Haematology of the New Zealand freshwater eelsAnguilla australis schmidtiiandA. dieffenbachii

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Abstract
The morphology and probable lines of development of blood cells are described for the short‐finned eel, Anguilla australis schmidtii (Phillips), and the long‐finned eel, Anguilla dieffenbachii (Gray). Erythrocytes, neutrophils, lymphocytes, thrombocytes, and varying numbers of their precursors occur, the erythrocytes retaining their ribosomal reticulum and nuclei. Some fish were found to have peroxidase in their neutrophils, but the specificity of the enzyme is questionable. The high glycogen content of the thrombocytes is consistent with their function. When glass eels enter the river they have only scattered blast cells and macrophages in their blood. The retention of ‘larval’ blood characteristics depends on the size of the fish rather than their age. Haemopoiesis occurs in the kidney, spleen, and liver, but is most intense in the kidney, which is distinctly ‘myeloid’. The spleen is ‘lymphoid’ in character, and enlarges when 25% of the total blood volume is removed, indicating some functional interdependence between the spleen and kidney. Red and white pulps can be distinguished in the spleen, but there is no clear line of demarcation. Thick‐walled capillaries distort red cells markedly, and provide a possible means for their destruction. The thymus, which is not divided into cortex and medulla, is well developed in post‐larval glass eels, but begins to involute when eels attain a length of 7.5 cm. Lymphopoiesis begins earlier in the thymus than in the kidney, and it is suggested that the thymus ‘seeds’ the other haemopoietic organs or releases some triggering factor. The wide haemostatic tolerance found renders isolated measurements of cell numbers valueless in assessing fish health. Howell‐Jolley bodies, which were found in erythrocytes, may be valuable as an indicator of fish health, and should be further investigated. Cellular debris was more abundant in fish infected with Myxobolus than in uninfected fish.