Hypoxia impairs visual acuity in snapper (Pagrus auratus)

We investigated the effect of environmental hypoxia on vision in snapper (Pagrus auratus). Juvenile snapper inhabit estuarine environments where oxygen conditions fluctuate on a seasonal basis. Optomotor experiments demonstrated that visual acuity is impaired by environmental hypoxia, but not until levels approach the critical oxygen tension (P _crit) of this species (around 25 % air-saturated seawater). In 100, 80, and 60 % air-saturated seawater, a positive optomotor response was present at a minimum separable angle (M _SA) of 1°. In 40 % air-saturated seawater, vision was partially impaired with positive responses at M _SAs of 2° and above. However, in 25 % air-saturated seawater, visual acuity was seriously impaired, with positive responses only present at M _SAs of 6° and above. Snapper were found to possess a choroid rete, facilitating the maintenance of high ocular oxygen partial pressures (PO₂) during normoxia and moderate hypoxia (PO₂, between 269 and 290 mmHg). However, at 40 and 25 % water oxygen saturation, ocular PO₂ was reduced to below 175 mmHg, which is perhaps linked to impairment of visual acuity in these conditions. The ability to preserve visual function during moderate hypoxia is beneficial for the maintenance of a visual lifestyle in the fluctuating oxygen environments of estuaries.