Macrostomum cf. rubrocinctum Ax, 1951

Schockaert, Ernest R., 2014, Marine Macrostomorpha (Platyhelminthes, Rhabditophora) from the Algarve (Southern Portugal), Zootaxa 3872 (5), pp. 577-590 : 579-580

Macrostomum cf. rubrocinctum Ax, 1951

Fig. 1 C

Localities in the Algarve. Faro ( Portugal), Ria Formosa, at the entrance of the so-called Garganta channel. Higher littoral, in the outlet of a tidal pool with very coarse shell debris, mixed with coarse sand, detritus and silt (13 September 2011).

A B c b D Α ⊂ Material. One whole mount (HU nr. VI.3.19).

The stylet of this species is 73 µm long (or 65 µm measured along line b), the proximal opening 35 µm wide, the stylet tapering to 5 µm width at some 55 µm, where it bends over 90° to the distal end. The terminal opening is at the convex side, about 10 µm wide.

The shape and size of the stylet correspond with those in M. rubrocinctum (50–75 µm in Ax 2008). However, since no data is available on the living animal, and the pigmentation of the head, which is characteristic for this species, is not seen in the whole mount with certainty, I hesitate to assert that this species indeed belongs to M. rubrocinctum . The shape of the stylet is also very similar to that of M. (Archimacrostomum) sublitorale Faubel & Warwick, 2005 , but the measurements do not correspond: those authors give a stylet length of 105 µm (to compare with the length along line b), much longer than the stylet in the Algarve specimen. As thoroughly discussed by Papi (1959) the genera Archimacrostomum and Inframacrostomum , as proposed by Ferguson in 1954, are not acceptable.

The three species found in the Ria Formosa belong to the group of species with a hook-shaped stylet. Schärer et al. (2011) mapped a number of reproductive traits on the molecular phylogeny of 16 species of Macrostomum , five of which have such a hook-shaped stylet. It appears that two species with an nearly identical stylet are not closely related to each other: M. hystricinum marinum Rieger, 1977 from the open beach with high salinity of Grado near Venice ( Italy) and M. hystrix Ørsted, 1843 sensu Luther, 1905 from an irrigation channel with low and variable salinity in the same area. The authors consider it very difficult—if not impossible—to identify some of these species based on stylet morphology alone. Since insufficient data on the living animals, and no molecular data are available, I do not want to add to the already existing taxonomic confusion in this group of species by naming the species from the Ria Formosa (see e.g. the discussion by Schärer et al. (2011) on M. hystrix ).