Protozoan classification: Organisms that form spores in their life cycle and lack specialized locomotory organelles (no cilia, no flagella, no pseudopodia) belong to which phylum?

Question

Accepted Answer

Introduction: Classification questions in protozoology often test whether you can link a hallmark biological trait to the correct taxonomic group. Here, the key clues are the presence of a spore-forming stage and the absence of specialized locomotory organelles such as cilia, flagella, or obvious pseudopodia. These features point to a very important group of parasitic protozoa associated with diseases in humans and animals. Given Data / Assumptions: Organisms have a spore-forming stage in their life cycle. They lack specialized locomotory organelles (no cilia, no flagella, no obvious pseudopodia). We must identify the correct protozoan phylum based on these traits. Concept / Approach: Members of the phylum Apicomplexa are obligate intracellular parasites characterized by an apical complex used for host cell invasion and by complex life cycles that include spore-like stages (sporozoites). They do not possess locomotory organelles like cilia or flagella in their typical trophozoite stages (some gametes may be flagellated, but that is not the defining rule). Well-known examples include Plasmodium, Toxoplasma, Cryptosporidium, and Eimeria. Step-by-Step Solution: Step 1: Map the trait “spore-forming stage” to protozoan groups known for sporogony (production of sporozoites). Step 2: Exclude groups that use cilia or flagella for movement (Ciliophora have cilia; many Euglenozoa have flagella). Step 3: Recognize that Apicomplexa characteristically lack specialized locomotory organelles in trophozoites and undergo sporogony. Step 4: Select Apicomplexa as the only option aligning with both clues. Verification / Alternative check: Standard parasitology texts define Apicomplexa by the apical complex, obligatory intracellular lifestyle, and complex cycles that include a spore-forming phase. The locomotion is typically via gliding mechanisms rather than cilia/flagella. Why Other Options Are Wrong: Microspora (Microsporidia): Now often considered highly derived fungi; obligate intracellular but not grouped with Apicomplexa in classical protozoology. Myxozoa: Parasitic cnidarians, not true protozoa in modern classification. Ciliophora: Possess cilia; contradicts the “no specialized organelles” clue. Euglenozoa: Frequently flagellated; do not fit the spore-forming, non-motile description. Common Pitfalls: Lumping all spore-bearing parasites together without checking locomotory features; forgetting that Apicomplexans glide and rely on an apical complex rather than cilia or flagella. Final Answer: Apicomplexa is the correct phylum.

Protozon Questions