The lancelet Branchiostoma lanceolatum Pallas, 1778, is the only leptocardian species that appears in all the European seas, everywhere in the Mediterranean, in all areas of the Adriatic Sea, and in the Black Sea. Due to the unique environmental conditions Adriatic lancelets were noted most in the northern part of Venice Bay. In other parts of the eastern Adriatic Sea lancelets have been recorded rarely, and always at locally well limited sites. Our records in the Mljet National Park seem to be the first in the island’s waters. Branchiostoma was located at four stations only, at depths of from 1 to 40 metres.

Kopljača Branchiostoma lanceolatum Pallas, 1778, jedini je naš predstavnik razreda Leptocardia. Nalazimo je u svim morima Europe, pa tako i kod nas. Najčešća je u sjevernom Jadranu, a drugdje njezina su naselja vrlo ograničena. Našim je nalazima utvrđeno prisustvo ove vrste u posebnim uvjetima podmorja Nacionalnog parka Mljet. Na nju su ronioci naišli na četiri mjesta, na dubinama od 1 do 40 m.