Northern Movements
Dusky Shark (from Blueplanetarchive.com)
A recent science paper noted Dusky Sharks feeding on Gray Seals off Cape Cod. The Dusky Shark, Carcharhinus obscurus, is a subtropical to temperate species that roams further north during the warm months; it is not known to enter the icy northern waters north of the Gulf of Maine. Expectably, they tend to be rare north of the New England coast. That was then; this is now. We have already entered a new world of warming.
Dusky Sharks can grow to be quite large, reaching lengths of over 3 meters, or more than 10 feet. They are feeders on fish, lots of fish, and will, apparently, also go in for those nice succulent seals. Thisis not something they were thought to feed on much. However, those cheeseburgers dotting the coastline, hauling out on rocks, must be a pleasant sight for a hungry shark.
The abundance of seals makes them a central target for shark species. These marine mammals are abundant in late August all along the coast and must be the draw for the increasing abundance of white sharks and other species. Adult seals are large and, for most sharks, a dangerous prey to catch. But this time of year, again, in August, the new youngsters are learning to swim and catch food, so there are plenty of smaller-sized cheeseburgers dotting the coastline.
Dusky Sharks are being seen more often. This is a species of shark I knew next to nothing about. So I dived in. At first glance, they are a very nice sharky shark specimen. They have a distinctive shark silhouette, very nicely proportioned fins, and a nice pointed snout. Their first dorsal fin has a slightly rounded tip, but still an impressive silhouette.
Their teeth are a fantastic example of an adaptation for a diverse diet. The top jaw primarily houses triangular teeth, great at slicing through tough tissue; not quite the spitting image of White Shark triangles, but close. The bottom jaw has more pointed teeth, particularly good at poking into slimy fish, the mainstay of their diet, or at least what was thought to be their dietary mainstay.
One of the astounding things is that these sharks not only give birth to live young, which is actually quite common among sharks, but also develop a placental connection after the yolk is exhausted.
It goes like this: the young first develop the nutrients in the yolk, the momma shark having laid eggs inside its oviduct. After the yolk is gone, or maybe once it’s mostly gone, a transition takes place.
This was first recognized in... well, it seems a little mysterious who first recognized that some sharks form a placental connection, for the Dusky Shark, it goes something like this:
Hamlett and others in the 1980s and 1990s conducted detailed histological and ultrastructural studies of shark placentas. See the book cited at the end for supreme detail if interested. They were able to describe the cellular structure of the yolk-sac placenta and confirm its role in providing nutrients to the developing embryos after the initial yolk reserves were depleted.
These studies revealed how the uterine lining of the mother interdigitates with the fetal yolk sac to form a functional placenta. WHOA!! The yolk sac connects, and the uterine lining grows into it.
Earlier comparative studies of reproduction in different shark species also contributed to a broader understanding. Researchers recognized the existence of placental viviparity in many members of the family Carcharhinidae, to which the dusky shark belongs, long before the detailed mechanisms were fully understood. Some of the earlier researchers include Stewart Springer (1940) and Eugenie Clark and Kenneth von Schmidt (1965). Understanding that there was a placental contribution came sometime before the detailed studies that showed how it was done. Thus it often is in sciences, and thus it should be.
The Dusky Shark carries from 3 to 16 little pups, between 8 and 12 is most common. Sixteen is a lot of pups to carry.
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| Litter sizes for Dusky and Spinner sharks (Hussey et al 2009) |
Most requiem sharks (Carcharhinidae) and hammerhead sharks do this, that is, form a placental connection. Why, you may ask, or you may not ask, but I’m asking, do not all of the requiem sharks do this? For example, I found references that the Blue Shark, Prionace glauca, does not have a placental stage. But is this true? As with other challenging things to study, it may not be true as some references suggest they do, others suggest they do not. Next time, maybe I’ll look into more about the Blue Shark.
Back to shark sightings:
Other species besides the Dusky Shark, and also more recognizable as southern denizens, are showing up, too: Tiger sharks, oooo, one of my favorite sharks. Additionally, hammerheads of two species have been sighted off Cape Cod in the past, and they have also been spotted there recently. The differences include more sightings, greater regularity, and, particularly for the seals, increased danger. Danger also increases for humans, but shark attacks on humans still are, and likely always will be, rare. However, it is time for increased diligence.
Also check out the Spinner Shark shiver off mantauk recently.
Sources and further Readings:
Buddle AL, Van Dyke JU, Thompson MB, Simpfendorfer CA, Whittington CM. 2019. Evolution of placentotrophy: Using viviparous sharks as a model to understand vertebrate placental evolution. Marine and Freshwater Research 70, 908-924.
Hamlett WC, JS. Hudson, and J.P. Wourms. 1981. Ultrastructural analysis of protein absorption in shark yolk-sac placenta. A.S.B. Bull 28.
Hamlett WC, Jones CJP, & Paulesu LR. 2005. Placentatrophy in sharks. In WC Hamlett (Ed.), Reproductive Biology and Phylogeny of Chondrichthyes: Sharks, Batoids, and Chimaeras (Vol. 3, pp. 463–502). Enfield, NH: Science Publishers.
Romine JG, Musick JA, & Burgess GH. 2009. Demographic analyses of the dusky shark, Carcharhinus obscurus, in the Northwest Atlantic incorporating hooking mortality estimates and revised reproductive parameters. Environmental Biology of Fishes 84: 277–289.
Winton M, Novak A, Migneco V, Chisholm J, and Skomal G. 2025. Dusky sharks actively prey on gray seals. Environmental Biology of Fishes. 10.1007/s10641-025-01736-1.
Wourms JP. 1977. Reproduction and development of chondrichthyan fishes. Amer. Zool. 17:379-410.
Wourms JP. and Hamlett WC. 1978. Cell surfaces during development of shark yolk sac placentae. American Zoologist 18: 642. (Abstr.)
Wourms JP. 1981. Viviparity: The maternal–fetal relationship in fishes. American Zoologist 21(2): 473–515. https://doi.org/10.1093/icb/21.2.473
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