Axinella arctica

Axinella arctica (Vosmaer, 1885)

(Figure 2, Table 1)

Original description: Phakellia arctica Vosmaer, 1885. 25, Pl. V, fig. 25-26

Synonyms: Axinella calyciformis (Lamarck, 1814), Spongia calyciformis Lamarck, 1814, Spongia pocillum Lamouroux, 1816, Tragosia arctica (Vosmaer, 1885), Tragosia calyciformis (Lamarck, 1814).

Material examined

North Labrador Sea: CMNI 2018 -0094, 60.468 N, 61.287 W, 412 m depth; CMNI 2018 -0099, 60.466 N, 61.278 W, 452 m depth. Western Greenland Shelf: CMNI 2018 -0146 67.967 N, 59.484 W, 877 m depth. Larne, Antrim, Northern Ireland: Axinella infundibuliformis BELUM.Mc38, 54.93 N, 5.742 W, 23 m depth. Near Lundy, Bristol Channel, England: Axinella infundibuliformis BELUM. Mc 41, 51.178 N, 4.683 W, 23 m depth.

External appearance (Figure 2 A-D)

Axinella arctica (Vosmaer, 1885) is a cup-like or flabellate shaped sponge. The species often forms an inverted, hollow cone that ranges 5–25 cm in diameter, and has a solid stalk. The sponge is pale yellow or buff to white in colour. The surface of the inner portions has many pin-hole sized openings; smaller openings are present on the outside surface. Dense longitudinal ribs extend from the stalk to the distal portions on the outside surface. The consistency is firm, and pieces will break off when bent more than 45°. The walls of the specimens are up to 0.5 cm thick. The distal lip is hispid from protruding spicules.

Spicules and skeleton (Figure 2 E–F)

Megascleres are styles 401– 467 –561 x 13– 17 –25 μm, and oxeas 329– 386 –443 x 13– 18 –24 μm. No microscleres are present. The skeleton consists of dense axial tracts of oxeas with loose extra-axial fibres consisting of styles which form long spicule brushes at the surface. Oxeas connect the extra-axial fibres.

Genetic data

COI for CMNI 2018–0099 (GenBank MK570860) was sequenced. The species is a perfect match to GenBank sequence for Axinella infundibuliformis BELUM:Mc4438 (GenBank HQ379410.1) based on 539 of 584 nucleotides. Although no apparent genetic difference in the COI mtDNA region exists between the two specimens, the lack of trichodragmata spicules and skeleton are characteristic of Axinella arctica; therefore until more comprehensive DNA–based distinctions in a less-conserved gene region can be made, the specimens collected from eastern Canada are considered here to be Axinella arctica based on morphology.

Distribution and ecology

Axinella arctica (Vosmaer, 1885) is found on rocky bottoms attached to hard substrates. In the North Labrador Sea and Baffin Bay the species was collected in deep water> 412 m. The species was also collected in the North Labrador Sea by Murillo et al. (2018), and reported from the Barents Sea, Nordic Seas, and European Waters (Stephens 1921; Van Soest et al. 2000, 2019). This species has a more northern distribution and was not collected in the Gulf of St. Lawrence.

Remarks

The specimens considered here were previously described briefly by Dinn & Leys (2018), which was the first georeferenced record of the species in the North Labrador Sea and Baffin Bay.

The feature of this species that distinguishes it from other cup-shaped specimens in the region is the presence of oxeas and styles. The growth form and shape in situ is similar to Plicatellopsis bowerbanki (Vosmaer, 1885) comb. nov., but A. arctica can be differentiated by its dense stalk and thicker tissue.

A. infundibuliformis was apparently collected by Whiteaves (1874) from the Gulf of St. Lawrence, but no spicule measurements or description were given. It is probable that the specimen collected by Whiteaves was another flabelliform species because A. infundibuliformis is not otherwise recorded from the western Atlantic (Van Soest et al. 2019). By contrast A. infundibuliformis has styles 260–360 µm and oxeas 210–280 µm, considerably smaller than the specimens collected in the North Labrador Sea. A. infundibuliformis also has trichodragmata microscleres, though these may be scarce (Ackers et al. 1992). The skeleton of A. infundibuliformis is also much more dense than A. arctica, with oxeas forming thick axial tracts and styles in multiple, tightly arranged extra-axial fibres. Interconnecting oxeas form a more noticeable reticulation in A. infundibuliformis and extra-axial fibres of styles ramify closer to the choanosome than in A. arctica, forming less obvious spicule brushes. Although A. arctica and A. infundibuliformis are difficult to distinguish from outer morphology, the size of the megascleres and skeletal arrangement are distinct. The genetic variation between the two species is, however, not discernable using the COI Folmer fragment gene region.

The spicule measurements from the North Labrador Sea specimens are only slightly shorter than those of the type specimens of Phakellia arctica (Vosmaer, 1885) measured by Stephens (1921) where the styles are 300–700 x 20 µm, and the oxeas are 300–460 x 14 µm. Stephens (1921) also collected specimens from Ireland, which have styles 300–750 x 20 µm and oxeas 375–600 x 20 µm. According to Stephens (1921), the Irish specimens agree more with Norwegian specimens collected by Topsent (1913) than the original description by Vosmaer (1885). Stephens (1921) reassigned P. arctica to the genus Tragosia rather than Phakellia or Axinella based on the arrangement of spicules in the skeleton. This placement was later refuted by Dendy (1922) because trichodragmata spicules were suggested to be a unique character of Tragosia. However, the presence of trichodragmata does not appear to be unique to any one genus of the Axinellidae and thus this species, as well as other members of Tragosia, were transferred to the genus Axinella (Alvarez & Hooper 2002).