Extremely early maturity found in Okinawan gobioid fishes Takeshi Kon* * and Tetsuo Yoshino Department of Marine Sciences, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara Okinawa 903-0213, Japan (e-mail: TY, b985005@sci.u-ryukyu.ac.jp) *Present address: Department of Marine Bioscience, Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan (e-mail: kon@ori.u-tokyo.ac.jp) Received: September 27, 2001 / Revised: January 26, 2002 / Accepted: February 20, 2002 Abstract Extremely small animals including fishes have been reported with discussion of the causes and consequences of their miniaturization. Here we demonstrate, for the first time, very early (i.e., 23–60 days old and 42–67 days old) sexual maturity in two groups of gobioid fishes (Schindleria and Paedogobius, respectively) in warm water, based on the otolith increments. The generation time of Schindleria is the shortest known among vertebrates under natural conditions. We discuss the occur- rence and evolutionary significance of the progenesis found in gobioid fishes. Key words Gobioidei · Progenesis · Warm waters · Otolith · Daily increments Ichthyological Research ©The Ichthyological Society of Japan 2002 Ichthyol Res (2002) 49: 224–228 W hat is the smallest vertebrate? Many biologists have reported extremely small animals, including fishes from Southeast Asia (Roberts, 1986; Parenti, 1989), South America (Roberts, 1984; Weitzman and Vari, 1988), and Africa (Grande, 1994), and have discussed the causes and consequences of their miniaturization (Hanken and Wake, 1993; Miller, 1996). Miniaturization is assumed, without sub- stantial age data (especially in fish), to accompany early maturity (or progenesis in paedomorphosis) being one of the effective strategies for the increase of r (Gould, 1977). In addition, early maturity is expected to accelerate speciation, leading to an increase in species taxonomic diversity (Marzluff and Dial, 1991; Munday and Jones, 1998). The suborder Gobioidei (Teleostei, Perciformes) com- prises more than 2000 species that are distributed world- wide and adapted to various environments. Of these, some progenetic gobioids, Schindleria spp. and Paedogobius kimurai, are known to inhabit coral reefs and muddy bottoms, respectively, in the Ryukyu Islands. The two spe- cies of the gobioid genus Schindleria, S. praematura and S. pietschmanni, broadly distributed in coral reefs of the Indo-Pacific region and characterized by a slender trans- lucent body and absence of the first dorsal and pelvic fins, are known as one of the smallest vertebrates in the world (Johnson and Brothers, 1993). In Okinawa Island, Schindleria is observed throughout the year, and one type of female similar to S. praematura in having 17–22 dorsal fin rays, 10–13 anal fin rays, and a urogenital papilla with two projections is very common. We herein call the female as Schindleria type 1. Another species, Paedogobius kimurai, is a diandric goby distributed in the Ryukyu Islands, Thailand, and Australia (Neira et al., 1998; Iwata et al., 2001). The female is planktonic, lacking a pelvic fin, whereas the sec- ondary male is benthic, having a pelvic fin and a large mouth with strong canine-like teeth. In recent years, many studies have demonstrated that the examination of otolith microstructure and counting of daily growth increments allow the age of individual fish, including that of gobioid fishes, to be determined with great precision (Campana and Neilson, 1985; Iglesias et al., 1997; Hernaman et al., 2000). Using this method, we obtained convincing data for the age of maturity in Schindleria type 1 and P. kimurai in Okinawa I. and thereby estimated the genera- tion time for the first time. We then compared generation times between progenetic and neotenic gobioids in relation to seasonal trends in water temperature and, especially in Schindleria type 1, discuss the bearing of shortened genera- tion time on the species diversification as recognized. Materials and Methods Sampling materials.—Examined specimens of Schindleria type 1 were collected at high tide of every month (new moon) from June 1999 to July 2000, at Motobu, Okinawa I., Ryukyu Is., by hand net or bucket with a night lamp. We examined 131 ripe specimens of Schindleria type 1 (of 1209 specimens of Schindleria spp.). Paedogobius kimurai were collected on 21 May 1998 and 12 February 2000, at Nakagusuku Bay, Okinawa I., by seine net. The seine net, which consisted of two 56-m-long wings and a 24-m-long  10-m-high  11-m-wide pocket, was fished to a depth of about 10 m and swept an area of about 1700 m 2 . We exam- ined 7 ripe specimens (of 23 specimens). Sex and develop- mental stage of gonads were determined by direct observa- tion of ovarian eggs visible through the translucent body wall. Records of surface water temperature in the study sites were obtained from JODC (Japan Oceanographic Data Center) as monthly averages between 1906 and 1994 at the western coast of Okinawa Island.