Potamon fluviatile (Herbst, 1785)

Klaus, Sebastian & Brandis, Dirk, 2011, Evolution of sperm morphology in potamid freshwater crabs (Crustacea: Brachyura: Potamoidea), Zoological Journal of the Linnean Society 161 (1), pp. 53-63 : 57-60

POTAMON FLUVIATILE ( FIG. 1F–J; TABLE 2)

As stated above, this specimen of Potamon fluviatile was frozen after death, which elicited acrosome reaction. However, there were still a few sperm cells that remained intact. The operculum is rather thin. Lateral of the operculum there are electron-lucent structures that resemble a vestigial periopercular rim as it occurs in Potamonautidae and Gecarcinucidae . The acrosome ray zone is prominent and bulging outwards. There are many laterally situated nuclear arms. Potamon fluviatile shows both coenospermia and cleistospermia, with the coenospermic spermatophores being identical to those of the Potamiscinae .

PHYLOGENY OF THE POTAMIDAE

Here, phylogenetic inference is not performed to reveal potamid relationships, but to be used in the subsequent ancestral character state reconstruction. Thus, it is important to note that the topology of the 16S rDNA analysis (although several splits have only minor support; Fig. 2 ) is widely in agreement with that of Shih et al. (2009), which was based on a large data set of 72 species and 49 genera. The major difference is the position of Malayopotamon brevimarginatum (representing the ‘Sunda Shelf islands’ subclade), which clusters in our analysis with a very short branch and low posterior probability as sister group to the subfamily Potaminae , whereas in the analysis of Shih et al. (2009) it is monophyletic with the other Potamiscinae (however, only weakly supported in their minimum-evolution approach). Apparently the ‘Sunda Shelf islands’ subclade corresponds to the Isolapotamidae of Bott (1970a), and might be recognized as a third potamid subfamily Isolapotami-

nae (see Shih et al., 2009). We did not force monophyly on the Potamiscinae in the phylogenetic analysis, but kept this incongruence during the ancestral state reconstructions as none of the two topolo-

gies can currently be excluded. In summary, we can recognize the ‘ Potamiscinae s.s.’ (including the genus

Socotrapotamon , but excluding Malayopotamon ), the subfamily Potaminae , and a group [ Malayopotamon

+ Potaminae ], with the possibility that the latter is not monophyletic (the respective node is present in

49% of the 1000 trees fed into the ancestral character state analysis).

ANCESTRAL CHARACTER STATE RECONSTRUCTION

Reconstructions of ancestral character states under a parsimony model are shown in Figure 5A–D. The ancestral character state of the operculum (perforate or imperforate) at the base of the Potamidae remains equivocal. In total, 25.2% of reconstructions support the imperforate state and 19.5% of reconstructions support the perforate state at this node (Fig. 5C). The same ambiguous reconstruction of operculum states occurs in the ‘ Potamiscinae s.s. ’, the Potaminae , and the [ Potaminae + Malayopotamon ].

A true middle acrosomal zone only occurs in the

Potamiscinae and in the genus Malayopotamon

(Fig. 5D). Consequently, the absence of the middle acrosomal zone belongs to the ground pattern of the

Potaminae . However, because of its absence in

Socotrapotamon , the ancestral state reconstruction finds this character in only 58% of the analysed trees at the base of the ‘ Potamiscinae s.s.’.

The presence of the ‘tongue-and-groove’ connection that connects the operculum and acrosomal zones situated beneath is most probably an apomorphy for the whole Potamidae (occuring in 71.1% of

·

Figure 5. Reconstruction of ancestral sperm characters under a parsimony model over 1000 trees from the Bayesian inference (MESQUITE 2.6). A, operculum perforate or imperforate. B, ‘middle acrosome zone’ present or absent.

C, ‘tongue-and-groove’ connection present or absent. D,

coenospermia present or absent. Values at nodes indicate the percentage of trees with the respective state reconstructed or percentage of equivocal reconstructions. The proportion of trees that failed to find a respective node is the same for all four reconstructions, and the exact values are given in Figure 5B.

reconstructions; Fig. 5A). Secondary reduction of this character took place in Socotrapotamon socotrense , and in the lineage leading to Parathelphusula panningi and the genus Potamon within the Potaminae .

The coenospermic spermatophores are reconstructed as the ground pattern of the Potamidae (reconstructed in 80.3% of the trees analysed; Fig. 5B). Several reductive events led to cleistospermia in the genus Potamon , in Johora singaporensis , and in Thaiphusa sirikit . Also for the ‘ Potamiscinae s.s. ’ (79.1%), the [ Potaminae + Malayopotamon ] (40.0%), and for the Potaminae (81.7%), coenospermia most probably represents the ancestral character state.

Figures

Figure 1. Transmission electron microscopy (TEM) of potamid spermatozoa. A–E, Socotrapotamon socotrense. A, longitudinal sagittal view; the white line points to the perforate operculum. B, coenospermic spermatophore. C, cross section. D, electron-lucent ring around the opening of the perforatorial chamber. E, spermatophore wall. F–J, Potamon fluviatile. F, spermatophore wall. G, coenospermic spermatophore. H, longitudinal sagittal section (slightly transverse) and cleistospermic spermatophore. J, cross section. Scale bars: 1 Mm, or as indicated. Abbreviations: ar, acrosome ray zone; ia, inner acrosomal zone; nu, nucleus; oa, outer acrosomal zone; op, operculum; pc, perforatorial chamber; sw, spermatophore wall.

Figure 2. Maximum clade credibility tree based on Bayesian inference (BEAST 1.4.8) of a 16S rRNA fragment (523 bp) including potamid species with known sperm ultrastructure and Gecarcinucidae as the out-group. The nomenclature of subclades is as suggested by Shih et al. (2009). European Molecular Biology Laboratory (EMBL) accession numbers of sequences obtained in this study are shown in bold.