Lophogaster typicus, M. Sars, 1857

Grams, Markus, Klinger, Michael & Richter, Stefan, 2023, Neither leg nor jaw-nor always the same: a critical revision of the eumalacostracan maxilliped, Zoological Journal of the Linnean Society 197 (4), pp. 965-1004 : 981-984

LOPHOGASTER TYPICUS

The first pair of thoracopods (Thp1) is highly modified compared with the posterior pairs (Thp2–8) of pediform thoracopods. Owing to this strong morphological difference, the following descriptions relate only to Thp1, based on a detailed examination of the right Thp1 of three adult males of L. typicus .

Cuticle and skeletal structures of Thp 1 in L. typicus

The more or less rectangular protopod (more than twice as wide as long) of L. typicus consists of the coxa (twothirds) and basis (one-third of the total length of the protopod). The thorax connects and articulates with the coxa by a cuticular half-ring, leaving the lateral area membranous (Th in Fig. 10A, B ). Posteriorly, the cuticle of the thorax is invaginated as a lateromedial fold, forming a short roof-like apodeme (black arrows in Fig. 10A ; partly dashed curved arrows and dashed line in Fig. 10B ).

The coxa does not form a complete ring but has the medial to anteromedial area membranous (Co in Fig. 10A ). A bicondylar articulation with the thorax is formed by an anterolateral and posteromedial articulation point, respectively (*l, *m and ax Th-Co in Fig. 10A ). Posteriorly, the coxa forms an invagination, which appears as a linear lateromedial furrow (white arrows in Fig. 10A ) that ends posterolaterally in a roundish depression. Distally, the podomere articulates anterolaterally and posteromedially with the basis (*l, *m and ax Co-Ba in Fig. 10B ).

Laterally, a huge lamellar epipod (Ep in Fig. 10A ) is attached to the coxa. It is broadly flattened anteroposteriorly and long, reaching at least double the length of the exopod (see below, this section). The epipod articulates with the coxa via two sclerites: anteriorly, a rod-shaped sclerite (S 1 in Fig. 11A, B ) forms a dorsal articulation point, while a second ventral sclerite (S 2 in Fig. 11A, B ) interlocks the epipod anteriorly on the coxa. The area between both sclerites is membranous. More distally on the epipod, several sclerites form the ‘frame’ of its lamellar distal portion (Ep in Figs 10A , 11B ).

The basis, from a dorsal perspective, appears roughly drop-shaped, with the medial side being the spherical part, narrowing towards the lateral side (Ba in Fig. 10C ). Here, the proximal margin of the basis widens again into a V-shape, ending in two fingers, of which the posterior one serves as attachment site for three muscles (TB1, CB2 and CB3; Fig. 11C ). The lateral wall appears membranous ( Fig. 10B, C ). The distal opening of the basis into the endopod is found in the medial third of the podomere ( Fig. 10C ). Here, a bicondylar articulation with the endopod (i.e. the ischium) is formed by an anteromedial and a posterior articulation point, respectively (ax Ba-Is in Fig. 10C ).

A lamellar exopod attaches at the lateral membrane of the basis via a short proximal stem (Ex in Fig. 10A ) that articulates with the anteromedial finger-like extension of the basis via a hook-like structure ( Fig. 10C ). The stem is followed by a much longer distal portion (almost as long as the endopod), which is shaped as an anteroposteriorly flattened blade with a setose distal tip.

Table 3. Continued

The crescent-shaped endopod consists of the ischium, merus, carpus, propodus and dactylus. In relationship to the following podomeres, the ischium appears short (Is in Fig. 10A ). An articulation with the merus is created by an anteromedial articulation point. Although the podomeres are in close contact posterolaterally, no clear articulation point can be made out. Nonetheless, this closeness might be sufficient to achieve the functionality of a bicondylar joint.

The merus (Me in Fig. 10A ) measures about twice the length of the ischium. Its distal opening is not oriented distally but is displaced ~90° in the lateral direction. Anteromedially, the podomere is covered with strong, cuspidate setae. A bicondylar articulation with the carpus is formed by an anteromedial and posterolateral articulation point, respectively.

The carpus (Ca in Fig. 10A ) is about twice the length of the merus. Four long cuspidate setae are attached medially. Its proximal margin (connected to the merus) faces medially, whereas its distal margin faces in the distal direction, articulating with the propodus by an anterior and posterolateral articulation point, respectively.

The propodus (Pr, dotted outline in Fig. 10A ) is shorter than the carpus and roughly as long as the merus. Three long, cuspidate setae are attached medially. The distal margin of the propodus is slightly oriented in the medial direction, where the podomere is connected with the dactylus by an anterolateral and a posterior articulation point, respectively.

The dactylus (Da in Fig. 10A ) is almost equal in length to the propodus. The podomere tapers towards its distal tip, where a strong and cuspidate seta is present, while four smaller cuspidate setae cover the dactylus medially.

Figures

Figure 10. Cuticle of macerated thoracopod 1 (Thp1; right) of Lophogaster typicus (male); micro-computed tomography volume rendering; colour code of podomeres as in Figure 1. A, posterior view; epipod damaged; dashed line indicates the concealed distal margin of the carpus; dotted line indicates the outline of the concealed propodus; black arrows show the posterior thorax invagination; white arrows show the posterior coxa invagination. B, C, podomeres distal to the ischium blanked out. B, dorsal view of protopod; partly dashed curved arrows emphasize the posterior invagination of the thorax; white line outlines the proximal/dorsal margin of the thoracomere; dashed line indicates the concealed posterior distal/ ventral margin of the thoracomere; left half of thoracomere/sternum blanked out.C, dorsalview ofbasis–ischium articulation; dashed line emphasizes the proximal margin of the basis; thorax and coxa blanked out. Abbreviations: ant, anterior; ax, articulation axis between […]; Ba, basis; Ca, carpus; Co, coxa; Da, dactylus; dis, distal; Ep, epipod; Ex, exopod; Is, ischium; *l, lateral articulation point; lat, lateral; *m, medial articulation point; Me, merus; med, medial; Pr, propodus; Th, thorax.

Figure 11. Muscles in protopod and exopod of thoracopod 1 (Thp1; right) of Lophogaster typicus (male); micro-computed tomography volume rendering. A, C, white lines emphasize the margin of the thorax and the outlines of the basis and the epipodal sclerites.A, anterior view. B, macerated sample; anterior view of coxa–epipod articulation; colour code of podomeres as in Figure 1. C, posterior view; dashed line indicates continuation of the thoracic margin in the background. Abbreviations: Ba, basis; Co, coxa; dis, distal; Ep, epipod; Ex, exopod; lat, lateral; S1, dorsal sclerite of epipod; S2, ventral sclerite of epipod; Th, thorax. Numbered muscle labels are as in Table 4.