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Most fish species spawn eggs that are fertilized externally, typically with the male inseminating the eggs after the female lays them. These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest treefrog, Chiromantis xerampelina.
A female produces an ovum with a full set (two sets of genes) provided solely by the mother. Thus, a male is not needed to provide sperm to fertilize the egg. This form of asexual reproduction is thought in some cases to be a serious threat to biodiversity for the subsequent lack of gene variation and potentially decreased fitness of the offspring.
When mating, the female seahorse deposits up to 1,500 (average of 100 to 1,000) eggs in the male's pouch, located on the ventral abdomen at the base of the tail. Male juveniles develop pouches when they are 5–7 months old. The male carries the eggs for 9 to 45 days until the seahorses emerge fully developed, but very small.
The egg is not retained in the body for most of the period of development of the embryo within the egg, which is the main distinction between oviparity and ovoviviparity. [1] Oviparity occurs in all birds, most reptiles, some fishes, and most arthropods. Among mammals, monotremes (four species of echidna, and the platypus) are uniquely oviparous.
The paternal chromosomes are therefore excluded from nascent eggs, without recombination having typically occurred. [8] In some cases, such as Pelophylax esculentus , there is also endomeiosis prior to cell division, which means that the maternal chromosomes are duplicated and each egg contains identical pairs of chromosomes. [ 8 ]
About 50% of female sea turtles complete "false crawls," which occur when they crawl onto the beach but return to the water without laying eggs.
During courtship, the male betta may exhibit aggressive behavior towards the female by acts of chasing or nipping at her fins. [15] The act of spawning itself is called a "nuptial embrace", for the male wraps his body around the female; around 10–40 eggs are released during each embrace, until the female is exhausted of eggs.
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