Three species of Pseudoprotella have been described so far: Pseudoprotella phasma (Montagu, 1804), Pseudoprotella inermis Chevreux, 1927 (redescribed by Guerra-García & Takeuhi, 2000) and Pseudoprotella bogisa (Mayer, 1903), which was recently transferred from the genus Noculacia to Pseudoprotella (see Guerra- García, 2002a). A detailed comparison of the three species is provided by Guerra-García (2002a). Pseudoprotella inermis is restricted to the Strait of Gibraltar area and North Atlantic coast of Morocco (Guerra-García & Takeuchi, 2000; Guerra-García et al., 2014) and it is characterised by total absence of dorsal body projections. Pseudoprotella bogisa has been collected so far only from Thailand waters (McCain & Steinberg, 1970; Guerra- García, 2002a) and Pseudoprotella phasma [sensu lato] is distributed along Mediterranean Sea and Atlantic Ocean.
Pseudoprotella phasma was initially described by Montagu (1804) as Cancer phasma. Mayer (1882) transferred it to the genus Pseudoprotella and included description and figures of the species. Mayer (1890, 1903) considered four forms (f. typica, f. minor, f. quadrispinis and f. bispinis) based on differences in the arrangement of body dorsal projections and differences in the male gnathopod 2. Chevreux & Fage (1925) and Krapp-Schickel (1993) provided a key to identify the four varieties. Recently, Zeina et al. (2015) studied material from Azores, reporting the forms typica and quadrispinis. They found distinct and constant differences between these two forms. Therefore, they pointed out that these two forms deserve the rank of distinct species and probably, rank of species should also be assigned to forms minor and bispinis. Guerra-García (2004) figured a female of P. phasma [sensu lato] from deep-sea waters of Azores that seem to be closer to the forms typica or minor. The two species found during the present study (figs. 22 and 23) are very similar in having a dorsal projections pattern of 1-1-2+1 (as f. typica and minor) but can be clearly distinguished by the following characteristics: (1) gills of Pseudoprotella sp. 1 are medially curved while in Pseudoprotella sp. 2 are elongate; (2) pereopods 3 and 4 are about ¼ of gills length in P. sp. 1 while they are minute in P. sp. 2; (3) pereonite 3 is provided with anterolateral acute projections in P. sp. 1, which are absent in P. sp. 2; (4) P. sp. 1 is considerably smaller (adult males shorter than 12 mm) than P. sp. 2 (adult males longer than 16 mm) and (5) P. sp. 1 pereonites are covered by fine setulae, which are absent in P. sp. 2. Both species are closer to f. typica, but they are totally different from the f. typica described by Mayer (1890, 1903) and figured by Zeina et al. (2015) from shallow waters of Azores. Hence, P. phasma [sensu lato] is probably including at least six different species, which are still waiting for description and/or new appraisal of species ranking. A detailed morphological and molecular study is, therefore, mandatory to clarify the taxonomical status of P. phasma [sensu lato] from shallow waters of Mediterranean and Atlantic, as has been done for other species such as Caprella californica [sensu lato] (Takeuchi & Oyamada, 2013).
Pseudoprotella phasma is normally associated with hydroids (Guerra-García et al., 2014; Zeina et al., 2015: fig. 18).