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Gill rakers in fish are bony or 
Gill rakers in fish are bony or cartilaginous
Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...
processes that project from the branchial arch (gill arch) and are involved with suspension feeding tiny prey. They are not to be confused with the gill filaments that compose the fleshy part of the gill used for gas exchange. Rakers are usually present in two rows, projecting from both the anterior and posterior side of each gill arch. Rakers are widely varied in number, spacing, and form. By preventing food particles from exiting the spaces between the gill arches, they enable the retention of food particles in filter feeders.
The structure and spacing of gill rakers in fish determines the size of food particles trapped, and correlates with feeding behavior. Fish with densely spaced, elongated, comb-like gill rakers are efficient at filtering tiny prey, whereas carnivores and omnivores often have more widely spaced gill rakers with secondary projections. Because gill raker characters often vary between closely related taxa, they are commonly used in the classification and identification of fish species. Much of the variation in gill raker morphology is thought to be due to adaptation to optimize the consumption of different diets.
To prevent the potentially damaging passage of solid material through the gill slits and over the gill filaments, early gill rakers strained large particles from the water and diverted them to the esophagus
The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...
. Since an appreciable fraction of this material was nutritious, rakers subsequently evolved as food-trapping mechanisms in filter feeders. Gill rakers, when long and closely set, play the same role in suspension-feeding fish such as herring
Herring are various species of forage fish, belonging to the Order (biology), order Clupeiformes.
Herring often move in large Shoaling and schooling, schools around fishing banks and near the coast, found particularly in shallow, temperate wate ...
, mullet, megamouth, basking and whale shark
The whale shark (''Rhincodon typus'') is a slow-moving, filter feeder, filter-feeding carpet shark and the largest known Extant taxon, extant fish species. The largest confirmed individual had a length of . The whale shark holds many records for ...
s, as baleen
Baleen is a filter feeder, filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by th ...
in the filter-feeding whales.
References
{{diversity of fish Fish anatomy