SHORT REPORT Group formation and anemone use in captively reared anemonefish (Amphiprion frenatus) Eric K. Bollinger Æ Paul V. Switzer Æ Jesse Pfammatter Æ Jason Allen Received: 16 August 2007 / Revised: 20 January 2008 / Accepted: 24 January 2008 Ó The Ichthyological Society of Japan 2008 Abstract We studied group formation and anemone use in captively reared anemonefish Amphiprion frenatus. Four small fish (23–32 mm total length) were simultaneously released into a central area of the aquaria (ca. 160 l) that was divided in half with an anemone placed on each half, with the opportunity to settle on either side. Four-day trials were conducted for analyzing settlement patterns. Because of the monogamous mating system, we predicted that small fish should settle in pairs. However, all four small fish settled on the same side in 7 of 14 replicates, which was a higher than expected value under random settlement. Keywords Amphiprion frenatus Entacmaea quadricolor Group formation Introduction Anemonefish (genus Amphiprion) typically occur in groups consisting of a monogamous pair and a variable number of nonbreeding subordinates in symbiotic association with sea anemones (Allen 1975; Fricke and Fricke 1977; Ross 1978; Hattori 1991, 2000; Fautin and Allen 1992; Buston 2003a). These fish are typically protandrous hermaphrodites in which sex change from male to female is prevented by social dominance of the larger female over the male (Fricke and Fricke 1977; Moyer and Nakazono 1978; Ross 1978a; Buston 2003b). When females die or are experimentally removed, males (released from this dominance) often grow rapidly, change sex, and assume the role of the dominant female (Fricke and Fricke 1977; Fricke 1979; Godwin 1994). The largest nonbreeding subordinant then becomes a sexually mature male to reform a monogamous pair. This breeding system is thought to have evolved as a mechanism to allow for expeditious pair formation given that sea anemones are generally sparsely distributed and movement among anem- ones by anemonefish is considered to be both rare and risky (Allen 1975; Moyer and Sawyers 1973; Ross 1978b; Hattori 1991). Larval recruitment to anemones is thought to be a largely random event, leading ultimately to random pair formation (Fricke and Fricke 1977; Kuwamura 1988; Ku- wamura and Nakashima 1998; but see Hattori 2005). However, exact mechanisms involved in both juvenile recruitment to anemones and pair formation have not been extensively studied (Hattori 2005). Furthermore, recent studies have found that movement among anemones may be more common than assumed, that pair formation may be nonrandom (Hattori 2005), and that in areas of high anemone density, at least one species of anemonefish (Amphiprion clarkii) may seldom change sex (Ochi 1989a, b; Hattori and Yanagisawa 1991; Hattori 1994; Hattori and Yamamura 1995). Thus, additional studies of pair forma- tion in anemonefish have the potential to offer further insight into this system. We were specifically interested in patterns of pair for- mation in Amphiprion frenatus. Breeding groups in this species typically consist of a female, a male, and one or two small nonbreeders (Hattori 1991). They are one of the largest species of anemonefish and also have one of the largest average size differences between the sexes (Hirose 1995). We used a laboratory system to release groups of four small A. frenatus into aquaria with two anemones of similar size on each side of the tank. Since only the monogamous pair breed in anemonefish groups, we predicted that these fish should initially settle in pairs on each side of the tank. E. K. Bollinger (&) P. V. Switzer J. Pfammatter J. Allen Department of Biological Sciences, Eastern Illinois University, 600 Lincoln Ave, Charleston, IL 61920, USA e-mail: ekbollinger@eiu.edu 123 Ichthyol Res DOI 10.1007/s10228-008-0047-0