Fig 1.
Schematic diagrams of a human eye (A) and the conventional aqueous flow pathway (B).
The human eye (A), which is fairly representative of the vertebrate eye, is composed of concentric layers of tissue enclosing a fluid filled chamber. Light is scattered towards the back of the eye by the cornea and lens. Phototransduction is carried out in the retina. Most of the light is focused on an area centralis, which here coincides with the fovea. The intraocular pressure is maintained by the equilibrium between the formation of aqueous humour and the resistance to its outflow from the eye. Produced by the ciliary body, the aqueous humour flows around the iris into the anterior chamber and is drained through the trabecular meshwork and Schlemm’s canal (B).
Table 1.
List of notations used in the scaling and statistical analyses.
Fig 2.
Estimated intraocular pressure () in amphibians.
IOPs were collected through a systematic review (S1 Table and Table A in S1 File). The studies the data was extracted from, the sample size N, the observed IOP and the 95% confidence interval (CI) are indicated for each species.
Fig 3.
Estimated intraocular pressure () in birds.
IOPs were collected through a systematic review (S2 Table and Table B in S1 File). The studies the data was extracted from, the sample size N, the observed IOP and the 95% confidence interval (CI) are indicated for each species.
Fig 4.
Estimated intraocular pressure () in fish.
IOPs were collected through a systematic review (S3 Table and Table C in S1 File). The studies the data was extracted from, the sample size N, the observed IOP and the 95% confidence interval (CI) are indicated for each species.
Fig 5.
Estimated intraocular pressure () in mammals.
IOPs were collected through a systematic review (S4 Table and Table D in S1 File). The studies the data was extracted from, the sample size N, the observed IOP and the 95% confidence interval (CI) are indicated for each species.
Fig 6.
Estimated intraocular pressure () in reptiles.
IOPs were collected through a systematic review (S5 Table and Table E in S1 File). The studies the data was extracted from, the sample size N, the observed IOP and the 95% confidence interval (CI) are indicated for each species.
Fig 7.
Estimated intraocular pressure () across vertebrates.
IOPs were collected through a systematic review (S1–S5 Tables and Tables A–E in S1 File). The number of species included in the study, the estimated IOP and the 95% confidence interval (CI) are indicated for each class of vertebrates.
Fig 8.
Log-log plot representing the evolution of the IOP (Π) with typical body mass in vertebrates.
The IOP was found to correlate weakly with body mass across all vertebrates (τ = 0.296, p < 0.001).
Table 2.
Kendall’s rank correlation (τ) and 2-sided p-value for IOP and body mass among vertebrates.
Kendall’s rank correlation could not be computed for fish because typical IOPs were found in three species only.
Table 3.
Means of the rate of formation of aqueous humour (Q) and the volume of the anterior chamber (Vac) with respective standard deviations in various vertebrates.
The systematic review yielded measurements of Q and Vac in eight species only, all of them mammals. Only one study per species (apart from human) satisfied the inclusion criteria.
Fig 9.
Log-Log plot representing the evolution of the rate of formation of aqueous humour (Q) with the volume of the anterior chamber (Vac) in various vertebrates.
A fit (plain line) was obtained through model averaging. The rate of formation of aqueous humour was found to scale as , which is in agreement with scaling Eq (10).
Fig 10.
Simplified phylogenic tree of the vertebrate family (A) and estimated IOP within each group (B).
The diagram was not plotted to scale. The sample size N and the 95% confidence interval (CI) is indicated for each group. The estimated IOP appears to have increased with the evolution of terrestrial animals.
Fig 11.
Observed IOP in vertebrates adapted to aquatic vision.
The IOP appears to be lower in aquatic animals adapted to freshwater as compared to seawater. However, the group of animals adapted to saltwater is here limited to mammals. Among all the species adapted to fresh and saltwater the residual heterogeneity is I2 = 90.09%. Including saltwater and freshwater adaptations to the model as moderators accounts for 88.95% of the heterogeneity in effect.