Macrostomum tuba Graff, 1882

Reyes, Jhoe, Binow, Daniela, Vianna, Rogério T., Brusa, Francisco & Martins, Samantha E., 2021, Fig. 12. Majella pristis n in Periclimenaeus djiboutensis Bruce 1970, Zoological Studies 60 (22), pp. 1-33 : 10-14

Macrostomum tuba Graff, 1882 ( Fig. 3J )

Synonyms: Macrostomum tuba gigas Okugawa, 1930 .

Macrostomum tuba bulbostylum Ferguson and Jones, 1940 . Macrostomum tuba minuta Luther, 1947 .

Macrostomum tuba verbekei Marcus and Marcus, 1957 .

Studied material: A single live whole-mounted individual studied ( MZUSP PL 2235).

Localities: Site 1 (1/07/2019), associated with phytal microhabitat.

Description: Macrostomum tuba is a well-known species due to its simple and recognizable penis stylet. The slightly curved shaft is tube-shaped (209.2 μm long) and it narrows from the base (12.2 μm wide) to the distal tip (5.5 μm wide). Moreover, the distal tip has bulb-shaped thickened walls, which is a distinguishable feature from other species of Macrostomum ( Fig. 3J ).

Taxonomic remarks: The stylet morphology of this species may vary according to body size, being straight in smaller animals and slightly curved in larger animals ( Reyes and Brusa 2017) as it has seen in other regions of the world ( Hyman 1936; Ferguson 1939 1940; Papi 1951; Du Bois-Reymond Marcus 1957; Young 1976).

Distribution: Worldwide distributed (cosmopolitan) (Ferguson 1940; Marcus 1946; Papi 1951; Hyman 1955; Young 1976; Noreña et al. 2003; Gamo and Leal-Zanchet 2004; Wang et al. 2004; Adami et al. 2016; Reyes and Brusa 2017).

Macrostomum itai n. sp. Reyes and Brusa ( Figs. 4–5 ) urn:lsid:zoobank.org:act:0EACDBC3-FC14-47B0-8AE1-4D9FA7BADBDD

Studied material: Holotype: one mounted specimen ( MZUSP PL 2193 ) . Paratypes: three mounted specimens ( MZUSP PL 2194–2196).

Type locality: Site 1 (32°33'18.07"S; 52°31'18.50"W) (19/11/2018), associated with benthal microhabitat at the ESEC Taim.

Other Localities: Site 1 (19/11/2018; 1/07/2019) and Site 2 (9/10/2018), associated with benthal and floating vegetation microhabitats.

Etymology: The name of this species is originated from the name of the goddess Itaí. This deity was adored by the first indigenous people (Charrúas, Minuanos and Guaranís) who lived in the southern region of Brazil and, as the legend says, gave the name to the Taim area ( Crivellaro et al. 2004).

Diagnosis: Prostatic vesicle globular or invert “U”-shaped, with globular prostatic glands presenting small refractive corpuscles. Penis stylet “J”-shaped, base with thick walls, gradually becomes thinner until half of the stylet length. Stylet base is ± 17 μm in length, the shaft gradually narrows ending in a curve in hook-shaped. Penis stylet opening is subterminal, on the convex side of the curvature of the stylet.

Description: Body of living mature individual 1138.9 ± 185.3 μm (957.4–1327.8 μm; n = 3) long and 328.9 ± 104.3 μm (211.6–411.1 μm; n = 3) wide, greatest width in the mid-length. Body dorsoventrally flattened (adult) ( Figs. 4A , 5A ). Anterior end rounded, somewhat truncated. Posterior region of body spatulashaped, narrow. When adhered to the substratum the posterior end is truncated. Adhesive papillae present. The body is whitish in reflected light ( Figs. 4A, D– E , 5A, D ). The whole-body surface is covered by the epidermis of ± 10 μm thick. Epidermis with abundant locomotory cilia. Sensory cilia sparsely distributed ( Fig. 5D ). Rhabdites are ± 10.86 μm in length, rod-shaped. Three-to-ten packed rhabdites located through the body, especially in the dorsal surface. Cerebral ganglia paired at the anterior region of the body, linked by a weak commissure, sometimes, difficult to observe ( Figs. 4B , 5A ). Two rounded eyes, overlapping to the cerebral ganglia, separated each other by 50.2 ± 11.8 μm (40– 63.12 μm; n = 3) ( Figs. 4A, D–E , 5A, D ). A mouth is 67.8 ± 22.6 μm (42.2–85 μm; n = 3) long, posterior to the eyes, opening to the pharynx simplex, which is surrounded by several thin pear-shaped pharynx glands ( Figs. 4B , 5A ). Blind-sac intestine, which extends towards almost the posterior end of the body.

Male reproductive system ventral. Two elongated testes, 287 ± 50.4 μm (204.7–322.4 μm; n = 5) long, are located at the anterior region of the body, with a smooth surface, they are posterior to the pharynx and latero-ventrally to the intestine ( Figs. 4A, C , 5A, D ). A deferent duct was not observed. Seminal vesicle globular, 38.6 ± 6.1 μm (31.6–43.2 μm; n = 3) in diameter, muscular thick-walled, on the central body axis, posterior to the female antrum ( Figs. 4A, E , 5A, C–D ). A seminal vesicle, full of spermatozoa, connects through a thick and short intervesicular duct to a smaller muscular prostate vesicle. Prostatic vesicle globular or invert “U”-shaped, 21 ± 8.2 μm (12–27.9 μm; n = 3) in diameter ( Fig. 5C ). Globular prostatic glands inside the prostate vesicle, presenting small refractive corpuscles without a particular arrangement. Also, digitiform glands in the distal region of the prostate vesicle ( Fig. 5C ). Stylet total length is 47.3 ± 9.6 μm (35–56.4 μm; n = 4) (“a” sensu Schockaert (2014)), whereas the total length from the distal tip of the stylet to the base is 46.7 ± 9.5 μm (34.9–57.5 μm; n = 4). The ratio of the proximal diameter (base of the penis stylet) to the stylet length is 36%. Penis stylet in “J”-shaped, base with thick walls, gradually becomes thinner until almost half of the stylet length ( Figs. 4F , 5B–C ). Proximally, the stylet has a slightly crenelated base. Stylet base is 17 ± 3.4 μm (13–21.4 μm; n = 4) in length, the shaft gradually narrows ending in a curve in hook-shaped, which presents an angle of ~ 73.1 ± 17.8° (50.5–91°; n = 4) (sensu Ferguson (1940)). Penis stylet opening is 3.6 ± 0.7 μm (2.8–4.4 μm; n = 4) long, subterminal, on the convex side of the curvature of the stylet ( Figs. 4F , 5B ).

The female reproductive system. Ovaries are 236.1 ± 17.5 μm (214–253.5 μm; n = 4) long, posterior to the testes on both sides of the body ( Figs. 4A, C–E , 5A, D ). Big light-brown mature oocytes can be observed containing abundant yolk. Female antrum located ventrally to the intestine and surrounded by numerous cement glands. The female antrum is connected to the ventral female gonopore ( Fig. 5A ).

Taxonomic remarks: As it was stated by Ferguson (1954) and Rieger (1977), the most remarkable feature for species identification within Macrostomum is the penis stylet. So, we discuss on the 13 species whose penis stylet resembles the morphology of the male copulatory structures of M. itai n. sp. ( Table 1).

The above-mentioned species have a “J”-shaped stylet. However, some species within this group have a false seminal vesicle ( M. axi Papi, 1959 , M. balticum Luther, 1947 , M. majesticis Schmidt and Sopott-Ehlers, 1976 , M. shenda Wang and Xin, 2019 , M. thingithuense Young, 1976 ), while M. itai n. sp. lacks this vesicle. Moreover, these species present the total stylet lengths further longer than in M. itai n. sp. In addition, the lengths of the penis stylet base of M. majesticis and M. thingithuense ( Papi 1959; Schmidt and Sopott-Ehlers 1976; Young 1976) are shorter than in M. itai n. sp.

A comparison of the length from the distal tip of the stylet to the base present notable differences between M. distinguendum Papi, 1951 and M. barigoense Young, 1976 , when compared to M. itai n. sp. Also, M. distinguendum , M. barigoense and M. kepneri Ferguson and Jones, 1940 present total lengths of the stylet longer than M. itai n. sp. Also, dissimilarities are observed when comparing the distal penis stylet opening, in M. distinguendum ( Papi 1951 1953; Ax 2008), M. barigoense ( Young 1976) and M. kepneri ( Ferguson and Jones 1940) the lengths are higher than in M. itai n. sp.

Macrostomum species 2 Schockaert, 2014 and M. troubadicis, Schmidt and Sopott-Ehlers, 1976 , both inhabitants of marine environments ( Schmidt and Sopott-Ehlers 1976; Schockaert 2014) are also similar to M. itai n. sp. The distal tip of the stylet of M. species 2 spirals in a counter-clockwise direction ( Schockaert 2014), while the distal tip of M. itai n. sp. comes from a conical straight stylet (“J”-shaped) and simply bens to a hook-shaped tip. Further, in M. species 2 the distal stylet opening was not observed ( Schockaert 2014), in M. itai n. sp. it is present on the convex side. As for M. troubadicis , it presents a triangular seminal vesicle ( Schmidt and Sopott-Ehlers 1976) which is not observed in M. itai n. sp. Besides, M. troubadicis has a slightly “S”-shaped stylet with the base of the stylet of 4–5 μm long ( Schmidt and Sopott-Ehlers 1976), much smaller than in M. itai n. sp. Also, the distal end of the stylet of M. troubadicis is truncated, while M. itai n. sp. has its distal end hook-like.

Macrostomum silesiacum Kolasa, 1973 is different from M. itai n. sp. due to: the first has a slightly spirallytwisted stylet, a rounded distal tip and the distal stylet opening is on the concave side ( Kolasa 1973b); while the new species, lacks these features. Compared with Macrostomum schmitti Hayes and Ferguson, 1940 , the main difference is observed in the length of the penis stylet opening, being M. schmitti bigger than M. itai n. sp. Moreover, the base of the stylet of M. schmitti is highly crenate ( Hayes and Ferguson 1940), whereas M. itai n. sp. is slightly crenelated.

Also, Macrostomum platensis Adami, Damborenea and Ronderos, 2012 , is similar to the new species, but the distal end of the stylet of M. itai n. sp. bends to a hook-shaped tip, while the distal end of the stylet of M. platensis is beveled ( Adami et al. 2012). As so, the morphological differences observed, principally, in the penis stylet of the 13 above-mentioned species indicate that M. itai n. sp. is a new species within the genus Macrostomum .

Distribution: Southern Brazil, Rio Grande do Sul state, Taim strict nature reserve (ESEC Taim).

Figures

Fig. 3. Live specimens of Stenostomidae and Macrostomidae. (A–C) Stenostomum cf. pegephilum: (A) chain with two zooids, (B) schematic representation, (C) detail of the muscular pharynx. (D1–2) Stenostomum saliens: (D1) detail of the anterior region of the body and (D2) dorsal view of the habitus of a specimen. (E) Stenostomum simplex (scale bar not available). (F–G) Stenostomum tuberculosum: (F) individual with three zooids, (G) schematic representation. (H–I) Macrostomum quiritium: (H) dorsal view of a mature specimen, (I) detail of the penis stylet. (J) Macrostomum tuba, detail of the penis stylet.

Fig. 4. Live specimen of Macrostomum itai Reyes and Brusa n. sp. (A) Dorsal view of a mature individual. (B) Detail of the anterior body region. (C) Detail of the testes and ovary. (D) A free-swimming specimen. (E) Posterior body region with details of the copulatory organs. (F1–2) Detail of the penis stylet in two different focal planes (black arrowhead indicating the penis stylet opening).

Fig. 5. Macrostomum itai Reyes and Brusa n. sp. (A) Schematic reconstruction of the body. (B) Schematic representation of the penis stylet (from the holotype). (C) Detail of the male copulatory system. (D) Free-hand drawing of a swimming individual.