OXYGEN AVAILABILITY AND EMBRYONIC-DEVELOPMENT IN SAND SNAIL (POLINICES SORDIDUS) EGG MASSES

The oxygen transport physiology of sand snail Polinices sordidus egg m asses was investigated using oxygen microelectrodes and open-how respi rometry, P. sordidus eggs are laid in a jelly matrix that rapidly abso rbs water acid swells into a horseshoe-shaped sausage, The average dia meter of these sausages is 37 mm, Eggs are enclosed in capsules that a re distributed throughout the jelly matrix, but 65 % of the eggs are l ocated within 3 mm of the outer surface, There is no circulatory or ca nal system within the matrix so all gas exchange between developing em bryos and the environment must occur by diffusion through the jelly ma trix, Oxygen tension in the outer layer remains moderately high (P-O2> 10 kPa) throughout incubation but decreases rapidly in more centrally located regions, so that by day 4 embryos in this region are exposed t o extremely hypoxic conditions (P-O2<1 kPa). This hypoxia limits oxyge n consumption of embryos to low levels and appears to slow embryonic d evelopment or even to arrest it, From day 4 onwards, the central regio n gradually become less hypoxic because the hatching of peripherally l ocated embryos causes the outer layers of the jelly matrix to disinteg rate and thus reduces the diffusion distance for oxygen between the ce ntrally located embryos and the surrounding sea water, As the oxygen t ension rises, development accelerates and the embryos eventually hatch as viable veligers, apparently unharmed by their prolonged exposure t o hypoxia.