parasite can leave his host and go looking for another one, in case the fish dies of becomes too weak to guarantee the parasite’s survival.

Even 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 one in case of necessity. An exception is the “tongue-biter”, isopod that infects many species of fish and recently he became famous for some clownfish images, replacing completely the host’s tongue.

Copepods and isopods don’t save even other animal groups, including crustaceans as well.  Isopods of the Bopyridae family are ectoparasites of crabs and shrimps, living on the carapace or inside the gills chamber, externally observable because of the formation of a characteristic “bulge”. Probably shrimps are very vulnerable to parasitic infection: even big cladocerans can attach to their body. Maybe the absence of cleaning buddies for these small shrimps is an important factor in this vulnerability, as nobody can clean them. Of course 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 (Amphipods) on gorgonians. Just 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 gorgonians polyps, and some intensive infections drive to a fast death of the gorgonian colony.


Out of the classics: some strange infections


A commensal shrimp (Ancyclomenes magnificus) infected by a Bophirid parasite, forming the characteristic “Bulge”

Gorgonians are also subjected to another kind of parasites, but definitely less deadly than caprellids: is the case of Egg Cowries, gastropods of the Ovulidae family. These small and astonishing mimic animals, with egg or spindled shape, are found in gorgonians, sponges or soft coral all along the Indo-Pacific , but it is possible to find some ovulid species even in the Caribbean and in the Mediterranean. They perform something very similar to a grazing activity, eating host’s tissues, polyps and mucus, and absorbing pigments to closely match the host’s color. The host continually regrows the lost tissues, so this grazing activity never leads to the host’s death, and the cowry never risks getting short on food. In this case the word “infection”, usually associated to dangerous diseases, assumes another, lighter meaning.

The Parasitic Cap Shell (Thyca cristallina) lives on some starfish with the same “grazing” activity on the host. I’ve personally observed a blue sea star with more than 10 little shells, quite an infection!

Another kind of strange parasitism is the acoel flatworms infection on the bubble coral (Pleurogyra sinuosa). Apparently not harmful, probably these animals feed on organic detritus deposited on the host’s tissues. These small worms can duplicate themselves just with fragmentation, and every little piece can forms another individual: that’s why sometimes this kind of infection is really huge and can arrive to cover the host entirely. Sea anemones can compete for the space with other animals, same as the growth of some Parazoanthus species on sea fans, sponges, corals or even hydroids.

The Animal Kingdom is almost full of parasites. No animal group can be parasite free, and parasitism plays a very important role in controlling populations of wild species and driving evolution towards the selection of the fittest, the individual that can resists even to parasites.

Almost every animal group can be parasite of something else, even vertebrates: birds can be parasites of other birds’ nests, fish can inhabit the internal organs of big holothurians, just to make some examples. All of these parasites utilize the host resources leaving him poorer, tired, with less possibility of resist to other dangers. Brief final reflection: can be human parasites of the whole Earth? And, since we’re driving our “host” towards to a fast breakdown, maybe we’re neither the smartest parasite out there?


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