910 Hydrodynamic and phylogenetic aspects of the adipose fin in fishes T.E. Reimchen and N.F. Temple Abstract: The adipose fin on fishes is a highly conserved and enigmatic, small, non-rayed fin that has persisted from the Mesozoic on some basal teleosts such as salmonids. Using juvenile steelhead, Oncorhynchus mykiss (Walbaum, 1792), ranging from 5 to 18 cm standard length, we experimentally test the effects of adipose fin removal on swim- ming performance in a variable velocity flow chamber and quantify, with seven independent trials, amplitude and fre- quency of caudal fin movement at multiple flow velocities (range 10–39 cm·s–1). Results demonstrate that adipose fin removal on smolts produces an average 8% (range –3% to 23%) increase in caudal fin amplitude relative to unclipped fish across all velocities. However, we observed no effects in trials with smaller fish (<7 cm) or larger fish (>12 cm). Consistent with speculations in the literature, our results show that the adipose fin may function to control vortices en- veloping the caudal fin during swimming or, alternatively, function as a passive precaudal sensor of turbulent flow. Phylogenetic persistence of this trait among multiple groups of early bony fishes is probably due to its hydrodynamic attributes rather than developmental constraints, and the current widespread practice in fisheries of removing the adi- pose fin as a marking technique may have significant biological costs. Résumé : La nageoire adipeuse chez les poissons est une petite nageoire sans rayons qui est énigmatique, mais forte- ment conservée au cours de l’évolution, car elle persiste depuis le mésozoïque chez quelques téléostéens primitifs, tels que les salmonidés. Nous avons vérifié expérimentalement les effets de l’ablation de la nageoire adipeuse sur la perfor- mance de la nage chez de jeunes truites arc-en-ciel anadromes, Oncorhynchus mykiss (Walbaum, 1792), de longueur standard de5à18cmdans une enceinte à débit variable; nous avons quantifié, dans sept essais indépendants, l’amplitude et la fréquence des mouvements de la nageoire caudale dans une gamme de plusieurs vitesses de courant (étendue 10–39 cm·s–1). Les résultats indiquent que l’ablation de la nageoire adipeuse chez les saumoneaux cause un accroissement moyen de 8 % (étendue de –3%à23%)del’amplitudedumouvementdelanageoire caudale, par comparaison à des poissons ayant conservé leur nageoire adipeuse, et ce à toutes les vitesses de courant. Cependant, il n’y a aucun effet discernable chez les poissons plus petits (<7 cm) ou plus grands (>12 cm). En accord avec les spécu- lations trouvées dans la littérature, nos résultats montrent que la nageoire adipeuse peut servir à contrôler les tourbil- lons qui entourent la nageoire caudale durant la nage; elle peut aussi servir de senseur passif du débit turbulent en position pré-caudale. La persistance au cours de l’évolution de ce caractère chez de nombreux groupes de poissons os- seux primitifs s’explique probablement par ses qualités hydrodynamiques, plutôt que par des contraintes de développe- ment. La pratique courante et répandue d’enlever la nageoire adipeuse comme technique de marquage peut probablement entraîner des coûts biologiques significatifs. [Traduit par la Rédaction] Reimchen and Temple 916 Introduction non-rayed fin usually located medially between the dorsal and caudal fins and has a very restricted taxonomic occur- Small plesiomorphic and seemingly trivial structures orig- rence (Nelson 1994). It occurs among eight extant groups inating in ancestral forms and persisting through extended of basal euteleosts (Fig. 1), including the Characiformes geological periods provide a continued focus for discussion (characins), the Siluriformes (catfish), the Salmoniformes on the role of conserved characters in evolution. Persistence (salmonids), and the Myctophiformes (lanternfish); and within can result from developmental constraints (Schlichting and some of these groups, the adipose fin is variable in expres- Pigliucci 1998) or comprise an evolutionary trade-off be- sion among closely related taxa. It is uniformly absent in all tween constraints and functionality (Futuyma 1998; Galis et modern teleosts. No function has yet been identified for this al. 2001). The adipose fin of fishes is an enigmatic, small, small fin and it represents an important trait for distinguish- ing major taxonomic groups and affinities (Helfman et al. 1997). Received 15 November 2003. Accepted 21 May 2004. Published on the NRC Research Press Web site at The small size of the adipose fin and its passive motion http://cjz.nrc.ca on 12 August 2004. during swimming appear to limit its role in stability, drag, or 1 thrust, which are the major functions of medial fins in fishes T.E. Reimchen and N.F. Temple. Department of (Webb 1975; Aleyev 1977). It may simply be a phylogenetic Biology, University of Victoria, PO Box 3020, Victoria, vestige of a formerly larger posterior dorsal fin (Sandon BC V8W 3N5, Canada. 1956; Kosswig 1965) that has persisted among extant groups 1Corresponding author (e-mail: [email protected]). owing to epistatic effects and constraints (for review see Can. J. Zool. 82: 910–916 (2004) doi: 10.1139/Z04-069 © 2004 NRC Canada Reimchen and Temple 911 Fig. 1. Abbreviated phylogeny of Teleostei (modified from Nelson 1994). Taxa with an adipose fin shown with bold lines. Representa- tive adipose fin shown on salmon illustration (reproduced with permission from S.D. Douglas). Schlichting and Pigliucci 1998). Gosline (1971) suggests 1999), and it seems probable that over the 70+ million years that the adipose fin may be important in juvenile age classes of history of the groups there would be sufficient variability for generating a dorsal thrust vector symmetrical to that of to favor further reduction in size or elimination of the adi- the ventral anal fin during swimming. Webb (1975), Aleyev pose fin if it was metabolically costly to produce. Secondly, (1977), and Blake (1983) speculate that the adipose fin may any medial appendage would not persist from the Mesozoic influence vortices and flow regime to the caudal fin and po- to the present in multiple taxonomic groups unless it pro- tentially reduce cross-flow and boundary layer separation vided a benefit that offset the presumed drag-related costs. around the caudal peduncle just like the dorsal and ventral Thirdly, in salmon and trout, there is sexual dimorphism in finlets of tuna-like fishes. The position and passive nature of the size of the adipose fin (Beacham and Murray 1983) that the adipose fin could be equivalent to the 5th finlet of scom- is accentuated prior to migration up the spawning rivers brids, which redirects cross-peduncular flow during steady (Beacham and Murray 1986). In this paper, we present new swimming (Nauen and Lauder 2001). Whatever the function experimental evidence that adipose fin removal on juvenile of the adipose fin might be, the benefits are thought to be salmonid results in reduced swimming efficiency relative to small and, as in juvenile salmonids in Europe and North control fish in flow chambers across multiple flow veloci- America, the routine clipping of the adipose fin is the domi- ties. nant marking technique in fisheries science (Hammer and Blankenship 2001). Materials and methods Several lines of evidence encouraged us to further develop some of the speculations regarding the functionality of the To evaluate swimming behaviour, we videotaped using a adipose fin. Firstly, there is size variation in the adipose fin Sony Handycam at 30 frames/s swimming juvenile steel- within species (Martinez 1984; Nelson 1994; Petersson et al. head, Oncorhynchus mykiss (Walbaum, 1792), in a rectangu- © 2004 NRC Canada 912 Can. J. Zool. Vol. 82, 2004 Table 1. Trial characteristics of protocol and results. Trial Protocol A B C D E F G Number of fish 9 21 11 10 16 8 37 Mean standard length 11.8 (10.3–13.3) 8.5 (7.3–9.6) 11.0 (8.1–12.8) 10.7 (8.0–12.9) 16.4 (13.9–17.9) 6.7 (6.0–7.6) 5.4 (4.5–6.2) (cm) (range) Flow velocity 9.7, 11.3, 15.6, 9.7, 11.3, 15.8 and 27.1 17.4 and 27.6 21.8, 31.7, 13.1, 14.8, 15.8, 10.4 and 14.7 categories (cm·s–1) 20.3, and 15.6, 20.3, 36.7, and 18.7, 21.8, 25.3 and 25.3 39.2 and 27.1 Protocol differences MS222 MS222 Temperature Temperature Temperature Temperature Temperature controlled;* controlled;* controlled;* controlled;* controlled;* MS222; fish eugenol; eugenol eugenol; MS222; restricted to turbulent small fish includes injury the front of flow regime treatment group the chamber introduced Average percent change 17 (13–23) 17 (16–18) –1 (–3 to 1) 10 (9–12) 3 (–3 to 6) 1 (0–5) 0 (–1 to 0) in amplitude† (range) Significance P < 0.001 P < 0.001 P = 0.77 P = 0.002 P = 0.02 P = 0.25 P = 0.81 Note: MS-222 and eugenol were the anaesthetics used in the trials. *Temperature was controlled using a cooling unit (Universal Marine Industry Inc., San Leandro, California) and a pump to maintain water temperature at 12 ± 1 °C. †Relative increase in amplitude among clipped fish relative to the fishes in the control group after treatment. lar plexiglass flow chamber (96 cm long × 10 cm wide × for recovery (range 48–56 h). After this duration, arbitrarily 15 cm high) over seven trials. Water turbulence from the chosen fishes were individually placed in the flow chamber, pump was greatly reduced by forcing water through two se- identified from pigmentation, and videotaped at each of the quential plates of thin tubes (20 mm long, 2 mm diameter) at velocity categories. Experiments were replicated after 24 h. the entrance of the flow chamber. A 2.5-mm grid was placed We analyzed swimming behaviour on video-playback on a on the underside of the flow chamber against which each high-resolution monitor (73.7-cm RCA Colortrak).