The diploid number and karyotype of three endemic and highly distributed species from the Qatari shores: Brachidontes variabilis (Krauss, 1848) (Mytilidae), Isognomon nucleus (Lamarck, 1819) (Pteriidae) and Neotrapezium sublaevigatum (Lamarck, 1819) (Trapezidae), are described for the first time. Brachidontes variabilis presented a diploid number of 2n = 30 and the karyotype included 2 metacentric, 6 submetacentric, 3 subtelocentric and 4 telocentric chromosome pairs. Isognomon nucleus had a diploid number 2n = 28 and the karyotype was comprised of 3 metacentric, 4 submetacentric and 7 subtelocentric chromosomal pairs and the karyotype of Neotrapezium sublaevigatum, with 2n = 36, included 5 metacentric, 9 submetacentric, 2 submetacentric/subtelocentric and 2 subtelocentric chromosome pairs. A review of the previously published cytogenetic studies in all three families was also performed, as well as a comparative karyological analysis within those families.