Pectoral fins: Found in pairs on each side, usually just behind the operculum. Pectoral fins are homologous to the forelimbs of tetrapods, and aid walking in several fish species such as some anglerfish and the mudskipper. A peculiar function of pectoral fins, highly developed in some fish, is the creation of the dynamic lifting force that assists some fish such as sharks in maintaining depth and also enables the "flight" for flying fish. Certain rays of the pectoral fins may be adapted into finger-like projections, such as in sea robins and flying gurnards. Froese, Rainer; Pauly, Daniel (eds.) (2006). " Periophthalmus barbarus " in FishBase. November 2006 version. N. A. Campbell and J. B. Reece (2005). Biology, Seventh Edition. Benjamin Cummings, San Francisco, California.

Under a tough membranous shell, the tunica albuginea, the testis of some teleost fish, contains very fine coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells ( germ cells) that from puberty into old age, develop into sperm cells (also known as spermatozoa or male gametes). The developing sperm travel through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly created sperm cells mature (see spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions. A neuron is "identified" if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties. [65] In vertebrate nervous systems, very few neurons are "identified" in this sense (in humans, there are believed to be none). In simpler nervous systems, some or all neurons may be thus unique. [66]Tetrapods ( amphibians, reptiles, birds and mammals) emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish ( pisces or ichthyes) are rendered paraphyletic by excluding the tetrapods, and are therefore not considered a formal taxonomic grouping in systematic biology, unless it is used in the cladistic sense, including tetrapods, [6] [7] although usually " vertebrate" is preferred and used for this purpose (fish plus tetrapods) instead. Furthermore, cetaceans, although mammals, have often been considered fish by various cultures and time periods.

Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then enter the ostium, then through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the coelom from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct. [56] Gymnovaries are the primitive condition found in lungfish, sturgeon, and bowfin. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct. [50] Secondary gymnovaries are found in salmonids and a few other teleosts. EU fish quota deal hammered out". BBC News. 21 December 2006. Archived from the original on 26 December 2006 . Retrieved 18 January 2006. DWDS – Digitales Wörterbuch der deutschen Sprache". DWDS (in German). Archived from the original on 31 July 2020 . Retrieved 21 January 2023. In the Book of Jonah, a work of Jewish literature probably written in the fourth century BC, the central figure, a prophet named Jonah, is swallowed by a giant fish after being thrown overboard by the crew of the ship he is travelling on. [112] [113] [114] The fish later vomits Jonah out on shore after three days. [112] [113] [114] This book was later included as part of the Hebrew Bible, or Christian Old Testament, [115] [116] and a version of the story it contains is summarized in Surah 37:139-148 of the Quran. [117] Early Christians used the ichthys, a symbol of a fish, to represent Jesus, [110] [111] because the Greek word for fish, ΙΧΘΥΣ Ichthys, could be used as an acronym for "Ίησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ" (Iesous Christos, Theou Huios, Soter), meaning "Jesus Christ, Son of God, Saviour". [110] [111] The gospels also refer to "fishers of men" [118] and feeding the multitude. In the dhamma of Buddhism, the fish symbolize happiness as they have complete freedom of movement in the water. Wegner, Nicholas C.; Snodgrass, Owyn E.; Dewar, Heidi; Hyde, John R. (15 May 2015). "Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus". Science. 348 (6236): 786–789. Bibcode: 2015Sci...348..786W. doi: 10.1126/science.aaa8902. ISSN 0036-8075. PMID 25977549. S2CID 17412022.Acoustic communication in fish involves the transmission of acoustic signals from one individual of a species to another. The production of sounds as a means of communication among fish is most often used in the context of feeding, aggression or courtship behaviour. [3] The sounds emitted can vary depending on the species and stimulus involved. Fish can produce either stridulatory sounds by moving components of the skeletal system, or can produce non-stridulatory sounds by manipulating specialized organs such as the swimbladder. [66] Stridulatory French grunts – Haemulon flavolineatum The heart has two chambers and pumps the blood through the respiratory surfaces of the gills and then around the body in a single circulatory loop. [3] The eyes are adapted for seeing underwater and have only local vision. [ definition needed] There is an inner ear but no external or middle ear. Low-frequency vibrations are detected by the lateral line system of sense organs that run along the length of the sides of fish, which responds to nearby movements and to changes in water pressure. [2] Hartman, K.H.; etal. (2004). "Koi Herpes Virus (KHV) Disease: Fact Sheet VM-149" (PDF). University of Florida Institute of Food and Agricultural Sciences. Archived (PDF) from the original on 6 February 2007.

a b Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp.367–376. ISBN 978-0-03-910284-5. Fish may have evolved from a creature similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today).a b c d Oliveira, T.P.R.; Ladich, F.; Abed-Navandi, D.; Souto, A.S.; Rosa, I.L. (26 June 2014). "Sounds produced by the longsnout seahorse: a study of their structure and functions". Journal of Zoology. 294 (2): 114–121. doi: 10.1111/jzo.12160. Diphycercal: vertebrae extend to the tip of the tail; the tail is symmetrical and expanded (as in the bichir, lungfish, lamprey and coelacanth). Most Palaeozoic fishes had a diphycercal heterocercal tail. [27] Shi Z, Zhang Y, Meek J, Qiao J, Han VZ (2008). "The neuronal organization of a unique cerebellar specialization: the valvula cerebelli of a mormyrid fish". J. Comp. Neurol. 509 (5): 449–73. doi: 10.1002/cne.21735. PMC 5884697. PMID 18537139.