130 COPEIA, 1994, NO. 1 haviourand copulationin Dracodussumieri Dum. & REYES,A. Y. 1968. Food habits of Draco volans Lin- Bib. (Reptilia: Sauria).J. Bombay Nat. Hist. Soc. naeus. SillimanJ. 15:353-356. 64:112-115. ROSE,B. 1981. Factorsaffecting activityin Sceloporus -. 1967b. Activity rhythm and thermoregula- virgatus. Ecology 62:706-716. tion in the South Indian flying lizard, Draco dus- SCHMIDT,K. P. 1935. Notes on the breeding behav- sumieriDum. & Bib.J. Anim. Morphol. Physiol. 14: ior of lizards. Zool. Ser. Field Mus. Nat. Hist. 20: 131-139. 71-76. MXGDEFRAU,K. 1991. Haltung, Verhaltenbeobach- SMITH,M. A. 1935. The Fauna of British India, in- tungen und Zuchtversuche von Draco spilopterus cluding Ceylon and Burma. Reptilia and amphibia. (Wiegmann, 1834). Herpetofauna 13:29-34. Vol. II. Sauria. Taylor and Francis Ltd., London, MARTIN,P., AND P. BATESON. 1986. Measuring be- England. haviour: an introductory guide. CambridgeUniv. STAMPS, J. A. 1977. Social behavior and spacing pat- Press, Cambridge,England. terns in lizards, p. 265-334. In: Biology of the Rep- MORI, A., AND T. HIKIDA. 1993. Natural history ob- tilia. Vol. 7. Ecology and behavior. A. C. Gans and servationson the flying lizards,Draco volans suma- D. W. Tinkle (eds.). Academic Press, London, En- tranus (Agamidae, Squamata)from Sarawak,Ma- gland. laysia. RafflesBull. Zool. In Press. MUSTERS, C. J. M. 1983. Taxonomy of the genus Draco L. (Agamidae, Lacertilia, Reptilia). Zool. DEPARTMENT OF ZOOLOGY, FACULTY OF SCIENCE, Verhandelingen No. 199:1-120. KYOTO UNIVERSITY, SAKYO, KYOTO, 606-01 P. 1962. Parades et terri- PFEFFER, comportement JAPAN.Submitted 5 Sept. 1992. Accepted 29 torial chez trois especes de Dragons-volants(Agam- Jan. 1993. Section editor: W. J. Matthews. idae) d'Indonesie. Terre Vie 109:417-427. Copeia, 1994(1), pp. 130-135 Why Swim Upside Down?: A Comparative Study of Two Mochokid Catfishes LAUREN J. CHAPMAN, LES KAUFMAN, AND COLIN A. CHAPMAN Some catfishes in the genus Synodontis and its allied genera (family Mochok- idae) swim upside down and exhibit reverse countershading. We demonstrate a potential respiratory function for this behavior through laboratory observations of upside down (Synodontis nigriventris) and right side up (Synodontis afrofischeri) species exposed to low PO2. Both species used aquatic surface respiration (ASR) at the air-water interface when PO, was <15 mm Hg. With decreasing PO,, S. nigriventris increased the percentage of ASR time spent upside down, did not emerse its body during ASR, and had very modest surface activity levels. Syn- odontis afrofischeri used a vertical posture for ASR that caused emersion of its snout and required constant swimming to maintain position; it used active for- ward motion during ASR at very low PO, and made repeated trips to the bottom. The vertical posture and increased swimming activity associated with ASR by S. afrofischeri is probably less efficient than the inverted ASR of S. nigriventris. M ANY catfishes of the family Mochokidae terning may allow the fish to merge with their are benthic in habit, feeding on bottom- background of light when their ventral surface dwelling organisms or detritus with their ven- is directed upward. Apparently, swimming with tral mouths (Lowe-McConnell, 1975). A few the ventral surface uppermost is used in a feed- species, however, can be found swimming in- ing context, facilitating the exploitation of verted, a habit often associated with exploita- plankton and fine detritus from the surface wa- tion of the water surface. Although similar mor- ters (Bishai and Abu Gideiri, 1963; Holden and phologically to the other mochokids, some Reed, 1972; Lowe-McConnell, 1975). Howev- species that exhibit upside down habits, such as er, a respiratory function has also been ascribed Synodontis nigriventris and Hemisynodontis mem- to this unusual behavior (Holden and Reed, branaceus, are characterized by reverse coun- 1972; Roberts, 1975). tershading (Daget, 1948; Poll, 1971). This pat- Many fish species use aquatic respiration at ? 1994 by the American Society of Ichthyologists and Herpetologists CHAPMAN ET AL.-WHY SWIM UPSIDE DOWN? 131 the surface (Aquatic Surface Respiration, ASR; species, an experimental tank (90 x 30 x 37 Kramer and Mehegan, 1981) when inhabiting cm, maintained at 32 cm of depth) was divided hypoxic waters. ASR works because diffusion into two major compartments (36 x 30 x 37 from the atmosphere provides a thin, well-ox- cm) using a screen partition that limited visi- ygenated microlayer of surface water, even bility and eliminated fish movement between though deeper waters may be hypoxic (Gee et compartments but permitted some water ex- al., 1978; Kramer and McClure, 1982; Kramer, change. Two additional partitions divided the 1983). The upside down habit of certain mo- major compartments from the two ends of the chokid fishes could also provide surface access tank. These smaller end compartments con- for purposes of ASR, an activity that would oth- tained heaters and air stones. Each major com- erwise be difficult for a fish having a ventral partment contained a box filter and two pieces mouth. There is little quantitative data on the of cover that consisted of strips of plastic con- respiratory responses of the upside down cat- nected to a small plexiglass tube. The two spe- fishes to hypoxia or their use of ASR. cies were placed in separate compartments, Here we compare the behavioral responses maintained at 23-26 C, and fed Tetramin food to hypoxia of two mochokid catfish (Synodontis flakes. nigriventris and Synodontis afrofischeri). Unlike Fish were exposed to various concentrations S. nigriventris, S. afrofischeridoes not habitually of oxygen between 0.15 and 8.3 mg/L at 23- swim upside down and, thus, may not respond 26 C (3-154 mm Hg). Twenty-one trials were to hypoxia by swimming ventrally at the surface. conducted over 16 days, with the level of oxy- The percentage of time spent at the surface and gen concentration selected randomly from pre- the behavior of both species is described in re- determined values. No more than two trials were lation to PO2. In addition, we examine behav- conducted each day, with an average of 3 h ioral parameters related to efficiency of respi- between trials on the same day. Fish were main- ratory use of the surface layer. tained at normoxia overnight, and PO2 was re- turned to normoxic levels between trials con- ducted on the same day. Fish were starved for METHODS several hours before a trial to ensure that trips to the surface were not associated with feeding Study species.-Wild-caught S. afrofischeri were at the surface. PO2 was reduced by bubbling N2 obtained from Old World Exotics of Florida through an air stone in each of the two end from a private collector who works on the compartments. To create extreme hypoxia (<1 northern shoreline of Lake Victoria between mg/L), small amounts of sodium sulfite were Entebbe andJinja. The four smallest individuals added to the water (Lewis, 1970; Kramer and were captive spawned from the wild-caught Mehegan, 1981; Gee and Gee, 1991). Oxygen group in the Lake Victoria exhibit at the Frank- content was measured (?0.1 mg/L) with a YSI lin Park Zoo, Boston, Massachusetts. Synodontis (Model 57) 02 meter and converted to P02 us- nigriventris used in this work were purchased ing standard tables (Davis, 1975). For each trial, from Metro Pets, New Jersey. oxygen concentration was reduced over 40-90 Synodontisafrofischeri occurs in the Nile basin, min. Fish were then allowed to acclimate for 30 primarily in Lake Victoria and its affluent riv- min. ers; the Victoria Nile; and lakes Nabugabo, The average amount of time spent at the sur- Kyoga, and Ihema (Greenwood, 1966; Poll, face was determined for each species by an ob- 1971). It is benthic, retains a normal orienta- server who sat behind a blind 0.5 m from the tion, feeds on insect larvae and molluscs (Cor- tank. The blind was necessary to prevent dis- bet, 1961), and rarely attains a total length of turbance that would result in fish retreating to over 15 cm. Synodontis nigriventris reaches a cover. A continuous record was kept of the maximum total length of 9.6 cm and occurs in number of fish and their orientation at the sur- streams and rivers of the central Congo (Poll, face for 30 min. Video recordings were used 1971). It has reversed countershading related for assessment of posture, positioning of the to its habit of routinely swimming inverted. barbels, and emersion of the snout. Gill ventilation rates/15 sec were determined The effectofPO2.-Four small S. afrofischeri(mean in relation to the method of ASR employed by Total Length = 64.9 + 4.7 mm) and four S. the fish. Oxygen was lowered slowly. Gill ven- nigriventris (mean Total Length = 64.3 ? 4.1 tilation rates were recorded four times on each mm) were selected from a stock aquarium (59 fish just prior to the ASR threshold level and x 37 x 39 cm, 23-26 C). To determine the after ASR was initiated for each species in dif- effect of hypoxia on the behavior of the two ferent postures. 132 COPEIA, 1994, NO. 1 100 individuals at the surface) was significantly high- er for S. nigriventris (mean = 116 sec) than for S. = 22 sec, t-test, t = 80- Synodontisnigriventris afrofischeri(mean paired 3.80, P = 0.005). 60 Respiratory behavior.--Synodontis afrofischeriwas 40 * never observed to swim on its back. ASR was achieved by positioning the body nearly per- < 20 pendicular to the surface (Fig. 2A). Active movements were required to maintain this pos- I 0 ture. ASR was performed in two ways. The first 100 was to perform ASR while hovering in a vertical position against the wall of the aquarium.