Trikentrion Ehlers, 1870

Soest, Rob van, Carballo, Jose Luis & Hooper, John, 2012, Polyaxone monaxonids: revision of raspailiid sponges with polyactine megascleres (Cyamon and Trikentrion), ZooKeys 239, pp. 1-70 : 31-32

Genus Trikentrion Ehlers, 1870

Type species:

Spongia muricata Pallas, 1766 (by monotypy).

Definition

(emended). Cyamoninae with reticulate skeleton containing polyactine spicules of which the basal cladi are provided with hook-like spines in mature condition, and if present choanosomal oxeas. Microscleres trichodragmas. Additional longer and shorter thin styles are usually present in peripheral regions.

Remarks.

Polyactine spicules are genuinely polyaxone, with axial canals visible in all of the predominantly three, occasionally four- or two cladi. As will be demonstrated below, none of the specimens of the type species we were able to examine, including the neotype, possess the raspailiid synapomorphy of peripheral long styles surrounded by short styles, despite Hooper’s (2002) description of the type species where such spicules were mentioned. Possibly, but unlikely, these spicules are present in living condition, because we only had dry old specimens available and the peripheral skeleton may have become abraded. It seems likely that Hooper’s (2002) description was based on a contaminated spicule slide. All other Trikentrion species do have the long and short styles as a peripheral skeletal feature, and in that sense the type species appears a deviating representative of the genus.

Trikentrion differs from Cyamon in its possession of choanosomal oxeas (whereas Cyamon has styles), but several species, Trikentrion catalina , Trikentrion helium Dickinson, 1945 and Trikentrion africanum sp. n., are lacking these spicules. The polyactines of Trikentrion differ from those of Cyamon in having only the basal clade provided with strong hook-like spines, with the lateral cladi smooth; also the shape is often Y- or T-shaped. As demonstrated above, these differences are not entirely exclusive, because Cyamon arguinense sp. n. and Trikentrion quinqueradiatum also have only the basal cladus spined, whereas Y- and T-shaped polyactines occur in Cyamon neon and Cyamon argon . Finally, all species of Trikentrion described below have abundant trichodragmas, which are entirely lacking in Cyamon species.