Apicomplexa

The Apicomplexa constitute a large phylum of unicellular, spore-forming eukaryotic organisms that are predominantly parasitic. They form part of the Alveolata and are characterised by a distinctive array of organelles collectively referred to as the apical complex. This complex, located at one end of the infectious stage known as the sporozoite, enables the parasite to invade host cells efficiently. Most apicomplexans are obligate parasites of animals, although rare exceptions occur in symbiotic or photosynthetic lineages closely related to this group. The phylum includes several organisms of major medical and veterinary significance.

General Characteristics

Apicomplexans possess an apicoplast, a non-photosynthetic plastid enclosed within multiple membranes. This organelle derives from ancient symbiosis and plays crucial roles in metabolic pathways such as lipid production. Only a few taxa lack the apicoplast entirely. The apical complex itself typically contains a conoid, rhoptries, micronemes and polar rings, all of which participate in host penetration.
Members of this phylum show considerable morphological diversity across species and life stages. While their motile structures such as flagella or pseudopodia are restricted to gamete stages, many apicomplexans exhibit a specialised gliding motility that enables movement across tissues. This locomotion relies on the interaction of adhesins with small myosin motors.
Apicomplexan cells share many features with other eukaryotes, including a nucleus, endoplasmic reticulum, Golgi apparatus and mitochondrion. The mitochondria are distinguished by tubular cristae. A trilaminar pellicle composed of alveoli supports the cell membrane. Feeding occurs through micropores instead of oral structures. Chloroplasts and other ejectile organelles are absent.

Diversity and Taxonomic Context

The phylum encompasses several major groups, including coccidians, gregarines, haematozoans and piroplasms. Well-known genera include Plasmodium, Babesia, Cryptosporidium, Cyclospora, Cystoisospora and Toxoplasma. Diseases caused by these parasites range from malaria to toxoplasmosis and cryptosporidiosis. Historically, Apicomplexa formed the core of the group once referred to as the Sporozoa, defined by the absence of flagella and cilia. Modern classification has reassigned several former sporozoans to unrelated lineages.
Certain marine symbionts, such as Nephromyces, were once misclassified within Apicomplexa but do not share the defining parasitic lifestyle. Chromerids, close relatives of apicomplexans, possess photosynthetic capabilities and provide insight into the evolutionary origins of the apicoplast.

Structure of the Apical Complex

The apical complex is the hallmark of the phylum. It typically consists of:
• Polar rings, which act as structural anchors• Conoid, a cone-shaped assembly of microtubules in some groups• Rhoptries, large secretory organelles releasing enzymes for host invasion• Micronemes, smaller secretory bodies essential for motility and attachment• Dense granules, scattered throughout the cell and active after invasion
These components facilitate entry into host cells by deploying enzymes and adhesins. The apical region’s spirally arranged microtubules contribute to the organism’s rigidity and penetration capability. After invasion, dense-granule secretion supports the formation of the parasitophorous vacuole in which the parasite resides and multiplies.

Reproduction and Cell Biology

Apicomplexans undergo complex life cycles that generally incorporate both asexual and sexual reproduction. Replication frequently involves schizogony, a form of multiple fission producing numerous daughter cells within a host cell. The resulting merozoites continue infecting other cells until sexual forms appear. Gametes fuse to form a zygote, which develops into an oocyst containing sporozoites. The zygote is typically the only diploid stage in the life cycle.
Cell division often involves a closed intranuclear spindle, although some taxa show open spindle formation at the poles. Transposons are uncommon within the group, though identified in a few genera.

Mobility and Host Interaction

A defining feature of apicomplexans is their gliding motility, enabling them to traverse tissues and cross cellular boundaries. This motion is not reliant on cilia or flagella but is powered by the parasite’s actin-myosin system. Adhesive proteins secreted at the apical end interact with substrates, allowing the cell to propel itself forward.
The pellicle, made up of three membrane layers, provides structural support. Micropores serve as sites of nutrient uptake. Sexual reproduction, the production of oocysts and the reliance on sporozoites as the infectious stage are common across the phylum.

Life Cycle Patterns

Most apicomplexans share a pattern involving host invasion by sporozoites, rapid asexual multiplication, release of merozoites and eventual sexual reproduction. The bursting of infected cells releases new forms that spread throughout the host. In many species, different life stages occur in different hosts, resulting in multi-host life cycles.
The apical complex plays a central role throughout the infection cycle by facilitating attachment, penetration and intracellular establishment. Many variations occur among taxa, reflecting adaptation to specific hosts and environments.

Major Subgroups

CoccidiansCoccidians primarily infect vertebrates and are common parasites of intestinal epithelial cells, though some invade other tissues. Their life cycle includes merogony, gametogony and sporogony. Distinctive features include the formation of large stationary macrogametes and numerous motile microgametes. Fertilisation yields an oocyst that exits the host and sporulates externally.
GregarinesGregarines largely parasitise invertebrates such as annelids, arthropods and molluscs. They often inhabit the gut but may invade other tissues. A typical lifecycle features a trophozoite developing into a schizont, which divides by schizogony to form merozoites. Gametocytes arising later combine to form gametes, which fuse and produce oocysts that depart the host.
Haematozoans and Related GroupsThis subgroup includes parasites that infect blood cells or associated tissues. Piroplasms and certain other lineages share this general lifestyle. These organisms often rely on arthropod vectors and are important in both human and animal health.

Evolutionary and Biological Significance

Apicomplexans provide valuable insight into the evolution of parasitism, intracellular survival strategies and plastid biology. Their conserved cellular features, particularly the apicoplast and apical complex, highlight the adaptation of a eukaryotic lineage to obligate parasitism. The medical significance of many apicomplexans has made them central to research in cell invasion mechanisms, drug development and host–parasite interactions.