The Sporozoa are perhaps the most famous group of protozoan parasites. As well as being one of the few protozoan groups distinctive enough to be well-characterised prior to the advent of electron microscopy and molecular analysis (albeit with a few hangers-on such as microsporidians and myxozoans that have since been cast aside from the sporozoan community), distinguished by their lack of motile organelles and intracellular invasion of their hosts, sporozoans include the causative agents of such maladies as malaria and toxoplasmosis. In many references, you may find the Sporozoa referred to as Apicomplexa, a name that refers to the apical complex, an organelle at the front end of the cell that is used to invade the cells of the sporozoan's host. However, as the apical complex is also found in some flagellates that are closely related to sporozoans (such as Colpodella), the name Apicomplexa is better used for the larger clade including these taxa while the name Sporozoa is restricted to the nested aflagellate clade (Cavalier-Smith & Chao, 2004).
Sporozoans have been divided into three classes, the invertebrate parasites Gregarinae (or Gregarinea), Coccidia (intestinal parasites of vertebrates) and Hematozoa (parasites of vertebrate blood cells). Phylogenetic analyses have indicated that the basal division in Sporozoa is between the vertebrate-parasitic Coccidia and Hematozoa on one branch (the Coccidiomorpha), and the Gregarinae on the other, though the Gregarinae is less well supported as monophyletic than the Coccidiomorpha and may yet be paraphyletic (Leander & Ramey, 2006; Leander et al., 2006). One notable exception is that the vertebrate parasite Cryptosporidium, previously regarded as a coccidian, may in fact be related to the gregarines or even derived from within them. Not surprisingly, their choice of hosts means that the Coccidiomorpha are by far the better studied of the two clades, while the Gregarinae have kind of been the poor relation. Nevertheless, it is the gregarines that are my focus today.
Gregarines have been divided into three groups, the archigregarines, eugregarines and neogregarines (Leander, 2007), but it seems more than likely that these represent a series of grades, with eugregarines paraphyletic to neogregarines and archigregarines paraphyletic to the eugregarine + neogregarine clade. In contrast to the complex life-cycles of some coccidiomorphs, gregarines have fairly simple life histories with only a single host. Transmission from one host to another is usually via oocysts released with the host faecal matter, but some gregarine oocysts hitch a ride with their host's gametes during copulation as protozoan STDs. Once inside the host, the oocysts hatch out into infective sporozoites that attach to or invade host cells and develop into feeding trophozoites. Some gregarines can reproduce asexually; the majority cannot. Sexual reproduction occurs by two trophozoites joining together as reproductive gamonts and becoming enclosed within a gametocyst; within the gametocyst they will each divide into hundreds of gametes that will fuse to form oocysts, ready for release.
As I said before, the "archigregarines" probably represent the basal grade of gregarines. They are all intestinal parasites of marine invertebrates, and as such have been unfairly condemned as of little interest to anyone. Archigregarines have very similar sporozoites and trophozoites that are vermiform (worm-shaped) and generally move by wriggling (go here for videos of gregarine movement). Some archigregarines have cells with numerous longitudinal folds, others are smooth. Archigregarines also retain the ancestral characteristic of feeding on their host by using their apical complex to pierce the host cell and sucking out its contents.
The 'eugregarines' include the majority of gregarines (at least, the majority of described gregarines), and include parasites of freshwater and terrestrial as well as marine invertebrates. Again, their study has been biased towards those species that are parasites of insects, with the remainder being generally snubbed. Most marine eugregarines have been lumped together as the genus 'Lecudina', with little to unite them other than that they are marine eugregarines (Leander, 2007). Eugregarines differ from archigregarines in having morphologically quite distinct sporozoites and trophozoites. Their cell walls also became very rigid, and they lost the wriggling ability of archigregarines. Instead, eugregarines developed a system of gliding motility, with an actinomyosin skeleton running along the edge of the numerous surface folds. Ancestrally, eugregarines are intestinal parasites like archigregarines, but instead of the active feeding process of eugregarines, eugregarines absorb nutrients from the host through micropores on the cell surface (Leander et al., 2006).
Neogregarines are a derived subgroup of eugregarines with reduced trophozoites that specialise on terrestrial hosts (mostly insects) and mostly live in non-intestinal tissues. Another group of eugregarines, the urosporidians, became parasites of their host's coelom. Urosporidians lost their direct attachment to host cells, and became free-floating within the tissue as united gamont pairs. The gliding motility and longitudinal folds of other eugregarines were lost, and instead urosporidians move by pulsations of the cell walls. Cells became branched - many are V-shaped with two primary branches that each divide distally into a number of smaller "fingers".
Archigregarines in particular retain a number of features that are believed to be ancestral for sporozoans as a whole (such as sucking feeding), and Leander et al. (2006) suggest that they may constitute the ancestral group not just for eugregarines, but also for coccidiomorphs and hence sporozoans as a whole. Interestingly, gregarines (including archigregarines), so far as is known, lack the residual plastid found in coccidiomorphs. It is currently a subject of some debate as to whether the sporozoan (really coccidiomorph) plastid is homologous with that found in the related dinoflagellates or not (see here for an earlier take of mine on the issue), and the position of the plastid-less 'archigregarines' could have significant implications for this debate.
Cavalier-Smith, T., & E. E. Chao. 2004. Protalveolate phylogeny and systematics and the origins of Sporozoa and dinoflagellates (phylum Myzozoa nom. nov.) European Journal of Protistology 40: 185-212.
Leander, B. S. 2007. Marine gregarines: evolutionary prelude to the apicomplexan radiation? Trends in Parasitology 24 (2): 60-67.
Leander, B. S., S. A. J. Lloyd, W. Marshall & S. C. Landers. 2006. Phylogeny of marine gregarines (Apicomplexa) — Pterospora, Lithocystis and Lankesteria — and the origin(s) of coelomic parasitism. Protist 157: 45-60.
Leander, B. S., & P. A. Ramey. 2006. Cellular identity of a novel small subunit rDNA sequence clade of apicomplexans: description of the marine parasite Rhytidocystis polygordiae n. sp. (host: Polygordius sp., Polychaeta). Journal of Eukaryotic Microbiology 53 (4): 280-291.