Sycon Risso, 1826

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 : 218-220

Genus Sycon Risso, 1826

TYPE SPECIES. — Sycon humboldtii Risso, 1826 by subsequent designation (Dendy & Row 1913).

DIAGNOSIS. — Sycettidae with radial tubes partially or fully coalescent; distal cones are decorated by tufts of diactines. The inhalant canals are generally welldefined between the radial tubes and are often closed at the distal end by a membrane that is perforated by an ostium, devoid of a skeleton. There is no continuous cortex covering the distal ends of the radial tubes. Skeleton of the atrium and of the tubes composed of triactines and/or tetractines.

DESCRIPTION

The genus Sycon is cosmopolitan, and it is often considered to be a perfect example of the calcareous sponges ( Figs 16 ; 17 ). Many representatives have a simple radially organized body with a single osculum, occasionally with a short peduncle. Species that grow larger may be arborescent, with a peduncle and ramified body, each branch representing a complete syconoid organization. The radial tubes are generally simple, but in large specimens they can also be ramified. In this case, the branches remain parallel, and each branch ends by a peculiar terminal cone. In some species (e.g. Sycon elegans Bowerbank, 1845 ; Sycon gelatinosum Blainville, 1837 ), the distal cones have dense tufts of diactines, which terminate all at the same level, giving the external surface a smooth, tabulate appearance. This organization should not be misinterpreted as a cortex, which is always characterized by tangential triactine spicules.

A group of small representatives of the genus arise from solid or tubular creeping stolons. The stolons can produce terminal hollow spherical buds (e.g. Sycon sycandra Lendenfeld, 1885 ), which detach, and form propagules with a peculiar skeleton. They are usually hispid due to the presence of long diactines, which act as flotation devices and promote their subsequent anchorage, attachment to the substrate, and formation of the young sponge. These propagules can live for a long time in the water column, and are quite frequently collected in the mesopsammon. However, they cannot be identified as Sycon until they attach to the solid substrate and grow into the typical adult sponge. Alternatively, spherical propagules can be formed from the distal parts of the radial tubes through the constriction and subsequent detachment of the region just under the distal cones (e.g. Sycon frustulosum Borojevic & Peixinho, 1976 ).

Most species of Sycon are attached to hard substrates, but occasionally they can live on a soft bottom [e.g. S. villosum (Haeckel, 1872) , S. raphanus (Schmidt, 1862) ]. In this case, they are anchored by long and ornate spicules, which are not found on the specimens that are attached to hard substrates. Since all the other morphological characteristics are identical, and similar sponges can grow in close proximity on different substrata, we interpret the presence of these spicules to be a secondary adaptation to the type of substratum.

Sycon is a common genus which has been extensively studied. Following Laubenfels (1936), Burton (1963) revived the name Scypha Grant, 1821 which was described in the Flora of the British Plants, and which has recently been used by non-taxonomists. Since 1899, no taxonomist except Laubenfels and Burton has used this name and more than 25 works, and 10 authors have used the generic name Sycon since 1950. We wish to maintain the commonly used younger synonym to avoid confusion. As the rule 23.9.1.2 of reversal of precedence cannot be applied strictly, we refer the case to the Commission with an appropriate recommendation for a ruling under the plenary power (Art. 81). The use of the junior name is to be maintained while the case is under consideration (Art. 82).

Figures

FIG. 16. — Diagram of a transverse section through the wall of Sycon natalense Borojevic, 1967. Abbreviations: a, atrium; as, atrial skeleton composed of tangential triactines and tetractines; ss, subatrial spicules; ar, articulate choanosomal skeleton; dc, distal cone, with a short tuft of diactines (from Borojevic 1967b). Scale bar: 100 µm.

FIG. 17. — The organization of the aquiferous system in Sycon sycandra (SEM); A, tubes (t); B, distal chones (d); C, detail of tubes (t) and inhalant openings (i); D, apopyles (a). Scale bars: A, 160 µm; B, 65 µm; C, 13 µm; D, 43 µm.