A review of life history in serpulimorph polychaetes: ecological and evolutionary perspectives
The paper summarises information on the life history of tubeworms (Serpulidae and Spirorbidae). Topics reviewed are sexuality patterns, asexual development, gamete attributes, fecundity, spawning and fertilisation, larval development and morphology, larval ecology and behaviour (including larval swimming, feeding, photoresponse, and defences), brooding, settlement and metamorphosis, longevity and mortality. Gonochorism, sequential and simultaneous hermaphroditism are found in the group, the last pattern being apparently undereported. Asexual reproduction commonly leads to the formation of colonies. The egg size range is 40-200 ?m in serpulids and 80-230 ?m in spirorbids. The sperms with spherical and with elongated heads correspond, respectively, to broadcasting and brooding. Variability of brooding methods in serpulids has been grossly undereported and even exceeds that of spirorbids. Development is similar in feeding and non-feeding larvae and the developmental events are easily reproducible in the laboratory until the onset of competency, after which larvae require specific cues to proceed with settlement and metamorphosis. Settlement is affected by both non-specific and substratum-specific cues (conspecifics, microbial film, other organisms). Initial rapid juvenile growth slows down at later life stages. The growth rates are affected both by factors acting after the settlement and those experienced during the larval stage. Maturation is reached at a certain body size and depends on the factors controlling growth. Longevity varies from several months in small serpulids and spirorbids to 35 yr in the largest serpulids. Mortality is highest during the early embryonic and juvenile stages. The egg-size distribution in serpulimorph polychaetes is bimodal but the modes do not correspond to feeding and non-feeding development, and egg sizes of species with feeding and non-feeding larvae partially overlap. This pattern may be explained by high interspecific variability in the organic content of eggs and/or facultative larval feeding of some serpulids. Planktonic development is strongly correlated with larval feeding, and planktonic lecitotrophy is rare. The potential selective advantage of larval feeding is in the flexibility of the duration of the competent stage that increases the possibility to locate suitable substrata. As in other groups, small body size correlates with simultaneous hermaphroditism, brooding, and non-feeding development. Broader generalisations require better knowledge of the life-history of a greater number of species. Integration of phylogenetic analyses into life-history studies should help to clarify the direction of life-history transitions in this group and determine whether phylogenetic constraints can account for the observed life-history patterns.