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Symbiotic Relationships
| by Tania on 03/09/2010 08:33 AM |
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By Fiona Ayerst.
Fiona divulges interesting snippets about underwater symbiosis. Did you know that mimicry is when one species imitates another to gain the benefits enjoyed by that species? Read on for more interesting facts...
Symbiotic Relationships (continued)
Why do these relationships exist?
There is always a fight for survival and competition for food and territory in the animal world. To avoid competing with other species, it is expedient for an animal to find a specific niche within their environment. However, another way to avoid direct competition is for one species to form a stable relationship with another species, other than a predator-prey relationship. This allows two species to harmoniously share the same space and/or food supply. For example, the bull shark in the image (above) has a small “friend” that follows it around the ocean, usually hanging under its head but in some instances darting out to pick up pieces of food. These fish are varied and are pelagics but are usually kingfish or pilotfish of some description.
COMMENSALISM
Commensalism usually occurs between a species that is either vulnerable to predation or with an inefficient means of locomotion, and another species with a relatively effective system of defence.
In this relationship the host is neither advantaged nor disadvantaged. In the image to the right , a tiny starfish is hitching a ride on another larger one (the gold heart is all you can see of the larger starfish in the image), allowing it to cover a lot more space in a much shorter time and also receiving protection in the process.
Crabs and shrimps often form commensalistic symbiotic relationships with anemones in tropical waters, again for the purposes of protection from predation. For instance, the anemone crab (Neopetrolisthes oshimai), which is a filter-feeding porcelain crab, lives and captures its food from within the tentacles of anemones.
Apart from the anemone crabs, another interesting relationship is that of imperial shrimps (Periclimenes imperator) hitching a ride on sea cucumbers. The shrimps get transported through a large area of potential food by their host with only a minimal expenditure of energy on their part. They can be observed getting off their host cucumber to feed in productive areas, and back on for a ride to the next spot. Although this is currently classed as a commensalistic relationship, it is possible that the emperor shrimp may assist the sea cucumber by removing parasites, thus making the partnership more mutualistic. In the series of pictures to the right, you can see a shrimp has caught and is feeding on a worm from the sandy bottom whilst catching a ride on a sea cucumber. More than one symbiotic relationship can exist on and between groups of animals. There were many other small organisms living on this same cucumber, for example, a tiny shrimp about the size of a large flea!
PARASITISM
In a parasitic relationship, the host species is always exploited to some degree, although often in such a way that its health is gradually impaired. This allows the parasite to exploit its host over a longer period. Many parasites only spend a portion of their lives in the relationship, either to reproduce or during an initial growth stage. Parasites can be divided into two basic categories: ectoparasites and endoparasites, the former referring to external parasites and the latter, internal parasites.
External marine parasites are usually crustaceans from the order Isopoda or Copepoda and can often be seen by the naked eye. Although parasitism is an unpleasant concept to many people, the adaptations of parasites are quite amazing when viewed objectively. Isopods, for example, have a flattened body shape for streamlining against the body of their host, complex sucker-like organs for firm attachment and a set of sharp mandibles. An interesting adaptation of isopods is their ability to moult only half their exoskeleton at a time, unlike most crustaceans which shed their entire exoskeleton at once. This means the grip on the host species is not lost while they are in the soft state.
MUTUALISM
Mutualism is one of the most interesting forms of symbiosis, as it is beneficial to both species involved.
A particular type of mutualistic interrelationship that many divers have encountered is cleaning symbiosis, a widespread form of mutualistic relationship common to both temperate and tropical waters. Gobies, wrasses and shrimps are among the common fish-cleaning species. The fish being cleaned are often termed “client fish” and the sites they go to be cleaned are known as cleaning stations. In tropical waters, these areas of the reef are quite clearly defined by the numbers of stationary fish and their attendant cleaner organisms. In the image (left), a tiny goby takes refuge in the clamping folds of a giant clam and cleans the clam of parasites.
Colour and markings play an important role in the recognition of cleaner species. In fish, a horizontal stripe is often a cleaner sign and affords the owner some immunity to predation, with many cleaner fish even entering into the mouth and gills of large, otherwise carnivorous species. In some cases, notably with many of the wrasses, it is just the juvenile of a fish species that is a cleaner, while the mature fish progress onto a diet of larger invertebrates. Interestingly, some cleaner fish will even attend to human ”clients”.
As well as removing parasites, cleaners also remove dead skin, tissue and mucous, and in doing so, perform a valuable function in maintaining the health of marine populations. In fact, most reef fish spend a reasonably significant portion of their day at cleaning stations. Often, client fish undergo a colour change which is believed to be for one of two reasons: either as a signal that they require cleaning or so that external parasites stand out against the scales.
A common example is the various species of anemonefish. These depend heavily on their host, being unable to breed or survive predation without their host anemone. The anemone, on the other hand, can survive without its attendant clownfish, although it is hypothesised they may help aerate the tentacles of the anemone as well as get rid of parasites.
MIMICRY
The frogfish, a species of anglerfish, engages in aggressive mimicry. This species has a fleshy filament or lure just above its lip with which it mimics the movements of a small organism in order to attract its prey. When an organism mimics their environment, however, (such as a scorpionfish amongst bottom detritus or a seahorse entwined around a piece of Sargassum seaweed, as per the image below) it then becomes camouflage - a whole new topic!
Nature has a way of maximising utilisation of the environment so that every possible ecological niche is filled, but without any unsustainable overlap. There may be several species competing within a single micro-environment, but each can exist in a type of harmony with the other where a sustainable balance is reached. Symbiosis is a prime example of this paradigm. By living beside, on or even inside another organism, every potential use is made of the resources available in a sustainable fashion. No matter which type of relationship it is, this is an illustration of nature’s ability to co-achieve efficiency and equilibrium - something humans would be wise to start emulating as soon as possible.
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