Final answer:
Eimeria is the apicomplexan genus responsible for causing coccidiosis in chickens, which is a disease affecting the gastrointestinal tract.
Step-by-step explanation:
The apicomplexan genus that causes coccidiosis in chickens is Eimeria. Apicomplexans are a group of parasitic protists characterized by an apical complex used to enter and infect host cells. The genus Eimeria is specifically recognized for causing this disease, which can lead to severe gastrointestinal issues in poultry. While other apicomplexan genera such as Pneumocystis, Plasmodium, and Toxoplasma are also medically significant, they do not cause coccidiosis in chickens. For instance, Plasmodium is known for causing malaria, and Toxoplasma causes toxoplasmosis.