Introduction

Siphonophores are probably the most abundant organisms in the ocean that most people haven’t heard of. What makes these creatures extreme is that when first looking at them they appear to be normal organisms but on closer inspection they are actually a colony of multiple organisms that are all highly specialised. Each separate individual is called a zooid and although they are technically individuals they couldn’t survive if they became detached from the main colony. This is the human equivalent of having your arms and legs as different people! Most of them tend to be long thin and transparent floating in the water column. Most of the species live deep in the ocean and swim using the specialised individuals in the colony. Other species have been observed anchoring themselves to the ocean floor and one species the Portuguese man o’ war floats at the surface of the water column. It is equipped with a sail which allows it to be covered by winds across the ocean.  One of the longest creatures in the sea is actually a Siphonophore, the species Praya dubia measures between 40 and 50 metres, that longer than a blue whale! There are currently 175 species known to science and they are all predatory.

Are they only one organism?

Siphonophores are different from other types of colonial animals, in colonial species like coral all the polyps are morphologically and functionally the same this created the uniform shape that is common in most coral species. In Siphonophores however the individuals are highly specialised to perform a function. In terms of ecology a Siphonophores is an individual because it functions as an individual animal even though all the parts are actually separate organisms they are genetically identical. If a single individual was to become detached from the main colony then it wouldn’t be able to survive in the open ocean by itself. Another factor is that the whole colony behaves as a single organism. The movement of all the individuals is coordinated so the organism moves through the water column with a clear direction and purpose.

Structure

siphonophore-body-plan
Diagram of the basic structure of the body of a physonects Siphonophore. Original source:http://www.siphonophores.org/images/physonect_diagram.jpg

Three are three groups of Siphonophores the calycophorans, cystonects and physonects. The group will the simplest body shape are the physonects. They are arranged along a central stem that has a gas float at one end called the pneumatophore. Behind this float are the individuals that are specialised to move the colony through the water, these are called the nectophores. All of the individuals contract together to allow the colony to move in any direction. This area of specialised individuals for movement is known as the nectosome. The area after the nectosome is called the siphosome. This region contains all the rest of the individuals. This is where the zooid’s that are specialised for feeding and reproduction are.

Reproduction

Not a limited amount is known about Siphonophore reproduction because they often live in very deep water. The reproductive strategy varies amongst different species. Siphonophores can be either male or female, or may have the reproductive parts of both genders. The gametes mature inside specialised medusa. These can be let out into the water column, grown of polyps or they may be kept within the colony until it dies. The whole organism descends from one fertilised egg all of the individuals that make up the adult colony originate from this single fertilised egg.

Study sites

There are a few sites in the world where the conditions are right to collect species that are normally found in deep water at the surface. Villefranche-Sur-Mer, France is a very important location in the study of Siphonophores, in the spring of every year various species come to the surface and can be taken to a laboratory for examination. This is where most of the knowledge about Siphonophores comes from. As the technology of remote controlled submersible vehicles improves it becomes ever more viable to go and collect and observe these organisms in their natural habitat. This is a vast improvement from methods in the past that would trawl organisms up from the deep. This method didn’t produce many usable samples as a lot of the organisms collected would be reduced to a gelatinous mass because of the fragile nature.

Portuguese man o’ war

stinging-cell
Diagram of a nematocysts firing and releasing toxin original source: http://jcs.biologists.org/content/115/4/745

The most well-known species of this obscure group is the Portuguese man o’ war (Physalia physalis). There is also a species that lives in the pacific called Physalia utriculus. The reason that there are the most know species for two reasons. The first reason is that they have a large gas filled bladder so they float on the surface unlike their deep sea relatives. The second reason is because of their sting. They are responsible for around 10,000 stings each summer in Australia. The man o’ war’s tentacles consist of stinging cells called nematocysts. These cells can even release their sting when a tentacle has come off the main body or when the whole colony has died. The stinging cells are effective against small prey items. In humans the stings are known to cause a lot of pain and the victim might not know what has happen if they were stung by a tentacle that was dislodged from the main colony. In some rare cases however there may the symptoms may be more serious and could result in allergic reaction type symptoms, if in these cases the victim doesn’t receive medical attention then it may result in death.

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