Symbion
Encyclopedia
Symbion is the name of a genus
of aquatic animals, less than ½ mm wide, found living attached to the bodies of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own, new phylum
Cycliophora shortly after they were discovered in 1995. This was the first new phylum
of multicelled organism to be discovered since the Loricifera
in 1983.
and Peter Funch on the mouthparts of the Norway lobster
(Nephrops norvegicus), and other, related, species have since been discovered on:
The genus is so named because of its commensal relationship
with the lobster (a form of symbiosis
) – it feeds on the leftovers from the lobster's own meals.
The genus
Symbion are peculiar microscopic animal
s, with no obvious close relatives, and which was therefore given its own phylum, called Cycliophora. The phylogenetic position of Symbion remains unclear: originally the phyla Ectoprocta and Entoprocta
were considered possible relatives of Symbion, based on structural similarities. However, genetic studies suggest that Symbion may be more closely related to Gnathifera
.
. There are three basic life stages:
by budding
and sexually. In sexual reproduction the male attaches to a feeding stage and impregnates a budding female. The female then separates from the feeding stage and attaches herself to another host, where the larva in her develops. The female dies, and the larva escapes. The sexual reproductive cycle is triggered when the host crustacean molts its skin in order to grow.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of aquatic animals, less than ½ mm wide, found living attached to the bodies of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own, new phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Cycliophora shortly after they were discovered in 1995. This was the first new phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
of multicelled organism to be discovered since the Loricifera
Loricifera
Loricifera is a phylum of very small to microscopic marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more that have been collected and not yet described. Their size ranges from 100 µm to ca....
in 1983.
Scientific classification
Symbion was discovered in 1995 by Reinhardt KristensenReinhardt Kristensen
Reinhardt Møbjerg Kristensen is a Danish invertebrate biologist, noted for the discovery of three new phyla of microscopic animals: the Loricifera in 1983, the Cycliophora in 1995, and the Micrognathozoa in 2000. He is also considered one of the world's leading experts on tardigrades...
and Peter Funch on the mouthparts of the Norway lobster
Norway lobster
Nephrops norvegicus, known variously as the Norway lobster, Dublin Bay prawn, or scampi, is a slim, orange-pink lobster which grows up to long, and is "the most important commercial crustacean in Europe"...
(Nephrops norvegicus), and other, related, species have since been discovered on:
- the American lobsterAmerican lobsterThe American lobster, Homarus americanus, is a species of lobster found on the Atlantic coast of North America, chiefly from Labrador to New Jersey. Within North America, it is also known as the northern lobster or Maine lobster. It can reach a body length of , and a mass of over , making it the...
(Homarus americanus, host to Symbion americanus) - the European lobsterEuropean lobsterHomarus gammarus, known as the European lobster or common lobster, is a species of clawed lobster from the eastern Atlantic Ocean, Mediterranean Sea and parts of the Black Sea. It is closely related to the American lobster, H. americanus. It may grow to a length of and a mass of , and bears a...
(Homarus gammarus, host to an as yet un-named species of Symbion)
The genus is so named because of its commensal relationship
Commensalism
In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...
with the lobster (a form of symbiosis
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
) – it feeds on the leftovers from the lobster's own meals.
The genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Symbion are peculiar microscopic animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s, with no obvious close relatives, and which was therefore given its own phylum, called Cycliophora. The phylogenetic position of Symbion remains unclear: originally the phyla Ectoprocta and Entoprocta
Entoprocta
Entoprocta, whose name means "anus inside", is a phylum of mostly sessile aquatic animals, ranging from long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that draw food particles towards the mouth, and...
were considered possible relatives of Symbion, based on structural similarities. However, genetic studies suggest that Symbion may be more closely related to Gnathifera
Gnathifera (phylum)
Gnathifera is an assemblage of phyla of metazoans."Acanthognatha" is a similar grouping, including rotifers, acanthocephalans, gastrotrichs, and gnathostomulids....
.
Physical description
Symbion pandora has a bilateral, sac-like body with no coelomBody cavity
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...
. There are three basic life stages:
- Asexual Feeding Stage – At this stage, S. pandora is neither male nor female. It has a length of 347 μmMicrometreA micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
and a width of 113 μm. On the posterior end of the sac-like body is a stalk with an adhesive disc, which attaches itself to the host. On the anterior end is a ciliated funnel (mouth) and an anus. - Sexual Stage
- Male – S. pandora has a length of 84 μm and a width of 42 μm during this stage. It has no mouth or anus, which signifies the absence of a digestive system. It also has two reproductive organs.
- Female – S. pandora is the same size as the male in this stage. It does, however, have a digestive system which collapses and reconstitutes itself as a larvaLarvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
.
Reproduction
Symbion can reproduce both asexuallyAsexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
by budding
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...
and sexually. In sexual reproduction the male attaches to a feeding stage and impregnates a budding female. The female then separates from the feeding stage and attaches herself to another host, where the larva in her develops. The female dies, and the larva escapes. The sexual reproductive cycle is triggered when the host crustacean molts its skin in order to grow.