Rising to the top: Surfacing Behaviour Reflects Swimbladder Filling in Zebrafish

Abstract

In zebrafish, the swim bladder is filled by swallowing air at the water’s surface. Adjusting the
volume of the swim bladder allows the fish to achieve neutral buoyancy at a given depth.
Zebrafish lacking functional expression of the serotonin (5-HT) reuptake transporter Sertb
(sertb-knockout, sertb-KO; slc6a4b) exhibit “surfacing behaviour”, spending essentially all of
their time at the top of the water column. In the current study, we aimed to determine whether
surfacing behaviour in sertb-KO fish is associated with swim bladder filling and buoyancy. We
measured buoyancy by placing anesthetized sertb-KO zebrafish larvae at 7 days post-fertilization
(dpf) in a water column to assess the rate and direction of movement, and calculated swim
bladder volume from photographs. The sertb-KO larvae displayed higher buoyancy than
wildtype (WT) larvae, and higher buoyancy was associated with larger swim bladder volume.
Treatment of WT larvae with fluoxetine, a selective inhibitor of Sert, was also associated with
higher buoyancy and swim bladder volume. Conversely, treatment of sertb-KO larvae with the
serotonin synthesis inhibitor p-chlorophenylalanine (pCPA) resulted in decreased buoyancy and
smaller swim bladder volume. Further, WT larvae treated with an agonist of the 5-HT1A receptor
displayed higher buoyancy and swim bladder volume. These results implicate elevated swim
bladder volume in surfacing behaviour, and confirm a link between elevated serotonin levels and
swim bladder volume, likely mediated by the 5-HT1A receptor. A key remaining question is
whether surfacing behaviour is caused by serotonin-mediated swim bladder over-filling, or
whether sertb-KO larvae choose to be at the surface and over-fill the swim bladder to help them
remain there.