Remarks

The original description of Haliactis arctica is based on a small number of specimens from Greenland, Spitsbergen, Norway and Chukchi Sea collected at depths from about 7–9 to 380 m. The specimens described here are from the Chukchi Sea, White Sea and Franz Josef Land. Both external and internal features agree well with the original description of this species (Carlgren, Reference Carlgren1921). Body measurements in preserved condition indicated by Carlgren (Reference Carlgren1921) are 8–20 mm in height and 5–10 mm in diameter. The examined specimens slightly expand this range. One of the largest specimens is 25 mm in height, 13 mm in diameter in the widest part and 5 mm in diameter in the narrowest proximal part. The smallest specimen is 3 mm in height and the diameter of its pedal disc is 4 mm. The height of other specimens varies from 5–23 mm. Their width is also quite variable (5–15 mm) due to their different body shapes. Many of the examined specimens look identical to the specimens from Greenland, which were described and figured by Carlgren (Reference Carlgren1921, Pl. I, figure 3). In addition, they were similarly preserved in a contracted position with a wider distal or middle part; the surface of their column is similarly covered with numerous annular wrinkles. Like Carlgren (Reference Carlgren1921), I also found specimens whose proximal end is rounded. However, most of our polyps have a well-developed pedal disc, very often retracted. There are several differences in the number and size of the tentacles. Carlgren (Reference Carlgren1921) found 80–90 tentacles, but among the examined material the range is 40–87 due to the study of both adult sexually mature and juvenile specimens. In addition, I found a variation in the size of the tentacles, which is probably associated with a different degree of expansion when the sea anemones were fixed. Both inner and outer tentacles can be of approximately equal length. In addition, in specimens with fully retracted tentacles, the outer ones are usually longer and thinner, the inner ones are usually shorter and thicker. When the tentacles are extended, the length of both types can vary. It also cannot be said strictly that, for example, all the outer (or inner) tentacles are long, some of them will be short and vice versa.

The number of the cycles of the mesenteries and their arrangement correspond to Carlgren's (Reference Carlgren1921) description. Nevertheless, several juveniles have been found only with two cycles of the mesenteries, which may be complete or not. In addition, it was found that acontia and filaments can also be developed on mesenteries of the second cycle at the level of the actinopharynx and below its border. In the proximal part, they are usually absent. In rare cases, particular mesenteries of the third cycle have weak filaments, as well as acontia, a feature not mentioned by Carlgren (Reference Carlgren1921).

Mesenteric musculature often differs in appearance between individuals due to dissimilar degrees of contraction (compare Figure 5F–H). The mesenteric plate of some polyps has a thicker mesoglea. It gives large branches both in the area of the retractors and in the parietal part. A similar phenomenon is described by Carlgren (Reference Carlgren1921: 131). The structure of the retractors of the studied polyps is somewhat different from that described by Carlgren (Reference Carlgren1921). He described retractors in diagnosis (1921: 129) as ‘strong, furnished with high folds and rather richly ramificated, especially in the inner and outer parts’. In specimens examined, the entire retractor is characterized by branching high folds, and not only its outer and inner parts. In addition, on histological preparations, it is shown that the longitudinal retractor muscles of the directives and other perfect mesenteries often belong to different types (e.g. Figure 5D, F). Retractor muscles of the directives reniform and almost circumscribed in distal column, but below about the lower end of the actinopharynx, however, they become more elongated. Retractor muscles of other perfect mesenteries elongated throughout the column. Some polyps, however, do not have such a clear difference (Figure 4C). This was not recorded by Сarlgren (Reference Carlgren1921). His remark (1921: 131, textfig. 153) that the folds are very high and more branching in the reproductive area than in the lower part of the actinopharynx is not confirmed by my observations. The present research shows that towards the aboral end, on the contrary, the retractors are reduced more and more, almost completely being replaced by parietal muscles in the pedal disc area (Figure 7C, D). More branching folds can be observed in the reproductive region only in the retractors of the directive mesenteries. It should be noted, only one specimen shows the presence of weak retractors in the mesenteries of the second cycle in the distal part of the column (Figure 10A), but below about the middle of the actinopharynx, the retractors begin to decrease in some of the mesenteries, and the mesogloeal plate of the mesentery was completely occupied by clear parietal muscles (Figure 10B).

The parietal muscles of the specimens examined correspond to the description of Carlgren (Reference Carlgren1921): however, in a number of individuals, a mesogloeal thickening was found in the region of the mesenteric insertion. This thickening produces well-defined folds (thick and short or long and thin) on the side of the retractor, weak or absent on the opposite side. The folds are shorter between this thickening and the column wall (Figures 4С & 6B–D). Closer to the proximal part, the parietal muscles develop more strongly. Their folds become longer, thicker and more frequent. Mesogloeal thickening presenting in some specimens in the distal part disappears at this point (Figure 6E, F). Retractors are significantly reduced in size (Figure 7A, B). In the base area, parietal muscles displace retractors and almost completely occupy the mesenteric plate (Figure 7C, D). Of course, it should be noted that the changes in the musculature vary in different mesenteries. Nevertheless, the general trend is associated with a gradual reduction of the retractors and, on the other hand, the increasing development of the parietal muscles towards the aboral end. Mesenteries of the second and third cycle have well-developed parietal muscles with numerous folds, which can be thin or thick (Figures 8, 9 & 10). In addition, as in the case of retractors, the parietal muscles of the younger cycles have a slightly different structure in the distal and proximal parts of the column (Figures 8A, D, E & 9). The degree of development is variable both in different specimens and within one specimen (Figure 8B, C).

With respect to the acontia, Carlgren (Reference Carlgren1921) noted their abundance. The examined material exhibits that the number of acontia varies in H. arctica. Acontia are numerous (6–8) only in a few specimens. As a rule, acontia are few in number and most polyps have 1–3 acontia. Some specimens lack acontia that is also noted by Carlgren (Reference Carlgren1921) for the anemones, described by him as Acthelmis schaudinnii. The absence of acontia is also known for deep-sea Bathyphellia margaritacea (Danielssen, Reference Danielssen1890) (Bathyphelliidae Carlgren, Reference Carlgren1932) and Seepactis galkini Sanamyan & Sanamyan, Reference Sanamyan and Sanamyan2007 (Kadosactinidae Riemann-Zürneck, Reference Riemann-Zürneck1991) (see Riemann-Zürneck, Reference Riemann-Zürneck1997; Sanamyan & Sanamyan, Reference Sanamyan and Sanamyan2007; Sanamyan et al., Reference Sanamyan, Cherniaev and Sanamyan2009). An explanation of this phenomenon was given by Riemann-Zürneck (Reference Riemann-Zürneck1997) and Sanamyan & Sanamyan (Reference Sanamyan and Sanamyan2007). According to the assumptions of these researchers, acontia can be destroyed during the collection of samples from great depths. According to Stephenson (Reference Stephenson1920, Reference Stephenson1935), acontia could be repeatedly reduced in the course of evolution during the origin of Mesomyaria Stephenson, Reference Stephenson1921 from the representatives of Acontiaria Stephenson (Reference Stephenson1935).

Nematocysts were examined in specimens from all localities (in two from Franz Josef Land, in two from the White Sea, the Chukchi and East Siberian seas). Cnidom includes spirocysts, basitrichs and microbasic amastigophors (see Table 1 and Figure 11). Since the original description of Carlgren (Reference Carlgren1921) does not include data on filaments and acontia nematocysts, and, according to him, the material was in poor condition, I compared my material mainly with the clearer and more complete description of Carlgren (Reference Carlgren1940: 28). This comparison showed that the microbasic amastigophors of сolumn in the examined specimens have a smaller size, namely, 24–32 × 5–6 μm vs 25–36 × 5–6.5 μm (however, they exceed the size of the a- and b-nematocysts of the material in Carlgren (Reference Carlgren1921)). The size range of basitrichs of the tentacles turns out to be larger, namely, 15–26 × 2–3 μm vs 22–24 × 2.5 μm in Carlgren (Reference Carlgren1940). The size of spirocysts is similar to ones indicated by Carlgren (Reference Carlgren1921). The sizes of nematocysts in actinopharynx, filaments and acontia closely agrees with those in Carlgren (Reference Carlgren1940). Spirocysts from tentacles are numerous in all examined specimens. Basitrichs are smaller in number, and in some specimens they are rather rare, which is also noted for microbasic amastigophors in column. Microbasic amastigophors of the actinopharynx and filaments are of almost the same size, but the nematocysts of the filaments are more frequent. The frequency of nematocysts of the acontia is high, especially of microbasic amastigophors, and it is similar in all examined specimens. Several spirocysts are sometimes found in the ectoderm of column.

Thus, this material corresponds to the original description of Haliactis arctica given by Carlgren (Reference Carlgren1921), but shows wider variability in several characters.