title: The contribution of the swimbladder to buoyancy in the adult zebrafish (Danio rerio): a morphometric analysis.
abstract: Many teleost fishes use a swimbladder, a gas-filled organ in the coelomic cavity, to reduce body density toward neutral buoyancy, thus minimizing the locomotory cost of maintaining a constant depth in the water column. However, for most swimbladder-bearing teleosts, the contribution of this organ to the attainment of neutral buoyancy has not been quantified. Here, we examined the quantitative contribution of the swimbladder to buoyancy and three-dimensional stability in a small cyprinid, the zebrafish (Danio rerio). In aquaria during daylight hours, adult animals were observed at mean depths from 10.1 +/- 6.0 to 14.2 +/- 5.6 cm below the surface. Fish mass and whole-body volume were linearly correlated (r(2) = 0.96) over a wide range of body size (0.16-0.73 g); mean whole-body density was 1.01 +/- 0.09 g cm(-3). Stereological estimations of swimbladder volume from linear dimensions of lateral X-ray images and direct measurements of gas volumes recovered by puncture from the same swimbladders showed that results from these two methods were highly correlated (r(2) = 0.85). The geometric regularity of the swimbladder thus permitted its volume to be accurately estimated from a single lateral image. Mean body density in the absence of the swimbladder was 1.05 +/- 0.04 g cm(-3). The swimbladder occupied 5.1 +/- 1.4% of total body volume, thus reducing whole-body density significantly. The location of the centers of mass and buoyancy along rostro-caudal and dorso-ventral axes overlapped near the ductus communicans, a constriction between the anterior and posterior swimbladder chambers. Our work demonstrates that the swimbladder of the adult zebrafish contributes significantly to buoyancy and attitude stability. Furthermore, we describe and verify a stereological method for estimating swimbladder volume that will aid future studies of the functions of this organ.
authors.affiliation: Department of Anatomy and Neurobiology, Dalhousie University, Halifax, Nova Scotia B3H 1X5, Canada.
authors.forename: George N
authors.initials: GN
authors.lastname: Robertson
authors.forename: Benjamin W
authors.initials: BW
authors.lastname: Lindsey
authors.forename: Tristan C
authors.initials: TC
authors.lastname: Dumbarton
authors.forename: Roger P
authors.initials: RP
authors.lastname: Croll
authors.forename: Frank M
authors.initials: FM
authors.lastname: Smith
mesh_terms: D000400:Air Sacs / Q000033:anatomy & histology*; D000818:Animals; D001837:Body Weights and Measures; D005260:Female; D008297:Male; D011312:Pressure; D013550:Swimming; D015027:Zebrafish / Q000033:anatomy & histology* / Q000502:physiology