Bioacoustics of Dugesia saccata

Discussion. There is one character that immediately sets D. saccata apart from all of its known congeners, the sac-shaped expansion on the posterior section of the bursal canal, near the knee-shaped bend of the canal. This is slightly reminiscent of a situation in Dugesia aethiopica Stocchino, Corso, Manconi & Pala, 2002 , in which the posterior section of the bursal canal, immediately before receiving the separate openings of the oviducts, is expanded in lateral direction and gives rise to several large folds ( Sluys 2007 ). However, this is merely a superficial resemblance to the situation in D. saccata , and these two species also differ in many other features. For example, D. aethiopica shows a horizontal approach of the bursal canal to the atrium, which represents a rare feature among species of Dugesia and is also absent in D. saccata . In Dugesia arabica Harrath & Sluys, 2013 the bursal canal is considerably expanded as well as highly folded in the region of the oviducal openings ( Harrath et al. 2013 ). However, this situation differs from that in D. saccata in that the expansion sits near the oviducal openings, whereas in D. saccata the sac-shaped expansion occurs dorsally, or entally to the openings of the oviducts into the bursal canal. Another difference between these two species concerns the presence of a duct between seminal vesicle and diaphragm in D. saccata and absence of such a duct in D. arabica . Two characteristic features of D. saccata may be found also in other species of Dugesia , a ventrally displaced ejaculatory duct with subterminal opening and the presence of a duct between the seminal vesicle and the diaphragm. Besides D. saccata , these two character states are also expressed, among others, in the three Chinese species D. majuscula , D. umbonata Song & Wang, 2020 , and D. verrucula Chen & Dong, 2021 ( Song et al. 2020 ; Wang et al. 2021 a , b ). However, in contrast to D. saccata , in both D. majuscula and D. umbonata the ejaculatory duct has a subterminal dorsal opening at the tip of the penis papilla, while D. verrucula exhibits a permanent dorsal bump near the root of the penis papilla, which is absent in D. saccata . Another difference concerns the presence of a large muscularised hump on the dorsal surface of the bursal canal of D. umbonata , which is absent in D. saccata . Although these three species ( D. umbonata , D. verrucula , and D. saccata ) belong to the same clade, they are molecularly well-differentiated. On the other hand, while D. saccata , D. pendula , D. tumida , and D. majuscula belong to the same small clade, they are anatomically well differentiated. In Dugesia species, the basic chromosome number concerns three types , 7, 8, and 9. Previous studies have shown that in China number 8 is the most frequent type, while 7 is much rarer ( Wang et al. 2024 ). In that respect, it is noteworthy that the basic chromosome number in D. saccata is n = 7, which is shared only with D. pendula , D. hepta Pala, Casu, & Vacca, 1981 , D. batuensis Ball, 1970 , and D. ryukyuensis Kawakatsu, 1976 ( Kawakatsu et al. 1976 ; Pala et al. 1981 ; Khang et al. 2017 ; Wang et al. 2024 ). However, D. pendula exhibits an aneuploid plus mixoploid karyotype, with diploid (2 n = 2 x = 14 + 0-1 B-chromosome) and triploid (2 n = 3 x = 21 + 0-1 B-chromosome) sets, while D. batuensis exhibits six metacentric chromosomes and one subtelocentric chromosome, and D. ryukyuensis shows six metacentric chromosomes and one submetacentric chromosome, which is the case also in D. hepta . In contrast, D. saccata exhibits a mixoploid karyotype with diploid (2 n = 2 x = 14) and triploid (2 n = 3 x = 21) sets, with all chromosomes being metacentric, thus contrasting with the chromosome complements of the other species. In fact, D. saccata produced infertile cocoons and only showed asexual reproduction by means of fission, which corresponds with its poorly developed or hyperplasic ovaries and the triploid chromosome complement. It has been established that in such abnormal ovaries the oocytes are anomalous, thus preventing regular oogenesis ( Harrath et al. 2014 ).