Rossella (sponge)
Rosella is a genus of glass sponges in the family Rossellidae. It is found in the Antarctic and sub-Antarctic regions.
Description
Species are thick-walled saccular sponges. Calycocomes are always present. They are often accompanied by spherical discohexactines, mesodiscohexasters and microdiscohexasters.Dermalia are usually pentactines, sometimes with stauractines and hexactines. Prostalia lateralia, when present, are monaxons and sometimes pentactines. The hypodermal pentactine spicules may be differentiated into spicules with claw-like ends, serving as a means of attachment to the substrate the sponge is growing on. They commonly have paratropal and orthotropal tangential rays.
The choanosomal skeleton is made of diactines, and rarely hexactines. Atralia are mainly hexactines, rarely with pentactines or diactines.
Ecology
Rossella species are abundant on the Antarctic shelf, where they may cover as much as half of the sea floor. They therefore play an important role in forming the benthic communities of this region. They form biogenic structures, increasing the structural complexity of the sea floor of this region. This more heterogeneous space can be utilised by other species.A wide variety of invertebrate species also live within the sponge structure, including foraminiferans, polychaetes, amphipods, isopods, tanaids, copepods, ostracods, acari, pycnogonids, gastropods, bivalves, and nematodes. A sponge may host thousands of specimens and tens of species per 100 ml.
Species
The relationships between the species in this genus are not particularly well understood. The species composition has changed dramatically over the years, depending on which criteria are used. The external forms of this genus are highly variable, even within a single species. There is strong genetic evidence for a species flock in this genus, that is an accumulation of a large number of closely related species confined to a small area. These typically represent a rapid evolution of a dominant group in a region, resulting in many similar species. The current evidence suggests that this genus contains two clades corresponding to the well-defined species R. antarctica, and the diverse assemblage of species that has been termed the R. racovitzae flock. More research is needed to understand how many species fall in this genus and how they are related.The following species are, however, currently recognised: