Eimeria fraterculae is a species of protist

Protist

Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

 belonging to the family Eimeriidae

Eimeriidae

Eimeriidae is a family of Apicomplexa. It contains the following genera:*Alveocystis Bel'tenev, 1980*Atoxoplasma Garnham, 1950*Barrouxia Schneider, 1885*Caryospora Léger, 1904*Cyclospora Schneider, 1881*Diaspora Léger, 1898...

. It causes renal coccidiosis in the Atlantic puffin (Fratercula arctica).

Apart from its host-specificity, this coccidium can be recognised by the subspherical oocyte

Oocyte

An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...

s with unusually thick walls appearing distinctly greenish in colour. Coccidiosis caused by this parasite appears to be mild and nonfatal.