Hemoglobin enhances oxygen uptake in larval zebrafish (Danio rerio) but only under conditions of extreme hypoxia

The role of hemoglobin (Hb) in O-2 uptake by zebrafish larvae ranging in age from 5 to 42 days postfertilization was assessed under conditions of normoxia, moderate hypoxia and extreme hypoxia. This was achieved by exposing larvae with and without functional Hb to continuously declining oxygen levels (P-O2) in closed-system respirometers. Exposure to 5% CO for 2-4 h was used to render Hb effectively non-functional in terms of its ability to transport O-2. Routine metabolic rate (r(M) over dot(O2)), critical dissolved oxygen level (P-c) and residual oxygen level (P-r) were determined and used, respectively, as indicators of response in normoxia, moderate hypoxia and extreme hypoxia. r(M) over dot(O2) was defined as the average rate of O-2 uptake before O-2 became limiting (i.e. at high P(O2)s). P-c is the P-O2 at which r(M) over dot(O2) first becomes O-2-limited and P-r is the P-O2 below which larvae are no longer able to extract O-2 from the ambient medium. CO poisoning had no significant impact on r(M) over dot(O2) or P-c at any age, indicating that the lack of functional Hb does not impair routine O-2 usage in normoxia or at moderate levels of hypoxia [down to at least 25-50 torr (1 torr approximate to 0.133 kPa), depending on age]. P-r, however, was significantly lower overall for control larvae (6.7 +/- 1.1 torr; mean +/- 95%CI) than for CO-poisoned larvae (11.2 +/- 2.1 torr). It would appear that the presence of functional Hb allows zebrafish larvae to extract O-2 from water down to lower P(O2)s under conditions of extreme hypoxia. This is the first documented (as opposed to inferred) benefit of Hb in developing zebrafish. However, given the relatively small magnitude of the effect it is unclear if this benefit on its own is sufficient to balance the costs associated with Hb production and maintenance.