Reproductive behaviour in invertebrates

Protozoans and sponges

Most protozoans (one-celled organisms) reproduce asexually, usually by fission (splitting in two); in some species, however, sexual as well as asexual reproduction occurs and may be complex. The colonial organism Volvox, which may be either of one “sex” or composed of cells of both sexes, produces true eggs and sperm. A chemical substance released by “females” induces the production of sperm packets; following the union of the egg and sperm, the parent colony dissolves, and the zygote (fertilized egg) is released.

Another form of reproduction in protozoans is conjugation, in which organisms such as Paramecium fuse together briefly to exchange nuclear products. This results in a reshuffling of hereditary characteristics just as occurs in true sexual reproduction in higher animals. In some species of Paramecium, there are mating types, and an individual is of one type or the other. Opposite types apparently recognize each other by a chemical (pheromone) that is released on their body.

In the lower metazoans (multicellular organisms), reproduction is also by both asexual and sexual means. As befits their sessile life-style and low population densities, sponges that reproduce sexually are usually hermaphroditic; that is, each individual is capable of producing both sperm and eggs, but often at different times to prevent self-fertilization. The sperm are swept by water currents into another sponge, where they are picked up by specialized cells called choanocytes and carried to the egg. Fertilization takes place when a choanocyte fuses with the egg. The free-swimming larval stage that is produced is of short duration, after which the organism settles on the bottom and becomes a new adult sponge.

Coelenterates

Hydroids, jellyfishes, sea anemones, and corals of the phylum Coelenterata, or Cnidaria, reproduce by a variety of mechanisms. A familiar coelenterate animal, the freshwater Hydra, usually reproduces asexually by budding, a process by which small portions of the adult structure become new, but genetically identical, individuals. Hydras are also dioecious; that is, each individual produces either sperm or eggs. In many temperate-zone species of Hydra, sexual reproduction occurs during the autumn; the fertilized eggs enable the species to survive the winter.

Most of the other hydrozoans are colonial organisms, often occurring in polyp and medusal (umbrella-shaped) forms. In a colony, reproductive individuals called gonophores develop into free-swimming organisms (medusae) that reproduce sexually. Fertilization can be either external or internal; if external, the eggs are shed directly into the water. Internal fertilization results in larvae that swim out of the parent and soon settle on a surface, where they develop into another hydroid colony.

Sea anemones and the polyps of corals reproduce both asexually—by budding—or sexually. In the sexual mode, sea anemones have both dioecious and hermaphroditic species. One interesting aspect of sea anemones, which undergo internal fertilization, is that they are among the first lower animals known to provide parental care. The larvae of sea anemones remain inside the adult until they are ready to metamorphose (change in form), at which time they swim from the parent’s mouth and settle on its base, remaining there until they develop tentacles. When they have reached this stage of development, they move away from the parent’s protection.

Flatworms and rotifers

The reproductive structures of flatworms (phylum Platyhelminthes) resemble those found in the higher groups. Such flatworms as the land and freshwater planarians are hermaphrodites. Although some species can reproduce asexually by splitting in two, most engage in copulation. Some freshwater planarians can produce both thin-shelled summer eggs, which hatch in a short time, and thick-shelled winter eggs, which are resistant to freezing and hatch in the spring. An apparently unique situation in many planarians is that nutrition for the embryo is supplied by the addition of separate cells to the zygote, after which the entire mass is enclosed in the shell; more commonly, the yolk is incorporated within the structure of the zygote itself.

In the rotifers (phylum Aschelminthes), small but abundant freshwater animals, reproduction is usually sexual, and the sexes are separate. Copulation occurs by injection of sperm anywhere in the body wall of the female. Many species found in temporary ponds and streams exhibit a peculiar reproductive behaviour that is well adapted to their transient environment: they produce different kinds of eggs at different times of the year. One egg type, called amictic, is produced in the early spring. These eggs apparently cannot be fertilized, and the embryo develops without fertilization (parthenogenesis); the result is females with a life-span no longer than two weeks. When the population reaches a peak in the early summer, a second type of egg is produced. If unfertilized, this egg, which is called mictic, results in males. As the male population increases, most mictic eggs become fertilized, resulting in the production of a heavy-shelled dormant egg with much yolk. The dormant egg survives the winter and gives rise to the amictic females of the next spring. Thus, despite the many generations produced in the summer by so-called sexual means, the reshuffling and recombination of genetic material occurs only once a year.

Segmented worms

The marine worms of the class Polychaeta (e.g., clam worms and lugworms of the phylum Annelida) provide the first examples of a kind of courtship behaviour involving both visual and chemical displays initiated by some rather subtle environmental stimuli. Most polychaetes reproduce sexually, and there are two distinct sexes in most species. Either by transformation or budding, many polychaetes produce a reproductive form (epitoke). At a certain time of the year, the epitokes swarm to the ocean surface and engage in mass shedding of eggs and sperm. Some female epitokes of clam worms (Nereis) produce a chemical substance called fertilizin that attracts the male epitokes and stimulates the shedding of sperm. Male epitokes of a polychaete found in the Atlantic Ocean emit a flashing light; females emit a steady light. The light may serve to attract male and female and to aid in species discrimination. The swarming of the palolo worm Palola in parts of the South Pacific is apparently triggered by an annual and a lunar cycle; the epitokes separate from the parent (atoke) in October or November, during the last part of the lunar cycle.

The class Oligochaeta (phylum Annelida) contains a diversity of both aquatic and terrestrial worms, among which is the familiar earthworm, Lumbricus. Although some aquatic oligochaetes reproduce asexually, the majority are sexual, and all of these are hermaphrodites. At mating, two oligochaetes lie side by side so that the head of one is opposite the tail of the other. Sperm then pass reciprocally into small sacs, where they are temporarily stored. This transfer is more complex in the earthworms, however, because the respective male pores are not in direct opposition; each individual forms a temporary skin canal through which the sperm flow to their respective sacs for storage. The body of oligochaetes has a swollen girdle-like structure, the clitellum, which serves an important function in reproduction. After the eggs have matured, a mucous tube, secreted from the clitellum, slides along the body as the worm moves backward. The stored sperm are discharged into this tube, as are the eggs when the tube slides along the section containing them. As the worm literally passes out of the tube, a mucous, lemon-shaped cocoon forms around the now-fertilized eggs. This cocoon serves as a kind of primitive nest, in which the young hatch.

Many leeches (class Hirudinea), all of which are hermaphrodites, have copulatory behaviour much like that of earthworms. Cocoons are formed in a manner similar to that described above, but in some leeches the cocoon is transparent and remains attached to the parent in which the eggs were developed. After hatching, the young leeches remain attached to the “mother” until they become independent. One African leech gives birth to live young and even possesses a special incubating chamber in its body for the developing embryos.