Sexualdimorphismandgonadaldevelopmentofthe Australianlongfinnedrivereel C. T. WALSH *†‡, B. C. P EASE * AND D. J. B OOTH † *NSW Fisheries, P. O. Box 21, Cronulla NSW 2230, Australia and †University of Technology, Sydney, Department of Environmental Sciences, Westbourne Street, Gore Hill NSW 2065, Australia (Received 17 August 2002, Accepted 30 April 2003) The sex and stage of gonadal development of longfinned river eels Anguilla reinhardtii, captured from nine river catchments in New South Wales, Australia, between 1999 and 2001, were determined macroscopically. Sex was verified by histology. Histology was also necessary, how- ever, to accurately define stages of gonadal development, particularly in individuals <600mm in total body length. Anguilla reinhardtii displayed asynchronous gamete development. The most advanced cells present in migrating male and female A. reinhardtii were spermatocytes and pre- vitellogenic oocytes, respectively. Gonadal development stages were positively correlated with body size in both sexes. Females, however, were significantly larger than males and their gonads matured over a broader size range. Size at sexual differentiation (42–60cm for males and 50–76cm for females) was much larger than for most other anguillids that have been studied, with the exception of the New Zealand longfinned eel Anguilla dieffenbachii. Corresponding with its large range in size at sexual differentiation was a relatively large range in size at migration for both males (44–62cm) and females (74–142cm). # 2003 The Fisheries Society of the British Isles Key words: Anguilla reinhardtii; gonad development; sexual dimorphism. INTRODUCTION The Australian longfinned eel Anguilla reinhardtii Steindachner is predominantly a tropical to sub-tropical species found in the coastal catchments of eastern Australia from Cape York to Tasmania (Beumer, 1996). It is also known to occur in New Guinea, the Solomon Islands, New Caledonia, Lord Howe Island and New Zealand (Schmidt, 1928; Ege, 1939; Allen, 1991; Jellyman et al., 1996). Anguilla reinhardtii is a long-lived species that may attain a total length (L T ) of 16m and a mass of 20kg. Although it is the target of a significant trap and fyke net fishery in eastern Australia (Kailola et al., 1993), few biological and ecological studies have been done on this species (Sloane, 1984; Beumer, 1996) and none have focused on its reproductive biology. In early studies on gonad morphology and development of anguillids, staging was almost entirely based on the external appearance of the gonads. This is the ‡Author to whom correspondence should be addressed. Tel.: þ61295278411; fax: þ61295278576; email: walshc@fisheries.nsw.gov.au JournalofFishBiology (2003) 63, 137–152 doi:10.1046/j.1095-8649.2003.00136.x,availableonlineathttp://www.blackwell-synergy.com 137 # 2003TheFisheriesSocietyoftheBritishIsles