Episode 216: Gentle Giant Sharks

Let's learn about some of the biggest sharks in the sea--but not sharks that want to eat you! Further reading: 'Winged' eagle shark soared through oceans 93 million years ago Manta-like planktivorous sharks in Late Cretaceous oceans Before giant plankton-eating sharks, there were giant plankton-eating sharks An artist's impression of the eagle shark (Aquilolamna milarcae): Manta rays: A manta ray with its mouth closed and cephalic fins rolled up: Pseudomegachasma's tooth sitting on someone's thumbnail (left, photo by E.V. Popov) and a Megachasma (megamouth) tooth on someone's fingers (right): The megamouth shark. I wonder where its name came from? The basking shark, also with a mega mouth: The whale shark: Leedsichthys problematicus (not a shark): Show transcript: Welcome to Strange Animals Podcast. I’m your host, Kate Shaw. This week we’re going to look at some huge, weird sharks, but they’re not what you may expect when you hear the word shark. Welcome to the strange world of giant filter feeders! This episode is inspired by an article in the brand new issue of Science, which you may have heard about online. A new species of shark is described in that issue, called the eagle shark because of the shape of its pectoral fins. They’re long and slender like wings. The fossil was discovered in 2012 in northeastern Mexico, but not by paleontologists. It came to light in a limestone quarry, where apparently a quarry worker found it. What happened to it at that point isn’t clear, but it was put up for sale. The problem is that Mexico naturally wants fossils found in Mexico to stay in Mexico, and the authors of the study are not Mexican. One of the authors has a history of shady dealings with fossil smugglers too. On the other hand, the fossil has made its way back to Mexico at last and will soon be on display at a new museum in Nuevo León. Fossils from this quarry are often extremely well preserved, and the eagle shark is no exception. Sharks don’t fossilize well since a shark’s skeleton is made of cartilage except for its teeth, but not only is the eagle shark’s skeleton well preserved, we even have an impression of its soft tissue. The eagle shark was just slightly shorter than 5 ½ feet long, or 1.65 meters. Its tail looks like an ordinary shark tail but that’s the only ordinary thing about it. The head is short and wide, without the long snout that most sharks have, it doesn’t appear to have dorsal or pelvic fins, and its pectoral fins, as I mentioned a minute ago, are really long. How long? From the tip of one pectoral fin to the other measures 6.2 feet, or 1.9 meters. That’s longer than the whole body. Researchers think the eagle shark was a filter feeder. Its mouth would have been wide to engulf more water, which it then filtered through gill rakers or some other structure that separated tiny animals from the water. It expelled the water through its gills and swallowed the food. The eagle shark would have been a relatively slow swimmer. It glided through the water, possibly flapping its long fins slowly in a method called suspension feeding, sometimes called underwater flight. If this makes you think of manta rays, you are exactly correct. The eagle shark occupied the same ecological niche that manta rays do today, and the similarities in body form are due to convergent evolution. Rays and sharks are closely related, but the eagle shark and the manta ray evolved suspension feeding separately. In fact, the eagle shark lived 93 million years ago, 30 million years before the first manta remains appear in the fossil record. The eagle shark lived in the Western Interior Seaway, a shallow sea that stretched from what is now the Gulf of Mexico straight up through the middle of North America. Because it’s the only specimen found so far, we don’t know when it went extinct, but researchers suspect it died out 65 million years ago at the same time as the non-avian dinosaurs. We also don’t have any preserved teeth, which makes it hard to determine what sharks it was most closely related to. Hopefully more specimens will turn up soon. Now that we’ve mentioned the manta ray, let’s talk about it briefly even though it’s not a shark. It is big, though, and it’s a filter feeder. If you’ve never seen one before, they’re hard to describe. If it had gone extinct before humans started looking at fossils scientifically, we’d be as astounded by it as we are about the eagle shark—maybe even moreso because it’s so much bigger. Its body is sort of diamond-shaped, with a blunt head and short tail, but elongated fins that are broad at the base but end in drawn-out points. Manta rays are measured in width, sometimes called a wingspan since their long fins resemble wings that allow it to fly underwater. There are two species of manta ray, and even the smaller one has a wingspan of 18 feet, or 5.5 meters. The larger species can grow 23 feet across, or 7 meters. Some other rays are filter feeders too, all of them closely related to the manta. The manta ray lives in warm oceans, where it eats zooplankton. Its mouth is wide and when it’s feeding it moves forward with its mouth open, letting water flow into the mouth and through the gills. Gill rakers collect tiny food, which the manta ray swallows. It has a pair of fins on either side of the mouth that are sometimes called horns, but which are properly called cephalic fins. Cephalic just means “on the head.” These fins help direct water into the mouth. When a manta ray isn’t feeding, it closes its mouth just like any other shark, folding its shallow jaw shut. For years I thought it closed its mouth by folding the cephalic fins over it, but that’s not the case, although it does roll the fins up into little points. The manta ray is mostly black with a white belly, but some individuals have white markings on the back and black speckles and splotches underneath. We talked about some mysteries associated with its coloring in episode 96. The eagle shark isn’t the only filter feeding shark. The earliest known is Pseudomegachasma, the false megamouth, which lived around 100 million years ago. It was only described in 2015 after some tiny shark teeth were found in Russia. The teeth looked like those of the modern megamouth shark, although they’re probably not related. The teeth are only a few millimeters long but that’s the same size as teeth from the megamouth shark, and the megamouth grows 18 feet long, or 5.5 m. Despite its size, the megamouth shark wasn’t discovered until 1976, and it was only found by complete chance. On November 15 of that year, a U.S. Navy research ship off the coast of Hawaii pulled up its sea anchors. Sea anchors aren’t like the anchors you may be thinking of, the big metal ones that drop to the ocean’s bottom to keep a ship stationary. A sea anchor is more like an underwater parachute for ships. It’s attached to the ship with a long rope on one end, and opens up just like a parachute underwater. The tip of the parachute has another rope attached with a float on top. When the navy ship brought up its sea anchors, an unlucky shark was tangled up in one of them. The shark was over 14 ½ feet long, or 4 ½ m, and didn’t look like any shark anyone had ever seen. The shark was hauled on board and the navy consulted marine biologists around the country. No one knew what the shark was. It wasn’t just new to science, it was radically different from all other sharks known. Since then, only about 100 megamouth sharks have ever been sighted, so very little is known about it even now. The megamouth is dark brown in color with a white belly, a wide head and body, and a large, wide mouth. The inside of its lower lip is a pale silvery color that reflects light, although researchers aren’t sure if it acts as a lure for the tiny plankton it eats, or if it’s a way for megamouths to identify each other. It’s sluggish and spends most of its time in deep water, although it comes closer to the surface at night. The basking shark is even bigger than the megamouth. It can grow up to 36 feet long, or 11 meters. It’s so big it’s sometimes mistaken for the great white shark, but it has a humongous wide mouth and unusually long gill slits, and, of course, its teeth are teensy. It’s usually dark brown or black, white underneath, and while it spends a lot of its time feeding at the surface of the ocean, in cold weather it spends most of its time in deep water. In summer, basking sharks gather in small groups to breed, and sometimes will engage in slow, ponderous courtship dances that involve swimming in circles nose to tail. But the biggest filter feeder shark alive today, and possibly alive ever, is the whale shark. It gets its name because it is literally as large as some whales. It can grow up to 62 feet long, or 18.8 meters, and potentially longer. The whale shark is remarkably pretty. It’s dark gray with a white belly, and its body is covered with little white or pale gray spots that look like stars on a night sky. Its mouth is extremely large and wide, and its small eyes are low on the head and point downward. Not only can it retract its eyeballs into their sockets, the eyeballs actually have little armored denticles to protect them from damage. The body also has denticles, plus the whale shark’s skin is six inches thick, or 15 cm. The whale shark lives in warm water and migrates long distances. It mostly feeds near the surface although it sometimes dives deeply to find plankton. It filters water differently from the megamouth and basking sharks, which use gill rakers. The whale shark has sieve-like filter pads instead. The whale shark doesn’t need to move to feed, either. It can gulp water into its mouth by opening and closing its jaws, unlike the other living filter feeders we’ve talked about so far. We talked about the whale shark a lot in episode 87,...