Amblyraja radiata (Donovan, 1808)

Gordon, Cat A., Hood, Ali R. & Ellis, Jim R., 2016, Descriptions and revised key to the eggcases of the skates (Rajiformes: Rajidae) and catsharks (Carcharhiniformes: Scyliorhinidae) of the British Isles, Zootaxa 4150 (3), pp. 255-280 : 258-260

Amblyraja radiata (Donovan, 1808)

Distribution. Starry skate ( A. radiata ) is a boreo-Arctic species that, in the Northeast Atlantic, is common from the central North Sea to Iceland and the Barents Sea ( Stehmann & Bürkel, 1984). Verified records submitted to the Great Eggcase Hunt were usually collected along North Sea coasts, from north Norfolk up to the Shetland Islands. However, one unusually large specimen ( Figure 2 a) was reported in January 2015 from Borth, Cardigan Bay ( Wales), which is outside the expected geographic range (see below).

Material examined. Ninety-four eggcases were examined in total (excluding the unusual specimen collected from Borth ). Specimens were obtained primarily from North Sea trawl surveys (n = 79) with additional specimens available from beach collections (n = 10) and museum specimens (n = 5) historically collected from Scarborough ( North Sea coast, BMNH 1928.2.8.2.1–4, BMNH 1928.1.11.1). Specimens from the North Sea were captured at a range of trawl locations over the area from 55.6– 61.3°N and 0.5°W to 5.8°E, ranging from 37–170 m water depth.

Description. The eggcase of A. radiata ( Figure 2 b) is generally small. Excluding the large aberrant specimen from Borth, the mean total eggcase length is 101.2 ± 10.6 mm (range = 72.2–129.0), and eggcase length is 34.2 ± 1.1 mm (range = 32.7–36.7 mm) without horns. Eggcase width is 32.8 ± 2.6 mm (25.6–36.7 mm). The length and width of eggcases from historical museum specimens (n = 5) were within the size ranges recorded in contemporary samples. The dorsal surface is strongly convex and, when newly laid, covered in a thin fibrous layer. The ventral surface is slightly convex in shape but much flatter in comparison to the dorsal surface. Both capsule surfaces have longitudinal striations and latitudinal ridges that give a distinctive texture and pattern ( Figure 3 a), ranging from a delicate lattice appearance to a fine rippled effect. Lateral keels (which are often damaged or lost in older specimens) run the length of the capsule from the base of the anterior horns to the base of the posterior horns, and represent an average of 17% of the total capsule width. The anterior apron is straight while the posterior apron is much shallower and concave. The anterior horns taper to form fine, filamentous tubes while the posterior horns terminate in hooks.

Remarks. This is one of the smallest skate eggcases found in northern European shelf seas, and the textured surface on each side is highly distinctive. The fragile lateral keels are often broken in stranded specimens, and so this feature alone should not be used for identification. Many specimens collected by trawl surveys had small boreholes on the capsule surfaces, indicating predation by gastropod molluscs ( Cox et al., 1999). Templeman (1982) noted the small size of eggcases from North Sea populations, ca. 48 mm long by 34 mm wide, and even as small as 42 mm by 25 mm ( Clark, 1926). Spatial differences in eggcase size are known for this species, with eggcases from more northerly parts of the Northwest Atlantic waters 61–74 mm long and 41–60 mm wide, and eggcases from further south ranging from 71–96 mm long and 53–77 mm wide ( Vladykov, 1936; Templeman, 1982). Jensen (1914) provided data from Greenland waters, and these specimens were larger (44–68 mm) than in the North Sea. This author also provided more detailed measurements for a sub-sample, with these ranging from 56–68 mm long without horns, 127–172 mm long with horns, and 42–51 mm in width. Nordgaard (1917) described the eggcases and development of A. radiata from Trondheim Fjord (ca. 63.5°N), with these eggcases 50–60 mm long and 40–48 mm wide. The total eggcase lengths measured in the present study were all smaller than previous reports ( Table 1 ), although eggcase width was broadly similar.

The unusual specimen submitted to the Great Eggcase Hunt from Borth (total eggcase length = 174.9 mm; capsule length = 73.1 mm; eggcase width = 68.4 mm; capsule width = 52.8 mm) far exceeded the measurements of North Sea specimens in this study and even from those recorded in other parts of the Atlantic ( Table 1 ). Whilst early ichthyological accounts reported on the presence of A. radiata in the Irish Sea, there are no recent, authenticated captures ( Ellis et al., 2002, 2005), and so the origin of this eggcase is uncertain. Whilst other members of the genus Amblyraja may occur in deep waters north of the British Isles, none occur in the coastal zone. Jensen (1914) described the eggcase of Amblyraja hyperborea (Collett, 1879) , and two eggcases of this species (provided by A. Lynghammar, Arctic University of Norway) were available to measure during this study - both A. hyperborea specimens were much larger than documented A. radiata specimens (total eggcase length = 256.2 mm; eggcase length = 122.5 mm; capsule length = 75.3 mm; eggcase width = 66.0 mm; capsule width = 54.3 mm).

Figures

FIGURE 2. Eggcases of (a) Amblyraja radiata (large specimen from Borth, Cardigan Bay), (b) A. radiata (typical specimen from North Sea) and (c) Leucoraja naevus.

FIGURE 3. Eggcase capsule surface structures for (a) Amblyraja radiata (b) Dipturus batis (c) Raja clavata (d) Rostroraja alba (taken with USB Digital Microscope).