Magelona alexandrae, Magalhães & Bailey-Brock & Watling, 2018

Magalhães, Wagner F., Bailey-Brock, Julie & Watling, Les, 2018, Four new species of Magelona (Annelida: Magelonidae) from Easter Island, Guam and Hawaii, Zootaxa 4457 (3), pp. 379-396: 381-384

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Magelona alexandrae

sp. nov.

Magelona alexandrae   sp. nov.

Figures 1 View FIGURE 1 and 2A ‒B View FIGURE 2

Material examined. Holotype: Hawaii, Oahu, Mamala Bay , Cage , Sta. FWR2, 21°17'22.4" N, 158°00'36.7" W, June /2002, 39.6 m ( USNM 1494155 View Materials ) GoogleMaps   . Paratypes: Same station as holotype (2 complete specimens, USNM 1494156 View Materials ; 1 complete specimen, BPBM-R3870). Additional material examined. Hawaii, Oahu, Mamala Bay , Barbers Point outfall, Sta. HB 7R4, 21°15'30.0" N, 158°03'11.3" W, March /1998, 65 m (1 complete specimen with 58 chaetigers); Sta. HB 3R4, 21°16'52.0" N, 158°01'30.5" W, Mar /2016, 68 m (1 af); Sta. HB 1R1, 21°16'50.5" N, 157°59'20.3" W, Mar /2016, 64.6 m (1 af); Sta. HB 6R4, 21°16'33.1" N, 158°01'48.2" W, Mar /2016, 59.4 m (1 af). GoogleMaps  

Diagnosis. Prostomium longer than wide, with distinct prostomial horns. Notopodia of chaetigers 1‒8 with elongate foliaceous postchaetal lamellae with distinctly crenulate upper edges. Superior processes elongate and present from chaetigers 1‒8. Chaetiger 9 with only bilimbate simple capillaries. Abdominal hooded hooks tridentate. Posterior abdominal lateral pouches (C configuration) present from chaetiger 40, paired.

Description. Holotype: complete specimen with both palps attached; prostomium 0.64 mm long, 0.39 mm wide; thorax 3.34 mm long (including prostomium), 0.34 mm wide; abdomen 0.29 mm wide; total length 25.78 mm for 68 chaetigers. Paratypes: Three complete paratypes measured; prostomium 0.59‒0.67 mm long, 0.30‒0.36 mm wide; thorax 3.2‒3.76 mm long (including prostomium), 0.26‒0.30 mm wide; abdomen 0.28‒0.39 mm wide; total length 16.49‒25.80 mm for 60‒66 chaetigers.

Prostomium longer than wide (L:W ratio 1.64‒1.96), slender; anterior margin smooth and rounded triangular, lateral margins rounded, with conspicuous prostomial horns; horns distinctly separated from antero-lateral margins of prostomium; prostomium with lateral convoluted markings ( Figs 1A, B View FIGURE 1 , 2A View FIGURE 2 ). Two pairs of thick prominent longitudinal dorsal ridges, transverse ridges absent; outer pair, extending basally and approaching achaetous segment; inner pair diverging anteriorly into horn corners and posteriorly towards prostomial base, with slight medial gap along its whole length ( Fig. 1A View FIGURE 1 ). Burrowing organ partially everted in one paratype, saclike, ridges not observed. Slender palps arising ventro-laterally from base of prostomium and extending to chaetigers 20‒30 ( Fig. 1A View FIGURE 1 ); palps measuring 8.14 mm long in holotype and ranging from 8.43‒9.96 mm long in paratypes; non ‒papillated region short reaching anterior end of chaetiger 2 on holotype ( Fig. 1A View FIGURE 1 ). Papillae short proximally but long for majority of length, digitiform; proximally and medially with four rows of papillae, decreasing to two rows distally. Achaetous segment long, twice the size of chaetiger 1 ( Fig. 1A View FIGURE 1 ).

Thoracic segments slightly longer than wide; chaetigers 1‒8 similar; parapodia biramous; notopodia with low prechaetal lamellae confluent with foliaceous postchaetal lamellae of similar size throughout thorax; prechaetal lamellae joins far down postchaetal lamellae length ( Fig. 1D ‒F View FIGURE 1 ); notopodial postchaetal lamellae distally crenulate forming distinct bumps, elkhorn shaped, crenulations present along entire thorax and similar throughout ( Fig. 1D ‒F View FIGURE 1 ). Elongate prechaetal superior processes present (DML). Neuropodial pre- and postchaetal lamellae as low ridges on anterior thorax; ventral lobes (VNL) lobes elongate and decreasing in length on posterior thoracic segments; neuropodial postchaetal expansion as low ridge to chaetiger six, from chaetiger seven toward chaetiger nine forming a digitiform and distinct lobe ( Fig. 1J ‒M View FIGURE 1 ). Chaetiger nine constricted along its length and width; chaetiger nine ( Fig. 1G View FIGURE 1 ): notopodial prechaetal lamellae low, rounded, postchaetal lamellae slightly shorter than preceding thoracic chaetigers, not crenulate; superior processes absent. Neuropodia of chaetiger nine with digitiform prechaetal process and short triangular postchaetal lobes. Chaetae of all thoracic chaetigers bilimbate simple capillaries ( Fig. 1C View FIGURE 1 ); number of capillaries increasing posteriorly, 10‒12 notopodial and neuropodial capillaries anteriorly increasing to up to 20 bilimbate simple capillaries per ramus on chaetiger nine.

Anterior 10‒12 abdominal segments as long as wide, becoming longer than wide posteriorly. Abdominal parapodia with foliaceous lateral lamellae with low triangular expansions behind chaetal rows, edges crenulated in both rami ( Fig. 1H View FIGURE 1 ); DML and VML present, minute. Abdominal chaetae tridentate hooded hooks of similar size throughout; two same-sized teeth above main fang ( Fig. 1I View FIGURE 1 ); internal arcuate chaetae not observed. Hooks in two groups, main fangs vis –à –vis. Anterior abdominal segments with 5‒6 hooks per ramus, 3‒4 hooks more posteriorly.

Anterior abdominal lateral pouches (Σ configuration) absent and posterior abdominal lateral pouches (C configuration) present from chaetiger 40 of holotype, paired, present till chaetiger 61 and absent on last 6‒7 chaetigers. Additional specimens with posterior abdominal lateral pouches present from chaetigers 40‒49.

Pygidium rounded with a pair of digitiform, short, lateral anal cirri ( Fig. 1N View FIGURE 1 ).

All preserved specimens lacking pigmentation, pale yellow. Specimens previously stained with Rose Bengal presented pink patches intensified with methyl green staining (described below).

Methyl Green Staining ( Fig. 2B View FIGURE 2 ). Thoracic segments with dorsal transverse patches of green speckles, slightly arched; patches more evident from chaetiger 3 to posterior end of thorax. Transverse patches also present abdominally, more evident on anterior 20 abdominal segments. Abdomen with round pigmented region in between two subsequent parapodia (interparapodial). Thoracic and abdominal ventral region not distinctly stained.

Remarks. Magelona alexandrae   sp. nov. is readily distinctive from its congeners by the elkhorn-shaped thoracic notopodial lamellae. Other Magelona   species with distinct crenulations on thoracic notopodial lamellae are M. pectinata Nateewathana & Hylleberg, 1991   , M. johnstoni Fiege, Licher & Mackie, 2000   , M. marianae Hernández-Alcántara & Solís-Weiss, 2000   , M. montera Mortimer, Cassà, Martin & Gil, 2012   , and M. obockensis Gravier, 1905   . Magelona pectinata   and M. johnstoni   lack prostomial horns and have specialized chaetae on chaetiger 9. It differs from M. obockensis   which has distinct bi-lobed notopodial thoracic lamellae. Magelona alexandrae   differs from M. montera   in possessing bidentate and not tridentate abdominal hooded hooks and on the prostomial shape and nature of the frontal horns in M. montera   with bulbous distal tips. Magelona alexandrae   sp. nov. is most similar to M. marianae   by the distinct prostomial horns and shape of parapodial lamellae from chaetigers 1‒8. Magelona marianae   differs from M. alexandrae   sp. nov. in the presence of superior processes (DML) on chaetiger 9 and bidentate hooks instead of tridentate hooks.

Etymology. This species is named after Dr. Alexandra Rizzo, a polychaete researcher who has been an inspiration and friend to the first author (WFM) and has collaborated to the description of several new polychaete species to Hawaii.

Distribution. This species is only known from Mamala Bay, south shore of Oahu, Hawaii, 39‒ 68 m.