Last week provided another opportunity to love where we live and appreciate who we are.
Off-Island, rubbernecking occurs when there is a car accident or traffic stop. On-Island, it was a shark that made Islanders slow down, pull over or park, and line up and gawk not at an accident but at wildlife. Gotta love it and us: it is true Island style that the talk of the town is a wildlife sighting.
The shark that caused so many to stop and stare was a porbeagle. This species, lamna nasus — also known as blue dog shark, bottle-nosed shark and Atlantic mackerel shark — is not commonly seen, so it well earned the attention. Spending its time at the outer continental shelf, this special species does seasonally migrate between shallow and deeper waters. We know and appreciate a rarity when we see it so up close and personal.
Porbeagle sharks pose no problems for people. Though it is a predator of fish and cephalopods, humans have little to fear from this shark. While they occasionally jump out of the water after their prey, there have been few incidents of these sharks pursuing people. According to the International Shark Attack File database, only three instances of porbeagles coming after humans have been recorded. And it is likely that the incident reporters might have misidentified their shark predator.
These animals have more to fear from us. Their meat and fins are prized, and porbeagles have historically been commercially and recreationally fished. So much so that in the 1950s and 1960s their stocks collapsed commercially. The International Union for the Conservation of Nature has rated this shark as threatened, though it can still be caught legally under federal and state regulations.
Taking a deeper dive into the life cycle and habits of the porbeagle shark provides much wonder and admiration. An endotherm, or warm-blooded beast, porbeagle sharks can produce their own heat. Their core temperature is usually 14 to 18 degrees Fahrenheit higher than water temperatures. They accomplish this feat through the use of specialized blood vessels that serve as counter-current heat exchangers, keeping them warmer than the surrounding water.
Another fascinating feat is the porbeagle’s reproductive routine. These sharks are ovoviviparous, which means that they have eggs inside of their bodies but give birth to live young. Fertilization is internal and occurs after mating.
The eggs come from the shark’s single ovary and hatch inside one of the porbeagle’s two uteruses (or uteri), where they will grow and, after eight or nine months, be born alive. This is also called aplacental viviparous because the young are not connected via placenta or other way to the mother; they are just held in her uteri.
How do the young eat, then? Good question. In the case of the porbeagle, the embryo starts eating the yolk in its own egg sac. However, its mother produces a large number of eggs and, generally, only four will become embryos. The other eggs are unfertilized and their yolks will be used for food for the developing fetus, which will grow (and eventually lose) fangs to tear open the other egg capsules.
Porbeagles can reproduce at thirteen and eight years old, respectively, for females and males, and can breed only one time per year. This limits their lifetime fecundity, which is why recovery of the species has been slow.
It is not surprising that the sight of the shark fin in the harbor caused such a rumpus and so many references to Jaws. Rumors flew with ideas about its identification, its reason for being so close to shore and the health and wellness of the animal. Though the shark caused a spectacle, eventually the animal swam away, leaving the rubberneckers with nothing more than a fin-tastic memory and perhaps hopes for a seaside sequel.
Suzan Bellincampi is islands director for Felix Neck Wildlife Sanctuary in Edgartown and the Nantucket Wildlife Sanctuaries. She is also the author of Martha’s Vineyard: A Field Guide to Island Nature and The Nature of Martha’s Vineyard.
