Redescription of two species of Neanthes (Polychaeta: Nereididae) possessing a large notopodial prechaetal lobe

The family Nereididae consists of several large genera of which some are known to be heterogeneous groups of species. Among these genera, species that have been assigned to Neanthes Kinberg, 1865 have been delineated in informal groups based on chaetal and parapodial characters (Fauchald, 1972; Wilson, 1984). Examination of a number of nereidid taxa with paragnaths has shown that several morphological characters, especially parapodial characters, need to be emphasised to a SCIENTIA MARINA 70S3 December 2006, 27-33, Barcelona (Spain) ISSN: 0214-8358

SCIENTIFIC ADVANCES IN POLYCHAETE RESEARCH R. Sardá, G. San Martín, E. López, D. Martin and D. George (eds.)larger degree than has been usual in previous descriptions.Recent phylogenetic analyses of the traditionally recognised subfamily Nereidinae showed that Neanthes is not a monophyletic group (Bakken and Wilson, 2005).Targeting the subfamily Nereidinae as defined by Fitzhugh (1987), their results showed that Neanthes-species included in the analyses were paraphyletic, but also revealed that more knowledge of character homology is needed.Hence more data from described species of Neanthes is needed before phylogenetic analyses of Neanthes as a group can take place.
One parapodial character that has been shown to have important information is the notopodial prechaetal lobe, the size of the lobe and the range measured in a number of chaetigers along the body.The focus in this case is on the notopodial prechaetal lobe being "as large as dorsal and ventral notopodial ligules in anterior chaetigers", and if present "present throughout" or "restricted to anterior chaetigers" (Wilson et al., 2003).The common feature is that the size is reduced in posterior chaetigers.Similarly a prechaetal lobe can be smaller than the notopodial ligules in anterior chaetigers or absent altogether.Results from the nereidine phylogenetic analyses (Bakken and Wilson, 2005) illustrated how the included taxa with a large notopodial prechaetal lobe were placed in one clade, with uncertain relationships to nominal Neanthes taxa.Although no confirmed conclusions are drawn from this, as more taxa will have to be included to show the phylogenetic relationships between different taxa, it poses interesting questions about relationship and character homology.
Presence of a dorsal notopodial ligule expanded in breadth was demonstrated to be a synapomorphy for Alitta virens (Sars, 1835) and Alitta succinea (Leuckart, 1847) based on phylogenetic analyses (Bakken and Wilson in press: Fig. 6B).Along with the closely related species Alitta brandti Malmgren, 1866 and Alitta grandis (Stimpson, 1853) (Khlebovich, 1996) these two also share the presence of a large prechaetal lobe throughout the body.In Neanthes bongcoi Pillai, 1965 andNeanthes meggitti Monro, 1931 the dorsal notopodial ligule is not expanded in posterior chaetigers, and the prechaetal lobe is restricted to a number of anterior chaetigers.However, variation may be due to size of specimens, as has previously been demonstrated in specimens smaller than 1 mm in body width, where parapodial characters may be poorly developed (Wilson, 1984).
Being a sister group to A. succinea and A. virens (Bakken and Wilson, 2005) Neanthes cricognatha Ehlers, 1904 is similar to these in possessing a large prechaetal lobe throughout the body but does not have an enlarged dorsal ligule.A striking feature for N. cricognatha and two similar species that might be a species complex, Neanthes caudata (delle Chiaje, 1825) and Neanthes arenaceodentata (Moore, 1903), is the presence of paragnaths in Areas V-VIII joined in a single band (Wilson, 1984), a character that separates N. cricognatha from both A. virens and A. succinea, and also from N. boncoi.
Possession of a large notopodial prechaetal lobe is, however, not unique to taxa from Neanthes sensu lato, as it also occurs in Leonnates spp.demonstrating that it is a homoplasious character.This will not be discussed further here, as the aim of this paper is to redescribe N. bongcoi and N. meggitti to complement data for future phylogenetic analyses.

MATERIALS AND METHODS
Specimens were taken from museum material.The following abbreviations for museums and institutions are used: AM (Australian Museum, Sydney, Australia), MAGNT (Museum and Art Gallery of Northern Territory, Darwin, Australia), NHM (Natural History Museum, London, UK).
Descriptions were generated from a DELTA database (Dallwitz, et al. 1993 onwards) of Nereididae (Wilson, et al. 2003) updated with new information from examined material.
Notopodium with at least one distinct ligule or lobe.Dorsal notopodial ligule present, triangular with a pointed tip (Fig. 1B ventral ligule throughout, and of similar length throughout.Notopodial prechaetal lobe present on anterior chaetigers, approximately equal to length of dorsal notopodial ligule in anterior chaetigers (Fig. 1B); reducing in size from about chaetiger 20 (Fig. 1C), last present at about chaetiger 50 (Fig. 1D).
Remarks.The prostomium in the examined specimens has dark pigmentation often covering most of the prostomium, the peristomium has a transversal dark brown band, and intersegmental brown spots on the up to first 11 chaetigers.This pigmentation pattern varied from light to dark brown, or could be almost absent.Similarly the number of segments with pigmentation would also be subject to variation.No differences in morphological features were observed in the Australian material, and the Australian specimens agree well with the original description given by Pillai (1965).They differ only in the number of paragnaths in Area IV which are fewer in number (4 or 5) than the Australian material examined here (5-13).
It seems that the range of the notopodial prechaetal lobe is dependent of the size of the specimen.In larger specimens (2 mm body width at chaetiger 10 excluding parapodia) the prechaetal lobe is present throughout, but becomes smaller from chaetiger 20-25 and diminishes posteriorly to a very small lobe in the posteriormost chaetigers.In smaller specimens (up to 1 mm body width) the prechaetal lobe is absent from about chaetiger [45][46][47][48][49][50].
Neanthes bongcoi most closely resembles N. cricognatha to which it shares the characteristic preand postchaetal lobes in noto-and neuropodia respectively.Neanthes cricognatha, however, has paragnaths on the oral ring in one continuous band not separating Areas V-VIII.Neanthes bongcoi is also similar to Neanthes chilkaensis Southern, 1921, but can be distinguished from this by the shorter notopodial prechaetal lobe in the latter species, and the paragnath pattern (especially by the two rows in Areas VII-VIII in N. chilkaensis (Southern, 1921) compared with only a few conical paragnaths arranged in one row in N. bongcoi).The recently described Neanthes philippinensis de León-González and Salazar-Vallejo, 2003 is similar in having a large prechaetal notopodial lobe of the same size as the dorsal notopodial ligule, but is clearly restricted to a number of anterior chaetigers: unfortunately their description (de León-Gozalez and Salazar-Vallejo, 2003) fails to record this in detail.Neanthes philippinensis also has quite different heterogomph falcigers with the blade possessing a distal tendon, which is not observed in N. bongcoi.
It was not possible to arrange for a loan of the holotype of N. bongcoi for this study.A single specimen in the NHM representing the paratype must at some stage have been misplaced.The supposed paratype did not correspond to the original description (Pillai 1965), but rather represents an unidentified species.This fact was also reported by de León-Gozalez and Salazar-Vallejo (2003).Until the whereabouts of the holotype can be established no further action can be taken.
Habitat.The species occurs in sand with fine coral rubble, from the intertidal to a depth of 20 m, and on mudflats in front of mangroves.
Epitokal modified parapodia (observed in one male specimen) from chaetiger 18, anterior parapodia (from chaetiger 14) with a large neuropodial postchaetal lobe (Fig. 3E).Remarks.A lectotype is designated here for N. meggitti.All seven specimens in the type series are in very good condition.The specimens are mature, five atokous and in addition there is one specimen labelled "atokous female", and finally a specimen labelled "epitokous male".General appearance of the examined specimens is somewhat "bushy" at the anterior end with long parapodial appendages.The presence of long and slender notopodial prechaetal lobes as long as dorsal and ventral notopodial ligules in anterior chaetigers indicates that a large neuropodial postchaetal lobe should be present.This is usually observed in taxa with similar features starting on the first 2-5 chaetigers and is a common feature (personal observations).This is, however, not the case, a postchaetal lobe is present but only as a small lobe not projecting beyond the acicular ligule, and starting on chaetiger 15.It should be mentioned that most chaetae were broken in all specimens but it was still possible to observe chaetal characters, although observations on some variation might have been lost.
The parapodial characteristics of N. meggitti make it unique.This is illustrated by the large notopodial prechaetal lobe obviously restricted to a number of anterior chaetigers, in combination with the small neuropodial postchaetal lobe being restricted to a number of chaetigers commencing at chaetiger 15.