mesopelagic Syllabification: (mes·o·pe·lag·ic)Pronunciation: /ˌmezəpəˈlajik, ˌmē-/Definition of mesopelagic adjective Biology (of fish and other organisms) inhabiting the intermediate depths of the sea, approximately 650-3,300 feet (200-1,000 m) below the surface. mesopelagic in other Oxford dictionaries Definition of mesopelagic in the British & World English dictionary