Meganyctiphanes norvegica (M. Sars, 1857)
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https://doi.org/ 10.1093/zoolinnean/zlac083 |
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https://doi.org/10.5281/zenodo.7814232 |
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https://treatment.plazi.org/id/D96287D1-4B6A-321D-D476-16F9FEEC11A4 |
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Plazi |
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Meganyctiphanes norvegica |
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MEGANYCTIPHANES NORVEGICA View in CoL View at ENA
The first pair of thoracopods ( Fig. 7A View Figure 7 ) to Thp7 are all similar, whereas Thp8 is vestigial. The following descriptions are based on a detailed examination of the right Thp1 of three adult females of M. norƲegica .
Cuticle and skeletal structures of thoracopods in M. norvegica
The following description refers collectively to Thp1–7, if not specified otherwise. The ventral cuticle of the first thoracomere is coalesced with that of the cephalon. The density of the cuticle of this cephalothoracic sternum appears higher than in the posterior thoracomeres. Anteriorly, the thoracic cuticle articulates with the maxilla, which is tightly fitted against the anterior wall of the coxa and basis of Thp1. In all thoracomeres, the posterior thoracic cuticle is folded in an anterior direction, forming a broad roof-like apodeme (partly dashed curved arrow in Fig. 7B, C View Figure 7 ) of moderate density that hangs over the posterior half of the proximal foramen of the leg (i.e. of the coxa). Medially, the cuticle of the thorax forms a posterolaterally and dorsally directed finger-like extension (dashed black outline on Th in Fig. 7B View Figure 7 ).
The protopod of M. norƲegica comprises a coxa (onethird) and a subdivided basis (two-thirds of the total length of the protopod). The coxa is predominantly made up of its more solid distal margin, which forms a half-ring reaching from posterior over medial to anterolateral (Co in Fig. 7D View Figure 7 ), where it connects to a plate of less dense cuticle. From the anterolateral corner of this plate, a solid cuticular beam reaches dorsad (Cb in Fig. 7C, D View Figure 7 ) and articulates with the thorax (*l in Fig. 7B–D View Figure 7 ). Posterolateral on the coxa, an epipod arises, which on Thp1 is plate-like (Ep in Fig. 7A, B View Figure 7 ), extending somewhat dorsad, but mostly ventrad (almost reaching the exopod). On the posterior thoracopods, the epipod splits into two coiled branches (one dorsal and one ventral) that ramify into several smaller filaments; on Thp6–8, the epipods show the highest complexity, with an additional ventral off-branch that ramifies in a similar way. Medially attached to the coxa of Thp1, a setose endite is present (En, Fig. 7A, C View Figure 7 ). The endite forms the medial articulation point with the thorax, immediately beneath the finger-like extension (*m in Fig. 7B View Figure 7 ). On the following thoracopods, the endite is much smaller, if present at all ( Fig. 7E View Figure 7 ). Apart from the cuticular half-ring constituting the distal margin of the coxa, its anterior and posterior parts are mostly membranous, with two (posterior and posterolateral) invaginations. The posterolateral invagination forms a tight proximodistal fold, which expands into the central space of the coxa (curved arrows in Fig. 7D View Figure 7 ), where it forms a solid and highly complex endoskeletal structure (dark blue part of Co in Fig. 7B–D View Figure 7 ; see also the Supporting Information, Video S1). Starting from a posterolateral arm, the structure reaches into the centre of the coxa, where it expands into the dorsal and ventral directions. Dorsally, it reaches onto the roof-like apodeme of the thorax. Ventrally, it reaches onto the proximal margin of the basis by extending another two straight arms in anterior and posterior directions.
Table 2. Continued
The basis comprises a solid distal part (Ba in Fig. 7A–E View Figure 7 ), whereas its proximal part (InB in Fig. 7C–E View Figure 7 ) consists of three posterior and lateral plates and a broad anterior part. Owing to the resemblance to the anaspidan intrabasis, the proximal part(s) of the basis are referred to as the intrabasis in the following description. The anterior part of the intrabasis is separated from the larger distal basis by an inconspicuous furrow (dashed white line in Fig. 7C, E View Figure 7 ). Anteriorly, it forms a small bulb at its proximal margin ( Fig. 7E View Figure 7 ), into which the anterior arm of the coxal endoskeleton reaches. Proximal to the anterior bulb, at the proximal margin of the intrabasis, sits a somewhat triangular plate (white outline on InB and partly dashed curved arrow in Fig. 7C View Figure 7 ), which articulates with the coxa (a + in Fig. 7C View Figure 7 ). Another articulation point is realized between the coxa and the posteromedial plate of the intrabasis (p + in Fig. 7D View Figure 7 ), resulting overall in a bicondylar coxa–intrabasis articulation (ax Co-InB in Fig. 7B View Figure 7 ). The medial cuticle of the distal basis appears robust, whereas laterally it is mostly membranous. In this area, two rigid, finger-like structures emerge (one posterolateral and another distoanterolateral) and articulate with two hook-like structures of the exopod ( Fig. 7B, C View Figure 7 ; see also Supporting Information, Video S1). The posterolateral articulation is supported by a longish sclerite positioned immediately distal to the posterolateral finger. The medial cuticle of the distal basis appears robust and carries numerous setae. Distally, the basis is coalesced with the endopod (i.e. with the ischium), with a clear suture but no interpodomere membrane or articulation points ( Fig. 7C, D View Figure 7 ).
Within the lateral membrane of the distal basis, the exopod attaches (Ex in Fig. 7A–D View Figure 7 ). Its peduncle comprises a short posteroventral stem and a much longer distal portion, followed by the flagellum. The peduncle is anteroposteriorly flattened and has a prominent distodorsal pointed protrusion. Proximally, the peduncle articulates with the basis both anterolaterally and posterolaterally via hook-like structures (one from the peduncle stem and one from the distal portion) that are interlocked with the finger-like structures of the distal basis (short arrow in Fig. 7B View Figure 7 ; see also Supporting Information, Video S1).
The endopod consists of the ischium, merus, carpus, propodus and dactylus. The ischium (Is in Figs 7A View Figure 7 , 8A View Figure 8 ) is about twice as long as the protopod. Anteriorly, the ischium forms an articulation with the merus (arrow in Fig. 8D View Figure 8 ). However, posteriorly an articulation point is not clearly identifiable. Nevertheless, the proximal margin of the merus forms a prominent posterolateral protrusion, which overlaps slightly with the distal margin of the ischium (dashed line in Fig. 8D View Figure 8 ).
The merus (Me in Fig. 8A View Figure 8 ) appears slightly longer than the ischium. Two long sclerites/tendons emerge laterally and medially from the proximal margin of the merus and reach far into the ischium. A bicondylar articulation with the carpus is formed by a medial and a lateral articulation point, respectively (ax Me-Ca in Fig. 8A View Figure 8 ).
The carpus (Ca in Fig. 8A View Figure 8 ) measures approximately one-third of the total length of the merus. One short and two long sclerites/tendons emerge anteriorly (long) and posteriorly (long and short) from the proximal margin of the carpus and reach far into the merus. The distal margin forms anteriorly a noselike condyle that articulates with a broad anterior depression at the proximal margin of the propodus ( Fig. 8B, C View Figure 8 ). A thin, longish sclerite articulates laterally with the distal margin of the carpus and posteriorly with the proximal margin of the propodus ( Fig. 8B, C View Figure 8 ). Combined, these structures probably realize a somewhat less restricted bicondylar articulation.
the furrow between the intrabasis and basis; black lines emphasize individual plates of the coxa; white lines and partly dashed curved arrows emphasize the posterior thorax apodeme and the proximal anterior triangular plate of the intrabasis. D, posterior view; dashed white line emphasizes the posterior plates of the intrabasis; dashed white circle indicates the anterior bulb of the intrabasis; black lines emphasize individual parts of the coxa and the margins of the intrabasis; curved arrows indicate posterolateral coxa invagination. E, Thp3, anterior view; dashed line emphasizes the furrow between intrabasis and basis. Abbreviations: a*, anterior articulation point; ant, anterior; ax, articulation axis between […]; Ba, basis; Ca, carpus; Cb, proximolateral cuticular beam of coxa; Co, coxa; Da, dactylus; dis, distal; En, endite; Ep, epipod; Ex, exopod; InB, intrabasis; Is, ischium; l* and *l, lateral articulation point; lat, lateral; *m, medial articulation point; Me, merus; med, medial; p*, posterior articulation point; Pr, propodus; prox, proximal; Th, thorax.
The propodus (Pr in Fig. 8A View Figure 8 ) is about twice as long as the carpus. One short and three long sclerites/ tendons emerge from the anteromedial, posteromedial, anterolateral (all three long) and posterolateral (short) corners of the proximal margin of the propodus and reach into the carpus. The propodus articulates with the dactylus by a medial and a lateral articulation point, respectively (ax Pr-Da in Fig. 8A View Figure 8 ).
The dactylus is about half the length of the propodus in Thp1 (Da in Figs 8A View Figure 8 , 9E View Figure 9 ) and somewhat longer than half the propodus in the posterior thoracopods. Two long sclerites/tendons emerge anteriorly and posteriorly from the proximal margin of the dactylus and reach into the propodus. The podomere tapers from its proximal to its distal margin, except for Thp2, where it is more lateromedially flattened, forming a slight distoposterior bulge before tapering towards a distoanterior tip.
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.
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