O comportamento respiratório e a cascata de O2 no cascudo de respiração bimodal Pterygoplichthys anisitsi (Teleostei, Loricariidae)
Resumo
The condition of poor oxygen, called hypoxia, can affect aquatic environments.
Their cause can be natural or, more frequently, a consequence of anthropogenic action.
Among fish, the Siluriformes group (catfishes and armoured catfishes) contains
hypoxic-tolerant species, being considered interesting experimental models. This study
utilized the air-breathing armoured catfish Pterygoplichthys anisitsi, with the objective
to analyze its respiratory physiology, at aquatic and air respiration, submitted to
experimental conditions of hypoxia and air exposure to compare its respiratory
behaviour and oxygen transport cascade steps (ventilatory, morphofunctional,
haemoglobin and metabolic aspects). From all these experiments it can be concluded
that gills respiratory surface is much more developed than stomach s surface, which
indicates dependency on aquatic respiration. Furthermore, the air-breathing organ serves
as an air reservatory that can assure the tolerance to reduced oxygen availability in the
water. It was verified that the air-breathing behaviour is continuous, in non regular
periods, but not obligatory and, even in normoxia, its more frequently occurs at night
and in synchronic episodes, probably consisting in an antipredatory behaviour. In
exposure to hypoxia and without access to atmospheric air, the reduction in metabolic
rate and O2 extraction, the stomach supports the oxy-conformist behaviour that is
characterized by apnoea with the gradual reduction of oxygen tension. The association
among a stomach air reservatory, the continuous, but non-obligatory air breathing, an
oxy-conformist behaviour characterized by apnoea, the presence of an haemoglobin
cathodic component, and a higher haemoglobin-oxygen affinity in hypoxia perform
necessary adjustments to hypoxia conditions. Moreover, the haematological parameters
increase hypoxia tolerance, since hemoconcentration contribute to the enhanced
capacity of O2 transportation in response to the increased demand and, therefore, result
in a higher haemoglobin-oxygen affinity. With the decrease of metabolic rate and O2
extraction, there is the activation of anaerobic metabolism by fermentative process to
maintain the energetic supply. Differently from hypoxia condition, the gills do not seem
to be functional during air exposure, given that it was not observed a water flow through
branchial filaments and, thus the stomach surface can be considered the respiratory
organ in this condition. When exposed to air, periods of intermittent inspiration and
expiration behaviour through the mouth are verified in order to supply air to the
stomach. Additionally, haemoglobin-oxygen affinity is similar to that observed in
normoxia and, in the same way, there is not alteration on haematological parameters.
These observations indicate the functionality of stomach as a respiratory surface and the
maintenance of aerobic metabolism. However, air exposure is a stressful condition, as it
can be proved by the reduction of hepatic and muscular glucose and, the increase of
plasma glucose, and may, as a consequence, be utilized by species only in emergence
situations.