Thysanozoon brocchii (Risso, 1818) Grube, 1840

Gammoudi, Mehrez, Ahmed, Raja Ben & Tekaya, Saïda, 2016, Taxonomy of subtidal marine Polyclads from Tabarka (northwest Tunisia) with remarks on their habitat preferences, Ecologica Montenegrina 9, pp. 1-9 : 6

Thysanozoon brocchii (Risso, 1818) Grube, 1840

General features. This polyclad is broadly oval shaped. It uses undulations of its body margins to swim actively. The dorsal face is cream to brownish and provided with several black papillae. The number and size of papillae decrease going from median region to the marginal zone ( Fig. 4a ). Some papillae are provided with a white spots. Tentacles are developed and appear as expansion of anterior margin. They contain two clusters of marginal eyes ( Fig. 4c ). Behind marginal eyes, are located in the median region, the two semicircular clusters of cerebral eyes. The ventral surface is whitish and transparent and shows a ruffled pharynx located in the first third of body. In ventral surface of fixed specimens, different prominent structures are located in the anterior half of body ( Fig. 4b ). The mouth opens in the posterior part of pharyngeal cavity. Two symmetric male gonopores are situated in the same horizontal level and in both side of median line of body. A unique female gonopore lies behind the two male gonopores. Posteriorly, is located a developed sucker characteristic of cotylea sub-order ( Fig. 4b ).

Reproductive apparatus. The testes follicles are numerous and located more in the ventral parenchyma then in dorsal one. The ovaries are less numerous and bigger in size than testes ( Fig. 4e ). They are located in the dorsal and median parenchyma. There are two male copulatory apparatus. Each male gonopore leads to a male atrium in which is projected a penis papillae armed with stylet. The penis papillae are surrounded by a penis sheath. The ejaculatory duct leads to elongated seminal vesicle filled usually with sperm ( Fig. 4d ). A free prostatic vesicle is connected to the ejaculatory duct via a narrow prostatic canal. The eosinophilous secretions are detected in the two prostatic canals and in male atrium. The single female apparatus comprise a narrow female atrium that extends vertically to form a widened chamber corresponding to cement pouch characteristic of cotylea sub-order. The vagina curve then anteriorly where is located the entrance of oviducts to form a section oriented anteriorly.

Habitat. The two specimens of Thysanozoon brochhii were collected from a depth of 16 m among a dense sea grass meadows of of Posidonia oceanica (Linnaeus) Delile extended in rocky substrate. These flatworms were found associated with the sponge Dysidea tupha , the sea urchin Paracentrotus lividus , the crustacean Achaeus cranchii Leach, 1817 and the bryozoans Electra posidoniae Gautier, 1954 . Thysanozoon brocchii is a good swimmer. It move using undulations of its flat body.

Figures

Figure 4. Thysanozoon brocchii a Dorsal view. Scale bar = 1mm. b Anterior dorsal region showing marginal tentacular eyes te. Scale bar=200µ. c Ventral view in fixed specimen showing the mouth m, the two male gonopores mg, the female gonopre fg and the developed sucker s. Scale bar = 100µ. d Section through the elongated seminal vesicle sv. Scale bar = 100µ. e Section through ovary ov. Scale bar =100µ.