Sexual life cycles
|Because the life cycles of most animals are not complicated by such features as asexual reproduction and the alternation of haploid and diploid generations, many have rather simple life cycles. But the life cycles of many animals are complicated by the existence of specialized larval stages. Some types of lobsters, for example, have a dozen or so larval stages that drift free in the ocean for over a year. Think of the difficulties encountered by aquaculturists who would like to raise these lobsters! We will examine simple and complex life cycles, both to illustrate different kinds of life cycles and to consider the causes and consequences of these differences.
(A) Simple life cycles
Mammals, birds, and reptiles have rather simple life cycles, as do some invertebrate animals. In simple life cycles, the embryo develops directly into a form that resembles the adult. Hence a puppy is clearly recognizable as a dog, a nestling bird as a bird, and a newborn snake as a snake. Such life cycles proceed as follows:
|The embryo is a life stage that develops within the egg membrane; it is incapable of making a living on its own and depends on the provisions of the egg or upon direct parental care. Juveniles are functional organisms (though they, too, may need some parental care), and more or less resemble the adult form. Adults are capable of reproduction. Note that the possibility of death is included for each stage in the life cycle. See the examples of simple life cycles, and note the difference between precocial and altricial young.
What is the main function of each life stage? At what stage does most of the development take place? At what stage(s) does most of the growth take place? Where does reproduction take place? Where can dispersal take place? (Dispersal is the more or less permanent movement of potential reproducers from their place of birth.)
(B) Complex life cycles
Most species of animals have complex life cycles. Complex life cycles contain stages that do not resemble the juvenile/adult form, and which experience very different ecological conditions. Life stages that have hatched from the egg but do not resemble the juvenile or adult are called larval stages. Tadpoles are larvae of frogs and toads, and caterpillars are larvae of butterflies and moths. Larvae undergo metamorphosis (be it sudden or gradual) to the juvenile/adult form. Complex life cycles often take the form:
|Adults, juveniles, and embryos each perform similar functions in simple and complex life cycles. The functions of the larval stage are quite varied, but usually include continued morphogenesis (which may occur in a rapid metamorphosis at the end of the larval stage) and growth. Since these processes also occur in the embryo and juvenile stages, what makes the larval stage different? The larval stage differs from the embryonic stage in that the larva fends for itself while development continues. It is not provisioned by its parents (or, it has used up these provisions), so it obtains food on its own. This being the case, a parent might get one fully developed juvenile for a smaller investment in energy and time with a larval stage than without. As a result, the parent can produce more offspring for the same investment if there is a larval stage. (But note that the chances of survival will be smaller for those animals with less parental investment.) The larval stage differs from the juvenile stage in that it inhabits a different environment, or consumes different types of food than the adult.
Despite some of the obvious attributes of a larval stage, we find no general rules that explain why larval stages occur in some species but not in others. It appears that larval stages may exist for several different reasons (or combinations of reasons) in different kinds of animals.
(1) Examples of Complex Life Cycles
(a) Frogs lay eggs in water, and have larvae capable of and generally restricted to living in water.
Examine the life cycle of the frog. What is the larval stage? Most amphibians lay eggs in moist places or in water, since the eggs usually can't retain moisture. Not surprisingly, then, they have larvae that are adapted for aquatic life. The existence of an aquatic larval stage may also make some different resources available in the life cycle of a frog; tadpoles grow quickly on a diet of plants.
(b) Similar larval stages in different organisms suggests conservative developmental programs.
Particular kinds of larvae may occur in two or more groups of animals that are rather dissimilar as adults. On display are pictures of a trochophore larva and a leptocephalus larva. Each of these larval stages occurs in more than one kind of animal. The trochophore occurs in marine annelid worms (segmented worms related to earthworms) and in many molluscs (clams, chitons, snails, etc.). The leptocephalus larva occurs in two rather different looking kinds of fish: eels on one hand and tarpons and bonefish on the other. Nauplius larvae occur (either free-living or in the egg) in almost all crustaceans, from tiny barnacles and brine shrimp to large crabs and lobsters. Why should similar larval stages occur in such different groups of animals? One suggestion is that annelids and molluscs, eels and tarpons, etc., evolved from common ancestors that possessed a trochophore or leptocephalus larval stage, and that the larval stage is an essential step in the sequence of development. Many other examples of conservatism in development exist, such as the retention of gill slits in the development of birds and mammals. Thus the existence of certain types of larvae indicates severe developmental constraints.
(c) Animals with a non-living covering must molt in order to grow and change form.
Arthropods (insects, crabs, shrimp, spiders, and many, many others) all have hard, nonliving, external skeletons. This exoskeleton presents some problems in development and growth, since it completely encloses the animal and cannot change shape. In their growth and development, then, all arthropods go through a series of molts, in which the old exoskeleton is shed, and a new one, larger and perhaps of different shape, forms to replace it. The life stages between molts in arthropods are known as instars, and most arthropods pass through a specific number of instars during their lives. In some arthropods, eggs hatch into young that resemble their parents, and the main changes between instars are in size and the formation of reproductive organs. In most arthropods, however, instars differ in form, so that early instars can be considered larval stages.
(d) Planktonic larvae of marine animals offer ecological advantages.
Most "cold-blooded" marine animals, from tiny crustaceans to huge tunas, have larval stages. These larval stages are commonly planktonic, meaning that they drift free in the ocean. You have seen examples of planktonic larval stages in crustaceans, annelids and molluscs, and fish. On display is a chart illustrating the larvae of several other kinds of marine animals. Can you match the larvae with the corresponding adults? Are any adaptations for planktonic life evident in any of the larvae you have seen?
Why are planktonic larval stages so common in marine animals? In many cases, developmental constraints, as discussed in b and c above, may exist. But the preponderance of planktonic larvae in the sea suggests a common function. One consequence of planktonic larvae is certainly dispersal. Planktonic larvae drift from their place of birth and (a) may come upon unoccupied habitats, (b) may avoid competition or inbreeding with siblings, and (c) (especially important from the parents' point of view) may reduce the chance of complete reproductive failure in the event of a local catastrophe. As noted above, the larval stage is also self-sufficient, so that growth and development can continue in the absence of parental provisioning. In general, then, what is the advantage to a parent in producing tiny planktonic larvae instead of large, better-developed young? What are the disadvantages? How many must be produced, compared to the number of well developed young that can be produced? The young of most freshwater and terrestrial animals are much better developed at hatching or birth than the young of marine animals. Why might this be? Are freshwater and terrestrial animals "better" or "more advanced", or are they missing a good bet, just because of their habitat?
Oviparity, Ovoviviparity, and Viviparity
These three terms apply to both (a) where embryos undergo development, and (b) how parents provision their embryos. In oviparous animals, embryos develop outside of the parent, and obviously receive no nutrition other than yolk (at least until after hatching from the egg). Most animals are oviparous. In ovoviviparous animals, embryos develop inside a parent but receive no nutrition beyond the yolk, at least before birth. Ovoviviparity is surprisingly common; several examples are on display. In viviparous animals, young develop inside the parent and receive nutrition above and beyond that present in the yolk. Mammals (other than the Monotremes [duckbill platypus and spiny anteater]) are the most familiar examples of viviparous animals, but are certainly not the only ones. See the examples of viviparity on display.
Some biologists find the line between ovoviviparity and viviparity tenuous, preferring to designate all forms of internal bearing as viviparity, and distinguish between lecithotrophic (yolk-feeding) vs. matotrophic (mother-feeding) viviparity. Further, they distinguish two types of matotrophic viviparity: placental vs. aplacental. Placental matotrophy involves provisioning nutrients to the embryo through the circulatory systems of mother and embryo (as in placental mammals, but not limited to them). See an example of placental matotrophy in the viviparous surfperches that occur off the California coast. Aplacental matotrophy may involve feeding young in various ways. Young rockfish (also common off our coast) seem to ingest protein particles before they are born. More bizarre are some sharks, the young of which seem to gain their nutrition by eating their siblings within the uterus. People wondered for a long time why the unborn young of some sharks had such well-developed jaws and teeth!