User:Bpigz/Anglerfish

The anglerfish are fish of the teleost order Lophiiformes (/ˌlɒfiɪˈfɔːrmiːz/). They are bony fish named for its characteristic mode of predation, in which a modified fin ray (the esca or illicium) that can be luminescent acts as a lure for other fish. The source of luminescent comes from symbiotic bacteria, which is thought to be acquired from seawater, that dwell in and round the esca.

Some anglerfish are notable for extreme sexual dimorphism and sexual symbiosis of the small male with the much larger female, seen in the suborder Ceratiidae, the deep sea anglerfish. In these species, males may be several orders of magnitude smaller than females.

Anglerfish occur worldwide. Some are pelagic (dwelling away from the sea floor), while others are benthic (dwelling close to the sea floor). Some live in the deep sea (such as the Ceratiidae), while others on the continental shelf, such as the frogfishes and the Lophiidae|monkfish or goosefish). Pelagic forms are most laterally compressed, whereas the benthic forms are often extremely dorsoventrally compressed (depressed), often with large upward-pointing mouths.^{[citation needed]}

The source of luminescence is symbiotic bacteria that dwell in and around the esca, enclosed in a cup-shaped reflector containing crystals, probably consisting of guanine. Only a handful of luminescent symbiont species can associate with deep-sea anglerfishes. In some species, the bacteria recruited to the esca are incapable of luminescence independent of the anglerfish, suggesting they have developed a symbiotic relationship and the bacteria are unable to synthesize all of the chemicals necessary for luminescence on their own. They depend on the fish to make up the difference. Electron microscopy of these bacteria in some species reveals they are Gram-negative rods that lack capsules, spores, or flagella. They have double-layered cell walls and mesosomes. A pore connects the esca with the seawater, which enables the removal of dead bacteria and cellular waste, and allows the pH and tonicity of the culture medium to remain constant. This, as well as the constant temperature of the bathypelagic zone inhabited by these fish, is crucial for the long-term viability of bacterial cultures.

The light gland is always open to the exterior, so it is possible that the fish acquires the bacteria from the seawater. However, it appears that each species uses its own particular species of bacteria, and these bacteria have never been found in seawater. Haygood (1993) theorized that esca discharge bacteria during spawning and the bacteria are thereby transferred to the eggs.

Some evidence shows that some anglerfish acquired their bioluminescent symbionts from the local environment. Genetic materials of the symbiont bacteria is found near the anglerfish, indicating that the anglerfish and their associated bacteria are most likely not evolved together and the bacteria take difficult journeys to enter the host. In a study on Ceratioid anglerfish in the Gulf of Mexico, researchers noticed that the confirmed host-associated bioluminescent microbes are not present in the larval specimens and throughout host development.The Ceratioids likely acquired their bioluminescent symbionts from the seawater. Photobacterium phosphoreum and members from kishitanii clade constitute the major or sole bioluminescent symbiont of several families of deep-sea luminous fishes.

It is known that genetic makeup of the symbiont bacteria has undergone changes since they became associated with their host. Compared to their free-living relatives, deep-sea anglerfish symbiont genomes are reduced in size by 50%. Reductions in amino acid synthesis pathways and abilities to utilize diverse sugars are found. Nevertheless, genes involved in chemotaxis and motility that are thought to be useful only outside the host are retained in the genome. Symbiont genome contains very high numbers of pseudogenes and show massive expansions of transposable elements. The process of genome reduction is still ongoing in these symbionts lineages, and the gene loss may lead to host dependence.