Mesophellia and Castorium are common hypogeous macrofungi in the karri (Eucalyptus diversicolor F. Muell.) and jarrah (Eucalyptus marginata Donn ex Sm.) forests of south-western Australia.
Sporocarps of Mesophellia and Castorium develop 5-20 cm below the soil surface in close association with eucalypt roots. During differentiation of the sporocarps, eucalypt roots become trapped within the peridium where they branch profusely and form a dense ectomycorrhizal layer. Mature sporocarps of M. trabalis nom. ined. contain approximately S m of roots of 45 cm2 surface area. Anatomical studies have shown that these roots have Hartig nets penetrating to the hypodermis and are similar to the superficial eucalypt ectomycorrhizas formed in soil and litter. The association of Mesophellia and Castorium sporocarps with tree roots suggests that these are important mycorrhizal fungi in forests of southern Australia.