Asexual reproduction in free-living species usually involves nuclear division and the division of the cell into two identical daughter cells of equal size by binary fission. In parasitic protozoans and some free-living species, multiple fission, resulting in the production of many offspring that may not resemble the parent cell, is normal. During the cycle of growth and division, the protozoan undergoes a series of identifiable phases: a division phase, a growth phase during which the cell increases substantially in size, a phase of DNA synthesis, and a phase of preparation for division, which extends from the end of DNA synthesis until the initiation of division. The division of the cytoplasm (cytokinesis) is preceded by the division of the nucleus or nuclei.

The plane of division in protozoan cells varies among the different groups and is of taxonomic significance. The ciliates normally divide in an equatorial, or transverse, plane, thereby maintaining the correct number of ciliary rows, or kineties. The cell mouth and any specialized cilia around it are replicated in different ways among the various ciliate groups, depending on the complexity of the cytostome. The replication of the cytostome precedes the division of the cytoplasm. Some ciliates (e.g., Colpoda) divide within thin-walled reproductive cysts into two daughter ciliates, each of which then divides so that the cyst contains four progeny, which are released when the cyst wall ruptures.

The sedentary suctorians do not reproduce by binary fission, because the production of an identical nonswimming offspring would rapidly lead to overcrowding. They instead produce single ciliated offspring, called swarmers, by a process called budding. Budding can occur endogenously, in which the bud forms within the parent and is ejected when mature, or exogenously, in which the swarmer is formed outside the parent. The swarmers swim away from the parent, settle on a substrate, lose their cilia, and develop feeding tentacles and an attaching stalk.

Naked amoebae have no fixed plane of division but simply round up and divide into two basically equal halves. The testate amoebae, which live in single-chambered shells, or tests, exude the daughter from the aperture of the shell. In species that have a shell formed from silica plates, the daughter contains the plates used to produce the shell but remains attached to the mother cell until the shell is fully formed, when the final severing of the cytoplasm between the individuals occurs. Some of the testate amoebae live inside proteinaceous shells. There too, the new shell is secreted before binary fission is completed.

The foraminiferan and radiolarian amoebae have evolved multiple fission. Both produce many flagellated swarmers, or zoospores. The common planktonic foraminiferan Globigerinoides sacculifer, for example, can produce 30,000 swarmers at one time. Each swarmer is about 5 micrometres (0.005 mm) long. In planktonic species the parent usually loses buoyancy and sinks by shedding spines and withdrawing the complicated pseudopodial network into the shell. The swarmers are produced in deep water and migrate upward as they mature. Each secretes a shell around itself, which is added to as the organism grows.

Mechanisms of sexual reproduction

The foraminiferans are unusual among free-living protozoans in that a sexual phase is a regular part of the life cycle, alternating with an asexual phase. During the life cycle two types of swarmers are produced. One type, zoospores, have half the number of chromosomes of the parent (i.e., they are haploid); they grow until they become mature adults and can produce and release large numbers of gametic swarmers. These gametes are identical (isogamous) but are functionally comparable to the eggs and sperm of higher organisms. The gametic swarmers fuse in pairs, thus restoring the full complement of chromosomes (i.e., they are diploid), and each individual grows, matures, and ultimately produces haploid zoospores.

Sexual reproduction among other protozoans is not widespread and can involve identical gametes (isogamy) or distinct male and female gametes (anisogamy or heterogamy). The female gametes are usually larger and are stationary, whereas the male gametes are smaller, produced in larger numbers, and motile.

Conjugation in ciliates

Sexual reproduction among the ciliated protozoans takes the form of conjugation. The process does not result in an increase in numbers but is a simple exchange of genetic material between two individual cells. Conjugation occurs only between compatible mating strains within a species, and each species may contain many mating strains. Before conjugation occurs, special chemical signals, called gamones, are released by some ciliates. The gamones cause compatible mating strains to undergo processes that facilitate conjugation. In other ciliates, such as Paramecium, gamones are bound to the cell surface and elicit their responses when the ciliates make physical contact.

During conjugation, two ciliates line up side by side. The macronucleus, which plays no part in the process, disintegrates. A series of nuclear divisions of the micronuclei in each ciliate then ensues, including meiosis, during which a number of haploid micronuclei are produced in both cells. All but one of these haploid micronuclei disintegrate. The remaining haploid micronucleus in each cell then divides through mitosis into two haploid nuclei (gamete nuclei). A bridge of cytoplasm forms between the two ciliates, and one gametic nucleus from each cell passes into the other cell. The two gametic nuclei in each cell unite, thus restoring the diploid number of chromosomes. The micronucleus undergoes two mitotic divisions to produce four micronuclei: two of these will form the new micronuclei of the cell, and two are destined to become the macronucleus. Following the process of conjugation, normal binary fission proceeds. The number of macronuclei and micronuclei formed is dependent on the species and remains the same as the original number.

Parasitic protozoan life cycles

As is common with other parasitic organisms, parasitic protozoans face the problem of how to disperse from one host to another. In order to increase the probability of finding more hosts, most parasitic protozoans reproduce in high numbers. A representative life cycle of a parasitic protozoan can be found in members of the parasitic group Apicomplexa. These protozoans have a complex life cycle that involves a series of stages characterized by episodes of asexual multiple division called schizogony. In the parasite Plasmodium, for example, this phase of the life cycle occurs in the liver and red blood cells of humans. The parasite (sporozoite) enters the host’s cells and grows while feeding on the cell contents. It then undergoes a multiple asexual division (schizogony) into many individuals (merozoites). The host’s cell wall ruptures, permitting each individual to invade a new red blood cell and repeat the process.

In certain merozoites a sexual cycle is eventually initiated inside the red blood cell, and male and female gametes are produced. The male gametes (microgametocytes) are small, while the female gametes (macrogametocytes) are larger. The life cycle continues if the gametocytes are taken up by a feeding female mosquito of the genus Anopheles. Only the gametocytes can infect the mosquito. Inside the mosquito’s gut the haploid gametes fuse to form a diploid zygote, which then undergoes sporogony, a process of multiple divisions in which many sporozoites are produced. The sporozoites migrate to the salivary glands of the insect and are injected into a new host when the mosquito feeds again. They are carried by the blood to the liver, where they undergo their first schizogony inside liver cells, thereafter invading the red blood cells for repeated cycles of schizogony.

Flagellated protozoan parasites reproduce almost exclusively by asexual means and do not appear to have a sexual phase in their life cycles. There is, however, evidence of genetic exchange between certain subspecies of Trypanosoma brucei.