Observations on reproductive features of three species of Eunicidae (Polychaeta) associated with Posidonia oceanica seagrass meadows in the Mediterranean Sea

Abstract

Observations on the reproductive features are provided for three species of Eunicidae (Lysidice ninetta Audouin and Milne Edwards, Lysidice collaris Grube and Nematonereis unicornis (Grube)). They all occur in Posidonia oceanica meadows as borers in the sheaths. The material examined was collected during several studies on P. oceanica meadows conducted at various times of the year and in different years along the coasts of Sardinia and the Ischia islands (Tyrrhenian Sea), and Otranto (Southern Adriatic Sea). All individuals of the three species were measured (width 3rd chaetiger, diameter of eyes), and checked for presence of gametes. In each of the females recognized, egg diameter was measured. To integrate the observations, living specimens were collected in March 2003 and reared in the laboratory until August 2003 to check for gamete maturation; mature males were fixed for analysis of the structure of the spermatozoa by transmission electron microscopy (TEM). All three species studied are gonochoric. For both species of Lysidice gametes occurred in individuals with a width greater than 0.8 mm. The overall scarcity of mature males and females observed with respect to the total number of specimens collected is probably due to the dimensional constraint exerted by the sheaths. In Lysidice spp. specimens with small developing oocytes were observed in January-February and reached their maximum diameter between June and August (157 µm). Gametes were located mainly in the posterior part of the body, and an enlargement of the eyes (almost doubled in size) characterized mature specimens close to spawning. N. unicornis is a smaller species, and it is less frequent in Posidonia sheaths than Lysidice spp.; individuals with gametes had a width greater than 0.55 mm. A few mature specimens were observed between March and May with a maximum egg diameter of about 155 µm. In this species maturation of the gametes is coupled with a huge enlargement of the eyes (ten-times larger than in immature individuals), while the posterior part of the body is swollen and full of mature gametes, indicating reproduction by schizogamy. Analysis of mature spermatozoa of the three eunicids revealed a similar morphology, which is that of an “ect-aquasperm” type, typical of many free-spawning species with external fertilization. The nucleus is round and globose (2.5 µm diameter) and the acrosome, slightly different among the three species, has the general shape of a truncate sub-cone.