Metamerism and life‐style within polychaetes: Morpho‐functional aspects and evolutionary implications

Abstract

An analysis of polychaete life‐styles and morpho‐functional adaptations, in relation to their metameric organization, is presented with the aim of summarising the main hypotheses thus far proposed as for their ancestral form, and of discussing the evolutionary implications of the metameric design. The metameric organization, which seems to have appeared early within the proto‐stome line, have reached a high diversification in marine polychaetes among the Annelida. This high diversification of ‘annelid design’ in polychaetes is probably linked to their ancient origin, their early rapid adaptive radiation, and their morphological and physiological adaptability to a great number of habitats. Primary heteronomy, i.e. the presence of larval segments in polychaetes, as an adaptation to pelagic development, as well as secondary heteronomy, i.e. the differentiation of trunk segments as adaptation to different adult life‐styles, are also discussed, stressing the necessity for a better definition of larval segments. The hypothesis of the evolution of a compartimented coelom as an adaptation to peristaltic movements in an oligochaetoid form is rejected, due to both its difficult application to vagile forms with parapodial development and complete septa, and the fact that most of the burrowing polychaetes lack complete septa. Reduction or loss of septa is accompanied by parapodia reduction in some forms, and by increased efficiency of parapodia in other. The occurrence of septa in polychaetes is probably a reflection of growth processes by which new segments are added posteriorly, it is therefore more likely that their loss, rather than their retention, requires a functional explanation. The whole body design and the occurrence of some structures seem in polychaetes strictly linked to life habit so obscuring any phylogenetic relationships. This, together with the scarcity of fossil evidence, gives rise to numerous problems in tracing a phylogenetic pattern and in hypothesising the ancestral form. Two main contrasting hypotheses have been formulated as regards the annelid ancestor: the first depicts it as errant and epibenthic, the second as burrowing or oligochaetoid form. Both hypotheses are discussed also in the light of larval morphology and reproductive biology. It is hypothesised that the ancestral form could be very close to a relatively large meiofaunal ‘polygordiid‐like’ annelid, with a biphasic cycle, in which metamerism arised for developmental constraints. From this ‘Bauplan’ an errant form may be derived, with development of parapodia to improve the efficiency of lateral contraction.