However, his studies were later delayed due to World War I, where he fought as a soldier, and his second work, a thesis on Coleoptera, was published only in 1919, when he received his doctorate.

This offer came due to the efforts of the Society for the Protection of Science and Learning, Ltd, a group that tried to find jobs for displaced Jewish scientist.

He started to teach zoology at the University of São Paulo, occupying the chair that was vacant by the death of Professor Ernst Bresslau, and began studying the Brazilian bryozoan fauna.

Later works were published in English and covered invertebrate groups such as flatworms, annelids, onychophorans, nemertines, phoronids, gastropods, and pycnogonids.

[citation needed] Several species have been named after Ernst Marcus, such as the land planarians Luteostriata ernesti and Imbira marcusi.