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Most male fish have two testes of similar size. In the case of sharks, the testes on the right side is usually larger [citation needed].The primitive jawless fish have only a single testis, located in the midline of the body, although even this forms from the fusion of paired structures in the embryo.
A gonad, sex gland, or reproductive gland [1] is a mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. [2] The male gonad, the testicle, produces sperm in the form of spermatozoa. The female gonad, the ovary, produces egg cells
The stout infantfish is the smallest and lightest adult fish and is in fact the smallest vertebrate in the world; the females measures 8.4 mm (0.33 in) and the male just 7 mm (0.28 in). [ 35 ] A rare giant oarfish ( Regalecus glesne ), 7-metre (23 ft) long, captured in 1996
In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads). Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus.
A small portion of fish species are either viviparous or ovoviviparous, and are collectively known as livebearers. [23] Fish gonads are typically pairs of either ovaries or testicles. Most fish are sexually dimorphic but some species are hermaphroditic or unisexual. [24]
Sea urchins are dioecious, having separate male and female sexes, although no distinguishing features are visible externally. In addition to their role in reproduction, the gonads are also nutrient storing organs, and are made up of two main type of cells: germ cells, and somatic cells called nutritive phagocytes. [20]
Fish that have live-bearing offspring include the guppy and mollies or Poecilia. Fishes that give birth to live young can be ovoviviparous, where the eggs are fertilized within the female and the eggs simply hatch within the female body, or in seahorses, the male carries the developing young within a pouch, and gives birth to live young. [43]
The male portion of the reproductive tract includes both a short posterior vas deferens and a longer anterior vas deferens. The posterior vas deferens is followed by the prostate, and the anterior vas deferens flows through the haemocoele , an enlarged blastula filled with blood, of the head and opens into a muscular penis which is engulfed in ...