Beroe cucumis Fabricius 1780

BEROE CUCUMIS FABRICIUS, 1780

The body shape is strait or slightly oval in the oral part and oval at the aboral side, and it is more flattened in the paragastral plane than B. pseudocucumis (Fig. 6C, D). The adult length varies from 50 to 150 mm. Its length to width ratio (l/w) ranges 1.6–2.2. This ratio is variable: we observed specimens, which were shorter and longer, wider or slenderer, but never less than for B. pseudocucumis. Juvenile individuals may be narrower in the oral part of the body. Similar individuals were illustrated by Mayer (1912). The meridional canals lie under eight rows of ciliary combs, which extend about three-quarters of the distance from the aboral pole towards the mouth or a bit longer, but not up to the mouth. Its meridional canals have numerous diverticulae, which may branch out in adult ctenophores, but do not anastomose with each other, and do not connect with paragastral canals. At the aboral end, two oval polar-plates (Fig. 6A) surround the sense organ at the oval aboral pole, and are fringed with a row of short, branched papillae. Macrociliaries have three-toothed macrociliary tips with a somewhat larger middle tooth (Tamm & Tamm, 1993).

Geographical distribution: Beroe cucumis was previously believed to be a widespread species, continuously distributed from the Arctic to the Antarctic (Pages & Orejas, 1999), but according to our genetic and morphological studies, and by comparison of published data, this species has a bipolar distribution, inhabiting cold polar and temperate waters, while being absent from tropical and subtropical zones.

Distribution in the Arctic: all Eurasian seas (Sirenko, 2001), including the Barents Sea (Manko et al., 2015; Bandara et al., 2016; this study), the White Sea (Kosobokova & Pertsova, 2018), the Kara Sea (Dvoretsky & Dvoretsky, 2017), the Laptev Sea (Abramova & Tuschling, 2005), the East-Siberian Sea and the Chukchi Sea (Ershova et al., 2015); Canada Basin (Raskoff et al., 2005; Purcell et al., 2010).

Distribution in the Atlantic Ocean: individuals of Beroe cucumis were sampled from the North of Norway to the south-east of the North Sea and analysed genetically (this research). In the Atlantic Ocean Beroe cucumis individuals were sampled from the northern part of Norway to the south-east of the North Sea and analysed genetically (this research). Also, B. cucumis is known to occur in the northern and north-eastern Atlantic (Fabricius, 1780; Van Soest, 1973; Granhag et al., 2012; Licandro et al., 2015; Knutsen et al., 2018), and along the eastern coasts of Canada and the USA (Mayer, 1912; Harbison et al., 1978).

In the Mediterranean Sea, and the subtropical, tropical and equatorial regions of the Atlantic Ocean, it is replaced by B. pseudocucumis. Additional genetic studies are required to clarify the boundaries of the B. cucumis occurrence off the coast of South America, but presumably B. cucumis occurs also in the temperate and subpolar waters of the South Atlantic and Antarctic (Siegel & Harm, 1996; Mianzan 1999; Mianzan & Guererro, 2000; Pakhomov et al., 2000; Flores et al., 2010, 2011). There is evidence of B. cucumis occurring in the Benguela Current near South Africa (Gibbons et al., 1992).

In the Pacific Ocean, Beroe cucumis occurs in cold waters: north-west Pacific (Kasuya et al., 2000; Napazakov & Chuchukalo 2011); north-east Pacific, eastern Bering Sea (Hoff et al., 2011); and the southeast Pacific off Chile (Pages & Orejas, 1999; Oliveira et al., 2016). It frequently occurs in southern Australia (Stiasny, 1931; Edgar, 1997; Gershwin et al., 2010) and in New Zealand waters (Ralph, 1950).

Now that we have genetic confirmation of the presence of two species with a similar morphotype (Beroe cucumis and B. pseudocucumis), it is necessary to conduct a detailed analysis of the ecology and distribution of these two species. It is possible that there are areas of overlap in their habitat and geography. According to previous records, B. cucumis occurs in the areas between temperate and subtropical climatic zones, like the east coast of the USA (Mayer, 1912; Harbison et al., 1978), the west coast of the USA (Wrobel & Mills, 1998) and the Yellow and East China seas (Liu, 2013; Yin et al., 2017; Wang & Cheng, 2019). However, additional ecological and genetic studies of individuals in these areas are required to clarify the species identification and distribution.

Habitat: Cold-water boreal species, has bipolar distribution and inhabits cold polar and temperate waters, while absent in tropical and subtropical zones.