Comp. Biochem. Physiol. Vol. 119A, No. 1, pp. 17–26, 1998 ISSN 1095-6433/98/$19.00 Copyright  1998 Elsevier Science Inc. All rights reserved. PII S1095-6433(97)00410-8 Structure and Function of the Axillary Organ of the Gulf Toadfish, Opsanus beta (Goode and Bean) J. N. Maina,* , † C. M. Wood,‡ C. Hogstrand,§ T. E. Hopkins,  Y.-H. Luo, ¶ P. D. L. Gibbs ¶ and P. J. Walsh ¶ *Department of Veterinary Anatomy, University of Nairobi, Nairobi, Kenya; ‡Department of Biology, McMaster University, Hamilton, Ontario, L8S 4K1 Canada; §T. H. Morgan School of Biological Sciences, University of Kentucky, Lexington, KY 40506-0225, U.S.A.;  Rookery Bay National Estuarine Research Reserve, Naples, FL 33962, U.S.A. and ¶ Division of Marine Biology and Fisheries, NIEHS Marine and Freshwater Biomedical Sciences Center, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, U.S.A. ABSTRACT. The structure of the axillary organ of a batrachoidid species, the gulf toadfish (Opsanus beta Goode and Bean 1879), has been examined and several simple experiments designed to elucidate its function performed. Electron microscopy (EM) studies revealed cells and structures suggesting secretory and iono regula- tory roles (e.g., abundant intracytoplasmic secretory particles, rough endoplasmic reticulum, sparse Golgi bodies, indented epithelial cells with microvilli, numerous endocytotic vesicles, etc.). Our physiological experiments allowed us to reach several conclusions: the organs do not excrete significant quantities of urea relative to other areas of the fish (head and gills), the organs do not secrete a substance that is toxic to a teleost test fish (Gambusia affinis), the secretions do not induce short-term modifications in locomotory activity of other gulf toadfish (e.g., by pheromonal means) and the secretions do not inhibit the growth of several species of microorganisms in culture. The function of the organ and its secretions remains unknown, representing a fertile area for research on structure and function in comparative physiology. comp biochem physiol 119A;1:17–26, 1998.  1998 Elsevier Science Inc. KEY WORDS. Axillary organ, gulf toadfish, Opsanus beta, pheromones, anti-microbial compounds, toxins INTRODUCTION (the axillary gland) underlies the pore. The genus Batrachus was later reclassified into several genera and species, but it In the family Batrachoididae (toadfishes and midshipmen), is likely that Wallace was examining the abundant local there are approximately 69 species in 19 genera and 3 sub- oyster toadfish, Opsanus tau, whose distribution extends families (12). Several species of the subfamily Batrachoidi- from Maine to Central Florida (13). The gland is described nae possess an axillary foramen, a pore located between the as a pouch-shaped sac with numerous infoldings of epider- body wall and the pectoral fin. In the family in general, mal tissue and is typically divided into several chambers. the pelvic fins are typically small in size and located in the Through a variety of staining methods, Wallace established ‘‘jugular’’ position anterior to the pectoral fins, which are that at least one of the six cell types she described had vacu- large and used extensively during routine locomotion. The oles. The gross morphology of the gland and the presence presence or absence of this axillary foramen (and other of vacuoles strongly suggested a secretory function. At the ‘‘glandular’’ pores) has been a very useful characteristic for time, through the work of a French scientist, Alphonse Bot- systematic studies [e.g., (3)], but the anatomy and function tard (2), it was thought that many members of the family of the structure has not been especially well studied. were venomous, so Wallace reported that ‘‘the results The earliest reported description of the axillary pore is by gained from a few simple experiments with the living fish, Louise B. Wallace (15) of the Marine Biological Laboratory, do not seem to favor the idea of a poisonous secretion,’’ who demonstrated in Batrachus sp. that a glandular structure although she presented no data. She concluded on the de- scription of the gland by saying, ‘‘My work, however, has Address reprint requests to: P. J. Walsh, Division of Marine Biology and not been sufficient to determine the nature of the secretion Fisheries, NIEHS Marine and Freshwater Biomedical Sciences Center, Ro- which still offers an interesting problem to the physiolo- senstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, U.S.A. Tel. (305)-361-4617; Fax (305)-361-4600; E- gist.’’ mail: pwalsh@rsmas.miami.edu. A second early reference to the axillary gland was made †Present address: Department of Anatomical Sciences, Medical School, by Gill (7), who described the secretory cells as being ‘‘club- University of the Witwatersrand, Johannesburg, Republic of South Africa. Received 25 June 1996; accepted 4 November 1996. shaped, cylindrical and uncommonly large (0.275 mm