Plectroninia Hinde, 1900

Borojevic, Radovan, Boury-Esnault, Nicole & Vacelet, Jean, 2000, A revision of the supraspecific classification of the subclass Calcaronea (Porifera, class Calcarea), Zoosystema 22 (2), pp. 203-263 : 257-258

Genus Plectroninia Hinde, 1900

TYPE SPECIES. — Plectroninia halli Hinde, 1900 by monotypy.

DIAGNOSIS. — Minchinellidae with a choanoskeleton composed of tetractines, the basal actines of which are fused with the basal actines of adjacent spicules, while the apical actines remain free and point outward. This choanoskeleton is made up of two layers, an outer layer of large tetractines and a basal layer of small tetractines. The cortical skeleton consists of free spicules arranged tangentially. A perioscular circlet of tetractines, and rarely of triactines, is usually present.

DESCRIPTION

The genus Plectroninia , which was erected for fossil sponges, is difficult to distinguish from several other fossil genera, such as Bactronella Hinde, 1884 ; Porosphaera Steinmann, 1878 ; Tretocalia Hinde, 1900 ; Porosphaerella Welter, 1910 ; Sagittularia Welter, 1910 . The type species of Plectroninia , P. halli Hinde, 1900 from the mid- Miocene, was turbinate in shape and was probably free-living on a muddy sand bottom (Pickett, pers. comm.). It was therefore quite different from the Recent species first allocated to this genus by Kirkpatrick (1900) ( Figs 49-51 ). Kirkpatrick concluded that his specimen of P. hindei was a juvenile that had not reached its ultimate shape. The relationships of the Recent species, which are encrusting, with the massive Petrostroma Döderlein, 1892 are also unclear at present.

This genus contains 12 known Recent species that have an unusually large depth distribution, being known from shallow water caves down to 1600 m in depth.

Figures

FIG. 49. — Transverse section of Plectroninia hindei Kirkpatrick, 1900 (light micrograph). Abbreviations: p, papilla; o, osculum; c, choanosome; cs, cortical skeleton; lt, large fused tetractines of the choanoskeleton; st, small fused tetractines of the choanoskeleton. Scale bar: 330 µm (from Pouliquen & Vacelet 1970).

FIG. 50. — View of the choanoskeleton of Plectroninia sp., showing two layers of tetractines fused by their basal actines (SEM). Scale bar: 50 µm (from Vacelet 1991).