Insemination and embryonic development of some Mediterranean polyclad flatworms

We document the specific patterns of egg plates, the numbers and size of embryos per cocoon and the major stages of the spiralian embryonic development in four Mediterranean polyclads. These include the two cotylean species Prosthiostomum siphunculus (Delle Chiaje, 1828) Lang, 1884 and Thysanozoon brocchii (Risso, 1818) Grube, 1840, both with Muller's larvae, the directly developing acotylean Echinoplana celerrima Haswell, 1907 as well as the acotylean Imogine mediterranea (Galleni, 1976) Jennings and Newman, 1996 with Gotte's larva. The duration of embryonic development in three polyclads was tested at different temperatures, suggesting a strong decrease in developmental time with higher temperature (17 degrees C instead of 15 degrees C). In P. siphunculus, we also controlled salinity and light conditions, observing slightly faster development at salinity of 3.5%, and no major effect of light on development. In polyclads, insemination can occur by true copulation, dermal impregnation or hypodermic insemination. We used live observations and sagittal serial sections to detect sperm in tissues of the recipient animal. We found that there is no clear relationship between the presence or absence of a penis or Lang's vesicle and the mode of insemination, which can vary considerably even between species of the same genus.