Ptychogena A. Agassiz 1865
Creators: name: Peter Schuchert
Publisher: name: Zenodo
Year: 2017
Subjects: Biodiversity
Taxonomy
Animalia
Cnidaria
Hydrozoa
Leptothecata
Laodiceidae
Ptychogena

description: Genus Ptychogena A. Agassiz, 1865
Diagnosis: Laodiceidae medusae with large, prismatic manubrium, 4 radial canals connected to stomach via large, funnel-shaped, mesentery-like basal extensions of the manubrium. Along radial canals lateral, transverse, fold-like diverticula on which the gonads are located; bell margin with club-shaped cordyli, no cirri, no ocelli. Hydroids where known like Stegomoma plicatile (M. Sars, 1863). Colony erect, branching, polysiphonic, with tendency to ramification in one plane. Hydrothecae on stem and branches, all pedicellate. Hydrothecae tubular, curved outwards gradually narrowing downwards, ending in a gable-shaped operculum. Gonothecae larger than hydrothecae, elongated, almost completely adnate with branches and axis, with or without operculum.
Remarks: The diagnosis given in Kramp (1959a) and subsequent authors (e.g. Bouillon et al., 2006) had to be slightly modified. Based on observations on Ptychogena lactea A. Agassiz, 1865, influential authors like Haeckel (1879), Mayer (1910), and Kramp (1919, 1933) interpreted the perradial funnelshaped structures bearing the gonadal folds as widened proximal parts of the radial canals. However, especially in the case of P. crocea, these structure are intuitively better interpreted as pockets of the manubrium as seen in many other hydromedusae [see also Russell (1953: 224) who expressed the same opinion for the similar Chromatonema rubrum Fewkes, 1882]. These pouches have the same characteristic colour and tissue density as the manubrium, while the radial canals connecting the end of the funnel to the circular canal are thin and transparent like the latter. Whatsoever, they are gastrodermal structures and as such homologous. Interpreting the funnels as radial canals was perhaps influenced by the general diagnosis of the leptomedusae which should have gonads on the radial canals.
The only Ptychogena species for which the polyp stage is known is Ptychogena crocea (Schuchert et al., 2017), but the other species of the genus could have a similar or identical hydroid.
The medusae of this genus are rather similar or hardly distinguishable from the genera Modeeria Forbes, 1848 and Chromatonema Fewkes, 1882, although the latter genus has been placed in a separate family, Tiarannidae Russell, 1940. Currently, the only feasible trait to distinguish Chromatonema from Ptychogena is the shape of the cordyli: they are club-shaped in Ptychogena and spindle-shaped in the Chromatonema. It is possible that there are significant differences in the polyp-stage, but the polyps of Chromatonema remain unknown. Molecular phylogenies have to re-address the problem. Preliminary data (figs 1-2 in Schuchert et al., 2017) suggest that they are closely related.
intense yellow-orange colouration; bulbs whitish; canals slightly opaque.
The hydroid stage of this species is Stegomoma plicatile (M. Sars, 1863) (Schuchert et al., 2017).
Remarks: The nematocysts have been described in Russell (1940). The yellow colour is lost in preserved animals. For the difference to Chromatonema rubrum and other similar medusae see Schuchert et al. (2017).
description: Published as part of Peter Schuchert, 2017, Systematic notes on some leptomedusa species with a description of Neotima galeai n. spec. (Hydrozoa, Cnidaria), pp. 351-375 in Revue suisse de Zoologie 124 (2) on pages 355-356, DOI: 10.5281/zenodo.893549