The distinctive morphology of the collected specimens prevented confusion with any other hydromedusa species in the area and allowed for a straightforward identification of the animals as T. musculosa Beyer, 1958, subsequently confirmed through a detailed comparison with the holotype of the species. The morphology of the medusae is in close agreement with both the original description by Beyer  and the subsequent account by Hesthagen , except for the coloration pattern. Previous accounts on the coloration of T. musculosa include the presence of white to yellow epidermal spots located at the perradial junctions of the radial canals with the ring canal . In the individuals from Raunefjord, however, the entire ring canal and the distal parts of the radial canals were pigmented, with eight additional interradial pigment patches not associated with the radial canals present on the umbrella margin. A potential role of exumbrellar pigment spots as photoreceptors has been discussed previously for this species, since the medusae are known to spawn in response to strong illumination under the microscope in laboratory conditions , but further research is needed to determine the degree of interaction between light cues and ectodermal pigments in T. musculosa. When examined alive under the direct light of the microscope at different intensities, medusae from both Raunefjord (present study) and Oslofjord  did not show any phototaxic response, suggesting that these bottom-living medusae may not depend on light for orientation and movement. It has been suggested that the adult medusae permanently remain on the sea bottom, with the possible exception of swimming as a flight response [6, 8].
The number and position of the statocysts are diagnostic characters separating Tesserogastria from all other genera in family Ptychogastriidae [12, 21]. However, some confusion exists regarding the exact position of these structures in T. musculosa, as they have been alternatively described in scientific accounts as adradial  or interradial . The original description of the species does not include information on the position and number of statocysts because Beyer was unable to find any in formalin-preserved animals, presumably due to the disintegration of the statoliths in this fixative . Nevertheless, the statocysts from medusae from Raunefjord fixated with neutral-buffered formalin were evident even after 6 months of preservation, allowing us to confirm that in T. musculosa the statocysts are invariably interradial and can be used as a reliable character separating Tesserogastria from both Ptychogastria and Glaciambulata.
For both the 16S and COI markers, the sequences generated for this study show a large degree of differentiation (> 20%) between T. musculosa and individuals of the other two species in the family for which genetic data are available (i.e. Glaciambulata neumayeri Galea et al., 2016 and P. polaris), highlighting the value of these two markers as DNA-barcodes for this species. At genus level, all three ptychogastriid genera show a similarly large degree of differentiation, which is comparable to the distances observed by Grange et al. between individuals identified as P. polaris from the Antarctic Peninsula and the Sea of Japan . Our P. polaris individuals from Hjeltefjord are again clearly different from both the Antarctic and Japanese specimens, and the preliminary phylogenetic analysis suggests that the taxonomy of the Ptychogastriidae, and in particular the hypothesis of monophyly of genus Ptychogastria, needs to be revised, as the intraspecific degree of differentiation within this genus appears comparable to the degree of differentiation between genera. It is unlikely that DNA sequences from either the holotype or the paratypes of T. musculosa will be available in the near future, since in both cases the specimens have been subjected to fixation and long-term storage in formalin. The 16S and COI sequences provided here for T. musculosa constitute thus a useful tool for identification, as well as the first molecular evidence of the relationships of the genus with both Glaciambulata and Ptychogastria.
Tesserogastria musculosa was a common component of the epibenthic community in the sampled locality, with the estimated density (ca. 0.5 ind/m2) similar to what has been reported for other ptychogastriid medusae in boreal waters (0.01–0.91 ind/m2 for P. polaris in northeast Greenland, and 0.01–0.76 ind/m2 for the same species in the Barents Sea) [22, 23]. The density of T. musculosa was however considerably lower than the maximum densities reported for ptychogastriid medusae in Antarctic waters (up to 13 ind/m2 for P. polaris in Antarctic fjords) . In Digerud and neighbouring localities, T. musculosa has reached maximum values of > 10 ind/m3, being the most abundant organism in at least one sampling station , while subsequent observations in Oslofjord have shown that the species is common and, at times, highly abundant in soft bottoms [9, 24,25,26]. The medusae from Raunefjord and Oslofjord shared similar size ranges and unimodal distribution of bell height, but the mean umbrella height in this study (1.71 mm) was higher than the mean height (0.96–1.03 mm) reported for the same species in the type locality at any given time of the year .
The high numbers of medusae in some localities along Oslofjord allowed Beyer to analyze the local distribution of the species. This lead him to suggest that T. musculosa is an indicator of non-polluted bottoms, with the number of individuals rapidly increasing towards the open sea . Later on, the consistent patterns of decreasing abundance towards the inner parts of the fjord caused Beyer and Indrehus to advocate the species as an indicator of oceanic and unpolluted conditions in the area . Despite this general pattern, the population has fluctuated during the recent decades, with a dramatic population reduction in the inner fjord starting in the 1960s and continuing until the mid-1990s . Remarkably high numbers of medusae were observed again in 1996 , and the species has since been recorded rather consistently in Oslofjord until as recently as 2012 , but without information on population density or abundance.
High densities of ptychogastriid medusae in soft sediments have been linked to high productivity in subantarctic fjords [3, 28], and to the accumulation of organic and inorganic debris in the seafloor of relatively isolated deep environments [4, 29]. Raunefjord’s sediment characteristics and oceanographic processes (e.g. enhanced benthic productivity, vertical flux, trapping of detritus) are likely to provide a suitable habitat for epibenthic medusae in an analogous way to the observed abundance of P. polaris in subantarctic fjords , Ptychogastria asteroides (Haeckel 1879) in Mediterranean canyons , and T. musculosa in Oslofjord .
Contrary to the interannual variation, significant seasonal population fluctuations have not been observed for T. musculosa in its type locality, with high numbers of medusae collected throughout the year in seasonal surveys . In West-Norwegian Fanafjord, however, strong seasonality in T. musculosa was reported by Kaartvedt, who observed the highest abundances in June and much fewer specimens in April, September and December . Unpredictable, sporadic and episodic population fluctuations are widely documented for planktonic jellyfish [30, 31] and some benthic hydrozoans , but the mechanisms behind these remain poorly understood [31, 33]. Episodes of bloom-and-bust have never been reported for benthic ptychogastriid hydromedusae and these organisms may not be part of the subset of medusozoan species with life-cycle attributes that predispose them to bloom . Existing evidence appears to suggest that epibenthic medusae in genera Ptychogastria and Tesserogastria do not form blooms with abrupt periods of presence in the environment followed by completely disappearance, and instead maintain rather stable large populations in habitats such as the bottom of Boreal, Arctic and Antarctic fjords, and deep submarine canyons [3, 6, 22, 23]; although the observed variations in the abundance of T. musculosa in Fanafjord  could instead indicate that seasonal dynamics differ widely among populations of this species in different habitats.
For a relatively easy-to-identify species distributed in the well-studied vicinities of active marine biological stations in both Oslofjord and Raunefjord, T. musculosa has a puzzlingly low number of records in the published, peer-reviewed scientific literature. Subsequently, it has been generally considered a rare and geographically restricted species. Based on our data and a thorough review of the “gray” literature, this perception is most likely incorrect, and T. musculosa in fact appears to be a relatively common and widely distributed component of the epibenthos in several fjords along the North Sea coast. The lack of published records for T. musculosa does not result from the rarity of the species, neither can it be attributed to undersampling of its habitat, as numerous studies have been conducted on the benthic fauna of the areas surrounding both the Marine Biological Station in Drøbak and the Espeland Marine Biological Station in Raunefjord since their founding in 1894 and mid-1950s, respectively [35, 36].
More than with other epibenthic animals, our ability to detect the presence of T. musculosa—and most likely other gelatinous epibenthos—appears to depend on a combination of the chosen sampling gear and a careful processing technique. In the original description, Beyer stressed the importance of the sampling gear in finding T. musculosa in Oslofjord, stating that his specially designed sledge, known as the Beyer sledge, consistently yielded catches of T. musculosa . In the localities where abundant medusae have been caught with sledges, sampling with plankton net hauls from above the bottom has never yielded a single specimen, either in Oslofjord  or Raunefjord and the surrounding fjords . Beyer suggests that “the species has probably been caught many times in grab and Mortensen dredge samples, but has then been disregarded together with the plankton inevitably caught in these apparatus”, thus highlighting the importance of gentle sampling, adequate sample processing techniques, and correct identification for observing this species . Prior to the current study, all specimens of T. musculosa ever collected have been obtained with Beyer’s epibenthic sledge (illustrated in ) with a 50 cm closing plankton net mounted on a steel toboggan, designed for the purpose of catching organisms on or immediately above the bottom [5, 6, 8, 9, 27, 39]. In the current study, a slightly less gentle and much larger Rothlisberg and Pearcy (RP) sledge was used, but this was compensated for by the careful processing of the samples, as well as the very slow transect and retrieval speed. It is not surprising that careful processing of samples will result in higher numbers of species found: in Kiel Bay, Remane found > 300 new species after modifying his sampling techniques and adopting a more careful handling approach of the samples .
The population assessment of benthic ptychogastriid medusae has only recently become feasible thanks to improved sampling techniques and in situ observations by divers or ROVs, resulting in the description of new species and several new records for the already-known ones [3, 12]. Other multidisciplinary approaches (e.g. sediment traps) have also been successfully used for the collection of well-preserved ptychogastriid medusae in remote locations such as submarine canyons , and thus represent a potential source of valuable information for this taxon. Ptychogastriid medusae were rarely reported before the late 1990s [22, 23], partly due to the difficulties of sampling deep marine environments, but also because the medusae, often damaged beyond recognition, were readily considered planktonic contaminants. The standard processing of benthic samples with sieves often results in the gelatinous species getting extruded and destroyed, leading to them remaining unreported. The lack of records for T. musculosa and other benthic and benthopelagic medusae  may thus be an artefact of the commonly used sampling techniques.
A second issue contributing to the perceived rarity of T. musculosa is the poor accessibility of the bulk of the records, which consist of technical reports, unpublished theses, and museum collection catalogues. Many basic and applied studies in biogeography, phylogenetics, and ecology rely on species distribution data compiled exclusively from the published scientific literature but, for some species, such as T. musculosa, these data are incomplete and may contain serious biases. While the use of unverified anecdotal occurrences and unpublished observations for the assessment of the current and historical ranges of rare species can certainly lead to large errors of omission and commission , for some taxa, such as T. musculosa, inclusion of records from the “gray” literature is crucial for a realistic estimation of their distribution. Common complaints against the inclusion of “gray” literature data in distributional analyses include the low quality and low accessibility of the sources. While the latter is also an issue in the case of T. musculosa (e.g. all records are in languages other than English and several unpublished documents are not available online), the quality of the data they contain is in general quite high: all observations were made by trained professionals working in the field of marine biology/ecology, in many cases in collaboration with the original author of the species or with one of his students, using established and well documented sampling protocols.
In the particular case of T. musculosa, reviewing and including occurrence records from “gray” literature considerably alters the perception of its distribution and commonness. Furthermore, due to the hyperbenthic habitat and the gelatinous morphology of the medusa, the species is unlikely to be collected or recorded by the most commonly used pelagic or benthic sampling methods. A similar methodological bias applies to other epibenthic and benthopelagic medusae, which remain a poorly known group . Targeted sampling with suitable gear and processing protocols, or using ROVs and other optical platforms, is required to establish a better understanding of diversity and ecology of ptychogastriids and other gelatinous benthos.
In addition to updating our knowledge of the distribution of the species, the reconstruction of the taxonomic history of T. musculosa also allowed us to clarify the existing confusion regarding its date of description. Although the paper by Beyer was actually published in 1958, some authors have mistaken the date of this publication as 1959 [2, 12, 21] probably based on a series of copies of Vol. 6 of the Nytt Magasin for Zoology printed in that year. The error has become widespread in the peer-reviewed scientific literature, although it has not completely permeated the “gray” literature produced in Norway. Beyer himself reported the date as 1958 in subsequent works, as did his students and collaborators (e.g. Hesthagen clearly states that the species was first described by Beyer in 1958) . An enquiry into the publications of the University of Oslo (publisher of Nytt Magasin for Zoology) has confirmed that the correct date of publication is 1958, the correct date for the genus is Tesserogastria Beyer, 1958, and the correct name of the species is Tesserogastria musculosa Beyer, 1958.