Raja clavata Linnaeus 1758

Raja clavata Linnaeus, 1758

Distribution. Thornback ray (R. clavata) is one of the most abundant elasmobranchs in the Northeast Atlantic and has a broad distribution from Iceland and Norway to Northwest Africa and the Mediterranean Sea (Stehmann & Bürkel, 1984; Serena et al., 2010). The eggcases of R. clavata were reported frequently to the Great Eggcase Hunt, with submissions from nearly all of the British coastline, albeit with only scattered reports from the northeast coast of England.

Material examined. Fifty-two eggcases were examined in detail, of which the majority (n = 38) were collected by the Great Eggcase Hunt, with other specimens from trawl surveys (n = 4), museum collections (n = 3; BMNH 1927.3.25.1, BMNH 91.6.18.66–67) and public aquaria (n = 7).

Description. The eggcase of R. clavata (Figure 5 b) is robust and of moderate size, with an eggcase length of 67.2 ± 10.0 mm (32.7–83.3 mm) and eggcase width of 37.0 ± 8.2 mm (32.7–69.0 mm). These values, however, mask a diversity in both size and the length to width ratio, with eggcases varying from rectangular to square, as shown by the width of the eggcase ranging from 62.3–91.0% of the eggcase length. This is demonstrated by one aberrant museum specimen (BMNH 1927.13.25.1), which measured 76 mm eggcase length by 69 mm eggcase width (Figure 5 c). One of the museum specimens examined (BMNH 91.6.18.67) was the largest R. clavata eggcase measured (eggcase length = 83.3 mm), with the two others within the upper limits of the sizes observed in contemporary material.

The dorsal surface of the capsule is distinctly more convex than the ventral. Freshly-laid specimens are covered with thin fibres (Figure 3 c) and although these are often lost by the time of stranding, evidence of remnant fibres can be visible on the keels and less abraded areas. The lateral keels are well-formed and sturdy, often remaining intact in strandline specimens. The keels extend from the base of the anterior horns, down the length of the capsule and terminate where the posterior horns join the capsule. The anterior apron is marginally larger than the posterior and is straight, while the posterior apron is slightly convex, but often broken away neatly in stranded samples, giving the appearance of a very narrow apron. The anterior horns are marginally longer than the posterior and curve inwards slightly. The lengths and widths of the eggcases recorded in the present study extended the reported size range for this species.