Rostroraja alba (Lacepede, 1803)

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 : 271

Rostroraja alba (Lacepede, 1803)

Distribution. The biogeographic range of white skate ( R. alba ) extends from the southern British Isles to South Africa, including the Mediterranean Sea ( Stehmann & Bürkel, 1984; Serena et al., 2010). This species is now exceedingly rare in northern European seas ( Rogers & Ellis, 2000). Consequently, few verified eggcases have been reported by the Great Eggcase Hunt. Most verified records have been received from southwest Ireland, however single specimens have also been verified by photograph from the north coast of Devon ( England), Peterhead ( Scotland) and Fair Isle (Northern Isles).

Material examined. Only five specimens of R. alba eggcases were available for examination. Three specimens were submitted to the Great Eggcase Hunt from County Kerry, Ireland, and single specimens were examined from a museum collection ( BMNH 98.2.26.33) and a public aquarium.

Description. R. alba has a very large eggcase ( Figure 6 b), with a mean eggcase length of 148.1 ± 24.2 mm (127.8–190.0 mm), total eggcase length of 272–506 mm when horns are fully extended and eggcase width of 126.7 ± 17.8 mm (104.3–145.0 mm). One historic specimen measured (BMNH 98.2.26.33) was particularly large, measuring 190.0 mm without horns, and 506.0 mm with horns fully extended. The eggcase is distinctly convex on the dorsal side and almost flattened on the ventral. The capsule surface is striated with densely-packed, fine, longitudinal ridges; on occasion the transverse ridges are more pronounced, giving a ‘beaded’ and lattice-like appearance ( Figure 3 d). Attachment filaments are usually absent in strandline specimens although may be present in freshly laid specimens. The thick lateral keels are striated longitudinally and run the length of the eggcase. Given the size of the eggcase, the anterior and posterior aprons are both surprisingly shallow. The keels taper into long, ribbon-like anterior horns which curve inwards towards each other while the posterior horns are relatively short, each terminating with a distinct hook which is orientated towards the flat surface of the eggcase.

Remarks. Along with Dipturus cf. intermedia , this is one of the largest eggcases found in UK and Irish waters. Holt (1898) described the eggcase of R. alba , citing a midline eggcase length of 174 mm, eggcase width of 138 mm, and with the capsule 133 mm by 105 mm.

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).