Even though previous researches have been led on the acid-base balance of the coelomic fluid of sea urchins, most of them were carried out on regular euechinoids. To this date, very little is known on the acid-base status of cidaroids and irregular euechinoids. We have investigated this status within regular euechinoids, irregular euechinoids, and cidaroids from different regions including the temperate intertidal zone, the tropical subtidal zone and the Antarctic waters. Our survey pointed out the same pattern within all regions: 1) Regular euechinoids have a coelomic fluid (CF) pH of about 7.2, an increased buffer capacity of the CF due to an increased concentration of bicarbonate ions and high molecular weight compounds; the HCO3- ions most probably originate from the surrounding seawater (witnessed by a similar isotopic signature of the carbon), 2) Irregular euechinoids and Cidaroids have a CF pH of about 7.0, a buffer capacity and a concentration of bicarbonate ions of the CF similar to that of seawater and an isotopic signature of the carbon different from that of the surrounding seawater. This pattern is quite regular in its occurrence and shows a difference of baseline physiology within the different echinoid groups. These differences lead to different responses when these animals face hypercapnia and/or acidosis, resulting in different sensitivities. More research on the physiology and acid-base balance and regulation needs to be undertaken in order to understand the occurrence of sea urchins in variable or unfriendly environments, as well as how they will be able to cope with climate change.