Why only chimaera eggs? According to Finucci, there's no definitive answer.
"Any answer to that question is purely speculative," she told Discovery News. "Prickly dogfish don't look like they're built to be active predators capable of chasing down their prey, so presumably these eggs are an easy and readily available source of food."
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Adding to the puzzled picture is that there's no shortage of other kinds of food in its range for the small sharks to eat.
"One area where we collected prickly dogfish, Chatham Rise, is known for its very productive waters and supports a very diverse collection of taxa," Finucci noted, "so prickly dogfish have no need to be picky eaters."
What's more, it's possible that the prickly dogfish's selective dining might be a trait seen in other species of rough shark.
"This specialized diet preference may be common across the Oxynotus [rough shark] genus," Finucci said. "I say 'preference' because in the case of Oxynotus centrina, a European relative of prickly dogfish, researchers found captive individuals to feed only on skate eggs. However, previous reports found wild specimens ate a diverse range of prey."
"It may be a case of when chondrichthyan [sharks, rays, skates, and chimaera] eggs are available, Oxynotus may choose to feed only on the eggs," Finucci said, "but will consume other prey items if their main food source becomes scarce or is unavailable."
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In addition to the specialized diet uncovered, the researchers also highlighted a potential problem for both the prickly dogfish and chimaera, going forward.
Prickly dogfish are by-catch of commercial fisheries, they explained. The dogfish are considered "low productivity" -- they don't make tons of babies. That lack of reproductive vigor, coupled with a specialized diet and a risk of becoming by-catch, puts the prickly dogfish "at higher risk from the impact of fishing than currently estimated," the team wrote.
Meanwhile, chimaera, they added, "may be similarly vulnerable, and there may be damage to egg cases and egg-laying habitat by bottom trawling and exploratory mineral mining."