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Symbiotic relationships effect the evolution of organisms. Many symbiotic events can be spotted between marine invertebrates and bacteria. Symbiosis helps a host utilize high-energy compounds to generate energy. It is suggested that while vertebrates usually house thousands of different prokaryotic phylotypes, invertebrates are often selective for single phylotype.

Symbiosis with bone eating worm
Osedax, also called the bone eating worm is a siboglinid worm from polychaete genus. It was discovered in a whalefall community on the surface of bones, in the axis of Monterey Canyon, California, in 2002. Osedax lacks a mouth, a functional gut and a trophosome. But female osedax have a vascularized root system originating from their ovisac which contains heterotrophic endosymbiotic bacterial community dominated by γ-proteobacteria clade. They use the vascularized root system to access the whale bones. The endosymbionts help the host utilize nutrients from the whale bones.

Symbiosis between Hawaiian squid and Vibrio Fischeri
Hawaiian sepiolid squid Euprymna scolopes and bacterium Vibrio Fischeri also show symbiosis. In this symbiosis, symbiont not only serve the host for defense, but also shapes the host morphology. Bioluminescent V. Fischeri can be found in epithelial lined crypts of the light organ of the host. Symbiosis begins as soon as a newly hatched squid finds and houses V. fischeri bacteria.

The symbiosis process begins when Peptidoglycan shed by the sea water bacteria comes in contact to the ciliated epithelial cells of the light organ. It induces mucus production in the cells. Mucus entraps bacterial cells. Antimicrobial peptides, nitric oxide and sialyted mucins in the mucus then selectively allow only V. fischerii which encode gene rscS to adhere and win over gram positive and other gram negative bacteria. The symbiotic bacteria are then guided up to the light organ via chemotaxis. After successful colonization, symbionts induce loss of mucus and ciliated sites to prevent further attachment of bacterial cells via MAMP (microbe associated molecular pattern) signalling. Also, they induce changes in protein expression in the host symbiotic tissues and modify both physiology and morphology of light organs. After bacterial cells divide and increase in population, they begin expressing enzyme luciferase as a result of quorum sensing. Luciferase enzymes produce bioluminescence. Squids can then emit the luminescence from the light organ. Because Euprymna scolopes emerges only during night time, it helps them avoid predation. The bioluminescence allows them to camouflage with the light coming from moon and stars to ocean and avoid predators.

Chemosynthetic symbiosis
Marine environment consists of a large number of chemosynthetic symbioses in different regions of the ocean: shallow-water coastal sediments, continental slope sediments, whale and wood falls, cold seeps and deep-sea hydrothermal vents. Organisms from seven phyla (ciliophora, porifera, platyhelminthes, nematoda, mollusca, annelida and arthropoda) are known to have chemosynthetic symbiosis till now. Some of them include nematode, tube worms, clam, sponge, hydrothermal vent shrimp, worms mollusc, mussels and so on. The symbionts can be ectosymbionts or endosymbionts. Some ectosymbionts are: symbionts of polychaete worm Alvinella which occur in their dorsal surface and symbionts occuring on the mouthparts and gill chamber of the vent shrimp Rimicaris. Endosymbionts include symbionts of gastropod snails which occur in their gill tissues. In the siboglinid tube worms of the groups Monilifera, Frenulata and Vestimentifera, symbionts can be found in an interior organ called trophosome.

Most of the animals in deep-sea hydrothermal vents exist in a symbiotic relationship with chemosynthetic bacteria. These chemosynthetic bacteria are found to be methane or sulphur oxidizers.