Tmetonyx cicada (Fabricius, 1780)

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 : 480-482

Mouthparts: Tmetonyx cicada

Epistome and upper lip

The epistome is produced, widely conical with a rounded apex and lacks any spines or setae ( Figure 1 (d)). The epistome and the upper lip are separate. Posterolaterally on the oral surface of the epistome is a pair of small depressions (ed, Figure 1 (d)) that receive the articulating knobs of the mandibles.

Mandibles

The hinge line of the mandible is only slightly diagonal (almost transverse) and is located at the posterior end of the mandible (hl, Figure 2 (f)). In the resting position, the palps are held anteriorly below the antennae, with article 3 directed laterally ( Figure 5 (g)). Palpal article 2 is slender and about 4 times as long as article 1 ( Figure 2 (g)). The mandibles are asymmetrical in the morphology of the incisor, setal row and lacinia mobilis. The large, triangular molar processes (mo, Figure 2 (f)) are oriented obliquely to the mandibular body in the posteromedial direction and have narrow, slightly convex triturating surfaces (ts, Figure 2 (g)) with ca. 50 transverse ridges carrying densely packed columnar teeth ( Figure 2 (k)). The triturating surface also bears three crater-like structures (arrowheads, Figure 2 (k)) arranged equidistant along the midline of the surface. The setal row (sr, Figure 2 (g)) extends from the incisor to the molar process along the medial margin of the mandible and then continues along the medial side of the molar process and the aboral side of the triturating surface ( Figure 2 (k)). The three apical setae (ar, Figure 2 (h)) on each mandible are very large and spine-like, with curved, hooklike tips. In mandibular adduction, these setae on one mandible fit into the gaps between those on the other mandible. The remaining setae of the setal row are thin and hairlike, they have about the same size throughout the length of the row and are especially densely spaced along the distal side of the molar, where they form a thick brushlike coat. In the resting position, the triturating surfaces of the molar processes and the spine rows of the opposite mandibles are tightly pressed against each other ( Figure 2 (f)). A low ridge (mmr, Figure 2 (g)) carrying a row of simple setae extends along the middle of the oral side of the molar process from the edge of the triturating surface to the base of the molar. The whole oral surface of the molar process is pubescent, with denser pubescence on the proximal side of the molar ridge. The left incisor ( Figure 2 (f,j)) has a wide and straight cutting edge and bears a single small tooth at its ventral end. The cutting edge of the right incisor (in, Figure 2 (g,h)) is shorter, curved, and in some specimens is armed ventrally with two small teeth ( Figure 2 (h)). The cutting edges of the incisors are oriented in a diagonal plane; in the adducted position, the left incisor overlaps the right incisor anteriorly. Midway between the incisor and the palp is a small, rounded articulating knob that fits into the corresponding depression on the oral side of the upper lip (see above). The lacinia mobilis is present only on the left mandible and has been reduced to a small, curved peg (arrowhead, Figure 2 (i)).

Paragnaths

A pair of paragnaths ( Figure 1 (f)) 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 left paragnath is slightly larger than the right and the distal tip of the right paragnath lies below the tip of the left paragnath. The paragnaths are slightly longer than wide and are fused only at their bases. Proximolateral surfaces on the oral side of each paragnath are finely pubescent. These fields of pubescence are bordered medially by an arch-shaped longitudinal row of longer simple setae (as, Figure 1 (f)). Lateral to this setal row at the distal end of each paragnaths is a small knoll (pk, Figure 1 (f)) that fits into the depression on the mandible posterior to the incisor. A small, conical spine (sp, Figure 1 (f)) with a pointed tip projects from the medial face of each paragnath, about one-third of their length from the distal tips. The posterior face of each paragnath forms a posteriorly directed lateral lappet with a rounded tip (lp, Figure 1 (f)).

Maxillulae

The maxillulae lie in a horizontal plane (ml, Figure 5 (g)) 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 (g)). The inner plate (ml ip, Figure 3 (e,f)) is small, cylindrical, about twice as short as the outer plate. Its blunt apical tip bears two large flexible pappose setae with curved tips (pi, Figure 3 (e,f)). The basipod extends widely from the coxa and has an additional medial joint with the inner plate. The outer plate (ml op, Figure 3 (e,f)) is wide and leaf-shaped. 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 (e,h)): 7 teeth (st1–7) in an apical row (st1 1–2-cuspidate, st2 1–3-cuspidate, st3 2-cuspidate, st4 3-cuspidate, st5 3–4-cuspidate, st6 5-cuspidate, st7 4–6-cuspidate) and 4 teeth (stA-D) in a subapical row (stA 2-cuspidate, stB 3–4-cuspidate, stC and stD 3-cuspidate). The distal portion of the outer plate also has two longitudinal bands of simple setae (bo, Figure 3 (e,f,h)) located along the medial margin and on the oral surface of the plate. The palp (ml pl, Figure 3 (e–g)) is 2-articulate and lacks setation; article 1 is triangular, about as long as wide; article 2 is long and curved towards the midline. Wide distal ends of both palps are armed with eight short spine-like setae (smp, Figure 3 (g)) and with two flexible sensory setae (arrows, Figure 3 (g)) located between spines 6 and 7 and dorsally to the last eighth spine (counting spines ventral to dorsal). In the resting position, the opposite palps meet at the midline behind the outer plates of the maxillipeds, with the left palm slightly overlapping the right one anteriorly ( Figure 3 (e,g)).

Maxillae

The left and right maxillae are roughly equal in size and shape ( Figure 4 (e)). Small, triangular coxae (mx cx, Figure 4 (e)) 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 (g)) partly concealing the larger maxillulae from below. The anteromedially oriented basipod is basally wide ( Figure 4 (e)), then tapers apically and curves to form an anteriorly oriented inner plate (mx ip, Figure 4 (e,f)). The inner plate carries three closely spaced parallel rows of flexible setae along the medial margin ( Figure 4 (f,h)). The setae in the outer (ri3, Figure 4 (h)) and inner (ri1, Figure 4 (h)) rows are about twice as wide and long as those in the middle row (ri2, Figure 4 (h)). The setae in the outer row are unilaterally serrated in the distal half ( Figure 4 (h)); the remaining setae are simple. On the aboral surface of the inner plate near the setal row is a small field of sparsely spaced simple setae (fi, Figure 4 (f)). The outer plate (mx op, Figure 4 (e,f)) is lancet-shaped and carries distally small simple setae on the oral surface and two rows of flexible setae along the medial margin ( Figure 4 (g)): the setae in the outer row (ro2, Figure 4 (g)) are wide and unilaterally serrated in the distal half, those in the inner row (ro1, Figure 4 (g)) are thin and simple.

Maxillipeds

Maxillipeds consist of a fused coxa (mp cx, Figure 5 (g,h)), basipods (mp bs, Figure 5 (g,h)) with endites (inner plates), ischia (mp is, Figure 5 (g–i)) with endites (outer plates) and palps. The coxa is oriented in the vertical plane (mp cx, Figure 5 (g)), 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 (h)). An array of long, simple setae extends along the medial and distal margins on the aboral surface of basipods and there are tufts of simple setae at the distolateral corners of the basipods and ischia ( Figure 5 (h)). The inner plates (mp ip, Figure 5 (i,j)) gradually widen from the base to the apex; their anterior margins are obliquely bevelled. In the resting position, the opposite plates meet each other in the middle ( Figure 5 (i,j)). The medial portion of the anterior margin of each inner plate is extended into a triangular process that bears three nodular spines (ns, Figure 5 (k)). The anterior and medial margins on the oral surface of the plates carry a row of large setae; the medial of these setae are pappose (psi, Figure 5 (j)) and those at the anterior margin lack setules (ssi, Figure 5 (j)). The rest of the oral surface of the plates is evenly covered with thin simple setae. The outer plates (mp op, Figure 5 (g–i)) are about twice as large as the inner plates and have straight medial and arcuate lateral margins. The medial and distal margins of the outer plates are armed with small subquadrate teeth and covered with a row of simple setae on their aboral surfaces ( Figure 5 (l)); the remaining surface of the plates lacks setation. Numerous small irregularly spaced pores ( Figure 5 (l)) are located on the oral surfaces of the outer plates along their medial margins. The palps are 4-articulate ( Figure 5 (g–i)); their articles 1 and 2 are flattened and wide (but less so than in A. nugax ) and cover the maxillae laterally. The medial margins of all palpal articles bear a row of long simple setae. In the resting position, the palps do not cross or meet at the midline, but the right palp lies slightly anterior to the left palp ( Figure 5 (h)).

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.