Macrostomum quiritium Beklemischev, 1951

Reyes, Jhoe & Brusa, Francisco, 2017, Species of Macrostomum (Macrostomorpha: Macrostomidae) from the coastal region of Lima, Peru, with comments on M. rostratum Papi, 1951, Zootaxa 4362 (2) : -

Macrostomum quiritium Beklemischev, 1951

Localities. The specimens were found in the sampling sites coded as CR-4 (11°44’1.8’’S; 76°58’19.56’’W), CR-5 (11°44’4.38’’S; 76°58’20.52’’W) and CR-6 (11°43’30.9’’S; 76°57’54.1’’W)( Figure 1 , Table 1) in the Chillón River in Lima.

Other localities in the world. The species has a worldwide distribution; it occurs in Russia, Poland, Switzerland ( Schärer et al. 2011) and Argentina (Río de la Plata River and Paraná River) ( Adami et al. 2016).

Studied material. Ten specimens observed alive (squash method) and 20 specimens (MUSM 3414–3433) fixed in polyvinyl-lactophenol.

Description. Adult specimens are whitish in reflected light. The body is 0.9–1.9 mm long and 0.3–0.4 mm wide. The whole body is covered by cilia. Large amounts of homogeneously distributed rhabdites are observed on the surface of the body. The anterior region of the body has a rounded shape, while the posterior region has the shape of a rounded spatula. The mouth is just behind the kidney–shaped eyes, with many cilia in its outline. The simplex pharynx has sac-shaped pharyngeal glands that are arranged beside and behind the pharynx. The intestine occupies 3/4 of the total length of the body and extends almost to the posterior region of the body.

Male reproductive system: Two oval–shaped testes arranged at the sides of the body, behind the beginning of the intestine and anterior to the ovaries ( Figure 4A ). The copulatory apparatus is located posterior to the intestine, and has a false thin–walled seminal vesicle, which stores the spermatozoa. It is continued through a thin intervesicular duct with the seminal vesicle, which has thick muscular walls and is smaller than the false seminal vesicle ( Figure 4B–C ). The prostatic vesicle is larger than the seminal vesicle and communicates with it through its proximal end. Within the prostatic vesicle, granulations are observed. These are grouped in small and long sacs that distally enter the stylet. The stylet is tubular with a slight curvature. The length of the stylet is 144.9 µm (99.6– 181.9 µm; n = 20; sd = 21). The proximal base of the stylet is 25.7 µm wide (15.1–34 µm; n = 20; sd = 5.6) and narrows towards the distal opening, which is 5.4 µm wide (3.9–6.5 µm; n = 20; sd = 0.7) ( Figure 4C–E ). The distal tip of the stylet presents a slight curvature, without lobulation or expansion, but the walls of the stylet in the distal region terminate in a very slight slope ( Figure 4D–E ). The stylet protrudes through the male gonopore, which is covered with cilia and is located in the posterior part of the body, behind the female gonopore.

Female reproductive system: The ovaries are approximately located at the mid-part of the body and are arranged lateroventrally to the intestine. The female genital atrium is located anterior to the false seminal vesicle of the male reproductive system and ventral to the intestine. The atrium is lined with abundant cells corresponding to the cellular valve ( Ladurner et al. 2005) and in some cases, spermatozoa were observed anchored to this epithelium. The atrium communicates immediately with the female gonopore. In some adult specimens, egg formation was observed in the atrium ( Figure 4A ).

Remarks. The specimens described here represent the first record of M. quiritium for Peru, and the second for the Neotropical Region. M. quiritium has a tubular- or “J”-shaped penis stylet ( Beklemishev 1951; Adami et al. 2016), which resembles 16 species of Macrostomum ( M. amurense Beklemischev, 1950 , M. axi Papi, 1959 , M. balticum Luther, 1947 , M. bulbostylum Kepner & Stiff, 1932 , M. christinae Young, 1976 , M. coxi Young, 1976 , M. curvituba Luther, 1947 , M. dongyuanensis Wang & Sun, 2015 , M. georgeense Young, 1976 , M. longistylifernum Ax, 1956 , M. lignano Ladurner et al., 2005 , M. magnacurvituba Ax, 1994 , M. parmum Ball, 1977 , M. sinensis Wang, 2005 , M. spec 1 Ax, 2008, and M. spec 2 Ax, 2008). Among them, five species, namely M. axi , M. balticum , M. sinensis , M. spec 1 and M. spec 2, have the distal part of the stylet ended in a sharp point. In contrast, M. quiritium has the stylet with its distal region terminating in a very slight slope. The stylet of M. bulbostylum , M. christinae , M. coxi , M. curvituba , M. longistylifernum , M. lignano , and M. magnacurvituba has notably thickened walls at its distal end, which is not observed in the distal part of the stylet of M. quiritium found at the Chillón River. As for M. amurense , M. georgeense and M. parmum , the penis stylet is clearly bent; in contrast, in M. quiritium , the distal part of the penis stylet is slightly bent ( Young 1976, 2001; Sun et al. 2015).

The internal morphology, i.e., the penis stylet, of M. dongyuanensis is very similar to that of M. quiritium ; however, M. dongyuanensis has a “U”-shaped prostate vesicle ( Sun et al. 2015), while M. quiritium has a sacciform prostate vesicle ( Beklemishev 1951; Adami et al. 2016). Moreover, the shape of the stylet and the fact that the female atrium is lined with a cellular valve suggest reciprocal insemination ( Schärer et al. 2011).

M. quiritium was found associated with algae such as Spirogyra sp., Hydrodictyon sp., Enteromorpha sp. and Cladophora sp. The average water temperature was 24.3 °C, the pH was 8.34 and the conductivity was 0.85 mS/cm ( Table 2).

Sites Conductivity pH Temperature Type of Type of vegetation Other invertebrates in the (mS/cm) (C°) bottom sites

Figures

FIGURE 1. Sample sites where species of Macrostomum were found. A, M. quiritium and M.tuba in Chillón River (sites 4, 5 and 6). B, M. rostratum in Humedales de Ventanilla (sites 1, 2 and 3).

FIGURE 4. Photographs of a live specimen of Macrostomum quiritium Beklemischev, 1951. A, Dorsal view of a mature specimen; B–C, Posterior body region with details of the copulatory organs observed in dark and normal field; D–E, Photograph and schematic reconstruction of the penis stylet.