Invertebrates: Reproduction and Life Cycle
Invertebrates: Reproduction and Life Cycle
Invertebrates display a wide variety of methods of reproduction. Some invertebrates reproduce by asexual reproduction, in which all offspring are genetically identical to the parent. Asexual reproduction methods include fragmentation, in which animals divide into two or more offspring, and budding, in which animals sprout buds that break away to take up life on their own. The majority of invertebrates reproduce sexually. The genes from two parents recombine to produce genetically unique individuals. For most invertebrates, sexual reproduction involves laying eggs. With a few exceptions, such as scorpions and spiders, most invertebrates abandon their eggs as soon as they are laid, leaving them to develop on their own.
When invertebrate eggs hatch, the animals that emerge often look nothing like their parents. Some are so different that, in the past, zoologists mistook them for entirely new species. Young like this are known as larvae. As they grow up, larvae change shape, a process known as metamorphosis. A larval stage enables invertebrates to live in different habitats at different stages of their lives. For example, adult mussels live fastened to rocks, but their larvae live floating among plankton. By having larvae that drift with the currents, mussels are able to disperse and find homes with new food sources for their adult life.
The change from larva to adult is quite gradual in many invertebrates, such as crabs and lobsters, but in insects it can be much more abrupt. Caterpillars, the larvae of butterflies and moths, often live for several months, but they take just a few days to turn into adults. During the transition stage, known as the pupa, the caterpillar’s body is broken down and reassembled, forming an adult insect that is ready to breed.
Most invertebrates are short-lived animals, but slow-growing species often break this rule. Wood-boring beetles can live well into their teens, while queen termites can live 40 years or more. But in the invertebrate world, the real veterans live in the sea. Growth lines on bivalve shells suggest that some clams can live to be 200 years old or more. A similar age has been claimed for pogonophoran worms living around hydrothermal vents in the darkness of the deep seafloor.
When invertebrate eggs hatch, the animals that emerge often look nothing like their parents. Some are so different that, in the past, zoologists mistook them for entirely new species. Young like this are known as larvae. As they grow up, larvae change shape, a process known as metamorphosis. A larval stage enables invertebrates to live in different habitats at different stages of their lives. For example, adult mussels live fastened to rocks, but their larvae live floating among plankton. By having larvae that drift with the currents, mussels are able to disperse and find homes with new food sources for their adult life.
The change from larva to adult is quite gradual in many invertebrates, such as crabs and lobsters, but in insects it can be much more abrupt. Caterpillars, the larvae of butterflies and moths, often live for several months, but they take just a few days to turn into adults. During the transition stage, known as the pupa, the caterpillar’s body is broken down and reassembled, forming an adult insect that is ready to breed.
Most invertebrates are short-lived animals, but slow-growing species often break this rule. Wood-boring beetles can live well into their teens, while queen termites can live 40 years or more. But in the invertebrate world, the real veterans live in the sea. Growth lines on bivalve shells suggest that some clams can live to be 200 years old or more. A similar age has been claimed for pogonophoran worms living around hydrothermal vents in the darkness of the deep seafloor.
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