Upstream eventReduced, Anterior swim bladder inflation
Reduced, Swimming performance
Key Event Relationship Overview
AOPs Referencing Relationship
|fathead minnow||Pimephales promelas||NCBI|
Life Stage Applicability
Key Event Relationship Description
Effects on swim bladder inflation can alter swimming performance and buoyancy of fish, which is essential for predator avoidance, energy sparing, migration, reproduction and feeding behaviour, resulting in lower young-of-year survival.
Evidence Supporting this KER
The weight of evidence supporting a direct linkage between these two KEs, i.e. reduced anterior swim bladder inflation and reduced swimming performance, is weak.
The anterior chamber of the swim bladder has a function in regulating the buoyancy of fish, by altering the volume of the swim bladder (Roberston et al., 2007). Fish rely on the lipid and gas content in their body to regulate their position within the water column, with the latter being more efficient at increasing body buoyancy. Therefore, fish with functional swim bladders have no problem supporting their body (Brix 2002), while it is highly likely that impaired inflation severely impacts swimming performance. Fish with no functional swim bladder can survive, but are severely disadvantaged., making the likelihood of surviving smaller.
Several studies in zebrafish and fathead minnow showed that a smaller AC was associated with a larger posterior chamber (Nelson et al., 2016; Stinckens et al., 2016; Cavallin et al., 2017, Stinckens et al., submitted) suggesting a possible compensatory mechanism. As shown by Stoyek et al. (2011) however, the AC volume is highly dynamic under normal conditions due to a series of regular corrugations running along the chamber wall, and is in fact the main driver for adjusting buoyancy while the basic PC volume remains largely invariable. Therefore, it is plausible to assume that functionality of the swim bladder is affected when AC inflation is incomplete, even when the PC appears to fully compensate the gas volume of the swim bladder.
- After exposure to 2-mercaptobenzothiazole, a TPO inhibitor, from 0 to 32 days post fertilization (dpf) in zebrafish, the swimming activity of fish was impacted starting at 26 dpf if the inflation of the anterior chamber of the swim bladder was impaired or had no normal structure/size (Stinckens et al., 2016).
- Methimazole (MMI) and propylthiouracil (PTU), two thyroperoxidase inhibitors, and iopanoic acid (IOP), a deiodinase inhibitor, each reduced both anterior chamber (AC) inflation and swimming distance in zebrafish exposed from fertlization until the age of 32 days (Stinckens et al., submitted). The current dataset provides evidence for a specific, direct link between AC inflation and reduced swimming performance. First, after 21 d of exposure to 111 mg/L PTU around 30% of ACs were not inflated and swimming distance was reduced, while by 32 dpf all larvae had inflated ACs and the effect on swimming distance had disappeared. The most direct way to assess the role of AC inflation in swimming performance, however, is to compare larvae with and without inflated AC at the same time point and within the same experimental treatment. Both in the PTU exposure at 21 dpf and in the IOP exposure at 21 and 32 dpf, swimming distance was clearly reduced in larvae lacking an inflated AC, while the swimming distance of larvae with inflated AC was equal to that of controls.
- It has also been reported that larvae that fail to inflate their swim bladder use additional energy to maintain buoyancy (Lindsey et al., 2010, Goodsell et al. 1996), possibly contributing to reduced swimming activity. Furthermore, Chatain (1994) associated larvae with non-inflated swim bladders with numerous complications, such as spinal deformities and lordosis and reduced growth rates, adding to the impact on swimming behaviour.
- An increasing incidence of swim bladder non-inflation has also been reported in Atlantic salmon (Poppe et al. 1997). Affected fish had severely altered balance and buoyancy, observed through a specific swimming behaviour, as the affected fish were swimming upside down in an almost vertical position (Poppe et al. 1997).
Uncertainties and Inconsistencies
After exposure to 100 mg/L MMI, 95% of the zebrafish larvae failed to inflate their AC at 32 dpf and swimming distance was reduced (Stinckens et al., submitted). On the other hand, there was no effect of impaired AC inflation on swimming distance in the MMI exposure of 50 mg/L. Also, inflated but smaller ACs did not result in a decreased swimming performance in the present study. A similar result, where non-inflated ACs did not consistently lead to reduced swimming performance, was previously found after exposure to MBT (Stinckens et al., 2016). In summary, the precise relationship between these two KEs is not easy to determine and may be different for different chemicals. Swimming capacity can be affected via other processes which may or may not depend on the HPT axis, such as decreased cardiorespiratory function, energy metabolism and growth.
As Robertson et al., (2007) reported, the swim bladder only starts regulating buoyancy actively from 32 dpf onward in zebrafish, possibly explaining the lack of effect on swimming capacity in some cases.
The anterior chamber is also important for producing and transducing sound through the Weberian Apparatus (Popper, 1974; Lechner and Ladich, 2008). It is highly plausible that impaired inflation or size of the anterior swim bladder could lead to a reduction in young-of-year survival as hearing loss would affect their ability to respond to their surrounding environment, thus impacting ecological relevant endpoints such as predator avoidance or prey seeking (Wisenden et al., 2008; Fay, 2009).
Quantitative Understanding of the Linkage
The direct quantitative evidence supporting the connection between anterior chamber impairment and swimming capacity is lacking.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Importance of swimming performance for natural behaviour is generally applicable to fish.
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