The species Krithe gnoma was described from Holocene sediments on the continental margin of Brazil, where it occurs on both shelf and slope. The shelf distribution of this species is restricted to those areas influenced by the Malvinas current between 32°11' and 22°31'S. K. gnoma was selected due to the excellent resolution of images of the anterior vestibule. So the range in size of the vestibula of this species was used to determine whether or not the dimensions of the anterior vestibule were to some degree related to amount of dissolved oxygen in the water at the sampling site. Female valves of the 7th and 8th instars were measured and their sizes and those of the anterior vestibule were compared. These valves and the reference carapace were collected at six localities in the southern shelf area. The anterior vestibule of right and left valves of the 7th instar and one of the 8th instar from discrete localities were measured. In the reference carapace, the larger left valve also has the larger vestibule. A gerontic 8th instar has valves larger than those of the previous instars, but its vestibula are smaller than those of the 7th instar. The vestibula of the left valves of the 7th instar collected at discrete localities showed consistent differences in size related to the oxygen content of the sampling site. The relationship between vestibule size and oxygen content is inversely proportional. However, the poor correlation factor (R2<<1) of these results are not precise enough to support Peypouquet's hypothesis. But the size of the vestibula of K. gnoma does show some degree of relationship to oxygen content. So it is important to measure the vestibula of species of Krithe using the procedure described here. Rigorous application of this procedure will eliminate the possibility of discrimination based on variations in size due to ontogeny and dimorphism leaving only those related to actual valve size. In any case, variations in the size of vestibula may have a phenotypic origin.