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Message definition (Fish ) The primary reproductive organs are paired [[testicle]]s and [[ovary|ovaries]]. Eggs are released from the ovary to the [[oviduct]]s. Over 97% of fish, including salmon and goldfish, are [[oviparous]], meaning that the eggs are shed into the water and develop outside the mother's body. The eggs are usually fertilized outside the mother's body, with the male and female fish shedding their [[gamete]]s into the surrounding water. In a few oviparous fish, such as the [[Skate (fish)|skates]], fertilization is internal: the male uses an [[Ichthyology terms#I|intromittent organ]] to deliver sperm into the female's genital opening of the female. Marine fish release large numbers of small eggs into the open water column. Newly hatched young of oviparous fish are [[Ichthyoplankton|planktonic larvae]]. They have a large [[yolk sac]] and do not resemble juvenile or adult fish. The larval period in oviparous fish is usually only some weeks, and larvae rapidly grow and [[metamorphosis|change in structure]] to become juveniles. During this transition, larvae must switch from their yolk sac to feeding on [[zooplankton]] prey. Some fish such as [[Embiotocidae|surf-perches]], [[Goodeidae|splitfins]], and [[lemon shark]]s are [[viviparous]] or live-bearing, meaning that the mother retains the eggs and nourishes the embryos via a structure analogous to the [[placenta]] to connect the mother's blood supply with the embryo's.
主要な生殖器官は対の精巣 と卵巣 である。卵は卵巣から輸卵管 へ放出される。サケやキンギョを含む97%以上の魚は卵生 であり、卵は母体外に放出され、水中で発生する。卵は通常、雌雄が配偶子 を周囲の水中に放出することで体外受精される。エイ類 のような一部の卵生魚では受精は体内で行われ、雄は交接器官 を用いて精子を雌の生殖口に注入する。海産魚は多数の小卵を外洋に放出する。卵生魚のふ化直後の仔魚は仔魚プランクトン であり、大きな卵黄嚢 をもち、稚魚や成魚とは似ていない。卵生魚の仔魚期は通常数週間にすぎず、急速に成長して形態変化 し、稚魚となる。この移行期には、仔魚は卵黄嚢から動物プランクトン 捕食への切り替えを行わなければならない。カワスズメ科 、グーデア科 、レモンザメ のような一部の魚は胎生 であり、母体が卵を保持し、胎盤 に類似した構造を通じて母体の血流から胚を栄養する。