Reproduction and Life Cycles in Invertebrates

Abstract

The patterns of reproduction among the invertebrates are much more varied than those of vertebrates. Male and female functions can occur in the same individual either sequentially or at the same time. Various patterns of asexual reproduction may be observed, although these are almost always combined with sexual reproduction at some stage in the life cycle.

Keywords: sexual reproduction; asexual reproduction; parthenogenesis; hermaphrodite; larva

Figure 1.

Diagrammatic representation of complex life cycles involving alternation of asexual and sexual reproduction. (a) Multiplication of embryos (polyembryony); (b) replication of larvae; (c) fission or budding; (d) parthenogenesis – each parthenogenic diploid female may produce further diploid eggs as shown. Eventually, in a sexual phase of the life cycle (not shown), meiosis may occur so that haploid eggs and sperm are produced and sexual reproduction involving the fusion of haploid gametes may occur.

Figure 2.

Life cycle of the scyphosoan cnidarian Aurelia auretia, the common European jellyfish. Many medusae are derived from a single fertilized egg, but each has the same diploid genotype.

Figure 3.

Life cycle of an aphid with primary and secondary host plants.

Figure 4.

Representation of the trade‐off between investment in male and female functions in relation to fitness benefit and the evolution of gonochorism and hermaphroditism. (a) Direct relationship between investment and benefit (no saturation). (b) Trade‐off curve concave; maximum fitness (*) associated with investment of resources in either male or female function. Under these circumstances natural selection favours gonochorism. (c) Declining relationship between investment and benefit (saturation of the male or female gain curve). (d) Trade‐off curve convex; maximum fitness (*) associated with allocation of some proportion of resources, qf, to female function and some proportion, qm, to male function. Under these conditions selection favours simultaneous hermaphroditism.

Figure 5.

Simultaneous hermaphroditism in the earthworm Lumbricus. (a) The internal anatomy includes male and female reproductive organs, ovaries and testes, as well as seminal vesicles for the storage of ‘self sperm’ and spermatheca for storage of a partner's sperm. To engage in mating behaviour, the worms leave the burrows in the soil to find a mate. (b) During the copulatory behaviour, mutual exchange of sperm occurs and sperm moves from the seminal vesicles of the donor worm to the spermatheca of the partner. (c) After mating the worms can fertilize their eggs with the stored sperm of a partner. The eggs are released from the oviducts and move back to the clitellum, which will form a protective covering containing albumen to sustain the developing worms. (d) After the cocoon has been secreted, it moves forwards and, as it passes over the spermatheca, spermatozoa are released to fertilize the eggs. (e) The cocoon is shed over the head of the earthworm and forms a waterproof protective environment in which the eggs develop into young worms.

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Further Reading

Barnes RSK, Calow P, Olive PJW, Golding DW and Spicer JI (2001) The Invertebrates: A Synthesis, 3rd edn. Oxford: Blackwell Science.

Roff DA (1992) The Evolution of Life Histories: Theory and Analysis. New York: Chapman and Hall.

Stearns SC (1992) The Evolution of Life Histories. Oxford: Oxford University Press.

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How to Cite close
Olive, Peter JW(Apr 2002) Reproduction and Life Cycles in Invertebrates. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0003649]