Biverticillus tenuissimus Payne & Samaai & Kelly 2022, gen. et sp. nov.

Biverticillus tenuissimus gen. et sp. nov.

(Fig. 1)

Material examined. Holotype. SAMC-A088843 (cross reference TS 2563; WSL-INV74(45)): Walters Shoal Seamount, Grid WSL024, Station ALG10956 (33.147° S, 43.818° E), 103–348 m, coll. R / V Algoa, dredge, 29 May 2014.

Type locality. Walters Shoal Seamount, south of Madagascar on the Madagascar Ridge, Western Indian Ocean (Fig. 1A).

Description. Very thin sponge encrusting rubble (Fig. 1B), length 20 mm, width 19 mm and thickness 0.5 mm. Surface undulating but smooth, with no ostia or oscules visible. Texture soft, ectosome tears easily. Colour in life and preservative black, dark brown under microscope.

Skeleton. Choanosome comprises thin megasclere tracts that diverge and radiate from base to surface. Ectosome a dense tangential layer of megascleres, above which sits an irregular palisade of microscleres oriented in different directions (Fig. 1C).

Spicules. Megascleres (Fig. 1F), subtylostyles, smooth, sometimes polytylote, slightly sinuous: 263 (225–285) × 5 (4–6) μm, n = 25. Microscleres (Fig. 1D, E) are anisospinodiscorhabds with a stout shaft, the apical whorl is composed of four, solitary, curved spines (very rarely bifurcate; 98% of spicules), that extend from the shaft at a shallower angle than do the basal spines. Centrally located median and subsidiary whorls are composed of bifurcate spines, mostly equidiametral, equally spaced between each other and apical and basal whorls. The basal spines are also bifurcate. All spines are heavily spined: 36 (26–40) × 4 (2–4) μm, width including whorls, 20 (8–24) μm, n = 25.

Substrate, depth range and ecology. Attached to rubble and associated with dead clams and hydrozoans, 103– 348 m.

Etymology. Named for the very thin encrusting form (tenuissimus, very thin; Latin).

Remarks. The establishment of a new monotypic latrunculid genus for a small, inconspicuous holotype, is not unprecedented: tiny (4–12 mm diameter) Bomba endeavourensis Kelly, Reiswig & Samaai in Kelly et al. (2016) was also described from a single location (2500 m deep on the Endeavour Ridge off the coast of British Columbia) and had microscleres very different from other Latrunculiidae (Kelly et al. 2016). Unfortunately, DNA extractions and Polymerase Chain Reaction amplifications were unsuccessful in the case of Biverticillus tenuissimus gen. et sp. nov. and discontinued due to the risk of destroying the small holotype.

However, in terms of morphology and spiculation, we can differentiate this species from other Latrunculiidae. Biverticillus tenuissimus gen. et sp. nov. is a thin encrusting sponge (unusual in the family) with a unique form of discorhabd microsclere and, with the exception of Latrunculia cratera Bocage, 1870 (anisostyles 160–190 µm), has the smallest megascleres recorded in species of Latrunculiidae (Samaai et al. 2006; Kelly et al. 2016; MK unpublished review of New Zealand and Antarctic species).

The anisospinodiscorhabds of Biverticillus gen. nov. are superficially comparable to the typical anisodiscorhabds of subgenus Latrunculia (Biannulata) (Fig. 1I), also with only four distinct substructures, but there are several characters that differentiate these anisodiscorhabds further: 1) the anisodiscorhabds in all Latrunculia (Biannulata) (and indeed all species of Latrunculia; see Fig. 1H–J for some examples) have a greater expanse of shaft above the undifferentiated basal whorl and manubrium, than in those of Biverticillus gen. nov. (hence the prefix aniso-in the term aniso-discorhabd). In Biverticillus gen. nov., the distance between each whorl is equal, in the anisospinodiscorhabds the prefix aniso- refers to the difference in the form of the apex and base, not the distribution of the whorls along the shaft; 2) the whorls in the microscleres of Biverticillus gen. nov. are equidiametral unlike those in genus Latrunculia, which frequently have a broader median whorl; 3) the pattern of development (ontogeny) of the anisodiscorhabd in Latrunculia differs significantly from that of Biverticillus gen. nov. The anisodiscoprotorhabd of Latrunculia is a straight uniform shaft with two acentric crenulated discs that develops from a straight protorhabd (see Fig. 2A in Samaai et al. 2004), whereas the anisospinoprotorhabd of Biverticillus gen. nov. has synchronous protorhabd development and equal distances between the central whorls (Fig. 1E).

The anisospinodiscorhabds of Biverticillus gen. nov. (Fig. 1D) are more closely comparable to the isospinodiscorhabds in the South African genus Cyclacanthia (Fig. 1G), as both have equidistant, equidiametral whorls and apical and basal substructures. The major diagnostic difference, however, is that there is only a single, central whorl of spines in the isospinodiscorhabds of Cyclacanthia, which in one species may even be missing (C. rethahofmeyri Samaai, Kelly, Payne & Ngwakum, 2020). While certainly closely comparable, the microscleres of Biverticillus gen. nov. are aniso- in their form because the apical and basal substructures differ in the angle of repose of their spines.

Until new material is discovered, the establishment here of the monotypic genus, Biverticillus gen. nov., rather than expansion of the diagnoses of other Latrunculiidae such as Cyclacanthia, is supported by: 1) the ontogeny of the anisospinoprotorhabd in Biverticillus gen. nov. which differs from that in Latrunculia; 2) the unprecedented form of the diagnostic microsclere compared to the microscleres of other Latrunculiidae; 3) the very thin encrusting morphology of the holotype, which is unusual for Latrunculiidae; 4) the isolated location of the holotype south of Madagascar, and; 4) the continuing discovery of new fossil and Recent diversity in family Latrunculiidae.