A functional analysis of the shift from gill- to lung-breathing during the evolution of land crabs (Crustacea, Decapoda)

Abstract

The evolution of air-breathing in land crabs is associated with a progressive shift in the primary site of respiratory gas exchange from the diffusion-limited gills used for water-breathing, via a simple ‘cutaneous’ lung surface to the perfusion-limited, invaginated lung described in the mountain crab, Pseudothelphusa garmani. The reduced diffusion limitation over the lungs facilitates oxygen transfer from air to the tissues at lower ventilation rates but is associated with accumulation of carbon dioxide. A potential respiratory acidosis is buffered by the respiratory pigment haemocyanin and by elevation of haemolymph bicarbonate levels. These changes parallel those described in vertebrates but air-breathing crustaceans maintain relatively low carbon dioxide levels in the haemolymph, either by retaining an aquatic route for its elimination over the reduced gills or by blowing it off across the lung. Maintenance of low carbon dioxide levels may be associated with a limited capacity to buffer against an acidosis due to low levels of circulating haemocyanin (i.e. crustaceans lack red blood cells). This may ultimately limit their survival in air as an acidosis will reduce oxygen transport due to a marked Bohr effect on haemocyanin. The primary role of an invaginated lung may be to reduce rates of water loss in air.