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Best Dogfish Shark Dissection: Part I – External (Jr. High, High School and College)

Best Dogfish Shark Dissection: Part I – External (Jr. High, High School and College)


(upbeat music) Hello, I’m Steven Rokusek with South Dakota Public Broadcasting. And I’m Dale Droge, professor of Biology at Dakota State University. And today we’re gonna dissect the dogfish shark. Yep. I’ve got a question for you. I know in the past, I remember on the website we have dissection of the perch. So why do you, two different fish? Yeah, well, they’re two very different lines of fish. Sharks are a member of a group called Chondrichthyes, and that line evolved first over 400 million years ago, and they have a cartilaginous skeleton we’ll look at later, a denser body, just some differences in terms of the various structures. But they have, what’s quintessential in all fish is these paired appendages, so they have the pectoral fins and the pelvic fins, and these help for maneuverability. So, the muscles, you know, the motions or the power for swimming comes from the tail going side to side, but it’s these paired fins that allow these animals to be much more maneuverable and to elevate themselves in the water. And then the other major innovation that sharks were the first vertebrate to show were jaws. And jaws were a huge change because jaws now allow you to eat larger and more varied food items. And so, those things two things together have pretty much led to an explosion of different types of vertebrates. Let’s say I bring in the river perch over here. I’ll bring one in. What’s some of the differences and similarities? Well, one of the differences is in the density of the body. Sharks do not have a swim bladder, where perch do. They have that internal gas-filled bladder which allows them to remain, it gives them buoyancy, lets them stay at a constant place in the water column, where sharks, if they’re not swimming, will sink. So, that was a huge change. They also have, sharks have the gill slits are external, you can see where the openings to the gills are external, and so, each gill area has a pouch, where the gills of bonny fish like perch are covered by this operculum, this bony structure. So that is also a pumping mechanism that allows them to breathe while they’re remaining fairly motionless. Again, sharks have to move to force water over the gills. Then of course, the fins themselves, the bony fish have ray fins, the fins are supported by these rays, where the fins of sharks are more fleshy. How about the outer surface here? The scales, yeah, scales are very different. The bony fish have bony scales, they’re tiny, bony scales that cover the body, where the shark skin is covered by tiny little tooth-like scales, they’re actually covered with enamel. And if you run your finger over it, you can hear or feel how raspy it is. At one time, before the modern invention of sandpaper and things, shark skin was actually used as a way of polishing a final finish of furniture. It’s also shiny too, which you don’t hear so much anymore but we used to talk about shark skin suits that people would wear, and shark skin suits had that kinda shiny appearance to them. Anything similar or different? Just the difference in the fins, it’s just more maneuverability, bony fish are much more maneuverable, they’re able to keep their place in the water column. This allowed to them to radiate out. You know, there’s probably 40 to 60000 different species of bony fish, while there’s less than 1000 species of Chondrichthyes left. And about 400 of the species of those are sharks, most of them are skates, and rays, and things like cousins of the shark. We should also probably bring skeletons. Oh, I did wanna mention too, sorry, the mouth is another difference too. The mouth, the shark mouth is on the bottom, the underside, and the bony fish, it’s terminal. For most bony fish, it’s terminal, it’s at the end of the jaws. We should bring in the skeletons, just kinda show that quick, just the difference there. Sure, I think we mentioned that bony fish have the typical, we refer to it as the bony skeleton, where sharks have a cartilaginous skeleton, so you can see, we even have to, it has to be preserved inside of water to keep it from drying out. And it’s thought that the ancestors of sharks actually had bony skeletons, but they have now lost that because it’s a much more flexible and lighter kind of skeleton. You see, one of sharks’ problems is that sinking in water, so a lot of things that they do allow them to lighten up the body and give them more buoyancy. That is one difference, though, big difference. We often talk about the cartilagenous fishes and the bony fishes. Okay, so there’s that over here. And now I guess we should go ahead and look at the internal structures? Or I guess we can look at the external structures first. Yeah, we can look at the, and that’s, you know, again, a 400 million year old design, but in many ways, sharks have changed very little over those intervening years because what they evolved is just an almost perfect aquatic predator. They have a long, streamlined body. You can see how they would cut through the water very easily. They have these powerful muscles to move the tail side to side, which propels them through the water, so they can swim long distances, and they’re fairly strong swimmers, and they can be fairly fast, you know, when they have to be. Again, the paired appendages give them maneuverability, the jaws give them the ability to catch prey and chew that prey up or bite it into pieces. They’ve got incredible sensory structures. They have, of course, large eyes, as you can see, large paired eyes here to allow them to tail. They have a very keen sense of smell, and they probably locate most of their prey by distance by smell. They have nostrils? Yeah, they have the nostrils here, I’m sorry, on the underside of the body here. They’re actually blind sacs, they don’t open into the nasal cavity, or into the mouth cavity like ours do, so the water kinda goes in and out, but there are sensory structures in there that will sense the chemicals. You mentioned something on the front here also? Oh yeah, they have, what’s unusual among vertebrates, there are just a few other vertebrates that are sensitive to electrical signals. Right here, there are some sensory structures called ampullae of Lorenzini, and these are sensitive to very slight electrical fields, and so, when a fish or some other prey item is moving, they’re generating electrical impulses in their muscles, and the sharks are capable of detecting that. Is it just like pores, or what is it? Yeah, they’re little pores, the things are little structures underneath there that just are sensitive to differences in voltage, so they can actually locate prey buried in the sand, for example, so there’s no way they could smell it or see it, and then they can locate it using these electrical sensors. They also, like other fish, have a lateral line system, so there’s a line running along here. These are pressure sensors, so they can detect changes in water pressure. So, waves pushing, something moving toward them or away from them would create water pressure differences, and they can detect that. And so, they really have, they have really strong, you know, you think about what a predator does. They can move, they can find their prey, they can eat their prey, you know, they really have a very classic design, but probably the reason why they’ve survived, you know, all of this time. I imagine this much movement must take quite a bit of nourishment then, I imagine? Yeah, of course, yeah. If you’re gonna move around a lot and things, they have to eat a lot, and they are, again, that’s part of their thing. They eat large things, and they hunt fairly voraciously and fairly frequently. What do we have right here, the little holes on top here? Oh, okay, yeah, these are also, these are called spiracles, and spiracles open down into the gill chambers. So, sharks don’t use them as much as rays are, but they actually are ways that water then can enter and go down through the gills here, so pass down, so water can bring oxygen in, pass it over the gills, where if it’s coming through the mouth, if the mouth, if you’re down on the bottom of the ocean, you know the sand and things, it’s gonna block that, so it’s essentially a second entrance for water to come in to provide oxygen to the gills. Did you mention the skin? Yeah, we should probably just name the fins again. Externally, you have the forward or the pectoral fins, which are near the shoulder area, and then in back are the pelvic fins, and right near the base of the pelvic fins is the cloaca. The cloaca, yeah, the cloaca is a common opening of the urogenital and digestive system. So, both, you know, the end of the, where the undigested food comes out, but also where babies are born and things like that come out that end. And then we talked about the unpaired, so those are the paired fins. The unpaired fins are the dorsal fins, famous in sharks for always breaking the surface, and then there’s a posterior dorsal fin too, in this case, and then the tail, which is a, in many sharks, is a so-called heterocircle, or has one lobe that’s much larger than the other. It’s usually the top lobe that’s larger, and as this moves through the water, it forces the head up, which is, you know, again, a problem sharks have with being so heavy that they need to keep swimming to maintain their position in the water. I noticed here there’s a couple of spots removed here? Oh, yeah, these are called, these actually, this fish we’re looking at today is the spiny Atlantic dogfish, and this is about all the bigger they get, just a little bit, maybe, might be a little bit larger than this, but they’re a small shark, but as their name implies, they’re spiny, so they’ll have large spines right near the beginning of these different fins, and the supply houses basically cut those off so that you don’t stab yourself with them when you’re working with them. We should mention too, you know, as far as sharks go, as far as the concerns in dissecting these? Yeah, I mean, with, well, any vertebrate, of course, we wanna be very careful about, you know, how many we use, because we don’t want to, you know, take too many lives for this purpose. I mean, there is a lot of value in doing the hands-on type of looking at things, but we try to use them as carefully as we can. And sharks, in particular, their numbers have been somewhat, they’re unstable over the last many decades, and this is largely due to a lot of human activities and different kinds of things changing in the ocean, so there’s certainly no fear that these animals are caught. Fishermen catch these, you know, when they’re fishing for other things, but it’s just, again, just being kind of, trying to be respectful if you’re, you know, use as few of them as you can, and use them, you know, only use them if you’re gonna spend a considerable amount of time looking at the structures and things like that. Maybe use the video instead. Yeah, hopefully a video like this or others would then provide some, you know, an alternative means of discovering shark anatomy and structures. I also noticed here that this one has a cut tail here. Yeah, everybody always asks about that. There’s a couple of reasons, as I understand it. One is that the cut, and if they’re going to inject the circulatory system, they can do that through a couple of veins and arteries that run through the tail here. Through the cut, you can actually see the vertebrae of this central axis of exoskeleton. And then also, this tends to make them easier to fit into containers, particularly when they’re in pails and things like that, they just pack a little bit better. So, there isn’t any major significance for the tail being cut, it’s more of a convenience thing. Anything else you can think of? I think we should go ahead and look at the inside. I can’t think of too much else. Yeah, no, they’re pretty, and then inside, they’re pretty much a very typical, again, vertebrate body plan. We see different kinds of structures, so it’s a pretty interesting group to look at. (upbeat music)

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