Anonyx nugax (Phipps, 1774)

Soldatenko, Elena V., 2021, Morphology of the mouthparts and digestive system in two species of Uristidae Hurley, 1963 (Amphipoda: Lysianassoidea), Journal of Natural History 55 (7 - 8), pp. 471-494 : 474-480

Mouthparts: Anonyx nugax

Epistome and upper lip

The epistome ( Figure 1 (a,b)) is produced, conical, subacute and lacks any spines or setae on the aboral surface, but is evenly covered with simple setae on the oral surface. The ventral margin of the epistome bears a rounded medial projection (pe, Figure 1 (a,b)) that in the resting position fits between the mandibular incisors. The epistome and the upper lip are separate. Posterolaterally on the oral surface of the epistome is a pair of deep depressions (ed, Figure 1 (a,b)) that receive the articulating knobs of the mandibles.

Mandibles

The hinge line of the mandible is slightly oblique and is located at the posterior end of the mandible (hl, Figure 2 (a)). In the resting position, the palps are held upward so that they lie between the antennae (md pl, Figure 5 (a)), with article 3 directed anteriorly. The mandibles are asymmetrical in the morphology of the incisor, setal row and lacinia mobilis. The molar process (mo, Figure 2 (a–d)) is large, roughly rectangular and is oriented obliquely to the mandibular body in the posteromedial direction. In the resting position, the tips of the opposite molar processes do not meet at the midline lying a short distance apart ( Figure 2 (a)). The tip of each molar process is slightly rounded; it is densely pubescent and lacks a triturating surface ( Figure 2 (d)). The setal row is located on the oral side of the mandible near its medial margin and is oriented perpendicular to the blade of the incisor. It starts about 100–150 µm from the incisor ( Figure 2 (b,c)) and reaches the basal portion of the molar process (sr, Figure 2 (d)). The three apical setae (near the incisor) on each mandible are very large and spine-like (ar, Figure 2 (a–c)); the remaining setae are much thinner and hairlike. Since the setal row is located some distance from the medial margin of the mandible, in the resting position the setae of the opposite mandibles are kept widely apart ( Figure 2 (a)), but it is clear that in mandibular adduction the three apical setae of one mandible interdigitate with the corresponding setae on the other mandible. The blades of the incisors are oriented in a nearly vertical plane; in the adducted position the left incisor overlaps the right incisor anteriorly ( Figure 2 (a)). Both incisors have a wide and straight cutting edge ( Figure 2 (a–c,e)), but the edge of the left incisor is slightly longer ( Figure 2 (a)). The medial ends of both incisors bear long conical teeth ( Figure 2 (a)), with two teeth located on the left (ti, Figure 2 (b)) and one on the right mandible (ti, Figure 2 (c)). The teeth are arranged such that in mandibular adduction the right tooth fits into the gap between the left teeth. Midway between the incisor and the palp is a large, conical articulating knob (mk, Figure 2 (b)) that fits into the corresponding depression on the oral side of the upper lip (see above). The lacinia mobilis (arrow, Figure 2 (b); lm, 2(e)) is present only on the left mandible and has been reduced to a small, curved peg ( Figure 2 (e)). The palp is 3-articulate and located opposite the molar; its article 2 is slender and about 3 times as long as article 1.

Paragnaths

A pair of paragnaths ( Figure 1 (c)) bound the mouth posteriorly; in the resting position the mandibular bodies are closely pressed against their lateral sides and the paragnaths cover completely their molar processes. The paragnaths are symmetrical, only slightly longer than wide and are fused only at their bases. Both oral and aboral surfaces of the paragnaths are densely pubescent, except their lateral margins, which lack setae. On the oral surface of each paragnath there is a depression (pd, Figure 1 (c)) into which the molar processes of the mandibles fit. The aboral face of each paragnath forms a posteriorly directed lateral lappet with a pointed tip (lp, Figure 1 (c)).

Maxillulae

The maxillulae lie in a horizontal plane (ml, Figure 5 (a)) and are closely pressed against the paragnaths and mandibles. A short triangular coxa is attached to the cephalothorax with a wide articulating membrane (mmp, Figure 5 (a)). The inner plate (ml ip, Figure 3 (a,b)) is about twice as short as the outer plate; it has an elongated narrow basis and an expanded club-shaped distal portion. The inner plate carries apically two large pappose setae with long needle-like setules (pi, Figure 3 (a,b,d)). The basipod extends narrowly from the coxa and is articulated medially with the inner plate by a cuticular membrane and a triangular knob. The outer plate (ml op, Figure 3 (a,b,d)) is wide, almost cylindrical in shape. In the distal portion of the plate, there are two rows of large slender spine teeth with a 7/4 crown arrangement diagnostic for the Uristidae ( Figure 3 (a–c)): 7 teeth (st1–7) in an apical row (st1–4 one-cuspidate, st5–6 two-cuspidate, st7 three-cuspidate) and 4 teeth (stA-D) in a subapical row (stA one-cuspidate, stB three-cuspidate, stC one-cuspidate, stD threecuspidate). The outer plate also bears a U-shaped band of simple setae (bo, Figure 3 (b–d)); the two arms of this band extend along the medial margin and oral surface of the plate and are linked just proximal to st7. The palp (ml pl, Figures 3 (a–c), 4(a) and 5(a)) is 2-articulate; article 1 is triangular, about as long as wide, article 2 is long and curved towards the midline. The surface of the palp lacks setae, except a small setal field on the oral side. Wide distal ends of both palps are armed with 8 short spine-like setae (smp, Figure 3 (c)). In the resting position, the opposite palps meet at the midline behind the outer plates of the maxillipeds, with the left palp overlapping the right one anteriorly ( Figure 3 (a)).

Maxillae

The left and right maxillae are roughly equal in size and shape ( Figure 4 (a)). Small, triangular coxae (mx cx, Figure 4 (a)) arise from a common extension of the sternum. The surface of the coxa lacks setation. The basipod and both inner and outer plates extend from the coxa anteriorly in a horizontal plane (mx, Figure 5 (a)) partly concealing the large maxillulae from below ( Figure 4 (a)). The anteromedially oriented basipod is basally wide ( Figure 4 (a)), then tapers apically and curves to form an anteriorly oriented lancet-shaped inner plate (mx ip, Figure 4 (a,b)). The inner plate carries three closely parallel rows of simple setae along the medial margin ( Figure 4 (b,d)): the setae in the inner (ri1, Figure 4 (d)) and outer (ri3, Figure 4 (d)) rows are spine-like and much wider and longer than those in the middle row (ri2, Figure 4 (d)). The setae progressively increase in size towards the apex, but the most proximal seta in the inner row (arrow, Figure 4 (b,d)) is markedly larger than the others. A U-shaped field of thin simple setae encloses the three setal rows on either side and proximally (bi, Figure 4 (b,d)). The outer plate (mx op, Figure 4 (a,b)) is lancet-shaped and carries distally a small field of simple setae on the oral surface and two rows of simple setae along the medial margin ( Figure 4 (c)): the setae in the outer row (ro2, Figure 4 (c)) are much wider and almost twice as long as those in the inner row (ro1, Figure 4 (c)).

Maxillipeds

Maxillipeds consist of a fused coxa (mp cx, Figure 5 (a–c)), basipods (mp bs, Figure 5 (a–c)) with endites (inner plates), ischia (mp is, Figure 5 (a–c)) with endites (outer plates) and palps. The coxa is oriented in the vertical plane (mp cx, Figure 5 (a)), but near its distal tip it curves forward so that the remaining maxilliped articles lie in the horizontal plane covering completely the other mouthparts. The basipods touch across the midline but do not fuse ( Figure 5 (b)). A few long simple setae are located along the distal and medial margins on the aboral surface of basipods and the distolateral corners of the basipods and ischia; a row of simple setae extends across the aboral surface of the ischia and continues along the medial margin of all palpal articles ( Figure 5 (b,c)). The inner plates ( Figure 5 (e)) are very small, subrectangular; in the resting position, the opposite plates do not touch across the midline. Each plate bears apically three large pappose setae (ppi, Figure 5 (e)) that project from a deep longitudinal notch on the oral surface of the plate. Lateral to these setae on the apical tip of the plate is a pair of larger and another pair of smaller cuspidate setae (cpi, Figure 5 (e,f)). The outer plates (mp op, Figure 5 (a–d)) are large, lancet-shaped, about twice as long as the inner plates, finely serrated along the medial and anterior margins ( Figure 5 (d)) and lack setation except for a row of small reduced setae along the medial margin ( Figure 5 (d)). The palps are 4-articulate ( Figure 5 (a–c)); their articles 1 and 2 are wide, flattened and cover the maxillae laterally. In the resting position, the palps cross each other at the midline ( Figure 5 (b)).

Figures

Figure 1. SEM images of the upper lip and paragnaths of Anonyx nugax (a–c) and Tmetonyx cicada (d– f). (a) Upper lip and epistome in ventral view. (b) Upper lip in side view. (c) Paragnaths in ventral view. (d) Upper lip and epistome in ventral view. (e) Upper lip in side view. (f) Paragnaths in ventral view. Abbreviations: as – arch-shaped setal row on paragnaths; ed – depressions on epistome articulating with mandibles; ep – epistome; lb – labrum; lp – lateral lappet of paragnath; pd – depressions on paragnath for molar processes; pe – medial projection on epistome; pk – anterior knoll on paragnaths; sp – median spine on paragnath. Scale bars: A–C, E, F: 500 µm; D: 200 µm.

Figure 2. SEM images of mandibles of Anonyx nugax (a–e) and Tmetonyx cicada (f–k). (a) Mandibles in situ in ventral view (maxillipeds, maxillulae and maxillae removed). Note that the left incisor has proximally two teeth, while the right incisor only one. (b) Left mandible in oral view. Arrow points to the lacinia mobilis. (c) Right mandible in medial view. (d) Molar and setal row of the left mandible. (e) Cutting edge of the incisor and the lacinia mobilis (lm) of the left mandible. (f) Mandibles in situ in ventral view (maxillipeds,maxillulae and maxillae removed). (g) Right mandible in oral view.(h) Incisor and the setal row of the right mandible. (i) Mouth region showing the incisors and lacinia mobilis (arrowhead) on the left mandible. (j) Incisor of the left mandible. (k) Triturative surface of the molar of the left mandible. Arrowheads show crater-like structures. Abbreviations: ar – apical setae in mandibular setal row; hl – hinge line; in – incisor; lb – labrum; lm – lacinia mobilis; md pl – mandibular palp; mk – articulating knob of mandible; mmr – median ridge on molar process; mo – molar; sr – setal row; ti – medial teeth of incisor. Scale bars: A, B: 1 mm; C, D, F, G: 500 µm; E, H, J: 100 µm; I: 200 µm; K: 30 µm.

Figure 3. SEM images of maxillulae of Anonyx nugax (a–d) and Tmetonyx cicada (e–h). (a) Head with maxillulae in situ shown in ventral view (with maxillae and maxillipeds removed). (b) Right maxilla in oral view. (c) Apical setation on the outer plate of the left maxilla in aboral view. (d) Setation on the inner plate and the base of the outer plate of the left maxilla in aboral view. (e) Head with maxillulae in situ shown in ventral view (with maxillae and maxillipeds removed). (f) Inner plate and the base of the outer plate of the right maxillula in aboral view. (g) Apical ends of maxillular palps in situ in ventral view. (h) Apical setation on the outer plate of the right maxillula in aboral view. Abbreviations: bo – setal band on outer plate of maxillula; lb – labrum; ml ip – maxillular inner plate; ml op – maxillular outer plate; ml pl – maxillular palp; pi – pappose seta on inner plate of maxillula; smp – spine-like setae of mandibular palps; st1–6 – spine teeth in apical row on maxillular outer plate; stA-D – spine teeth in subapical row on maxillular outer plate. Scale bars: A, B: 1 mm; C, E:500 µm; D, F, H: 200 µm; G: 100 µm.

Figure 4. SEM images of maxillae of Anonyx nugax (a–d) and Tmetonyx cicada (e–h). (a) Head with maxillae in situ shown in ventral view (with maxillipeds removed). (b) Left maxilla in medial view. (c) Apical setation on the outer plate of the left maxillae in medial view. (d) Setation on the inner plate of the left maxillae in medial view. (e) Head with maxillae in situ shown in ventral view (with maxillipeds removed). (f) Left maxilla in medial view. (g) Apical setation on the outer plate of the right maxilla in aboral view. (h) Apical setation on the inner plate of the left maxillae in medial view. Abbreviations: bi – setal band on inner plate of maxilla; fi – setal field on inner plate of maxilla; fo – setal field on outer plate of maxilla; lb – labrum; ml pl – maxillular palp; mx cx – coxa of maxilla; mx ip – inner plate of maxilla; mx op – outer plate of maxilla; ri1–3 – setal rows on inner plate of maxillae; ro1–2 – setal rows on outer plate of maxillae; str – sternum. Scale bars: A, B: 1 mm; C, D: 200 µm; E, F: 500 µm; G, H: 100 µm.

Figure 5. SEM images of the head and maxillipeds of Anonyx nugax (a–f) and Tmetonyx cicada (g–l). (a) Lateral view of the head with intact mouthparts. (b) Head with maxillipeds in situ seen in ventral view. (c) Right maxilliped in aboral view. (d) Distal portion of the left maxilliped in situ showing the palp and the outer plate in ventral view. (e) Inner plate of the left maxilliped in oral view. (f) Part of the distal tip in oral view showing two pairs of cuspidate setae. (g) Lateral view of the head with intact mouthparts. (h) Head with maxillipeds in situ seen in ventral view. (i) Maxillipeds in oral view. (j) Inner plates of maxillipeds in oral view. (k) Triangular processes at the anterior margins of the inner plates. (l) Oral surface of the outer plate showing pores along its medial margin in oral view. Abbreviations: cpi – cuspidate setae on inner plate of maxilliped; lb – labrum; ml – maxillula; ml pl – maxillular palp; md – mandible; md pl – mandibular palp; ml – maxillula; mmp – connecting membrane of maxilliped coxa; mx – maxilla; mp1–4 – palpal articles of maxilliped; mp bs – maxilliped basipod; mp cx – maxilliped coxa; mp ip – maxilliped inner plate; mp is – maxilliped ischium; mp op – maxilliped outer plate; mx – maxilla; ns – nodular spine; ppi – pappose setae on inner plate of maxilliped; psi – pappose setae on inner plate of maxilliped; ssi – simple setae on inner plate of maxilliped. Scale bars: A, C, G, H: 1 mm; B, D, I: 500 µm; E, J: 200 µm; F, L: 100 µm; K: 50 µm.