Mithraculus forceps A. Milne-Edwards, 1875

Mollemberg, Michelle, Lianos, Laira, Zara, Fernando José, Swenson, Rafael, Cobo, Valter José & Santana, William, 2021, Morphological changes during ontogeny of the male first and second gonopods of Mithraculus forceps A. Milne-Edwards, 1875 (Brachyura: Majoidea: Mithracidae), Journal of Natural History 55 (15 - 16), pp. 953-967 : 956-958

Gonopods of Mithraculus forceps A. Milne-Edwards, 1875

( Figures 3–7 )

In the size class 1, the first gonopod (G1) is slightly flattened dorsoventrally, with an apical curvature and initial formation of the mesial suture ( Figure 3 (a,b)). The apical surface is blunt, with rounded margins and a closed tip ( Figure 3 (b,c)). The apical plate has few and small spines arranged in a row around the tip ( Figure 3 (c,d)). Sub triangular sensillum is distributed parallel to the lateral margin of the apex ( Figure 3 (e)). The G1 base shows a few plumose setae ( Figure 3 (f)). The second gonopod (G2) is short, cylindrical, and with the enlarged basal region tapering in the last distal third ( Figure 3 (g)). Sparse plumodenticulate setae are present at G2 base ( Figure 3 (g) detail). The apical region is a wrinkled area and a blunt median process centrally located ( Figure 3 (h)).

In successive stages during ontogenetic development, G1 increases in length, and one can observe the dorsoventral flatness,and the marked basal curvature ( Figures 4–7 ). From the fifth size class ( Figure 4 ) it is possible to note the mesial suture extending longitudinally from the base to the apex, to form the aperture of the ejaculatory canal ( Figure 4 (a,b)). The setae gradually increase in quantity and size: In the smaller individuals they are visible only in the basal region ( Figure 3 (a,f)), in the larger individuals the setae extend through the G1 shaft until the second distal third ( Figures 6 (a); 7(a)).In the base of the G1, plumodenticulate setae can be observed ( Figures 3 (f), 4(f), 6(f)), while on the G1 shaft the lateral margin there are only simple setae arranged in rows inclined to the G1 distal region ( Figures 5 (f); 7(f)). The most significant ontogenetic changes are concentrated in the apical plate of the G1. In the apical plate we notice the transition from a blunt structure with rounded margins ( Figure 3 (a-c)) to the development of two distinct lobes (lateral and mesial) ( Figure 4 (a,b); 5(a,b); 6(a,b)), with the lateral lobe tapering along the development ( Figure 7 (a,b)). The aperture of the ejaculatory canal gradually increases in size, becoming diagonal to the apical plate ( Figure 4 (a-c); 5(a-c); 6 (a-c); 7(a-c)). Also, the number of spines bordering the ejaculatory canal aperture increases considerably ( Figure 4 (b-d); 5(b-d); 6(b-d); 7(b-d)), with some inclined to the canal interior in the larger specimens ( Figures 5 (c); 6(c);7(c)).The sensillum increases in number and reduces in size along the ontogeny ( Figures 3 (e); 4(e); 5(e); 6(e); 7(e)), being subtriangular in smaller individuals ( Figure 3 (e)) and smaller with an acute apex in larger individuals ( Figures 6 (e); 7(e)).

The second gonopod (G2) does not show abrupt changes during the ontogenetic development. A gradually formation of a girdle in the apical region, with the median process becoming more prominent and tapered ( Figure 3 (g,h); 4(g,h); 5(g,h); 6(g,h); 7(g,h)) is verified. The number of setae at the base of the G2 has a slight increase ( Figure 3 (g) detail; 4(g) detail; 5(g) detail; 6(g) detail; 7(g) detail), with spines appearing in the apical region, specifically around the apical girdle, and increasing in quantity during development ( Figures 4 (h); 5(h); 6(h); 7(h)).

The specimens are considered physiologically mature from size class 5, diagnosed by the presence of white testicles and ductus deferens with spermatophores/sperm inside. Until size class 2, the reproductive system is not found in the juvenile individuals, and from size class 3 are found only as translucent ductus and not clearly differentiated, therefore no spermatophores are present.

Figures

Figure 3. Gonopods of Mithraculus forceps A. Milne-Edwards, 1875 (CW 4.0 mm – size class 1). (a-f), first gonopod. (g, h), second gonopod. (a), general view. (b), distal view. (c), apex view. (d), spines of apex. (e), sensillum. (f), plumose seta of base; (g), general view, plumodenticulate setae in detail. (h), apex.

Figure 4. Gonopods of Mithraculus forceps A. Milne-Edwards, 1875 (CW 8.9 mm – size class 5). (a-f), first gonopod. (g, h), second gonopod. (a), general view. (b), apical plate. (c), ejaculatory canal aperture. (d), spines of apex. (e), sensilla. (f), plumose setae of base; (g), general view, plumodenticulate setae in detail. (h), apex.

Figure 5. Gonopods of Mithraculus forceps A. Milne-Edwards, 1875 (CW 14.8 mm – size class 9). (a-f), first gonopod. (g, h), second gonopod. (a), general view. (b), apical plate. (c), ejaculatory canal aperture. (d), spines of apex. (e), sensillum. (f), simple seta of shaft; (g), general view, simple setae in detail. (h), apex.

Figure 6. Gonopods of Mithraculus forceps A. Milne-Edwards, 1875 (CW 18.1 mm – size class 12). (a-f), first gonopod. (g, h), second gonopod. (a), general view. (b), apical plate. (c), ejaculatory canal aperture. (d), spines of apex. (e), sensillum. (f), plumodenticulate setae of base; (g), general view, plumose setae in detail. (h), apex.

Figure 7. Gonopods of Mithraculus forceps A. Milne-Edwards, 1875 (CW 21.9 mm – size class 15). (a-f), first gonopod. (g, h), second gonopod. (a), general view. (b), apical plate. (c), ejaculatory canal aperture. (d), spines of apex. (e), sensillum. (f), simple setae of shaft; (g), general view, simple setae in detail. (h), apex.