Artemisina apollinis: Lundbeck 1905, p. 114 – 116, pl. XIII, figs 4a – g; Hentschel 1929, p. 876, 939; Koltun 1966, p. 140 – 141, fig. 97

Material examined

The holotype was collected at the north-eastern part of the Laptev Sea ( 77.76°N, 131.86° E); deposited in the Edward Eversman Zoology Museum (identification number 2.2.8.442).

Paratype locality same as holotype locality ( 78.05°N, 133.41°E).

Description

Sponge of cup- or fan-like shape, up to 4.5 cm in height. Surface coarse-pored, slightly setose. Circular oscules about 2 mm in diameter scattered over the surface. The dermal membrane is a thin, translucent film. The consistency is soft, loose and quite fragile. Both examined specimens possess gemmules of roundish and slightly flattened shape (up to 650 µm in diameter), distributed throughout the interior. Colour from light to dark brown.

Skeleton

The main skeleton consists of multispicular, longitudinal tracts and disorderly scattered single spicules. The dermal skeleton is composed of a layer of small styles, lying more or less tangentially.

Spicules

( Figure 6 (a – d)). Large choanosomal styles, slightly curved and spined at the basal end, dimensions: 548.6 – 658.8 – 782.9 ( n = 20) × 12 – 16.5 – 20 ( n = 15) µm; small dermal styles straight, roughly spined at the basal end, dimensions: 310 – 434.5 – 487.2 ( n = 15) × 5.15 – 7.4 – 8.7 ( n = 15) µm; toxa (fully developed) with spined ends: 279 – 409 – 498.6 ( n = 10) × 3.8 – 5 – 5.7 ( n = 10) µm; palmate isochelae: 12.7 – 14.7 – 16.2 ( n = 10) µm.

Etymology

The species is named in honour of William Lundbeck, a Danish zoologist, whose brilliant monographs on sponges have remained a model of systematic description to be followed.

Remarks

The term ‘ bipolar ’ usually refers to a species found in both the Northern and Southern hemispheres, with a gap in distribution at lower latitudes (i.e. in temperate and tropical waters). Numerous hypotheses for this distribution have been proposed (see Ekman 1953). As Bergh (1920) pointed out (referring to the work of Charles Darwin), the general cooling of equatorial waters and extension of cold/temperate regions during glaciation resulted in some cold-water species becoming capable of crossing the equatorial belt and penetrating farther into the Southern Hemisphere. With the onset of gradual warming, some populations that inhabited temperate and tropical waters retreated to higher latitudes or became extinct. As a result, a bipolar distribution was established and the Northern and Southern Hemisphere species populations became isolated.

Recent studies support this point of view. Using molecular methods, Halvorsen (2010) investigated two closely related Micromesistius (Pisces: Gadidae) species with an ‘ antitropical ’ distribution, and found that the time divergence between them was about 2 million years, roughly coinciding with climatic changes.

In view of the above, it becomes evident that we should delimit the Antarctic and Arctic representatives of Artemisina apollinis ( Ridley and Dendy 1886) as separate species. Initially described by Ridley and Dendy from the vicinity of the Kerguelen Islands, this species has repeatedly been recorded in Antarctic and Subantarctic waters. Later, Lundbeck (1905), Hentschel (1929) and Koltun (1955) described a close species from the Arctic waters and synonymised it with A. apollinis despite some morphological differences between them. The main differences are in the morphology of choanosomal styles, which in Arctic species show slight spination at the basal end of spicules and are characterised by their greater length in general: 326 – 670 µm (Antarctic species) vs 500 – 842 µm (Arctic species). Lundbeck (1905) examined the holotype, and insisted on the presence of slight spination on the choanosomal styles; this statement needs to be verified since none of the other authors ever mentioned it in the Antarctic specimens. Also, Lundbeck did not attach any importance to the differences in size of the spicules. However, the studies of past decades have proved the significance of a morphometric approach in the analysis of sponge spiculation (see e.g. Sara et al. 1992).