Sexual Reproduction:
1. External Fertilization: Many bony fish practice external fertilization. In this process, males release sperm into the water, and females release eggs. Fertilization occurs outside of the body, usually in the water column. Species such as salmon, trout, and certain killifish exhibit this reproductive mode.
2. Internal Fertilization: Internal fertilization is also common in Osteichthyes. Male fish have specialized structures, such as an intromittent organ or gonopodium, to transfer sperm into the female during mating. The fertilized eggs are retained within the female's body until they are ready to hatch. Livebearer species like guppies and swordtails exhibit internal fertilization and give birth to live offspring.
3. Egg Laying: After fertilization, many fish species lay eggs. These eggs can be adhesive, attaching to plants or substrates, or they can be scattered freely in the water. The eggs may be covered with protective coatings or shells to ensure survival until hatching. Examples of egg-laying species include goldfish, zebrafish, and clownfish.
4. Parental Care: The level of parental care varies among different Osteichthyes species. Some species provide extensive parental care, such as building nests, guarding eggs or young, and even mouthbrooding. Mouthbrooding involves one or both parents carrying the fertilized eggs or fry in their mouths for protection until they hatch or are ready to be released. Cichlids and certain catfish species are known for their mouthbrooding behavior.
5. Livebearing: Some bony fish species are livebearers. In these species, the fertilized eggs develop and hatch within the female's body. The females give birth to live young, which are fully formed and capable of swimming and feeding independently. Livebearing species include mollies, platys, and swordtails.
Asexual Reproduction:
While the vast majority of Osteichthyes reproduce sexually, a few species exhibit asexual reproduction, where offspring are produced from a single parent without the involvement of gametes or fertilization. Examples include certain Amazon molly species (Poecilia formosa) and the mangrove rivulus (Kryptolebias marmoratus).
It's important to note that the reproductive characteristics of Osteichthyes can vary considerably among different species and families, with diverse adaptations and specialization in their reproductive strategies.