A set of related species living in different habitats is suitable to evaluate various adaptive characters to habitats, particularly in a such set where plural genera are recognized. Functional morphology of ostracods, especially their carapace shape, is thoroughly studied here through observation of the different mode of behavior between phytal and bottom-dwelling species of Loxoconcha and some other genera in seagrass beds. The phytal species living on smooth leaf-surface of seagrasses such as Zostera have vertically compressed carapaces with blade-shaped ventral part. On the contrary, the bottom-dwelling species in Zostera beds are cubic with flat ventral plane or streamlined in the carepace shape. The characteristics of the phytal is directly related to the style of copulation which is adopted (ventral-to-lateral position) to do not slip down to the bottom during copulation. The bottom-dwelling species copulate in ventral-to-ventral position and their shapes of the ventral part seem to be controlled by not the copulation style but the mode of behavior whether more epibenthonic or inbenthonic. Functional characters of soft parts, such as sexual organ and the tip of the appendages, are also documented, and the significance of analyses of “breeding”, in general, is emphasized to understand Ostracoda.