Polychaete systematics: Past and present

Abstract

In this paper, we first demonstrate the historical background for the current unsatisfactory state of systematics of the polychaetes. We then briefly discuss our knowledge of internal and external structures. A review of the polychaete families makes up the third section; 81 families are treated in detail. Five families have been recently synonymized with others, and six families are too poorly known to be sufficiently characterized. Fossil polychaetes are briefly mentioned, with specific attention to problems associated with incorporating them in recent systematics.The traditional separation in 'errant' and 'sedentary' polychaetes has increasingly become recognized as being unsatisfactory; however, the current trend towards grouping the polychaetes in many orders without specifying the relationships among the orders, is no more satisfactory. The lack of consistent morphological information is a major source of uncertainty. Intensive morphological studies should remove terminological ambiguities and alleviate some of the problems.

Structure of the nervous system of Myzostoma cirriferum (Annelida) as revealed by immunohistochemistry and cLSM analyses

“…Thus, the preparapodial and the postparapodial cirri of M. cirriferum can be homologized with ventral and dorsal cirri, respectively, of the polychaetes. Like M. cirriferum, many Myzostomida have 10 pairs of cirri (Prenant, 1959;Fauchald and Rouse 1997;Rouse and Fauchald, 1997;Eeckhaut et al, 1998), so that for each parapodium there is one ventral and one dorsal cirrus. In some species, however, the number of cirri is distinctly greater, and it remains for future studies to determine whether all of these are innervated by side branches of the parapodial nerves and how these nerves branch.…”

“…Wheeler (1897) considered all the myzostomid marginal cirri to be homologous with the dorsal cirri of polychaete parapodia. Accordingly, Fauchald and Rouse (1997) evaluated the reduction of the notopodia except for the dorsal cirri as an autapomorphy for the Myzostomida. Fedotov (1929) andJä gersten (1940), on the other hand, rejected a homology of polychaete dorsal cirri and the marginal cirri of the myzostomids.…”

“…As Pietsch and Westheide (1987) pointed out, they have been considered to be either 1) a taxon outside the Annelida (Turbellaria, Trematoda, Tardigrada, parasitic Crustacea) (see Stummer-Traunfels, 1927;Jä gersten, 1940); 2) Annelida but not Polychaeta (Mecznikow, 1866;Brusca and Brusca, 1990); 3) a sister group of the "true" Polychaeta (Fedotov, 1929;Kato, 1952); or 4) a subtaxon of the Polychaeta (Beard, 1884;Wheeler, 1897). The serial arrangement of their chaetaebearing parapodia, protonephridia, and lateral organs suggests a segmented organization of the Myzostomida; for this reason, in most modern systems they are treated as Annelida and sometimes specified as Polychaeta (Barnes and Harrison, 1992;Westheide and Rieger, 1996;Fauchald and Rouse, 1997;Rouse and Fauchald, 1997). But other characters, chiefly the undivided coelom, the sequence of parapodia formation (3, 4, 2, 5ϩ1, Jägersten 1940;Eeckhaut et al, 1990), the variety of lateral organs (ranging in number from zero to 20 in the different species), and the assumed lack (Haszprunar, 1996) of segmentally arranged organs (serially arranged protonephridia have so far been reported only in Myzostoma cirriferum by Pietsch and Westheide [1987]) are used by other authors to interpret the Myzostomida as nonsegmented animals (Jägersten, 1940;Salvini-Plawen, 1980a,b;Haszprunar, 1996).…”

Spermiogenesis and seminal receptacles inAeolosoma singulare(Annelida, Polychaeta, Aeolosomatidae)

“…However, a more recent molecular study based on a larger taxon sample contrasted these results, finding that aeolosomatids 'are not closer to the Clitellata than any other annelid group' (Rota et ai, 2001). At present, aeolosomatids are considered polychaetes incertae sedis (Rouse & Pleijel, 2001), on the basis of a large phylogenetic analysis of all polychaete families using morphological data (Rouse & Fauchald, 1997). A recent ultrastructural analysis (Hessling & Purschke, 2000) confirmed the presence of nuchal organs in the aeolosomatids as supposed by Fauchald & Rouse (1997), which definitely corroborates their exclusion from the Clitellata.…”

“…Based mainly on similarities in external features, the two groups were considered to be close relatives (Lastochkin, 1935); however further investigations based on the anatomy and histology of the genital organs (Bunke, 1967) and on the ultrastructural features of the spermatozoon of P. fluviatilis (Bunke, 1985) rendered this assumption less probable, ultimately suggesting that potamodrilids have no close affinities to any annelid species or group (Bunke, 1988). However, potamodrilids formed a monophyletic group with aeolosomatids in all phylogenetic analyses including all polychaete taxa based on morphological characters coded as absence/presence (Rouse & Fauchald, 1997).…”