Marine Biology (1995) 123:763-773 9 Springer-Verlag 1995 R. Bochert 9 A. Bick Reproduction and larval development of Marenzelleria viridis (Polychaeta: Spionidae) Received: 17 May 1995/Accepted: 23 June 1995 Abstract The pelagic larvae of the polychaete Maren- zelleria viridis (Verrill, 1873) are newcomers among the meroplankton of the Baltic Sea, where they sometimes achieve an abundance of up to 21 x 106 indm -3 near the coast, especially in late autumn and even in winter. Benthos samples were collected in the Darss-Zingst bodden chain from April 1992 to 1993 and used to ascertain the reproductive stages of individuals. Devel- opment from fertilized egg to benthic juvenile is de- scribed on the basis of field material. Details of the morphology of various pelagic stages and the young benthic worm are presented as drawings and scanning electron microscope photographs. In 1992, develop- ment of the gametes started in mid-May. The indi- viduals reached maturity in late September after about 20 wk. The first planktonic stage was the fertilized egg. The larvae have initially one and later two pairs of black eyes and, in the 2nd segment, a ciliated pit. Gastrotrochs are present on the third and thereafter every alternate segment. Neuropodial setae develop once the 7-setiger stage is reached. Palps appear at the 10-setiger stage, and neuropodial bidentate hooks from the 10th to llth setiger on. Metamorphosis into the juvenile benthic stage takes place at the earliest when the 15-setiger stage has been reached. The reproductive seasons of various populations of M. viridis and the developmental patterns of their larvae are discussed, and differences are compared with relevant findings for the spionids. Communicated by O. Kinne, Oldendorf/Luhe R. Bochert ([]) Universitiit Rostock, Wissenschaftsbereich Meeresbiologie, D-18051 Rostock, Germany A. Bick Universitiit Rostock, Wissenschaftsbereich Allgemeine und Spezielle Zoologie, D-18051 Rostock, Germany Introduction A great deal is known about reproduction and larval development in the family Spionidae. Most studies have concentrated on specific species (e.g. Dean 1965; Day and Blake 1979; Myohara 1979, 1980; Radashevsky 1983, 1988; Cazaux 1985; Gu6rin 1991; Hsieh 1994), only a few consider several species simul- taneously (e.g. SSderstrSm 1920; Hannerz 1956; Blake 1969). Reproductive types vary widely among the spionids (Wilson 1991). This variability of reproductive type and development pattern is found not only within genera, but even within the same species (Hannerz 1956; Simon 1968; Levin 1984). Morphologically identical species can only be distin- guished from each other by considering reproductive characteristics. Proof of reproductive isolation has been used to show that Malacocerosfuliginosus (Gu&in and Kerambrun 1984), Polydora ligni and Streblospio benedicti (Rice 1991) are in fact species complexes. Nine other suspected cryptic or sibling species among the spionids have been reviewed by Rice (1991). Originally inhabiting only North American estu- aries, the polychaete Marenzelleria viridis (Verrill, 1873) has been found in various North Sea (Essink and Kleef 1988; McLusky et al. 1993) and Baltic estuaries (Bick and Burckhardt 1989) since the mid-eighties. Bochert et al. (1995) reported that the M. viridis population in the Darss-Zingst bodden chain, a tideless estuary-like brackish coastal embayment of the southern Baltic, reproduces at a different time than described for popu- lations in North American (George 1966) and North Sea estuaries (Atkins et al. 1987; Essink and Kleef 1988). Bastrop et al. (1995) studied the enzyme patterns of North Sea and Baltic populations by electrophoresis. In view of the considerable differences they found, they suspect that the North Sea and Baltic populations may even belong to different species. The present study describes aspects of the reproduc- tion biology and larval development of a Baltic