Thysanozoon brocchii (Risso, 1818) Grube, 1840

Gammoudi, Mehrez & Tekaya, Saïda, 2017, Suborders Acotylea and Cotylea (Polycladida): Study on morphological, ecological and reproductive features of some representative species from Tunisian coasts (Mediterranean), Ecologica Montenegrina 10, pp. 58-70 : 64

Thysanozoon brocchii (Risso, 1818) Grube, 1840

Morphological features. Body is oval. Length can reach 40 mm. Two prominent marginal tentacles are present ( Fig. 1F ). The pharynx is ruffled and located in the anterior half of body. Dorsal face is provided with papillae ( Fig. 1F ) and ventral one is smooth. A well developed sucker is located behind the mouth and genital pores. According to our observation, as in Yungia aurantiaca , the animal used this structure to attach itself to a substrate fighting against strong action of waves.

Reproductive features. This species have two male copulatory apparatus, indeed there exists a pair of seminal vesicles, a pair of vesicles prostatic, a pair of ejaculatory ducts and a pair of male gonopores. Thee latters are located on the same horizontal line. Both of copulatory apparatus are functional as sperm is seen inside both seminal vesicles and vasa deferentia. The prostatic vesicles are of free-type. Each ejaculatory duct leads to a penis papilla armed with a stylet. The testes are usually ventral. The ovaries are located in dorsal parenchyma. Both uteri are filled with eggs. The vagina is oriented anteriorly and is provided with uterine ramifications. A widening of vertical section of vagina surrounded by glands is noted, this is the cement pouch characteristic representatives of the Suborder Cotylea .

Histological sections show a dorsal germinative zone containing young oocytes and ventral growing zone containing growing oocytes ( Fig. 3B ).

Associated fauna. The associated fauna included the sea urchin Paracentrotus lividus (Lamarck, 1816) , the crustacean Achaeus cranchii Leach, 1817 and the starfish Asterina gibbosa (Pennant, 1777) .

Figures

Figure 1. Anterior dorsal region in A: Echinoplana celerrima. See the tentacular (te) and cerebral eyes (ce) forming two paramedian bands, scale bar: 0.5 mm. B: Letoplana mediterranea. See clusters of tentacular (te) and cerebral eyes (ce), scale bar: 0.5 mm. C: Discocelis tigrina. Note the presence of cerebral and tentacular eyes clusters, scale bar: 0.5 mm. D: Imogine mediterranea. Note the presence of two everted nuchal tentacles (arrows), scale bar: 0.5 mm. E: Yungia aurantiaca. See the two marginal tentacles (arrows) and cerebral eyes spots (arrowhead), scale bar: 1 mm. F: Thysanozoon brocchii. Dorsal faces provided with papillae and developed marginal tentacles (arrows), scale bar: 1 mm. G: Prostheceraeus moseleyi. Note the presence of marginal tentacles (arrows) and yellow marginal line, scale bar: mm. H: Prosthiostomum siphunculus. The cerebral eyes (ce) are not included in the semi circle formed by marginal eyes (me), scale bar: 1 mm.

Figure 3. Ovaries in polyclads A: Section through ovary of Echinoplana celerrima showing a ventral growing zone (grz) and a dorsal germinative zone (gmz), scale bar: 0.01 mm. B: Section through ovary of the cotylean Thysanozoon brocchii showing a ventral germinative zone (gmz) and a dorsal growing zone (grz), scale bar: 0.01 mm.