Damselfish neurofibromatosis, DNF, is a condition affecting the bicolor damselfish, Pornacentrus partitus, on coral reefs of South Florida. The condition is a progressive, fatal neurofibroma located in the peripheral nerves of adult damselfish. DNF arises from the neoplastic transformation of nerve sheath cells. Externally, the tumors are observable as pigmented dermal and subdermal lesions which are non-encapsulated and locally malignant; there is also an unpigmented tumor. DNF may be demonstrated internally most often in the pronephros, reaching relatively large sizes - approx. 25% b/w, and is invasive and destructive to gut, myomere, spinal cord and spleen as well. Histopathologically, DNF is an elongation and cross-section increase in the myelin sheath, especially Schwann cells with involvement of fibroblasts. Distinctive elongate, spindle shaped cells arranged in a whorling pattern are typical. DNF when combined with ecological, zoogeographic and ethological information is highly suggestive of a transmissible etiologic agent. Experimentally, DNF may be transmitted by injection of tumor homogenate into unaffected fish.

The eosinophilic granule cell, EGC, is found in normal fish in a manner suggestive of immune system action. In DNF, EGCs assume a presently unknown role. The EGCs are morphologically similar in normal and affected fish. The location, abundance and distribution of these cells in affected fish are suggestive of immune reaction and invoke analogies with human mast cells. There are significant patterns to the abundance and distribution of the EGCs in three groups of damselfish; normal fish, naturally tumored fish, experimentally tumored fish. The cytochemistry of the EGC indicates a strong influence of histamine actions through exposure of fish to Compound 48/80, a histamine liberator, Fishes do not have identifiable mast cells leading to the question of the EGC performing the role of mammalian mast cells. This description of EGCs begins a definition of classification of the EGC, as possibly the fish analog of the mammalian mast cell. The study of DNF and EGCs presents an opportunity for study of both a marine disease condition and possible use of DNF as a model of human peripheral nerve tumors.