Halcampa arctica Carlgren, 1893

Halcampa arctica Carlgren, 1893 View in CoL

Table 3; Figs. 5–7 View Fig View Fig View Fig .

Halcampa arctica Carlgren, 1893: 45 View in CoL ; 1921: 120.

MATERIAL EXAMINED. Franz Jozef Land   GoogleMaps , Wilton Island, 80°34.5′ N, 54°19.9′ E, Station 30, 18–25 m, 23.08.2013, collector S.D. Grebelnyi, one specimen.

DESCRIPTION. The specimen is 19 mm in height and 7 mm in greatest diameter, strongly contracted, with the distal part deeply invaginated into the body. The column is divisible into the physa, scapus and scapulus. The physa is naked, not invaginated in preserved specimen and visible externally. The scapus is covered by cuticle densely incrusted by fine sand grains ( Fig. 5A View Fig ) and has numerous tenaculi ( Fig. 6F View Fig ). The scapulus lacks cuticle and sand incrustation and has six prominent scapular ridges ( Fig. 5C View Fig ).

The tentacles on preserved specimen are short and thick, not visible externally, 12 in number, arranged hexamerously in two cycles.

The marginal mesogloeal sphincter is small, 150 × 30 µm on longitudinal sections, reticular, situated in the outer side of the base of the tentacles ( Fig. 6A View Fig ). Longitudinal muscles of the tentacles and radial muscles of the oral disc are ectodermal. The columnar circular endodermal musculature is well developed.

The actinopharynx is transversely sulcated ( Fig. 5C View Fig ). The siphonoglyphs are shallow and poorly defined, supported by two pairs of the directives. The mesenteries arranged hexamerously in two cycles (6+6 pairs). The mesenteries of the first cycle are macrocnemes, those of the second cycle are microcnemes ( Fig. 5B View Fig ). The oral and marginal stomata were not found. The retractors of macrocnemes are very strong, restricted to circumscribed, reniform, with large pennon on its parietal side and with about 25 muscle processes, most of which are branched ( Fig. 6B View Fig ). Parietal muscles are small, oval or triangular on transverse sections, with few thick weakly branched lamellae ( Fig. 6E View Fig ). Parietal muscle fibers are expanded on the body wall. The microcnemes ( Fig. 6D View Fig ) resemble parietal parts of the macrocnemes.

All macrocnemes are fertile, containing ova up to 0.3 mm in diameter.

Cnidom. Cnidom includes spirocysts, basitrichs, p -mastigophores B1 (see Table 3 and Fig. 7 View Fig ). The ectoderm of the scapulus, the tentacles and the oral disc contains numerous spirocysts.

REMARKS. The original description of Halcampa arctica is based on several specimens from Spitsbergen collected at the depth from about 9 to 70 m. The present specimen from Franz Josef Land comes from about the same latitude. Both external and internal features agree closely to the original and so far the most detailed description of this species ( Carlgren, 1893). In particular, the shape of retractors ( Fig. 5B View Fig ) and the number of their mesogloeal processes are similar to those described and figured by Carlgren (1893, Taf. V, Fig. 6 View Fig ) for H. arctica , but differ from those of H. duodecimcirrata (which has smaller retractors with less numerous mesogloeal processes, see Carlgren, 1893, Taf. V, Fig. 3 View Fig ). Fautin (2015) provide photographs of some of Carlgren’s (1893) syntypes of H. arctica and some of them look almost identical to our specimen, they are similarly contracted in preservative, with the distal end invaginated into the body and being wider than the proximal end, similarly covered by sand and are of about the same size. Thus, although we have only one specimen of this species and cannot estimate degree of its variability, the morphological similarity and the fact that it comes from high Arctic location left no doubt that it is conspecific with Carlgren’s(1893) specimens and belongs to H. arctica .

Carlgren (1933) reported a smooth specimen (without sand incrustation on the body and without tenaculi) which he with some hesitation referred to H. arctica . He suggested that if the species lives on the bottom which is not sandy the tenaculi do not develop. Without further investigation is hard to say whether this statement is correct.

Carlgren (1921: 122) had some doubt if Halcampa arctica (from Arctic locations) and H. duodecimcirrata (mostly from European shores) are separate species (“it is possible we have to do with only one species having its habitation proper in the Arctic Sea [...] but also distributed at the shores of Norway and Sweden and the Eastern sides of Denmark ”) but preferred to retain them separate. Hand (1954) provide rather detailed discussion of several Halcampa species and also preferred to keep them separate. At our opinion H. arctica is a valid Arctic species, which differs from H. duodecimcirrata by larger retractors, by the body more heavily covered with sand grains and, probably, by arrangement of cinclides on the physa.

Severallivingspecimenswerephotographed underwater by O. Savinkin in coastal waters of Luidji Island, Franz Josef Land, 06 August 2013 ( Fig. 5D View Fig ). In its type locality, Spitsbergen, Halcampa arctica may form settlements of many specimens — several underwater photographs showing numerous specimens of this species on sandy and muddy bottom were taken on Spitsbergen by Dr. P. Bałazy ( Fig. 5E–G View Fig ). These photographs show the specimens burrowed in sandy bottom with the oral disc, tentacles and white bare scapulus exposed on surface. The oral disc and the tentacles are pale yellowishbrown, 12 darker reddish-brown spots are in the circle around the mouth on the disc. The specimens from Franz Josef Land have more contrast colour patter on the disc and tentacles ( Fig. 5D View Fig ).