similar in juveniles and adults. Thus, juvenile catfish generally resemble and develop smoothly into their adult form without distinct juvenile specializations. Exceptions to this are the ariid catfish, where the young retain yolk sacs late into juvenile stages, and many pimelodids, which may have elongated barbels and fin filaments or coloration patterns. Sexual dimorphism is reported in about half of all families of catfish. The modification of the anal fin into an intromittent organ (in internal fertilizers) as well as accessory structures of the reproductive apparatus (in both internal and external fertilizers) have been described in species belonging to 11 different families. Size Catfish have one of the greatest ranges in size within a single order of bony fish. Many catfish have a maximum length of under . Some of the smallest species of the Aspredinidae and Trichomycteridae reach sexual maturity at only . The wels catfish, Silurus glanis, and the much smaller related Aristotle's catfish, are the only catfish indigenous to Europe; the former ranges throughout Europe, and the latter is restricted to Greece. Mythology and literature record wels catfish of astounding proportions, yet are to be proven scientifically. The typical size of the species is about , and fish more than are rare. However, they are known to exceed in length and in weight. In July 2009, a catfish weighing was caught in the River Ebro, Spain, by an 11-year-old British schoolgirl. In North America, the largest Ictalurus furcatus (blue catfish) caught in the Missouri River on 20 July 2010, weighed . The largest flathead catfish, Pylodictis olivaris, ever caught was in Independence, Kansas, weighing . These records pale in comparison to a Mekong giant catfish caught in northern Thailand on 1 May 2005, and reported to the press almost 2 months later, that weighed . This is the largest giant Mekong catfish caught since Thai officials started keeping records in 1981. Also in Asia, Jeremy Wade caught a goonch following three fatal attacks on humans in the Kali River on the India-Nepal border. Wade was of the opinion that the offending fish must have been significantly larger than this to have taken an 18-year-old boy, as well as a water buffalo. Piraíba (Brachyplatystoma filamentosum) can grow exceptionally large and are native to the Amazon Basin. They can occasionally grow to 200 kg (440 lb), as evidenced by numerous catches. Deaths from being swallowed by these fish have