Reference: Biol. Bull., 160: 240 249. (April, 1981) THE LYMPHOMYELOID (HEMOPOIETIC) SYSTEM OF THE ATLANTIC NURSE SHARK, GINGLYMOSTOMA CIRRATUM RAGNAR FANGE AND ARTUR MATTISSON Department of Zoophysiology, University of Goteborg, Sweden ABSTRACT The lymphomyeloid system of nearly adult nurse sharks, Ginglymostoma cir-ratum, was investigated. Lymphomyeloid structures detectable by the naked eye at dissection are the epigonal organ and the spleen. Microscopic examination shows that the epigonal organ produces granulocytes and lymphocytes. The white pulp of the spleen is lymphoid, whereas the red pulp is mainly erythropoietic. Cells with the morphological characteristics of plasma cells occur in the epigonal organ and the spleen. Peroxidase-positive granulated cells are found in the epigonal organ. In contrast to many other elasmobranchs, the nurse shark lacks the Leydig organ, i.e. the lymphomyeloid structure of the esophagus; but the epigonal organ is well developed and averages 0.60% of the body weight. The spleen weighs about 0.26% of the body weight. The mode of life of the nurse shark in shallow tropical waters probably puts a considerable demand on its immune system. Cells produced by the epigonal organ may be important in immune responses and in inflammatory pro-cesses. INTRODUCTION Elasmobranch fish lack bone marrow and lymph nodes, but possess a thymus gland (at least in early life), a spleen, and voluminous lymphomyeloid tissues as-sociated with the gonads, esophagus, or both. Diffuse lymphoid infiltrations may occur in the intestine or elsewhere. The gonad-associated lymphomyeloid tissue, often called the epigonal organ, reaches considerable size in certain sharks, such as the basking shark, Cetorhinus maximus (Matthews, 1950). Elasmobranch spe-cies lacking epigonal organs possess a similar tissue, termed the Leydig organ, in the esophagus (Fange, 1977). The epigonal and Leydig organs, both relatively little known structures, resemble bone marrow and lymph nodes of higher vertebrates. Most previous studies of elasmobranch lymphomyeloid tissues concern species from temperate or cool waters. The present work deals with the lymphomyeloid system of the nurse shark, Ginglymostoma cirratum (family Orectolobidae), that lives in tropical and subtropical parts of the Atlantic Ocean. MATERIALS AND METHODS Six not-fully-adult specimens of Ginglymostoma cirratum trapped off Puerto Rico were kept in running seawater for several weeks and fed minced molluscs. The nurse shark is one of the few sharks that can be maintained in captivity for long periods (Clark, 1963). Five sharks were killed and perfused with formalin solution. The sixth animal was investigated immediately after killing, without fix-ation. The main lymphomyeloid structures were isolated by dissection and weighed. Histological sections were stained with eosin-haematoxylin. Due to the paucity of Received 14 October 1981, accepted 22 January 1981. Abbreviation: MGG, May-Griinwald-Giemsa. 240
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