Ridley and Dendy, 1887
Definition: Halichondrida with axially condensed and extra-axially plumoreticulate choanosomal skeleton (Van Soest et al., 1990).
Remarks: Characteristically oscules are star-shaped, but in view of its (non-homologous?) occurrence outside the group, that is a doubtful synapomorphy. Next to typical Axinellidae genera like Axinella , Phakellia , and Auletta , the family Bubaridae with valid genera Bubaris , Monocrepidium , Hymerhabdia and Cerbaris , are sometimes included based on the hypothesis that their encrusting growth form is supported by an axially=basally condensed spicule mass and an extra-axially = peripheral plumose skeleton. In contrast to Van Soest et al., 1990 the genera Acanthella and Ptilocaulis are also included in the family.
Further corroboration of the affinities of Axinellidae and Halichondriidae originates from chemistry: both groups have unique cyanide containing compounds (isonitriles) found only rarely elsewhere among sponges. Both also share pyrrole derivatives with Agelasida (source: Braekman et al., 1992).
Remarks of Alvarez et al., 1998: The phylogenetic relationships and generic content of the family Axinellidae, like many other families within the Demospongiae are currently controversial. After Lévi (1973 ) subdivided the Demospongiae into three subclasses he placed the family into the order Axinellida, subclass Tetractinomorpha; this was followed by Bergquist (1970, 1978). The reassignment of this family, which was considered a member of the order Halichondrida Vosmaer, 1886, by other authors before Lévi (i.e. Topsent, Burton, de Laubenfels), was based on reproductive and larval characteristics of a few species and supported later by skeletal characters and free amino acid patterns (Bergquist, 1967, 1970, 1978; Bergquist and Hartman 1969; Hartman 1982; Lévi 1973). This scheme of classification was criticized by van Soest (1991) who formally proposed to abandon the use of the order Axinellida. Based on the result of a phylogenetic analysis, he grouped several taxa allocated to the Tetractinomorpha and Ceractinomorpha into new assemblages. Van Soest et al. (1990) suggested that the family had affinities with members of the redefined order Halichondrida. According to van Soest et al. (1990), the
Axinellidae are closely related to the families Desmoxyidae, Dictyonellidae and Halichondriidae in terms of skeletal features. However, the synapomorphies that define these groups, including the Axinellidae, are not unequivocal. The axial condensation of the choanosomal skeleton and the extra-axially plumoreticulate skeleton, synapomorphies defining the Axinellidae, are homoplastic characters present in other Porifera, and at the same time they are not present in all genera assigned here to the family.
At the supra-specific level, the Axinellidae are also problematic. The morphological characters that define the genera are not discriminatory, which makes allocation of the species difficult (Hooper and Bergquist, 1992; Hooper and Lévi, 1993). Any effort to study phylogenetic relationships and to place the family within a scheme of classification must be approached with a revision of the genera included in the family and clear descriptions of the species within each genus. This is still in progress, thus some uncertainty remains of the generic content and the affinities of this family. Currently about 12 genera are considered valid.
Sources: Van Soest et al., 1990; Alvarez et al., 1998
Genera represented in the area:
Acanthella Schmidt, 1862 (type species: Acanthella acuta Schmidt, 1862): encrusting, branching, bushy, or reticulate growth forms, with cartilaginous consistency; surface is fleshy, porous, with conules raised into bushy folds; choanosomal skeleton is not obviously differentiated into axial and extra-axial components, but there are vestiges of this differentiation in the form of with a central skeleton of compressed branching and rejoining spongin fibres, cored by intermingled styles, sinuous strongyles and/or oxeas, in uni- or paucispicular columns, forming a compressed anastomosing network or completely compacted without any obvious separation of fibre and mineral components; ectosome is fleshy, without specialized megascleres, typically with choanosomal spicules projecting through the surface (Acanthella cavernosa, Acanthella Vosmaer).
Auletta Schmidt, 1870 (type species: Auletta sycinularia Schmidt, 1870): specialized hollow tubular, branching or cylindrical growth forms, with terminal oscules; choanosomal skeleton has a basally condensed layer of sinuous strongyles and styles lining the endopinacoderm, and radial plumo-reticulate extra-axial tracts of long styles of two sizes, embedded perpendicular to the axial skeleton; these extra-axial tracts ascending towards the surface in longitudinal bands, united by abundant fibre and collagenous spongin, interconnected by occasional uni- or aspicular fibres; ectosome lacks a specialized skeleton, but extra-axial spicules may piercing the surface singly or in brushes.
Axinella Schmidt, 1862 (type species:Axinella polypoides Schmidt, 1862) (syn. Chalinissa Lendenfeld, 1887; Astrospongia Gray, 1867; Pachaxinella Burton, 1930a; Teichaxinella de Laubenfels, 1936; Tragosia Gray, 1867 ): flabellate and digitate growth forms; surface typically hispid, conulose; choanosomal skeleton always with some axial compression of spongin fibres, with or without differentiated primary and secondary fibre elements; fibres usually cored by styles, sometimes oxeas, occasionally strongyles, or sometimes all three in various combinations; extra-axial skeleton plumose or plumo-reticulate; ectosome without special megascleres but often with single spicules or bundles of extra-axial spicules protruding through the surface; microscleres may include raphides or microraphides, but these are not widely distributed amongst all species (Axinella Vosmaer, Axinella polypoides long).
Phakellia Bowerbank, 1863 (type species: Spongia ventilabra Linnaeus, 1767) (syn. Querciclona de Laubenfels, 1936): compressed flabellate or cup-like growth forms; surface is smooth or microconulose; oscules often surrounded by stellate subectosomal drainage canals; choanosomal axial skeleton is a dense mass of interwoven spicules, typically composed of only interwoven styles, or may include intermingled sinuous strongyles and styles, or occasionally only strongyles, organized into multispicular-ascending and paucispicular-transverse tracts, together forming a compressed reticulation at the axis; spongin fibres and collagenous spongin are sparse; extra-axial skeleton consists of sparse plumose bundles or individual styles or oxeas standing perpendicular to the axis, with or without transverse connecting megascleres; ectosomal skeleton membraneous without specialized spiculation, usually fleshy, often with spicules protruding through the surface; microscleres absent (Phakellia Vosmaer).
Pseudaxinella Schmidt, 1875 (type species: Pseudaxinella sulcata Schmidt, 1875) (synonym: Dragmacidon Hallmann, 1917: encrusting, massive, subspherical, club-shaped or unbranched lobate growth forms; surface often finely conulose, tuberculate or corrugated; choanosomal skeleton plumo-reticulate, without a compresed axial region, or differentiated axial and extra-axial regions; skeletal tracts crowded, plumose, more-or-less parallel and anastomosing, composed of smaller, thicker oxeas (or anisoxeas) and styles in equal proportion; ectosome fleshy, with a unispicular core of thinner, slightly longer spicules than in axial region; megascleres typically include only (anis-)oxeas and styles in equal proportions; microscleres absent.
Ptilocaulis Carter, 1883 (type species: Ptilocaulis gracilis Carter, 1883b) (syn. Plicatella Schmidt, 1864): erect, cylindrical, clavate, bushy, and lamellate growth forms; surface prominently conulose, with elongate, overlapping papilliform projections, often bifurcate at their points; choanosomal skeleton plumo-reticulate, with clearly differentiated axial and extra-axial components; axis compressed, composed of irregularly anastomosing close-set spongin fibres cored by styles, subtylostyles, anisoxeas or strongyles (usually asymmetrical), sometimes including sinuous forms; extra-axial skeleton plumo-reticulate or plumose, with heavy fibres cored by ascending multispicular tracts of the same spicules, interconnected by paucispicular transverse tracts forming subisodictyal reticulation (s.s.), or without transverse spicule skeleton and simply with meandering, plumose extra-axial spicule tracts; ectosomal skeleton is fleshy, without specialized spiculation, but surface may be pierced by plumose brushes of choanosomal styles; microscleres absent.
Stylissa Hallmann, 1914 (type specices: Stylotella flabelliformis Hentschel, 1912) (syn. Siphonocalypta Burton, 1931)- massive, fan-, tube-shaped and foliose growth forms; surface smooth or shaggy, often with small papillae or grooved ridges; choanosomal skeleton disorganised plumo-reticulate, with slightly condensed axis and slight differentiation between axial and extra-axial skeletons; well developed spongin fibres cored by parallel tracts of styles of 1 or 2 sizes, more-or-less ascending and diverging towards the periphery; peripheral styles often slightly larger than those in the axis; ectosome is fleshy, without specialized spiculation, but with bundles of styles diverging and protruding through the surface; microscleres absent.
Species included:
Auletta grantioides
Axinella alba
Axinella arctica
Axinella damicornis
Axinella egregia
Axinella flustra
Axinella infundibuliformis
Axinella polypoides
Axinella pyramidata
Axinella rugosa
Axinella subdola
Axinella verrucosa
Phakellia robusta
Phakellia ventilabrum
Stylissa cribrosa
Species not treated here:
Acanthella multiformis Vosmaer, 1882, Arctic, deep water (uncertain Acanthella )
Axinella vellerea Topsent (1904), deep water
Phakellia hirondelli (Topsent, 1892), deep water
Phakellia lambei (Topsent, 1913), Iceland, deep water
Pseudaxinella sulcata Schmidt (1875), Norway, cf. Arndt, 1935: 91, fig. 193
Ptilocaulis spec., specimens from North Atlantic (W Africa).