Research Article |
Corresponding author: Víctor Manuel Conde-Vela ( victorconde2323@gmail.com ) Academic editor: Oana Teodora Moldovan
© 2017 Víctor Manuel Conde-Vela.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Conde-Vela VM (2017) The troglomorphic adaptations of Namanereidinae (Annelida, Nereididae) revisited, including a redescription of Namanereis cavernicola (Solís-Weiss & Espinasa, 1991), and a new Caribbean species of Namanereis Chamberlin, 1919. Subterranean Biology 23: 19-28. https://doi.org/10.3897/subtbiol.23.13701
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Most species belonging to Namanereis Chamberlin, 1919 live in freshwater and subterranean waters, even in water bodies several meters above sea level. A new species belonging to the stygobiont Namanereis group is described here; it shares the common morphological characters of absence of eyes and pigmentation, bifid jaws, elongation of chaetae and cirri, which have been recently regarded as troglomorphies. Because these features are used in evaluations of phylogenetic affinity in Namanereis, a review of these features was made for all known namanereidins, and it was extended to include species in Namalycastis Hartman, 1959. It is shown that elongation of tentacular and dorsal cirri, or elongation of upper sub-acicular falcigers in pre- or post-acicular fascicles, are not exclusive or restricted to species living in subterranean habitats or to Namanereis, because these features are also present in several Namalycastis species. However, the presence of bifid jaws, and the absence of eyes are exclusively found in namanereidins living in subterranean habitats. A hypothetical evolutionary derivation of bifid jaws is proposed, based upon observations of jaw morphology of several species. These exclusive troglomorphic characters (bifid jaws, eyeless) are regarded as convergent features to aphotic environments, and they should be discouraged as indicators of common ancestry. The new species, herein described as Namanereis christopheri sp. n., was collected in a cave 435 m above sea level in Saint Vincent, Caribbean Sea. The species resembles N. cavernicola but it differs because it has shorter tentacular cirri, margin of prostomium entire, rounded neuropodial lobes and broader dorsal cirri throughout body. A key to identify all known Namanereis species is included.
Troglomorphic features, convergence, elongation of appendages, evolution of jaws
Troglomorphism comprises a set of convergent features found in organisms living in subterranean, aphotic environments, presumably resulting from similar selective pressures (
In an ecological classification, obligate residents of subterranean habitats are called troglobionts, and specifically stygobionts if they live in aquatic systems; interestingly, not all organisms living in subterranean environments are troglomorphs, nor are all organisms showing troglomorphic features are troglobionts (
Oligochaetes are the most common annelid troglobionts, but polychaetes can also be so, especially those species belonging to the family Nereididae de Blainville, 1818 and specifically the subfamily Namanereidinae Hartman, 1959 (
A group of at least 12 stygobiont Namanereis (from a total of 18 in the genus) have interestingly shared morphological features, such as the absence of body pigmentation and eyes and the elongation of appendages (cirri and chaetae), including some other features not present in all species as the presence of distally bifid and edentate jaws, and heterogomph falcigers in sub-preacicular fascicles increasing their length posteriorly, even being replaced with spinigers (
However, the same troglomorphic features are present at least in one other epigean species from another genus living within mangrove leaves litter, Namalycastis occulta Conde-Vela, 2013, as already noted in a previous contribution (
On the other hand, in the Namanereis stygobitic group, three species were found in groundwater at several meters above sea level (asl): N. beroni (Hartmann-Schröder & Marinov, 1977) from Papua New Guinea, 1700 m asl, N. cavernicola (Solís-Weiss & Espinasa, 1991) from Mexico, 1650 m asl and N. gesae Fiege & Van Damme, 2002 from Yemen, 700 m asl, while the top record of altitude is for Lycastoides alticola, found at 2150 m asl in Mexico (
The main goal of this contribution is the re-assessment of troglomorphic morphology in namanereidins. As an introduction to the troglomorphic morphology in stygobitic namanereidins, the systematic section is presented first, addressing the description of a new species of Namanereis and the redescription of N. cavernicola, and including a key to identify all known species of the genus. After, the discussion of troglomorphic features is presented, focused on the elongation of cirri and chaetae, the arrangement of chaetae and the morphology of the jaws.
For the morphology re-assessment, specimens of 7 species were examined and are deposited in the National Museum of Natural History, Smithsonian Institution (USNM), the Natural History Museum of Los Angeles County (LACM-AHF), and in the Reference Collection of El Colegio de la Frontera Sur, Chetumal (ECOSUR). They include paratypes of Namanereis cavernicola from Mexico (USNM 136559), topotypes of Namanereis hummelincki from Bonaire (USNM 29715, 29716), holotype (USNM 178870) and paratypes (USNM 31011) of Namalycastis intermedia from the Gulf of Mexico, and topotypes of Namalycastis occulta (ECOSUR P-2649), and non-type specimens of Namanereis cf. amboinensis (ECOSUR P-2902) and Namalycastis borealis (ECOSUR P-2651) from Chetumal Bay.
Specimens were examined under stereomicroscope (Olympus SZ40) and compound microscope with differential interference contrast (Olympus BX51). The photographs were made with a digital camera (Canon T5i) with adaptor for both microscopes. Plates and images were made with Adobe Photoshop® and Illustrator®. If not everted, pharynx was dissected to examine interior structures and, in some specimens, jaws were removed, mounted and observed in compound microscope. Parapodia from anterior, middle and posterior chaetigers were removed and mounted in semi-permanent slides, and examined under compound microscope. Some specimens were whole-mounted for examination of chaetal changes along body.
For descriptions, parapodial and chaetal terminology provided by
For the relative length of parapodial cirri, the ratios between the length of dorsal cirri (Ld) and length of neuroacicular lobe (Ln), for anterior (ALd/Ln) and posterior (PLd/Ln) chaetigers were used. The subtraction of PLd/Ln and ALd/Ln (Dp−a) is less than or equal to zero if anterior cirri are equal or longer than posterior ones, indicating elongation of anterior cirri; and greater than zero if posterior cirri are longer than anterior ones, indicating elongation of posterior cirri. With ALd/Ln and PLd/Ln, two histograms were made, the first explores the distribution of elongation of dorsal cirri in both anterior and posterior chaetigers, whilst the second shows the same distribution but based on anterior chaetigers only.
For the relative length of tentacular cirri, the ratio between length of posterodorsal tentacular cirri (Lpt) and wide of prostomium (Wp) was used. Lpt/Wp was obtained from illustrations directly (excepting Namalycastis geayi, N. longicirris and N. siolii). This measure was preferred over the number of chaetigers reached when posterodorsal tentacular cirri are placed backwards (
For the relative length of blades of chaetae, the ratio between length of blade (Lb) and the width of shaft (Ws), standardized to chaetiger 10, was used; only the ratio of the dorsalmost (Dm) and ventralmost (Vm) falcigers in sub-acicular fascicle were considered. The subtraction of Dm and Vm (Dd-v) is zero or nearly so if both falcigers have subequal length, and greater than zero if Dm is greater than Vm, indicating elongation in Dm.
Lycastilla cavernicola Solís-Weiss & Espinasa, 1991: 632–635, figs 1a–e, 2a–f.
Namanereis cavernicola Glasby, 1999: 83–86 (partim).
Izote Cavern, Guerrero, Mexico, 1650 m above sea level.
Paratypes USNM 136559 (2), Izote Cavern, (18°36'40"N, 99°33'25"W), Guerrero, Mexico 1650 m above sea level, 20 November 1988, Coll. L. Espinasa.
Paratypes in excellent condition, one complete; 29 mm long, 1.1 mm wide at chaetiger 10, 69 chaetigers. Body pale, without pigmentation (Fig.
Parapodial cirri pattern: Dorsal cirri sub-equal to neuroacicular lobes in anterior chaetigers, becoming longer than neuroacicular lobes toward posterior end, basally inserted throughout body. Ventral cirri shorter than neuropodial lobes, basally inserted throughout body.
In anterior chaetigers (Fig.
Notochaetae absent. Neurochaetae in type D arrangement, i.e. supra-acicular chaetae heterogomph falcigers (Fig.
Pygidium tripartite; a pair of anal cirri cirriform, short, as long as pygidium (Fig.
Remarks.
Namanereis cavernicola (Solís-Weiss & Espinasa, 1991). A–H Paratype USNM 136559 A Anterior end, dorsal view B Close-up of prostomium C Posterior end, dorsal view D Supra-acicular falciger, chaetiger 62 E Sub-acicular falciger, chaetiger 62 F Chaetiger 5, right parapodium G Chaetiger 20, right parapodium H Chaetiger 62, right parapodium. Scale bars: 1 mm (A); 0.2 mm (B, F–H); 0.5 mm (C); 10 µm (D, E).
Namanereis
cavernicola
Glasby, 1999: 83–86, figs 8c, 35a–g (partim, non
Saint Vincent, Lesser Antilles.
The specific name is after Christopher J. Glasby, in recognition of his numerous contributions in polychaete taxonomy, especially about nereidid taxonomy, and because he identified this species as new after his first evaluation (see below).
Holotype LACM-AHF 1227 and paratypes LACM-AHF 1228 (1), and LACM-AHF 1229 (10), Golden Grove, near Chateaubelair Bay (13º17'18"N, 61º14'25"W), Saint Vincent, Saint Vincent and the Grenadines, 31 July 1972, 435 m above sea level, spring pool in Colocasia (Araceae) swamp, Coll. J.J. Rankin.
Holotype complete, 32 mm long, 1 mm wide at chaetiger 10, 95 chaetigers; body with several parapodia removed in middle region, otherwise in good condition. Paratypes complete, in good conditions, 10–30 mm long, 1–2 mm wide, 62–95 chaetigers. Body pale, without pigmentation (Fig.
Prostomium wider than long, anterior margin entire, groove present; antennae cirriform, as long as prostomium; eyes absent (Fig.
Parapodial cirri pattern: Dorsal cirri longer than neuropodial lobes, basally inserted throughout body. Ventral cirri shorter than neuropodial lobes, basally inserted throughout body.
In anterior chaetigers (Fig.
Notochaetae absent. Neurochaetae in type D arrangement, i.e. supra-acicular chaetae heterogomph falcigers and sesquigomph spinigers in pre- and post-acicular fascicles respectively; sub-acicular chaetae heterogomph falcigers with short and long blades in pre-acicular fascicles.
Supra-acicular sesquigomph spiniger pectinated, teeth minute, decreasing slightly in size towards tip (Fig.
Pygidium tripartite; anal cirri cirriform, short, as long as last chaetiger (Fig.
The material of this species was previously examined by
On the other hand, N. christopheri sp. n. and N. cavernicola share some features as having falcigers with relative long blades and blades with several minute teeth. However, they have some important differences. First, N. christopheri sp. n. has an anterior margin of prostomium entire and antennae are shorter than prostomium, while in N. cavernicola the anterior margin is incised and antennae are longer than prostomium. Further, tentacular cirri in N. christopheri sp. n. are smooth and reach chaetiger 3, while in N. cavernicola they are annulated and reach chaetiger 5–6. In addition, N. christopheri sp. has jaws much broader than N. cavernicola. N. cavernicola has neuropodial lobes tapered, subconical with pointed tips, two or three times longer than wide, while in N. christopheri sp. n. they are rounded and as long as wide; and parapodial cirri in N. cavernicola are thinner than in N. christopheri sp. n.
As indicated in the key below, N. christopheri sp. n. is also closely related to N. hummelincki, differing mainly in chaetal features as
Namanereis christopheri sp. n. A, B, F–O Holotype LACM-AHF 1227 C–E paratype LACM-AHF 1229 A Anterior end, dorsal view B Posterior end, dorsal view C Close-up of prostomium D Areas V and VI, pharynx dissected E Right jaw, dorsal view F Chaetiger 5, right parapodium G Chaetiger 21, right parapodium H Chaetiger 49, right parapodium I Chaetiger 90, right parapodium J Supra-acicular sesquigomph spiniger, chaetiger 49 K Sub-acicular heterogomph falciger, chaetiger 49 L Supra-acicular heterogomph falciger, chaetiger 49 M Sub-acicular heterogomph falciger, chaetiger 49 N Close-up of blade, supra-acicular sesquigomph spiniger, chaetiger 49 O Close-up of blade, supra-acicular heterogomph falciger, chaetiger 49. Scale bars: 0.5 mm (A–D); 0.1 mm (E); 0.1 mm (F–I); 50 µm (J, K); 30 µm (L, M); 10 µm (N, O).
(Modified after
1 | Four pairs of tentacular cirri | 2 |
– | Three pairs of tentacular cirri | 4 |
2 | Prostomium with entire anterior margin | N. quadraticeps (Blanchard in Gay, 1849) (Strait of Magellanes, Chile)1 |
– | Prostomium with cleft anterior margin | 3 |
3 | Dorsal cirri shorter than neuroacicular ligule on posterior chaetigers | N. minuta Glasby, 1999 (Grand’Anse, Haiti) |
– | Dorsal cirri longer than neuroacicular ligule on posterior chaetigers | N. stocki Glasby, 1999 (St. Ann’s Bay, Jamaica) |
4 | With antennae | 5 |
– | Without antennae | N. malaitae (Gibbs, 1971) (Malaita, Solomon Islands) |
5 | Eyes present | 6 |
– | Eyes absent | 11 |
6 | Eyes conspicuous, separate | 7 |
– | Eyes barely visible, coalesced | N. sublittoralis Glasby, 1999 (Smoke Alley Well, Sint Eustatius) |
7 | Supra-acicular spinigers present | 8 |
– | Supra-acicular spinigers absent | N. pontica (Bobretzky, 1872) (Bay of Sevastopol, Black Sea) |
8 | Falcigers with long, strongly falcate tips, one half to one third of cutting edge of blade without teeth | 9 |
– | Falcigers with short, weakly falcate tips, teeth on almost all length of cutting edge of blade | 10 |
9 | Falcigers with blades longer than boss of the joint (i.e. long blades) | N. amboinensis (Pflugfelder, 1933) (Ambon Island, Indonesia) |
– | Falcigers with blades as long as boss of the joint (i.e. short blades) | N. riojai (Bastida-Zavala, 1990) (La Paz Bay, Mexico) |
10 | Jaws with 9 subterminal teeth (6–14) | N. catarractarum (Feuerborn, 1931) (Bedali, Java) |
– | Jaws with 5 subterminal teeth (5–8) | N. littoralis (Müller & Grube in Grube, 1872) (Santa Catarina Island, Brazil)1 |
11 | Jaws with terminal and subterminal teeth | 12 |
– | Jaws with two bifid distal teeth and smooth cutting edge | 15 |
12 | Prostomium with anterior margin entire | 13 |
– | Prostomium with anterior margin cleft | N. tiriteae (Winterbourn, 1969) (Turitea Stream, North Island, New Zealand) |
13 | Dorsal cirri shorter or subequal than neuropodial lobes throughout body | 14 |
– | Dorsal cirri longer than neuropodial lobes throughout body | N. beroni Hartmann-Schröder & Marinov, 1977 (Bem Tem, Papua New Guinea) |
14 | Supra-acicular falcigers with several, minute teeth (ca. 30) | N. gesae Fiege & Van Damme, 2002 (Abd al-Kuri Island, Socotra Archipelago, Yemen) |
– | Supra-acicular falcigers with few, minute teeth (7–11) | N. pilbarensis Glasby, Fiege & Van Damme, 2014 (Pilbara Region, Australia) |
15 | Prostomium with anterior margin cleft | 16 |
– | Prostomium with anterior margin entire | 17 |
16 | Dorsal cirri longer than neuropodial lobes in first chaetigers | N. araps Glasby, 1997 (Nakhal, Oman) |
– | Dorsal cirri shorter than to subequal than neuropodial lobes in first chaetigers | N. cavernicola (Solís-Weiss & Espinasa, 1991) (Izote Cavern, Mexico) |
17 | Supra-acicular falcigers with pectinate, minute teeth | 18 |
– | Supra-acicular falcigers with serrated, coarse teeth | N. serratis Glasby, 1999 (Étang Saumâtre, Haiti) |
18 | Upper sub-acicular falcigers with blades two or more times longer than lower falcigers | 19 |
– | Upper and lower sub-acicular falcigers with subequal blades | N. socotrensis Glasby, Fiege & Van Damme, 2014 (Socotra Island, Yemen) |
19 | Supra-acicular falcigers with teeth increasing their length greatly basally | N. hummelincki (Augener, 1933) (Fontein, Bonaire) |
– | Supra-acicular falcigers with teeth increasing their length slightly medially | N. christopheri sp. n. (Saint Vincent) |
1Species groups of these species were not considered, but only information of type materials. |
Elongation of parapodial cirri. The results of elongation ratios of parapodial cirri are depicted in Fig.
Based on ratios of anterior chaetigers only (Fig.
Elongation of tentacular cirri. The resultant histogram of Lpt/Wp ratios is shown in Fig.
Elongation of chaetae. The results of Dd-v for most species is shown in Fig.
Histograms showing length ratios of parapodial cirri among Namalycastis (boldface) and Namanereis (lightface) species. A Ratios between length of dorsal cirri (Ld) and length of neuroacicular lobe (Ln) at anterior (ALd/Ln) and posterior chaetigers (PLd/Ln), and difference among them (Dp−a) B Ratios between length of dorsal cirri (Ld) and length of neuroacicular lobe (Ln) at anterior chaetigers (ALd/Ln) only. Sorted from highest to lowest ratios; asterisks highlight stygobiont Namanereis.
Histograms showing length ratios of some troglomorphic features among Namalycastis (boldface) and Namanereis (lightface) species. A Ratios between length of tentacular cirri (Lpt) and wide of prostomium (Wp) B Difference (Dd-v) between ratios of dorsalmost (Dm) and ventralmost (Vm) falcigers at chaetiger 10. Sorted from highest to lowest ratios; asterisks highlight stygobiont Namanereis.
Typical morphology of stygobiont Namanereis are depicted in figures of N. cavernicola Solís-Weiss & Espinasa, 1990 and N. christopheri sp. n. (Figs
Notably, N. hummelincki, N. occulta, N. cavernicola, and N. christopheri sp. n. share loss of both eyes and body pigmentation (Figs
A possible explanation of the similar ALd/Ln in stygobiont Namanereis and epigean Namanereis and Namalycastis species is related to the shape of neuroacicular lobes. In some species, neuroacicular lobes are subconical, i.e., longer than wide with pointed tips, while in other they are rounded, i.e., as long as wide with rounded tips; then, species with relatively short dorsal cirri and rounded neuroacicular lobes have high ratios because rounded lobes are shorter respect to subconical ones. The effect of this difference is appreciable in N. araps, N. amboinensis and N. christopheri sp. n.: they have similar ALd/Ln but with evident subconical neuroacicular lobes in the former species and rounded ones in the last two (
As expected, most Namalycastis had high values of Dp−a since elongation of dorsal cirri toward posterior chaetigers is usual of that genus (
Based on the results of the analysis performed (Fig.
As shown above, the elongation and articulation of appendages are not restricted neither to Namanereis species nor stygobiont ones, but are rather present in several species occurring in different habitats. Some confusion can be led by evaluating the elongation of appendages a priori as troglomorphic features. For example,
While the factors driving the elongation of appendages in namanereidins are elusive, the evidence in other groups is weak or inconclusive. For example, the elongation of some segments of legs seems to be unique among stygobiont millipedes, but not the elongation of antennae (
Neurochaetal arrangement. The arrangement of neurochaetae deserves additional comments.
By definition, types C and D do not have chaetae in postacicular fascicles (Fig.
On the other hand, type A arrangement, typical of Namalycastis species, is characterized by having only falcigers in both supra- and sub-preacicular fascicles, and only spinigers in both supra- and sub-postacicular ones (Fig.
Another species from the Gulf of Mexico, Namalycastis intermedia, was described as having a type A arrangement, but heterogomph falcigers in both supra- and sub-preacicular fascicles are replaced by heterogomph spinigers toward posterior chaetigers (
Summarizing, Namalycastis caetensis, N. intermedia and N. nicoleae are species with type A arrangements but also with elongated falcigers or “pseudospinigers” in pre- or postacicular fascicles, resembling the type D arrangement. However, in type A species, typically the chaetae are clearly positioned in their fascicles and are abundant (e.g.
Jaws. The jaws of nereidids are formed by a cross-linked matrix of proteins, where the hardness and stiffness properties are due to the presence of high levels of glycine and histidine, halogens (especially chlorine) and zinc; the distribution of these elements is not homogeneous throughout jaw, but is more concentrated at the tip (
It has been suggested that bifid jaws in stygobiont Namanereis are a derived condition from the typically serrated jaws in nereidids, implying that species with bifid jaws arise from other Namanereis or Namalycastis ancestor with serrated jaws (
Based upon these previous ideas, another plausible scenario leading to bifid jaws is a hypothetical reduction or loss of teeth (Fig.
Based on the literature already cited, Namanereis and Namalycastis include species in various stages along these hypothetical evolutionary stages. This pattern in jaw morphology has been explained as a shift towards deposit-feeding habits for shovelling (
Additional considerations. Although elongation of appendages is not restricted to stygobitic namanereidins, the bifid jaws and absence of eyes are the only features present in namanereidin species living in aphotic environments. Up to date, all bifid-jawed species are blind, whereas some blind species have serrated jaws. A possible explanation to the distribution of troglomorphic features among extant namanereidins is that such features appeared in their ancestors before reaching subterranean habitats, in intermediate habitats called superficial subterranean habitats (SSHs) (
However, the current epigean distribution of Namalycastis occulta in Yucatan Peninsula likely is due to a secondary invasion of those habitats. Most land of Yucatan Peninsula uplifted from Jurassic, but it was submerged from Upper Cretaceous through Eocene, and progressively emerged since Oligocene (
Moreover, this also means that the use of troglomorphic features as evidence of phylogenetic affinity must be avoided since they could be convergent features, in disagreement with previous studies (
The use of some disregarded morphological features, as the number of tentacular cirri, could help to find better delimited groups.
Among all nereidids, only some Namanereis species and Lycastonereis indica Rao, 1981 have three pairs of tentacular cirri (
Namalycastis occulta clearly resembles Namanereis minuta and N. stocki by having four pairs of tentacular cirri. Their main difference is the evident elongation of dorsal cirri toward posterior chaetigers and that it becomes flattened in the former species, whereas in the last species they are subequal and cirriform throughout body.
If Namalycastis occulta is to be regarded as belonging into Namanereis, some problems could arise, as the mixture of current diagnostic features for both genera, exacerbating the problem of delimitation. Indeed, the elongation of appendages, bifid jaws and absence of pigmentation as stygobitic adaptations could explain the observed morphology, but could not account for the elongation and flattening of dorsal cirri towards the posterior body region only, as well as the elongation of sub-acicular, dorsalmost neurochaetae also matches this anterior-posterior gradient, both occurring in Namalycastis species as well (see above). It is true that N. occulta has not been recorded in cave environments yet, but other Namanereis species regarded as having stygobitic adaptations such as N. hummelincki, N. serratis Glasby, 1999 and N. tiriteae are in the same condition.
Of course, the most reliable way to test these and other hypotheses is through new phylogenetic analyses; however, there are some problems preventing it. As
Morphological comparison among namanereidins. Namanereis hummelincki (A, E) (USNM 29715, 29716); Namanereis cf. amboinensis (B, F, I, L) (ECOSUR P-2902); Namalycastis occulta (C, G, J, M) (ECOSUR P-2649); N. borealis D, H, K, N. A–D (ECOSUR P-2651). Anterior ends, dorsal view E–H Right parapodia from anterior (10, left) and posterior (right) chaetigers I–K Left jaws, dorsal view L–M Dorsalmost (left) and ventralmost (right) sub-acicular, heterogomph falcigers from chaetiger 10. Scale bars: 0.5 mm (A–D); 0.1 mm (E–H); 50 µm (I–K); 10 µm (L–N).
Chaetal arrangement of some namanereidins referred in this study. A Scheme showing the parapodial fascicles and their positions in relation to neuroaciculum B Neuropodial lobe of right parapodium from chaetiger 10 of Namalycastis occulta, showing the position of chaetae in type D species when mounted C–E Chaetal arrangement of types A, C and E, and their variants found in literature (modified from
Proposed evolution of jaws in Namanereidinae. A Jaws with fully dentate cutting edge, layer almost inconspicuous in basal teeth B Layer increases their extension among basal teeth and reaches medial ones C Layer covers completely basal and medial teeth, and reaches subterminal ones D Teeth recede and become narrow, layer covers subterminal teeth excepting the last two E Just remnants of teeth are visible if any, layer increases its extension F Teeth fully replaced by the layer. Stages B and C in most Namalycastis species and Namanereis with serrated cutting edge; stage D as in Namanereis stocki and N. tiriteae; stage E as in Namanereis socotrensis; stage F as in Namanereis with bifid jaws and Namalycastis occulta.
The author is in debt to Leslie Harris and David Ocker for providing accommodation and facilities in Los Angeles, special thanks to Leslie for the loan of specimens and facilities given at the LACM, and to Geoff Keel and Karen Osborn for the facilities given during a stay at the USNM. Thanks are due Sergio I. Salazar-Vallejo and Citlalli de Jesús-Flores for reading an early draft, and to Sergio and Luis F. Carrera-Parra by providing support for a visit. The author is grateful for the valuable suggestions from two anonymous reviewers in an early submission in another journal; also, the recommendations from Robin Wilson and Christos Arvanitidis in SB greatly helped to improve the manuscript.