12 Reproductivestrategies for survival A colourful marine mollusc, the orange spotted clown nudibranch (Ceratosomaamoena), has laid a cluster of many eggs on a green alga.
Figure 12.3 Coral is the product of tiny animals known as polyps. This photograph of coral (Tubastrea spp.) shows the polyp arms extended. (See chapter 13, page 423, for a photograph of a coral polyp that has captured its prey.)
Figure 12.2 Some of the many forms of coral, including boulders and plates. Other corals are convoluted and yet others are columnar or highly branched. ODD FACT Coral polyps, along with jellyfish, sea anemones and hydra, are members of phylum Cnidaria. Their distinctive body plan includes (i) a single opening to the body (ii) a three-layered body wall and (iii) tentacles covered by stinging cells.
Figure 12.4 A large group of cardinalfish (family Apogonidae) gather for mass spawning among the coral of the Great Barrier Reef.
Figure 12.5 These are newly hatched fish larva (termed ‘alevins’). Their yolk sacs — the orange bubblelike shapes — are still present and when this food source shrinks and disappears, the juvenile fish will feed independently.
Different reproductive strategies exist in the living world. • Reproductive strategies seen in reef fish include broadcast spawning and demersal spawning. • Energy investments by different species into various aspects of reproduction differ. Most demersaleggs hatch in the early evening after dusk, a period when predators are least active. How might this behavioural feature assist survival?
A range of reproductive strategies type of reproduction – asexual – sexual gender system – sexes exist as separate male and female – hermaphrodite – parthenogenesis mode of fertilisation – external – internal mating system – single pair matings: monogamy – multiple matings of individuals (polygamy) – promiscuity numbers of offspring – r-selection – K-selection place of development and source of nutrition for the embryo – oviparity — egg laying – viviparity — giving birth to live young (several types) investment of parental care into offspring – nil – care by one or both parents – care by extended family.
Several mating systems occur in the animal kingdom. • Monogamy and polygamy are examples of different mating systems. • Several polygamous mating systems can be identified, including polyandry and polygyny. • Polygamy is seen in bird species that live in habitats with plentiful and reliable food resources.
Offspring: how many? how often? • Two reproduction strategies are r-selection and K-selection. • r-selection is characterised by large numbers of offspring produced quickly. • K-selection is characterised by fewer offspring produced more slowly. • r- and K-selection are the extremes of a continuum and species may show features of both strategies.
Eggs or liveborn young? • Two major reproductive strategies are oviparity and viviparity. • Oviparous species lay eggs and their young hatch from eggs outside the mother’s body. • Viviparous species give birth to live young that develop within the mother’s body. • Various kinds of viviparity occur that differ in how the embryo is nourished. • Reproduction in fish and amphibians is dependent on the presence of water. • Amniote eggs of reptiles and birds enable reproduction to occur away from water.
Parental care or not? • The amount of parental care invested by parents into the care of their eggs or of their offspring varies between species. • Among bird species, different patterns of parental care can be identified in terms of which parent(s) provide the care. • The reproductive strategy of Australian marsupial mammals differs from that of placental mammals. • Reproduction of Australian marsupials includes features such as suspended embryonic development that equips these species for life in drought-affected habitats.
How often do matings occur? • In most animal species, fertilisation must be shortly preceded by mating. • In some animal species, including insects, sperm is long lived and the female can store sperm in her reproductive tract. • Females of various mammalian species have regular oestrous cycles throughout the year. • Reproduction may occur annually or may be a ‘once-in-a-lifetime’ event. • Reproduction in wild species can be influenced by external environmental factors such as day length.
Figure 12.38 Adaptations for dispersal by various agents. (a) The honesty plant (Lunaria annua) has flat dry fruit, containing flat seeds. (b) The dandelion (Taraxacumofficinale) with its head of many dry fruit each with a parachute of fine hairs. (c) Dry fruit, commonly called a burr, with barbs on the surface. (d) Seeds in emu dung.
Sexual reproduction in flowering plants and conifers involves pollination. • Transfer of pollen involves vectors, such as animals or wind. • Pollination in conifers is exclusively by the wind. • Depending on the pollination vectors, flowers will show particular features. • Seeds are the stage of the life cycle of conifers and of flowering plants that enable dispersal.