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Symbion is the name of a genus
Genus

A genus is a low-level taxonomic rank used in the classification of living and fossil organisms. The taxonomic ranks are domain , kingdom , phylum, class , order , family , genus, and species....
 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

A phylum "Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states. is a taxonomic rank below Kingdom and above Class ....
 Cycliophora shortly after they were discovered in 1995.






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Symbion is the name of a genus
Genus

A genus is a low-level taxonomic rank used in the classification of living and fossil organisms. The taxonomic ranks are domain , kingdom , phylum, class , order , family , genus, and species....
 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

A phylum "Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states. is a taxonomic rank below Kingdom and above Class ....
 Cycliophora shortly after they were discovered in 1995. This was the first new phylum
Phylum

A phylum "Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states. is a taxonomic rank below Kingdom and above Class ....
 of multicelled organism to be discovered since the Loricifera
Loricifera

Loricifera is a small phylum of marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more which have been collected and not yet described....
 in 1983.

Scientific Classification


Symbion was discovered in 1995 by Reinhardt Kristensen
Reinhardt Kristensen

Reinhardt M?bjerg Kristensen is a Denmark invertebrate biologist, noted for the discovery of three new phylum of microscopic animals: the Loricifera in 1983, the Symbion in 1995, and the Limnognathia in 2000....
 and Peter Funch on the mouthparts of the Norway lobster
Norway lobster

The Norway lobster, Nephrops norvegicus, , is a slim, orange-pink lobster which grows up to 24 cm long  . It is found in the north-eastern Atlantic Ocean and North Sea as far north as Iceland and northern Norway, and south to Portugal....
 (Nephrops norvegicus), and other, related, species have since been discovered on the American lobster
American lobster

The American lobster, Homarus americanus, is one species of lobster found on the Atlantic Ocean of North America. Within North America, it is also known as the northern lobster, Atlantic lobster or Maine lobster....
 (Homarus americanus, host to Symbion americanus) and the European lobster
European lobster

The European lobster, Homarus gammarus, is a large European clawed lobster. It is difficult to distinguish from the American lobster ? the best distinction is the geographical location, with the European lobster in the eastern Atlantic Ocean and the American lobster in the western Atlantic, and by the lack of teeth on the underside...
 (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 benefits and the other is not significantly harmed or benefited....
 with the lobster (a form of symbiosis
Symbiosis

The term symbiosis commonly describes close and often long-term interactions between different biological species. The term was first used in 1879 by the Germany mycology Heinrich Anton de Bary, who defined it as "the living together of unlike organisms"....
) – it feeds on the leftovers from the lobster's own meals.

The genus
Genus

A genus is a low-level taxonomic rank used in the classification of living and fossil organisms. The taxonomic ranks are domain , kingdom , phylum, class , order , family , genus, and species....
 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....
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 is a phylum of small aquatic animals, ranging in size from 0.5 mm to 5.0 mm. They have a lophophore, and as their name suggests, are distinguished from other lophophorates by the position of the anus inside the ring of cilia rather than outside....
 were considered possible relatives of Symbion, based on structural similarities. However, genetic studies suggest that Symbion may be more closely related to Gnathifera
Gnathifera

Gnathifera may refer to:*a Scientific classification within the superphylum Platyzoa *a Genus of moths ...
.

Physical Description

Symbion pandora has a bilateral
Symmetry (biology)

Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or glide symmetry....
, sac-like body with no coelom
Body 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 µm
    Micrometre

    A micrometre or micron is one Micro- of a metre, or equivalently one thousandth of a millimetre. It is also commonly known as a micron....
     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
    • 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 larva
      Larva

      A larva is a young form of animal with indirect developmental biology, going through or undergoing metamorphosis .The larva can look completely different from the adult form, for example, a caterpillar differs from a butterfly....
      . (Funch & Kristensen, 1995)
    • 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.


Reproduction

Symbion can reproduce both asexually
Asexual reproduction

Asexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction....
 by budding
Budding

Budding is the formation of a new organism by the protrusion of part of another organism. This is very common in plants and fungi, but may be found in some animals as well, such as the Hydra ....
 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.

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