Gas Exchange and Gas Transport in a Water Breather of Millimetre Size

Abstract

Body size is highly correlated with the structure-function relationship of an animal. Whereas oxygen uptake and distribution occur by simple diffusion in micro-organisms, the development of internal and/or external structures for convective gas transport (circulatory and ventilatory systems) becomes necessary with increasing body size. This con-straint is of special importance for the embryonic and larval development of both vertebrates and invertebrates. Water-breathers with a body size of up to 1 mm belong to a group in which oxygen transport via diffusion is sufficient, at least at normoxia, and convection is not yet required. This limitation in body size is derived from the maximum penetration depth of diffusively transported oxygen, which can be calculated by a simplified model (Piiper and Koepchen, 1975). We have focused our interest on a fresh-water invertebrate, the crustacean Daphnia magna(Figure 1), which has great ecological importance and is also used as a bioindicating organism for water quality control. This species has a body length of 1-4 mm and possesses an open circulatory system with freely dis-solved haemoglobin (Hb). The body appendages (thoracic limbs) are used as a pump to move water through, and retain food by, the filter apparatus.