Biserova N.M.1*, Bannikova M.A.1, Nikolaeva O.V.2, Aleoshin V.V.2,3 2025. Do eutardigrades have a copulatory organ? The case of Halobiotus crispae Kristensen, 1982 (Tardigrada: Halobiotidae) // Invert. Zool. Vol.22. No.1: 8–20 [in English].
1 Invertebrate Zoology Department, Lomonosov Moscow State University, Leninskie Gory 1-12, Moscow 119234 Russia.
2 Belozersky Institute of Physicochemical Biology, Lomonosov Moscow State University, Leninskie Gory 1-40, Moscow 119991 Russia.
3 Institute for Information Transmission Problems (Kharkevich Institute), Russian Academy of Sciences, Bolshoy Karetny per. 19-1, Moscow, 127051 Russia.
Natalia Biserova: nbiserova@yandex.ru ORCID https://orcid.org/0000-0002-5481-0207
Maria Bannikova: bannikova_mariya_96@mail.ru ORCID https://orcid.org/0000-0002-5192-0873
Olga Nikolaeva: olga_popova92@inbox.ru ORCID https://orcid.org/0000-0003-2424-298X
Vladimir Aleoshin: aleshin@genebee.msu.su ORCID https://orcid.org/0000-0002-3299-9950
* Corresponding author
doi: 10.15298/invertzool.22.1.01
ABSTRACT: Reproductive strategies of tardigrades vary considerably among taxa. Species of Heterotardigrada have separate gonopore and anus, whereas Eutardigrada species have a single cloacal opening, which is a shared exit from the cloaca, hindgut, and Malpighian tubules. While internal fertilization has been hypothesized for certain species based on the presence of spermatozoa in female spermathecae, the mechanism of sperm transfer remains undocumented. Notably, sexual dimorphism has not previously been reported in eutardigrades.
In this study, males and females of the eutardigrade Halobiotus crispae Kristensen, 1982 were collected from sand samples in the intertidal zone of Olenevsky Island, White Sea, and analyzed using molecular, electron-microscopy and immunocytochemical methods. Our findings advance the understanding of H. crispae reproductive biology, including the first documented evidence of sexual dimorphism in eutardigrades. Males possess a protrusible cloacal cylinder, hypothesized to function as a copulatory organ. Distinct morphological differences in cloacal structure were observed in two sexes. Males have oval-shaped and smaller cloaca with smaller cloacal plates; females have larger, diamond-shaped cloaca, with acute-triangular cloacal plates. Additionally, sensory organs and musculature near the cloacal plates and in the fourth pair of legs have been found which suggest neural regulation during mating, potentially guiding copulatory interactions. The proposed role of the male cloacal cylinder as a copulatory organ supports the hypothesis of internal fertilization in this group, changing the current understanding of reproductive strategies in tardigrades. These findings underscore the need for deeper exploration of tardigrade reproduction, particularly to clarify the prevalence and mechanisms of internal fertilization across species.
KEY WORDS: Tardigrada, sexual dimorphism, reproduction, fertilization, molecular genetics, scanning electron microscopy, immunocytochemistry.