Magelona symmetrica Mortimer & Mackie, 2006

Mortimer, Kate, Cassà, Susanna, Martin, Daniel & Gil, João, 2012, New records and new species of Magelonidae (Polychaeta) from the Arabian Peninsula, with a re-description of Magelona pacifica and a discussion on the magelonid buccal region, Zootaxa 3331, pp. 1-43 : 27-32

Magelona symmetrica Mortimer & Mackie, 2006

Figures 12 , 13 L

Magelona symmetrica Mortimer & Mackie, 2006: 127 –128, fig. 2

Material examined. Persian Gulf, IRAN—Stn. 8 ( NMW.Z.2010.037.0044; 10 af, and 1 dissected af), 1998; Stn. 13 ( MNCN.16.01/13250; 3 af), 1998; Stn. 14 (MB29–000207; 7 af), 1998; Stn. 8(1) ( MNCN.16.01/13251, 11 af; MB29–000208, 9 af; NMW.Z.2010.037.0045, 9 af), 2002; Stn. 9(1) ( NMW.Z.2010.037.0046; 8 af), 2002; Stn. 13(1) ( NMW.Z.2010.037.0047; 21 af), 2002; Stn. B3–10C ( NMW.Z.2010.037.0048; 1 af), 2005; Stn. B4–10 ( NMW.Z.2010.037.0049, grab A, 4 af; NMW.Z.2010.037.0050, grab B, 5 af; NMW.Z.2010.037.0051 grab C, 10 af), 2005. QATAR—Stn. E72 ( NMW.Z.2010.037.0052, grab A, 1 af; NMW.Z.2010.037.0053, grab B, 1 af), 2005; Stn. I56 B ( NMW.Z.2010.037.0054; 2 af), 2005; Stn. SC −2000 ( NMW.Z.2010.037.0055; 1 af), 2007; Stn. SBW 9(1) ( NMW.Z.2010.037.0056; 2 af), 2007.

Diagnosis. Stout species. Prostomium wider than long, subtrapezoidal, without prostomial horns. Palps robust, densely papillated. Chaetigers 1 − 9 with slender, smooth-edged, triangular postchaetal lamellae. Thoracic chaetigers with capillary chaetae. Abdominal lateral lamellae long, pointed triangular. Hooded hooks tridentate, in two facing groups (vis-à-vis).

Description. A large, stout species ( Figure 12 A); thorax marginally wider, but thinner than abdomen; no distinct constriction at chaetiger 9. All specimens posteriorly incomplete. Dimensions of figured anterior fragment: prostomium 0.5 mm long, 0.6 mm wide; thorax (including prostomium) 2.8 mm long, 0.9 mm at maximum width; abdomen 0.85 mm wide; total length 13.0 mm for 39 chaetigers. Dimensions of longest specimen: 65 chaetigers for 23 mm; 1 mm wide. Other specimens with 9 − 29 chaetigers for 0.75 − 12 mm in length.

Prostomium wider than, or occasionally as wide as long ( Figures 12 B and C, 13L), (L:W ratio 0.63 − 1.00); subtrapezoidal; anterior margin smooth, medially indented in many specimens. Prostomium postero-lateral margins sometimes folded upwards giving the appearance of a much wider prostomium (see Figures 12 B and 13L); prostomial horns absent. Two pairs of prominent longitudinal dorsal muscular(?) ridges; inner pair anteriorly divergent and lightly ridged transversely; degree of separation variable, often entirely separated. Outer pair abutting inners. The proboscis is everted in nine specimens, oval when partially everted, heart-shaped when fully everted; longitudinally ridged inferiorly, superiorly appearing smooth. Palps attached in 29 specimens. Palps arise ventrolaterally from base of prostomium, short, reaching chaetigers 10 − 20; robust and heavily papillated. Non-papillated region reaching to chaetigers 2−3. Palps with four rows of papillae either side of groove (groove fairly conspicuous in most specimens), reducing to one row either side at distal tip (two to three rows either side medially); papillae long, length decreasing only at proximal end.

Achaetous region behind prostomium approximately 1 and half times the size of chaetiger 1. Chaetigers 1 − 8 similar; parapodia biramous ( Figures 12 D − H); low triangular notopodial prechaetal ridges confluent with slender triangular postchaetal lamellae, tips of which are pointed. Neuropodial postchaetal lamellae triangular with low prechaetal ridges; initially ventral in position, becoming entirely postchaetal from chaetiger 5 − 6. Thoracic lamellae reducing gradually in size along thorax.

Chaetiger 9: Notopodia similar to preceding chaetigers. Prechaetal lamellae low ridges in both rami ( Figure 12 I). Neuropodial lamellae triangular, superior lateral in position; low inferior postchaetal ridges often terminating inferiorly in small triangular processes (processes present in larger specimens, although very small, not seen in smaller material. Sometimes observed only on one side of a chaetiger). Parapodia of chaetiger nine often interposed in furrow between surrounding chaetigers, particularly those of the neuropodia. All thoracic chaetae simple capillaries.

Abdominal chaetigers with sharply-pointed triangular lateral lamellae, of about equal size in both rami ( Figure 12 J). Lateral lamellae do not extend postchaetally, hooks arising from definite ridge. No dorsal or ventral (DML and VML) processes observed on abdominal chaetigers.

Abdominal chaetae all tridentate hooded hooks ( Figures 12 K − M) of similar size. Hooks in two groups, main fangs vis-à-vis. Initially about 10 hooks per ramus.

Paired anteriorly open lateral pouches absent on anterior abdominal chaetigers. Posteriorly open pouches not observed. Pygidium unknown.

Colour. Observations made on preserved material; now cream-white in alcohol, some still stained with Rose Bengal. Several specimens with pale reddish staining in posterior thorax, not as marked as a pigment band. Glandular areas particularly noticeable in abdomen as large intense white speckled interparapodial patches (often starting from chaetiger 8). Methyl green staining rather diffuse, no obvious pattern. Slightly darker staining in mid-dorsal (chaetigers 4 to 7) and mid-ventral areas of thorax. White speckled areas noticeable, dorsally next to parapodia ( Figure 12 A) between chaetigers 1 − 5. Ventrally white speckles strongest between chaetigers 8–13 (associated with mid-ventral line) continuing into abdomen, with additional patches around chaetigers 2–5. Rose Bengal staining dorsally noticeable between chaetigers 2–5 around parapodia (not medially), strongest on 3–4, but also present between 4–5 (if present, seen as sparse speckled areas). Small interparapodial patches are present between chaetigers 6–7, and stronger patches between chaetigers 8–9. Abdominal interparapodial areas showing stain with Rose Bengal, as do the white speckled areas of the venter.

Habitat. 107 specimens found, distributed at eight stations from three different surveys off Iran, medium sand, and shelly muddy sand, 10–21 m, and four stations from three surveys off Qatar, coarse sand, medium sand, and muddy sand with shell debris 17– 58 m. Evidence of a tube present on several specimens, covered with minute sand grains.

Distribution. Iran, Qatar (present study), Seychelles ( Mortimer & Mackie 2006).

Remarks. The Persian material observed here conforms with the type specimen and description extremely well. The only difference observed is the presence of pale reddish pigment in the posterior thorax of some of the Persian material. Magelona symmetrica was originally described from one specimen in which pigment in the posterior thorax was not observed. However, the presence of thoracic pigment amongst the Persian material was sporadic. The fact that pigment in Persian material was pale (never forming a distinct band) even in material collected more recently, and was only present on a small number of specimens suggests that this is a variable character within this species. Observation of live or fresh material may help to resolve this. The holotype of M. symmetrica possessed small ventral processes on the neuropodia of chaetiger 9. These were not always observed in the Persian material, even on different sides of the same chaetiger. However even when present, they are minute and often difficult to discern. Their presence/conspicuousness may well be related to size of a specimen. This may also be resolved in the future by the examination of additional material.

Of all the Persian species observed here, M. symmetrica shares the most similarities with Magelona cf. cincta , and these two species can be difficult to separate in preserved material, particularly when specimens are small. However, the following differences between the two species allow separation. Magelona cf. cincta possesses slightly scooped shaped ventral neuropodial lamellae on chaetigers 1–3; the pigment seen in the posterior thorax is a distinct band of a dark reddish/brown; the thoracic neuropodial lamellae decrease sharply in size from chaetigers 1–5; the chaetigers of the mid-thorax are characteristic bulbous; the prostomium has limited ornamentation either side of the prostomial ridges; a distinct methyl green staining pattern is present, and abdominal lamellae are broader at approximately one third of their length, not at the base. All specimens of Magelona cf. cincta observed were small in comparison to the majority of M. symmetrica specimens observed.

In contrast, the generally larger M. symmetrica specimens possess: sharply pointed triangular lamellae on chaetigers 1−3; a prostomial margin which is often medially indented (prostomium sometimes curving upwards) and is much more highly ornamented; neuropodial lamellae are of a similar size throughout the thorax; the pale reddish pigment in the posterior thorax (if present) does not form a distinct band, even in newer material (when comparing material of the two species from the same survey, the pigment seen in Magelona cf. cincta is much stronger); M. symmetrica is a stout species, however does not possess characteristically bulbous chaetigers in the mid-thorax; a distinct methyl green staining pattern is not present; intense glandular areas interparapodially are present from chaetiger 8 and abdominal lamellae are sharply triangular. However, it is important to note that the pigment band of Magelona cf. cincta does fade over time, and was observed here to be paler in older material. It is likely that some of these observed differences may be due to the size of the species, such as the ornamentation of the prostomium, given that the specimens of Magelona cf. cincta are much smaller.

Figures

FIGURE 12. Magelona symmetrica (A, NMW. Z. 2010.037.0055; B, NMW. Z. 2010.037.0047 b; C – M, dissected 23 chaetiger specimen NMW. Z. 2010.037.0044 b): (A) anterior, dorsal view, showing methyl green staining between chaetigers 4 − 9, abdominal interparapodial glandular areas and dorsal white speckled areas between chaetigers 1 − 4 and 8 − 12 (note distal tip of prostomium curled downwards); (B − C) Prostomia dorsal views; (D − J) chaetigers 1, 3, 4, 7, 8, 9, 13 respectively (anterior views); (K − M) tridentate abdominal hooded hooks (oblique lateral, frontal and lateral views respectively).

FIGURE 13. Prostomia of Arabian Peninsula Magelona species, dorsal views: (A) M. obockensis, 39 chaetiger anterior fragment, proboscis not everted, both palps attached, syntype, MNHN Type 1357; (B) M. obockensis, BMNH: ZB. 1971.54; (C) M. pulchella, holotype, BMNH 1969.391; (D) M. cornuta, holotype, ZMUC-POL- 969; (E) M. montera sp. nov. MNHN A 895; (F) M. sinbadi sp. nov., NMW. Z. 2010.037.0001; (G) M. cf. agoensis, NMW. Z. 2010.037.0004; (H) M. cf. cincta, NMW. Z. 2010.037.0041; (I) M. conversa, BMNH. 2010.234; (J) M. crenulifrons, NMW. Z. 2010.037.0024; (K) M. cf. falcifera, NMW. Z. 2010.037.0010 b; (L) M. symmetrica, NMW. Z. 2010.037.0047 b.