Heterochrony in Haplomesus (Crustacea: Isopoda: Ischnomesidae): Revision of two species and description of two new species

Two new species of Ischnomesidae, Haplomesus celticensis sp. nov. and Haplomesus hanseni sp. nov. are described from the southwest of Ireland and the Argentine Basin respectively. Both species lack the expression of pereopod VII, a characteristic that we argue is produced by progenesis, not neoteny as suggested by Brökeland & Brandt (2004). Haplomesus angustus Hansen, 1916 and Haplomesus tropicalis Menzies, 1962, also lack pereopod VII and are revised from the type material. The original description of Haplomesus angustus Hansen, 1916 describes the adult type specimen as a juvenile; the original description of Haplomesus tropicalis Menzies, 1962 fails to mention the lack of pereopod VII. Progenesis is discussed for the above species and within the family Ischnomesidae as a whole.