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DescriptionA polychaete worm that grows up to 40 mm in length. It is blood-red in colour and has the most earthworm-like appearance of all polychaetes. It lacks gills, eyes and head appendages and has insignificant parapodia. The chaetae are hair-like spines near the head and have crochet hooks towards the rear. The species is sedentary and fragile, with a flexible body.Capitella capitata represents a complex (Grassle & Grassle, 1976) of up to 50 sibling species (Mendez et al., 1997). Although each species within the complex differs in size, reproductive strategy and larval characteristics (Pearson & Pearson, 1991; Mendez et al., 1997), many studies not accounting for this variation have extrapolated the characteristics of one member to the aggregate as a whole (Grassle & Grassle, 1977). Thus, many detailed Capitella capitata studies exist that fail to determine to which particular species of the complex measured data actually apply (e.g. Grassle & Grassle, 1974). Caution should therefore be used when applying characteristics to subsets of the aggregate complex, as most species contained within it are data deficient.
The species of this complex can be morphologically distinguished (with difficulty) by adult differences in chaetae structure and distribution, the number of segments possessing chaetae, the shape of the pro- and peristomium, the shape of the tail and average wet weight (Grassle & Grassle, 1977; Pearson & Pearson, 1991). Clearer differences may be observed in reproductive strategy and output, larval dispersal modes and timescales, responses to ecological disturbance, genetic profiles and through the inability of different forms to interbreed.