Biodiversity of Tunisian Planarians (Platyhelminthes, Tricladida)

• Saïda Tekaya and Fathia Zghal
• Département de Biologie - Université de Tunis El-Manar -Faculté des Sciences de Tunis.
  2092 El-Manar 1 - Tunis -Fax : 216 71 885 480 Email: Saida.Tekaya@fst.rnu.tn
• The Regional Workshop on Marine Sciences & Natural Resources 25-26 May 2004.

Abstract

This presentation is a first synthesis of researches made in Tunisia for the knowledge of free living platyhelminthes belonging to the Turbellaria class. In this paper we are interested to the Tricladida or Planarians. These animals have a phylogenetic importance since they are the lower triploblastics organisms presenting a cephalic structure. The aquatic species are bio-indicators of unpolluted water. Planarians have colonized all mediums and are ecologically divided into four Super Orders : Terricola, Cavernicola, Paludicola and Maricola. Our investigations were made on Paludicola or freshwater planarians and Maricola or marine planarians. Several stations are prospected. Planarians collected are identified and some aspects of their biology are studied mainly ecological features and reproduction modalities. The summary of our results is presented here (for more details, see references below) :

1. Fresh water triclads: Many asexual populations and only one sexualized one are found.

   - The asexual populations are signalized in many rivers (streams) of the north, the north-east, the medium and the south of the country (Tunisia). They belong to the Dugesia gonocephla group of species. All of them are fissiparous and reproduce only asexually. The fissioning cycle is studied under various conditions of temperature. In laboratory dark conditions, the induction of the sexualization is obtained for some populations with a variable and weak frequency. The structure of the induced copulatory apparatus has allowed us to identify the species Dugesia sicula. Sexualized specimens are kept in breeding for the study of eventual sexual reproduction (for more details see Charni et al, 2004).

   - The unique sexual population is identified as Schmidtea mediterrane. distribution and ecological data, are given (Harrath et al, 2004). Reproduction and development modalities are also studied (harrath in preparation).

2. Marine triclads : we have discovered four species of Maricola : Three hermaphroditic planarians, Procerodes littoralis, Procerodes lobata (Zghal et Tekaya, 1980) Cerbussowia cerruti ( Tekaya et al., 1996) and the gonochoristic Sabussowia dioica (Zghal et Tekaya, 1980). All of them reproduce sexually and are not fissiparous. P. lobata and S. dioica have a good regenerative power when they are cutted transversally at the level of the prepharyngeal area (Zghal & Tekaya, 1982). S. dioica is the unique and exceptional gonochoristic species. The geographic distribution of this particular and scarce species is given (Zghal & Tekaya, 1980). A detailed study of its reproduction is realized (Tekaya, 1999). The summary of results on gametogenesis, fertilization and embryonic development are presented in Tekaya et al, 1997; 1998; 1999a &b.

We notice the large biodiversity of this group of lower metazoan. The marine species are the more primitive. They reproduce only sexually. Fissiparity occurs for the main Tunisian Paludicola and is the unique reproduction mode. The same European species can reproduce by the tow ways, sexual and asexual. We suggest with some other authors, that the sexual reproduction is allowed by low temperature and in the opposite direction, increasing of temperature is favourable to the fissiparity mode. Tunisia is a country with a temperate climate and temperatures are higher than those in Europe, which can explicate the exclusively asexual reproduction way of Tunisian Dugesia.

S. dioica is the unique gonochoristic triclad and a precious biological material for eluciding many other features as sex determination.

Keywords: Platyhelminthes ; Planarians ; Paludicola ; Maricola ; Reproduction