Description of Ichthyosporidium

The type species (I. giganteum) lives in the subcutaneous connective tissue of corkwing, Crenilabrus melops (Pisces, Labridae); forms multiple xenomas which amass into a ventral swelling, which may extend from the head to the anal fin of the fish; within the connective tissue of the swelling are small xenomas in the form of 0.1-0.2 µm cysts with fibrous capsules containing meronts, and mature xenomas, 4mm in diameter, containing sporonts and spores; meronts measure 1.5 µm and divide by binary fission; may be diplokaryotic ; sporonts are diplokaryotic, moniliform cells which lie perpendicularly to the wall of the xenoma and divide into diplokaryotic sporoblasts; spores, 5 x 7-8 µm, are ovoid and diplokaryotic; polaroplast is lamellar; polar tube is isofilar with about 32 coils in several ranks; prominent ventral bulge on the fish is a strong indicator of the infection.