Dipturus cf. intermedia (Parnell, 1837)

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 : 261-262

Dipturus cf. intermedia (Parnell, 1837)

Distribution. Flapper skate ( D. cf. intermedia ) is Europe’s largest rajiform and thought to have been formerly widespread across much of the Northeast Atlantic, although it has now been extirpated from much of its geographic range ( Rogers & Ellis, 2000). Populations still occur off northwest Scotland and the northern North Sea, including the Orkney and Shetland Isles, with smaller numbers also found at least as far south as the Celtic Sea. Most eggcases submitted to the Great Eggcase Hunt were collected from around Orkney (particularly from west facing beaches) and the west coast of Scotland, with some confirmed reports from western Ireland.

Material examined. Thirty-three specimens were examined in total; the majority of which were strandline specimens collected from Orkney and submitted to the Great Eggcase Hunt (n = 29). Four museum specimens were also examined, one from southwest Ireland ( BMNH 1928.9.18.26) and three of unknown provenance ( BMNH 1932.7.27.1–3). All museum specimens were originally misidentified as belonging to R. oxyrinchus .

Description. One of the largest eggcases in the Northeast Atlantic, with a total eggcase length of 235.2 ± 41.8 mm (range 140.0–280.0 mm), eggcase length of 201.4 ± 27.4 mm (range 130.0–235.0 mm) and eggcase width of 121.3 ± 10.5 mm (range 100.0–144.0 mm). Although only four historical specimens were examined, these were generally within the size range recorded in contemporary samples. The dorsal side of the eggcase is convex, while the ventral side often curves upwards giving the eggcase a bowed appearance ( Figure 4 b). The capsule surface is covered with a dense fibrous layer, creating a bark-like appearance (as with D. batis , Figure 3 b), and the longitudinal fibrous strips can often peel away quite easily from the capsule edges. The outer layer is pale brown in colour, but beneath these fibres the capsule itself is much darker with faint longitudinal striations. Strong lateral keels (average width of 20.64 mm for both keels) run the length of the eggcase, encompassing the anterior and posterior horns. The anterior apron is broad and well-developed, also encompassing much of the anterior horns, although the upper end is often ragged (even in unhatched eggcases) and can be damaged easily. The posterior apron is slightly shallower than the anterior, but is still broad, often crenulated and extends the length of the posterior horns. The horns are often difficult to determine in dry, strandline specimens, however when hydrated the anterior horns are visibly longer than the stout posterior horns.

Remarks. This is one of the largest eggcases found around the British Isles, rivalled only by that of white skate Rostroraja alba (Lacepede, 1803) . Strandline specimens often have large areas where the pale close-felt fibres have worn away to reveal the darker capsule beneath, this leaves the surface appearing patchy. Boeseman (1967) described an eggcase collected in the Firth of Clyde, which was attributed originally to D. nidarosiensis . However, the size (eggcase length = 251 mm; eggcase width = 9 5–98 mm), location and description of this eggcase agrees more closely with that of D. cf. intermedia .

Figures

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