Echinocardium cordatum is likely the most widely distributed extant echinoid. The species status remains, however, puzzling as it includes several morphotypes that are not all geographically well delimited. It encompasses a cryptic species complex of at least five clades some of them partly living in sympatry and hybridizing. E. cordatum lives infaunally, buried up to 20 cm depth in the sediment and feeds selectively on particular organic matter occurring in the sediment. Burrowing, burrow construction, respiration, and food gathering are realized by specialized regions and appendages leading to a remarkable functional regionalization of the body. Moving while buried, E. cordatum contributes significantly to bioturbation by mixing sediment, enhancing seawater circulation in the sediment and recycling of the organic matter. Its reproductive cycle is annual. In European seas, the spawning period varies with increasing latitudes (end of the winter in Mediterranean Sea to end of summer off Sweden). Metamorphosis of the competent larvae relies on still unidentified cues emitted by the sediment but recruitment may be synchronized over wide areas. Growth rate rather than increased longevity determine the size of adult individuals. E. cordatum hosts mutualistic bacteria in digestive caeca and parasitic protozoans in its coelomic cavities while commensals (mollusks and crustaceans) live in its burrow.