Polar Biol (1998) 19: 193±197
Ó Springer-Verlag 1998
ORIGINAL PAPER
Dorothea StuÈbing á Dieter Piepenburg
Occurrence of the benthic trachymedusa Ptychogastria polaris Allman, 1878 (Cnidaria: Hydrozoa) off Northeast Greenland and in the northern Barents Sea
Received: 8 September 1997 / Accepted: 12 September 1997
Abstract The trachymedusa Ptychogastria polaris Allman, 1878 has been identi®ed in seabed photographs from high-Arctic shelves and upper continental slopes o Northeast Greenland and in the northern Barents Sea. It was found to be a common epifaunal element, being present at 34 of 57 stations in 40- to 495-m depth and at 7 of 11 stations in 70- to 330-m depth, respectively. All specimens recorded in the photographs either sat directly at, or hovered very closely (£ about 1 cm) over, the sea bed, indicating a primarily epibenthic life style of this hydrozoan species. The small-scale (i.e. within-station) distribution of medusae was rather patchy, with frequencies along photographic transects ± consisting of 35±73 pictures distributed over seabed strips of 150- to 300-m length ± varying from 1 to 58% o Northeast Greenland and from 1 to 34% in the Barents Sea. Absolute maximum density was 6 ind m)2, and station mean abundances ranged from 0.01 to 0.91 ind m)2 and from 0.01 to 0.52 ind m)2, respectively. Values tended to decrease with water depth, albeit signi®cantly only o Northeast Greenland. Otherwise, no clear relationships to environmental conditions, such as geomorphology or near-bottom water hydrography, were detected. Circumstantial evidence suggests that seabed granulometry and potential food supply are important distribution determinants. However, further investigations are required to identify more stringently the key factors controlling the distribution of P. polaris.
1985 (Piepenburg and Schmid 1996a,b). These specimens had diameters of about 1±3 cm and were characterized by long ®liform tentacles and eight radial deepred stomach lobes (Fig. 1). However, it was dicult to identify them to species level since no such medusae were found in trawl catches carried out concomitantly at many photographic stations. Recent careful comparative analyses of morphological features visible in the photographs and discernible in specimens collected during other Arctic expeditions con®rmed that these medusae were Ptychogastria polaris Allman, 1878. This is the only trachymedusan species that is characterized by morphological and functional adaptations to benthic life such as, for instance, adhesive disks on the tentacles (Larson et al. 1992), that is known to occur in Arctic waters (Stepanjants 1989). P. polaris is an Arctic circumpolar species (Kramp 1947; Stepanjants 1989), which only occasionally penetrates boreal waters (Mackie 1985; Larson et al. 1992). Because of its supra- or epibenthic life style, P. polaris is dicult to collect using conventional sampling gear and has, therefore, only rarely been caught (Larson et al. 1992; Sirenko et al. 1996). Consequently, current knowledge about this species is relatively scant. The objective of this paper is to disseminate the results of quantitative analyses of sea-bed photographs with regard to distribution, abundance, behaviour and body size of P. polaris in two high-Arctic regions.
Introduction
Materials and methods
Benthic medusae were common and conspicuous organisms when recorded in seabed photographs taken in high-Arctic waters of the Greenland and Barents Seas in
D. StuÈbing á D. Piepenburg (&) Institut fuÈr PolaroÈkologie der UniversitaÈt Kiel, Wischhofstrasse 1-3, GebaÈude 12, D-24148 Kiel, Germany Fax: +49-431-600-1210; e-mail:
[email protected]
Sea-bed photographs were obtained during ``Polarstern'' expeditions ARK III in July/August 1985 (Gersonde 1986), ARK VI in July/August 1990 (Krause 1991) and ARK IX 1993 in June/July 1993 (Hirche and Kattner 1994) o Northeast Greenland, as well as during ``Polarstern'' expedition ARK VIII (``Arctic EPOS'') in July 1991 (Rachor 1992) in the northern Barents Sea. O Northeast Greenland, a total of 57 stations at water depths of 35±580 m were covered (Fig. 2; see also Piepenburg and Schmid 1996a), and in the Barents Sea a total of 11 stations at depths of 70±350 m (Fig. 3; see also Piepenburg and Schmid 1996b). A photoprobe, basically
194
Greenland and 0.01 and 0.52 ind m)2 in the northern Barents Sea (Table 1). Small-scale distribution
Fig. 1 Detail of sea-bed photograph from the shelf o Northeast Greenland (Ob Bank, 81°00¢ N, 11°33¢ W, 100 m), depicting three specimens (see arrows) of the epibenthic trachymedusa Ptychogastria polaris. White scale bar in lower right corner 5 cm consisting of a vertically mounted still camera (Photosea 70D) and an obliquely oriented strobe (Photosea 1500D), was lowered from the drifting ship. On sea-bottom contact, a forerunner weight hanging approximately 1.5 m below the probe triggered the camera and strobe. The gear was then heaved for some 5 m and lowered again to take the next picture. Thus, a number of series of 35±73 colour slides per station, each depicting approximately 1 m2 of the sea-bed, was obtained along transects of 150±300 m length. From 1991, a compass of known size, just touching the sea bottom at the same time as the forerunner weight, provided a size scale in the photographs. For details about the camera system and its handling see Piepenburg and von Juterzenka (1994). The colour slides were examined for presence and abundance of P. polaris by means of a binocular microscope at magni®cations of ´ 6.4 and ´ 16. Medusae were counted and, if the compass was visible in a photograph, sized. Photographic counts were raised to abundance ®gures (ind m)2). Spearman's rank correlation coecient (rhoS) was used to measure the strength of the relationship between abundance of medusae and environmental parameters (water depths, temperature, salinity).
Results and discussion Our ®ndings con®rmed prior reports that Ptychogastria polaris inhabits both the northern Greenland Sea and the northern Barents Sea (Kramp 1947; Stepanjants 1989). However, we expand the current knowledge by providing quantitative information on its abundance, small-scale dispersion pattern, behaviour and body size. Abundance P. polaris was a common epifaunal element in the study areas, being recorded at 34 (i.e. 60%) of 57 stations o Northeast Greenland (Fig. 2a) and at 7 (i.e. 64%) of 11 stations in the Barents Sea (Fig. 3a). A total of 557 medusae were found in the total of 3,361 sea-bed photographs analysed. Station mean abundances of P. polaris varied between 0.01 and 0.91 ind m)2 o Northeast
At most stations, the small-scale distribution pattern of P. polaris was rather patchy, with frequencies along the photographic transects varying between 1 and 58% o Northeast Greenland and 1 and 34% in the Barents Sea (Table 1). Moreover, in most sea-bed photographs on which P. polaris was present, single specimens were encountered, and only rarely were several medusae (six at maximum) detected in one photograph (Fig. 1). The pronounced patchiness of individuals at a 100-m scale implies that P. polaris might also have been present at stations where no specimens were detected. Behaviour No specimens visible in the photographs threw a shadow on the sea bed. This implies that they either sat directly on, or hovered very closely over, the sea ¯oor. Within the 1.5-m-thick near-bottom water layer covered by our approach, we did not detect any specimen ¯oating higher than about 1 cm at maximum above the sea bed, indicating that this hydrozoan species spends most of its time attached to sea-bed substrata. Recent aquarium observations of live specimens caught during a ``Polarstern'' expedition to the Barents Sea in May 1997 also showed that P. polaris preferred to adhere to the bottom, making only short swimming excursions (D. StuÈbing, unpublished data). Our ®ndings thus corroborate the suggestion of Mackie (1985) and Larson et al. (1992) that P. polaris should be classi®ed as an epibenthic medusa. Body size Because all medusae were on or directly over the seabed, their diameters could be measured with high precision (1 mm) in the photographs. The diameters of the exumbrellas of the specimens sized in sea-bed photographs ranged between 7 and 29 mm o Northeast Greenland (n 54) and between 5 and 21 mm in the Barents Sea (n 76). The median diameter was nearly the same in both study areas (14 and 13 mm, respectively). According to the literature (Allman 1878; Browne 1903; Mayer 1910; Broch 1929), bell diameters of P. polaris usually vary from 10 to 15 mm. Maximum values of 18±24 mm were reported by Haeckel (1881). It should be noted, however, that all these prior measurements were carried out with preserved specimens. Numerous ribs and grooves on the exumbrellas suggest that these individuals were contracted and actual sizes were, therefore, very probably underestimated (Haeckel 1881; Broch 1929).
195 Fig. 2 Northeast Greenland. a Geographical distribution of Ptychogastria polaris. b Relationship between mean abundances of P. polaris and water depth
Relation to environmental conditions P. polaris has been reported from water depths ranging from 10 to 2500 m (Kramp 1947; Stepanjants 1989). In our study, we found it at depths between 40 and 495 m o Northeast Greenland (Fig. 2b) and between 70 and 330 m in the Barents Sea (Fig. 3b), i.e. along almost the entire depth range covered by our photographic census. Otherwise, P. polaris was not recorded at stations on the continental slope o Northeast Greenland, and there
was no clear preference in its geographical distribution for certain geomorphological features such as shelf banks or troughs (Figs. 2a, 3a). However, o Northeast Greenland abundances of P. polaris did decrease with water depth from almost 1 ind m)2 at about 50 m to very few or nil at depths >400 m (Fig. 2b: rhoS )0.549, P < 0:0001). Although this correlation was signi®cant, it should be noted that P. polaris was absent at several shallow stations (Fig. 2a,b). In the Barents Sea, there was also a trend that, on average, P. polaris was
196
Fig. 3 Northern Barents Sea. a Geographical distribution of Ptychogastria polaris. b Relationship between mean abundances of P. polaris and water depth
Table 1 Photographic inventory of Ptychogastria polaris. Minimum, median and maximum values of within-station frequencies (%) along photographic transects and mean abundances (ind m)2)
more abundant in depths
