Introduction
⌅The family Polystomatidae, which includes parasitic flatworms, is represented by two species in Cuba: Neopolystoma orbiculare (Stunkard 1916) and Polystoma stellai Pérez Vigueras 1955 (see Pérez Vigueras, 1955Pérez Vigueras, I. (1955). Contribución al conocimiento de la fauna helmintológica cubana. Memorias de la Sociedad Cubana de Historia Natural “Felipe Poey”, 22, 21-71.; Coy & Lorenzo, 1982Coy, A. and Lorenzo, N. (1982). Lista de los helmintos parásitos de los vertebrados silvestres cubanos. Poeyana, 235, 1-57.). Neopolystoma orbiculare parasitizes the freshwater turtle Trachemys decussata (Gray 1831), while P. stellai is known to parasitize the tree frog Osteopilus septentrionalis (Duméril & Bibron 1841). In total, about 253 parasitic flatworms have been recorded from Cuba, comprising 72 cestodes, 34 monogeneans, and 146 trematodes, distributed across marine, freshwater, and terrestrial environments (Coy & Lorenzo, 1982Coy, A. and Lorenzo, N. (1982). Lista de los helmintos parásitos de los vertebrados silvestres cubanos. Poeyana, 235, 1-57.; Mendoza-Franco et al., 2006Mendoza-Franco, E., Vidal-Martı́nez, V., Cruz-Quintana, Y. and Prats, F. (2006). Monogeneans on native and introduced freshwater fishes from Cuba with the description of a new species of Salsuginus Beverley-Burton, 1984 from Limia vittata (Poeciliidae). Systematic Parasitology, 64, 181-190.; Fernández et al., 2015Fernández, R.A., Corrada, R.I. and Armenteros, M. (2015). Variaciones de la infracomunidad parásita durante la ontogenia de Chromis cyanea (Perciformes: Pomacentridae) en la costa norte de La Habana, Cuba. Revista de Biología Tropical, 63(3), 717-726.).
The genus Polystoma, a species-rich taxon within Monogenea, encompasses 67 species (WoRMS, 2024WoRMS (February 8, 2024). Polystoma Zeder, 1800. https://www.marinespecies.org/aphia.php?p=taxdetails&id=1287285 ). Generally, polystomatids are regarded as cryptic taxa with challenging morphological identification (du Preez et al., 2007du Preez, L.H., Verneau, O. and Gross, T.S. (2007). Polystoma floridana n. sp. (Monogenea: Polystomatidae) a parasite in the green tree frog, Hyla cinerea (Schneider), of North America. Zootaxa, 1663, 33-45.). Polystoma stellai exemplifies such enigmatic characteristics, being known only from two specimens - the first collected in La Habana, Cuba (Pérez Vigueras, 1955Pérez Vigueras, I. (1955). Contribución al conocimiento de la fauna helmintológica cubana. Memorias de la Sociedad Cubana de Historia Natural “Felipe Poey”, 22, 21-71.), and the second in Florida, USA (Stunkard, 1959Stunkard, H.W. (1959). Induced gametogenesis in a monogenetic trematode, Polystoma stellai Vigueras, 1955. The Journal of Parasitology, 45(4), 389-394.). These specimens were not stored in any known collection, and the species was even omitted from the list of helminthic parasites of Cuban vertebrates (see Coy & Lorenzo, 1982Coy, A. and Lorenzo, N. (1982). Lista de los helmintos parásitos de los vertebrados silvestres cubanos. Poeyana, 235, 1-57.); hence, knowledge of the species remains quite limited.
Recently, a second specimen of P. stellai from Cuba was collected, infecting its type host O. septentrionalis. This description offers a comparison between the newly collected specimen and the two previously recorded from La Habana and Florida, contributing valuable morphological data to enhance understanding of the species.
Material and methods
⌅Ten specimens of the anuran O. septentrionalis were collected by Biology students of Universidad de Oriente, during dissecting practices. During the examination of the animals, one parasitic flatworm was detected in one frog collected in the locality of Santa María (Fig. 1), Santiago de Cuba, Cuba (February 22, 2022). This flatworm was isolated and studied alive under a microscope. Finally, the specimen was preserved as a whole mount with lactophenol. Hard structures of the whole mounted specimen were examined with a Leica DM 2500 LED microscope, using Nomarski interference contrast. The identification of the host frog was made on the base of description provide by Díaz and Cádiz (2008)Díaz, L.M. and Cádiz, A. (2008). Guía taxonómica de los anfibios de Cuba. Abc Taxa.. Drawings were built on Inkscape 1.2.2. Measurements were taken along the central axis of the measured object. The voucher specimen was stored in the Museum of Nature Hamburg (ZMH).
Results and discussion
⌅Polystoma stellai Pérez Vigueras, 1955Pérez Vigueras, I. (1955). Contribución al conocimiento de la fauna helmintológica cubana. Memorias de la Sociedad Cubana de Historia Natural “Felipe Poey”, 22, 21-71.
⌅Known distribution
⌅La Habana, Cuba (Pérez Vigueras, 1955Pérez Vigueras, I. (1955). Contribución al conocimiento de la fauna helmintológica cubana. Memorias de la Sociedad Cubana de Historia Natural “Felipe Poey”, 22, 21-71.) and Florida, USA (Stunkard, 1959Stunkard, H.W. (1959). Induced gametogenesis in a monogenetic trematode, Polystoma stellai Vigueras, 1955. The Journal of Parasitology, 45(4), 389-394.).
Studied material
⌅One adult specimen collected in Santa María (20.059248, -75.807753), Santiago de Cuba, Cuba (February 22, 2022), studied alive and whole mounted (ZMH V13666).
Host
⌅Osteopilus septentrionalis (Duméril & Bibron 1841) (Amphibia, Anura, Hylidae); female specimen; urinary bladder.
Morphological description
⌅Specimen 3.94 mm long and 1.88 mm at its widest part (Fig. 2A). Pharynx (Figs. 2A-B: ph) 53 µm long and 43 µm at its posterior widest point. Digestive tract diverticulated with prehaptorial anastomoses. Haptor (Figs. 2A & 2C: h) 1,008 µm long and 1452 µm wide. Six haptoral suckers (Figs. 2A & 2C: s) measuring 257-286 µm in diameter (x̄ = 274 µm; n = 6). Anchor hooks (hamulus) (Figs. 2A & 2C: ha, 2D-E) unequal, 407-435 µm long and 217-287 µm at the widest proximal part. One of the hooks with a distal flap (Figs. 2D-E: f) 50 µm long and 35 µm wide. No marginal hooklets present.
Male genital system not observed. Ovary (Figs. 2A-B: ov) 344 µm long and 124 µm at its widest point, J-shaped. Vitellaria (Figs. 2A-B: vi) lie along the body sides and is better distinguishable in the anterior part of the animal. Paired vagina, only visible on the live specimen, open on each body side anterior to the ovary (Fig. 2: va). Other structures of the female system were not observed.
Remarks
⌅This represents the second documented specimen of P. stellai since its original description in 1955. Despite a lack of studies on the infection prevalence within this species, our suspicion of a very low infection rate is based on the scarcity of specimens collected over the 68 years following its description. Within our collected samples, a solitary frog exhibited infection (10%); however, since the frogs were collected from various locations, this infection rate cannot be generalised. The recently collected specimen was found approximately 800 km away from the original collection site in La Habana (Pérez Vigueras, 1955Pérez Vigueras, I. (1955). Contribución al conocimiento de la fauna helmintológica cubana. Memorias de la Sociedad Cubana de Historia Natural “Felipe Poey”, 22, 21-71.), with Stunkard (1959)Stunkard, H.W. (1959). Induced gametogenesis in a monogenetic trematode, Polystoma stellai Vigueras, 1955. The Journal of Parasitology, 45(4), 389-394. reporting the sole other sighting of the species in Florida.
Due to the limited number of known specimens for this species, its morphology remains somewhat mysterious. Specifically, the descriptions by Pérez Vigueras (1955)Pérez Vigueras, I. (1955). Contribución al conocimiento de la fauna helmintológica cubana. Memorias de la Sociedad Cubana de Historia Natural “Felipe Poey”, 22, 21-71. and Stunkard (1959)Stunkard, H.W. (1959). Induced gametogenesis in a monogenetic trematode, Polystoma stellai Vigueras, 1955. The Journal of Parasitology, 45(4), 389-394. lack detailed information regarding the morphology of the anchor hooks. In fact, Stunkard (1959)Stunkard, H.W. (1959). Induced gametogenesis in a monogenetic trematode, Polystoma stellai Vigueras, 1955. The Journal of Parasitology, 45(4), 389-394. only provided measurements of specimen length and width, along with sucker diameter (Table 1). Consequently, we present, for the first time, detailed photographs depicting the morphology of these structures. Although the male reproductive system was not observed in the most recent specimen, its well-developed ovary and vitellaria suggest adulthood. Probably, the used fixation method, typically used for free-living microturbellarians, damaged these structures; unfortunately, no other fixation method was available at that moment. Notably, the specimen from Santiago de Cuba is the smallest, yet the proportion of its body length to width resembles that of the specimen from La Habana (L/W = 2.10 vs. 2.73, respectively). Conversely, the specimen from Florida is notably more slender (L/W = 4.53). Given the scant description provided by Stunkard (1959)Stunkard, H.W. (1959). Induced gametogenesis in a monogenetic trematode, Polystoma stellai Vigueras, 1955. The Journal of Parasitology, 45(4), 389-394. and the significant size disparity of their specimen, we suspect the Florida population may represent a distinct species of Polystoma or other related genera.
| Characteristics | La Habana | Santiago de Cuba | Florida |
|---|---|---|---|
| Body L (mm) | 7.10 | 3.94 | 4.30 |
| Body W (mm) | 2.60 | 1.88 | 0.95 |
| Body L/W | 2.73 | 2.10 | 4.53 |
| Haptor L (µm) | 1400 | 1008 | - |
| Haptor W (µm) | 2100 | 1452 | - |
| Hamulus L (µm) | 480 | 407-435 | - |
| Suckers D (µm) | 350-380 | 257-286 | 290 |
| Ovary L (µm) | 880 | 344 | - |
| Ovary W (µm) | 530 | 124 | - |
| Cirrus spines L (µm) | 21 | - | - |
Remarkable differences in ovary length are observed between the specimens from La Habana (880 µm) and Santiago de Cuba (344 µm). However, this variance, along with differences in haptor and sucker sizes, may be attributed to the smaller body size of the latter specimen. Although the anchor hooks exhibit similar sizes between the two Cuban specimens (Table 1), we identified two previously unmentioned characteristics for these structures: one hook in the eastern Cuban specimen is larger and features a distal flap. It is important to note that du Preez et al. (2024: Fig. 4.111)du Preez, L.H., Landman, W.J. and Verneau O. (2024). Polystomatid Flatworms. State of Knowledge and Future Trends. Springer. purportedly presented a depiction of P. stellai drawn over the original illustration by Pérez Vigueras (1955)Pérez Vigueras, I. (1955). Contribución al conocimiento de la fauna helmintológica cubana. Memorias de la Sociedad Cubana de Historia Natural “Felipe Poey”, 22, 21-71.. However, upon examination, it becomes evident that the new drawing does not match the one provided in the original description but rather resembles the illustration by Stunkard (1959)Stunkard, H.W. (1959). Induced gametogenesis in a monogenetic trematode, Polystoma stellai Vigueras, 1955. The Journal of Parasitology, 45(4), 389-394.. This discrepancy, unfortunately, adds further complexity to the comprehension of this species. Therefore, we believe it is pertinent this clarification.
Some diagnostic traits observed in representatives of Polystoma, as detailed by Price (1939)Price, E.W. (1939). North American monogenetic trematodes. IV. The family Polystomatidae (Polystomatoidea). Proceedings of the Helminthological Society of Washington, 6(2), 80-92. and Schell (1970)Schell, S.C. (1970). The Trematodes. WM. C. Brown Company Publishers., such as the presence of numerous post-ovarian testes (currently consider a single follicular testis) and a short, preovarial uterus, have not been confirmed in P. stellai. Du Preez et al. (2024)du Preez, L.H., Landman, W.J. and Verneau O. (2024). Polystomatid Flatworms. State of Knowledge and Future Trends. Springer. presented a more comprehensive depiction of representatives of Polystoma, highlighting additional pertinent characteristics: a pyriform ovary; the uterus containing few operculated eggs; and a follicular, medial, posteriorly located testis. Based on previous findings and the current limited data available for P. stellai, it is not definitive to assert that this species unequivocally belongs to Polystoma. However, until further morphological and/or molecular evidence becomes accessible, the most cautious approach is to maintain the current classification of the species and regard both Cuban populations as part of the same species.
In accordance with the International Code of Zoological Nomenclature, we refrain from designating our specimen as a neotype (see Articles 75.3.5 and 75.3.6). Our discoveries contribute to a better understanding of the distribution and morphology of P. stellai. However, further investigations are essential to confirm the intraspecific variability of the species or ascertain whether multiple species of Polystoma inhabit Cuba.