Vertebrates: The Structure, Function and Evolution (BIOEE 2740) Lab 3 Chondrichthyes, the cartilaginous fishes.
Major concepts The evolution of jaws allowed vertebrates to eat and specialize upon a diversity of foods The evolution of 2 sets of paired appendages lead to greater maneuverability and more efficient swimming Think about the innovations in morphology which allowed transitions in feeding ecology among taxa in last weeks and this weeks labs: 1. filter-feeding with cilia (urochordates and cephalochordates) 2. Filter-feeding with a muscular pharyngeal pump (larval lamprey) 3. Jaws and active predation = large active predators, like sharks (and other gnathostomes)! Goals for this lab: 1. Learn the morphological features that define the Gnathostomata, the jawed vertebrates. 2. Learn what morphological features define the Chondrichthyes, and how this group is related to other major vertebrate groups. 3. Explore the diversity, structure and function of the Holocephalans and the Elasmobranchs, the two major groups within the Chondrichthyes. 4. Become familiar with the internal anatomy of the Chondrichthyes through dissection of a spiny dogfish shark, Squalus sp. Station 1. The Gnathostomata phylogenetic context and synapomorphies Remember from last weeks lab that the Gnathostomata (=jaw mouth) include all of the Vertebrata except for the Petromyzontiformes, the lamprey.
Some major morphological synapomorphies of the gnathostomes are: 1. Jaws enable gnathostomes to seize and eat a wider range of food items than non-jawed craniates.
2. Calcified teeth allow gnathostomes to eat a variety of prey items, and to specialize their diet via morphological specialization of the teeth. Tooth replacement is also present. Remember from lecture that most cartilaginous fishes have polyphyodont teeth (= continuous replacement) and homodont dentition (= all teeth in the mouth are the same shape; note the interesting exception to this pattern at station #6!). In contrast, most mammals have diphyodont teeth (= 2 sets of teeth, milk and adult) and heterodont dentition (= differentiated tooth shapes within the jaw). Examine the representative jaws at this station to familiarize yourself with homodont vs. heterodont dentition. 3. Three semicircular canals these are the fluid-filled tubes in the inner ear involved in balance and movement. Having three tubes allows for threedimensional orientation (remember that having at least two semicircular canals is a synapomorphy of the Vertebrata lampreys have two canals; hagfish have only one). 4. Paired appendages increase stability, maneuverability, and efficiency in movement. Paired appendages are possible because of the evolution of the pelvic and pectoral girdles (you will learn more about the girdles at station #5). 5. Myelin on neurons -- the myelin sheath lies along the axial portion of the neuron, and decreases resistances for faster neuronal signaling. 6. Paired nostrils remember from last weeks lab that the Petromyzontiformes had a single naris. In Gnathostomes, paired nostrils are a synapomorphy. An example of the rearrangement in the olfactory sacs that occurred in the transition from a single nostril to paired nostrils. A) is the condition in a lamprey, Petromyzon, and B) is the condition in an eel, Anguilla, a member of the bony fishes, Osteichthyes (stay tuned for more on the Osteichthyes next week).
Station 2. The Gnathostomata the evolution of jaws Jaws are the major synapomorphy of the gnathostomes. The jaw bones are derived from the bones of the gill arches (= pharyngeal arches, branchial arches, visceral arches). Using the figures below, note that the first gill arch, known as the mandibular arch, is transformed in the gnathostome condition into the upper jaw (= palatoquadrate), and the lower jaw (= Meckels cartilage, or mandible note that although the lower jaw is the mandible, the lower and upper jaw together make up the mandibular arch). The second gill arch is the hyoid arch. Three major bones of the hyoid arch are, from dorsal to ventral, the hyomandibula, the ceratohyal, and the basihyal. The spiracle is an opening anterior to the hyoid arch that is used for water intake in many sharks, skates, and rays. It is thought to be homologous to the first gill opening in the agnathous ancestor of the gnathostomes.
Jaw suspension Understanding the ways in which the jaws are suspended to the cranium is important for two major reasons. 1) Different styles of jaw suspension have different ramifications for functional use of the jaws. 2) The bones of the head may be constrained by their connections to other bonesand conversely, if a functional connection is lost, bones may be free to diverge in morphology and be used for a different purpose (keep this in mind especially when we get to tetrapod jaws). There are three major ways that jaws are suspended in this weeks focal group, the Chondrichthyes. In amphistylic suspension, the palatoquadrate is attached to the chondrocranium and to the hyomandibula, a bone of the hyoid arch. The lower jaw was attached to the upper jaw and the hyoid arch. This is thought to be the ancestral condition for gnathostomes, and is seen in fossil sharks. In hyostylic suspension, the mandibular arch (i.e. upper and lower jaws) is supported via ligamentous connections to the hyomandibula. The hyomandibula is attached via ligaments to the chondrocranium. This style of jaw suspension allows protrusion of the palatoquadrate in feeding. This is
the most common form of jaw suspension in extant sharks. In holostylic suspension, the palatoquadrate is fused to the chondrocranium, and the lower jaw has direct attachment to the upper jaw through ligaments. The Holocephalans (see station #5) are the only gnathostome group with holostylic suspension. When we get to tetrapods, we will see another form of jaw suspensionstay tuned!
Hyostylic jaw suspension, as is typical in the Elasmobranchii (see station #4). The main diagram is a lateral view, and the small inset to the upper right is a crosssectional view of the head, where S=spiracle; Q= quadrate region of the palatoquadrate; H=hyomandibula; M=mandible. This diagram is of the catshark, Scyllium sp.. Figure from Pough et al 2009.
Station 3: Chondrichthyes synapomorphies and phylogenetic context Familiarize yourself with some of the synapomorphies of the Chondrichthyes, the cartilaginous fishes. 1. Pelvic claspers structures near the cloaca in male sharks that are inserted into the females cloaca during copulation. All chondrichthyans have internal fertilization. 2. Placoid scales these tough cusped scales are homologous with teeth. They have a hard enamel exterior, a layer of dentine, and an interior pulp cavity. Examine the structure of placoid scales using the slides and diagrams at this station. 3. Rectal gland the rectal gland aids in maintaining salt balance in chondrichthyans. It concentrates salt and excretes it as a hypertonic NaCl solution. You will identify the rectal gland in your dogfish dissection at station #7. The Chondrichthyes form the most basal branch of the Gnathostomata. Two major groups make up the Chondrichthyes, the Holocephali and the Elasmobranchii. The bony fishes, the Osteichthyes (including the tetrapods) will be our focus for the rest of the labs this semester. Place the synapomorphies of the gnathostomes and the chondrichthyes on the phylogeny below to help yourself remember them.
Station 4. External anatomy of the Chondrichthyes Examine the external anatomy of the dogfish (Elasmobranchii, Squaliformes), the ray (Batoidea) and the ratfish (Holocephali) on display to familiarize yourself with the diversity in external anatomy in the Chondrichthyes. Note the dorsoventrally flattened body shape in the ray compared to the dogfish and the ratfish. Identify the paired fins of all three specimens. These include the pelvic and pectoral fins. Remember from station #1 that paired appendages are a synapomorphy of the gnathostomes. The pectoral fin of the ray is fused with the bodythis is a distinctive character of the rays. Note that the rays pelvic fins are reduced in size. Next identify the unpaired medial fins of all three fishes. These are the dorsal fin and the caudal fin. The anal fin is also an unpaired medial fin, but it is not present on any of these fishes. Note the shape of the dorsal and caudal fins. The holocephalans first dorsal fin is erect and triangular in shape. The dogfishs caudal fin is heterocercal in shape. In some species of holocephalans and rays the shape of the caudal fin is heterocercal, but in many other species this fin has been extensively modified in these groups. The holocephalans in particular tend to have highly reduced caudal fins, hence the common name ratfish. Find the gill slits on all three fishes. Count the gill slits on each fish, and note their placement.
Basic caudal fin shapes: A) heterocercal, B) protocercal, C) homocercal, D) diphycercal.
Station 5. Skeletal anatomy of the Chondrichthyes Lets now look more closely at the skeletal anatomy of the dogfish. Remember that the cartilaginous fishes have a cartilaginous endoskeleton. The three main parts of the vertebral column are the centrum, which protects the notochord, the neural arch, which protects the nerve cord, and the hemal arches, which protect the caudal artery. Identify these portions of the vertebral column on the skeletal material of the dogfish and the diagrams on display. Each vertebra is associated with a myomere, the segments of striated muscle which make up the sharks body walls. Using the diagrams on display, and the skeletal dogfish material, identify the elements of the girdles. The coracoid bar forms the center of the pectoral girdle and joins the two lateral elements to which the fins are attached. The scapular and suprascapular cartilage projects laterally from the coracoid bar, and the basal elements of the pectoral fin articulate with the scapular element. The basal elements are the propterygium, the metapterygium, and the mesopterygium. The radial elements articulate with the basal elements and the ceratotrichia extend laterally from the radial elements.
The structure of the pelvic girdle is similar to the pectoral girdle. In the pelvic girdle the isopubic bar is the central element, and the basal elements of the pelvic girdle, the propterygium and metapterygium, articulate with its acetabular surface. The Iliac process is a protrusion on the dorsal surface of the isopubic bar. If you have a male shark, you will see that the pelvic claspers are modified ends of the metapterygium. As in the pectoral girdle, the radial elements articulate with the basal elements and lateral to the radial elements are the ceratotrichia. You familiarized yourself with a basic view of gnathostome jaw morphology at station #2, but lets now look at the dogfish jaw in particular. Using the skeletal material, identify the parts of the mandibular and hyoid arches. Remember that the lower jaw is the mandible, or Meckels cartilage, and the upper jaw is the palatoquadrate. In the dogfish, the hyoid arch is composed of the hyomandibula, the ceratohyal and the basihyal.
Station 6. Major Chondrichthyan Groups I: The Elasmobranchii The vast majority of chondrichthyan diversity is in the Elasmobranchii, the group of cartilaginous fishes that includes the sharks, skates and rays. There are eight orders of elasmobranch sharks that we will ask you to become familiar with. Phylogenetic relationships among these groups are not well known. The skates and rays appear to be a monophyletic group, known as the Batoidea. Other sharks can be roughly grouped into two divisions: the Galeomorpha have anal fins and largely inhabit shallow waters, and the Squalomorpha (not a monophyletic group; Batoidea may be nested within it) lack an anal fin, and predominantly live in deep water. Examine the representative specimens at this station, and make drawings and notes about the distinctive features of each order/group based on what you see and the descriptions below. We will ask you to identify specimens to their appropriate order on lab practical exams. The figure below is a KEY, it is NOT representative of phylogenetic relationships among these orders. Use it ONLY to help you distinguish among the orders you will need to be familiar with for the exam. From Tricas et al. 1997. The Nature Company Guides: Sharks and Rays.
Galeomorpha Heterodontiformes Heterodontiform sharks include the hornsharks, Port Jackson sharks, and bullhead sharks, totaling 8 extant species. Their most distinctive feature is their teeth. Heterodontiform sharks have heterodont dentition, a very unusual characteristic in fishes (as you know from station #1; heterodontiform = different tooth shape). They have blunt heads with ridges above their eyes, and obtain a maximum size of about 1.5 meters. Their nostrils are connected to their mouths by a deep groove. Orectolobiformes Orectolobiform sharks include the carpet, nurse or zebra sharks and whale sharks, and include about 30 described species. The whale sharks claim to fame is being the largest fish species, with a length of up to 40 feet and weight up to 15 tons. It is a filter feeder. Another notable clade within this group is the wobbegong, a group of carpet sharks that have cryptic coloration and body form and function as ambush predators. Note that Orectolobiform species have small gill slits, and that the fifth gill slit overlaps with the fourth behind the origin of the pectoral fin. Note also that they have a groove running from the nostril to the mouth, and that this groove has barbels.
Carchariniformes The carchariniform sharks include the requiem, cat sharks and hammerhead sharks. This is a diverse group with over 200 described species. A distinctive feature of this group is that their eyes have a nictitating membrane.
A hammerhead shark. Figure from Pough et al 2009.
Lamniformes The lamniform sharks include the great white shark, the sandsharks, the thresher sharks, the mackerel shark and the basking shark for a total of 16 extant species. It also includes the elusive megamouth shark. Note that these sharks have two dorsal fins without spines, and that the mouth extends beyond the eye.
A great white shark. Figure from Liem et al. 2001.
Hexanchiformes The hexanchiform sharks include the cow and frill sharks, of which there are five extant species. Note that these species have six or seven gill slits instead of the normal five. They have only one dorsal fin without spines, and they have an anal fin.
Squalomorpha Squaliformes The squaliform sharks include the bramble sharks, sleeper sharks, cookie-cutter sharks and the dogfish sharks, and include a total of 74 extant species. You will dissect a spiny dogfish shark, Squalus sp., at station #7. These sharks have two dorsal fins, with or without spines, no anal fin. Some squaliform species are very small, with maximum lengths of only 20 cm.
A cookie-cutter shark. Figure from Pough et al 2009.
Squatiniformes The squatiniform sharks are known as angel sharks, and comprise 15 extant species. They have a distinctive dorso-laterally compressed body form and expanded pectoral fins that look like wings. Although the pectoral fin shape and use is similar to some species of the Rajiformes, note by comparing the specimens at this station that the body in the Squatiniformes is not nearly so dorso-laterally compressed as in the skates and rays, and that the angel sharks have a small and nearly terminal (i.e. at the end of the
nose) mouth. Pristiophoriformes The pristiophoriform sharks are the sawsharks. They are very distinctive among the sharks because of their elongated rostrum with teeth extending laterally on each side. They use this snout as slashing predators in schools of fish. They look superficially similar to the sawfishes of the Pristiformes (see below), but sawfish teeth are firmly attached into the sockets on the rostrum, while sawshark teeth are not (note that most of the teeth on the sawshark specimen are gone). Note also that these specimens have a single pair of barbels anterior to their eyes.
Batoidea The Batoidea include the skates, rays, sawfishes and guitarfishes. The sawfishes look superficially similar to the Pristiophoriformes, the sawsharks, as both have an extended rostrum with protruding teeth. Sawfishes can grow much larger than sawsharks, and also note the difference in teeth described above. The skates and rays have a distinctive flattened body form, and greatly enlarged pectoral fins that are fused with the body. The most morphologically distinctive of the rays are the sting rays, eagle rays, manta rays and butterfly rays. The guitarfishes are so named because of their guitar-shaped body form.
Station 7. Major Chondrichthyan Groups II: The Holocephali The Holocephali are weird-looking cartilaginous fishes commonly known as chimeras, ratfish or rabbitfishes. The only extant order is the Chimeriformes, in which there are about 40 described species, many of them extinct. Holocephali means whole head, and is a reference to this groups holostylic jaw suspension. Remember from station #2 that in holostylic jaw suspension, the upper jaw (palatoquadrate) is fused to the chondrocranium. The holocephalans are the only gnathostome group with this kind of jaw suspension. Examine the specimens on display at this station and the figures below to familiarize yourself with the distinctive morphology of the holocephalans. Look into the mouth and notice that the teeth are modified as several large grinding plates. Find the cephalic clasper if the specimen is a male. Find the gills, and notice that there is only a single gill opening covered by a fleshy operculum. Note that the specimens placoid lack (or any form of) scales. If placoid scales are a synapomorphy of Chondrichthyes, as we told you at station #3, how can Holocephalans be naked?? (Note: although the ratfishs body is naked, its fin spines and the spines on its claspers are actually modified placoid scales)
These four Holocephalan species, top left, plownose chimera, Callorhinchus milii, middle left, longnose chimaera, Rhinochimaera, top right, the spotted ratfish Hydrolagus colliei, and bottom, the smalleyed rabbitfish Hydrolagus affinis exhibit some of the distinctive features of the Holocephali. Note the enlarged head and rounded body form, and the erect first dorsal fin. Take a few minutes to think about the morphological features that distinguish the Holocephali from Elasmobranchii. Be sure that you can easily tell these major groups apart. Here are some major differences to keep in mind: Elasmobranchii Placoid scales Teeth Jaw Stomach Gill slits Covered in placoid scales Modified placoid scales Hyostylic J-shaped Separate and exposed Holocephali Sparse placoid scales Modified as grinding plates Holostylic Absent Single opening covered by a fleshy operculum
Station 8. Dogfish shark dissection With a partner, you will dissect a dogfish shark, Squalus sp., at this station. First examine the external anatomy of your dogfish. Find and identify all of the fins on the dogfish. Remember that paired lateral appendages are an important synapomorphy of the gnathostomes. On the dogfish, these are the pelvic fins and pectoral fins. Also identify the unpaired medial fins, the dorsal fins and the caudal fin. Note the heterocercal shape of the caudal fin, and remind yourself of the different caudal fin shapes you learned at station #4. Although the dogfish does not have one, many fishes also have an anal fin. There are two spiny dorsal fins on the dogfish; number, shape, size and number of dorsal fin spines and rays vary a great deal among fish groups; noting these characters is often an important aid in identifying fishes. The relative positions of the pectoral and pelvic fins are also an important feature to take note of when identifying fishes. Find the spiracles and the external gill openings. How many gill openings are there? Pop quiz: If you had counted seven openings, which order of sharks would you have in front of you? Open the mouth and examine the teeth of your dogfish. The chondricthyes have regular tooth replacement throughout their lives they are polyphyodont. Remember that calcified teeth are a synapomorphy of the Gnathostomata. A cross section of a chondrichthyan jaw showing the replacement teeth posterior to the current functional tooth in a lower jaw. Tooth replacement happens at regular intervals throughout the lifetime of chondrichthyans. Figure from Pough et al 2009. Find the tiny openings on the top of your dogfishs head, located between the spiracles. These are the openings to the endolymphatic ducts, tubes connected to the inner ear. Also notice the tiny pores on the anterior portions of the head. These are the external openings of the ampullae of Lorenzini, the organs that allow sharks to detect electric fields. If you squeeze these a jelly-like substance will emerge. Identify the lateral line on the sides of the body. You will have a chance to learn more about the inner ear, the ampullae of Lorenzini and the lateral line at station #8. Identify the cloaca. This is the shared exit for the reproductive, excretory and digestive systems. If you have a male dogfish, also locate the claspers (and remember that these are a synapomorphy of the Chondrichthyes).
Now examine the internal anatomy of the buccal and pharyngeal regions. Make a cut from the posterior corner of the mouth through the gill slits and gill arches. Once past the pharyngeal region, turn your cut to slice through the pectoral girdle and into the mid-body. You will now have exposed the buccal cavity, the region between the jaws and the spiracle, and the pharynx, the region between the spiracle and the most posterior gills. Find the interior opening of the spiracles just behind the upper jaw. Find the gill slits from this new view of the internal pharyngeal cavity. Water enters the sharks head through the mouth and spiracles and passes into the pharyngeal cavity and out the gill slits. In between the gill slits are the gill arches. Make cuts at the most dorsal and ventral connections of a single gill arch to dissect it out and examine it more closely. Note that the gill arch is a flexible, jointed cartilaginous structure. This flexibility is critical as the animal opens and closes its jaws. Think here also about the transition from gill arch to jaws that you learned about at station #2does seeing the structure of the gill arch make that transition more believable for you? Identify the gill rakers, the pointy protrusions that are oriented laterally into the pharyngeal cavity. Find the gill filaments (= gill lamellae) and the gill rays, the structures that support the gill filaments. Given the morphology of the gill filaments, what do you think their function is? What about the gill rakers? Now move on to examine the digestive organs of your dogfish. Cut from the pharynx to the cloaca of your dogfish. Scissors will probably be the easiest way to make this cut. Extend the cut laterally at the anterior and posterior ends, towards the pectoral and pelvic fins, respectively. This will allow you to open up your dogfish to look inside more easily. The first organ you will likely see is the large liver, a three-lobed structure. Find the left, medial and right lobes. The gall bladder is a greenish sac contiguous with the median lobe of the liver. The liver produces bile, which is stored in the gall bladder and released through the bile duct into the intestine to aid in digestion. If you move the lobes of the liver aside, you will see the esophagus and stomach. Unlike the mammals you may have dissected, the esophagus and the stomach in the dogfish are similar in diameter, and thus difficult to tell apart. If you cut into this tube, you will be able to distinguish where one ends and the other begins by looking for the esophageal papillae in the esophagus. Follow the stomach posteriorly to find the small intestine. The anterior-most region of the small intestine is the duodenum, which leads to the illium. If you cut open the illium you will find the spiral valve. The spiral valve increases surface area within the intestine and thus aids in absorption of nutrients during digestion. Continuing to follow the intestine posteriorly, you will reach the colon, and at the juncture of the colon and the rectum you will find the small pouch that is the rectal gland. Remember from station #3 that the rectal gland is a synapomorphy of Chondrichthyes, and it aids in maintaining salt balance.
Find the pancreas, a two-lobed structure anterior to the intestine. The ventral lobe is oval-shaped and adheres to the anterior region of the small intestine. The dorsal lobe is elongate and extends anteriorly from near the spleen.
Now examine your dogfishs urogenital system. Find the kidneys, which form a band of grey-brown tissue on the dorsal body wall. Find the ducts through which waste passes out the kidney and to the cloaca. If your dogfish is male, the testes are located at the anterior end of the kidneysyes, thats right, the anterior end! The tubules from the testes join with the urinary tubules to form the mesonephric duct, the shared passage which transports both sperm and urine to the cloaca. If you have a female dogfish, find the ovaries, located in the same position as the testes are in the male, near the anterior end of the kidneys. The ova from the ovaries pass through the oviducts into the uterus, located just anterior to the cloaca. The oviducts are connected by tissue to the ventral surface of the kidneys. Dogfish are ovoviviparous, giving birth to live young after many months (18-24) of gestation. From one to three fertilized dogfish eggs become enclosed in a rudimentary egg case as they travel past the shell (or nidamental) gland, into the uterus. Many months later, the developing young hatch from that egg case but remain within the mothers uterus. Throughout gestation, the young rely on yolk. They eventually are born as miniature adults, about 10-15 cm long.
The dogfish has a two-chambered heart (remember from last week that the twochambered heart is a synapomorphy of craniates). Identify the atrium and the ventricle in your dogfishs heart using the diagrams on display and the heart model on the heart/brain model table. Your dogfishs circulatory system has been injected with latex so that the arteries, the vessels that go from the heart to the rest of the body, are red and the veins, the vessels that return blood to the heart are blue. The sinus venosus collects blood that will enter into the atrium. The blood moves from the atrium into the ventricle and then to the conus arteriosus and into the ventral aorta. The blood then passes into the afferent branchial arteries which supply the blood to the capillaries in the gill lamellae, where the blood is oxygenated. The blood moves into the efferent branchial arteries from the gills, and then into the dorsal aorta which supplies blood to the rest of the body. The common cardinal vein is the major vessel that returns blood back to the sinus venosus and the heart. Also identify your dogfishs spleen, located close to the stomach. The spleen is important in blood cell production.
Station 9. The Chondrichthyan sense organs The sense organs of the craniates are concentrated in the headremember from last weeks lab that cephalized sense organs are a synapomorphy of the craniates. The chondrocranium, the box that surrounds the brain, is modified to protect these important sense organs. The olfactory capsules, the orbits and the otic capsules are the portions of the chondrocranium that protect the organs of smell, the eyes and the organs involved in hearing, respectively. Identify each of these in the dissected chondrocranium of the dogfish on display. The foramen magnum is the large hole in the posterior of the chondrocranium where the nerve cord connects with the brain, and foramina are small holes in the chondrocranium where nerves and blood vessels connect through to the brain. Identify and observe the major parts of the brain in the demonstration dissection at this table, and using the brain model at the brain/heart model table. The dogfish brain is tripartite (remember from last week that this is a synapomorphy of the craniates), with the forebrain (prosencephalon) associated with smell, the midbrain (mesencephalon) associated with vision, and the hindbrain (rhombencephalon) associated with balance and movement. The forebrain includes the cerebrum, which is important in physiological control. The hindbrain includes the medulla oblongata and the cerebellum, which function to coordinate muscle movements. The hindbrain also regulates heartbeat and the function of some organs. The dogfishs brain, with the anterior end on the left and posterior on the right. Cranial nerves and their numbers are denoted in roman numerals. Figure from Liem et al. 2001.
The cranial nerves are nerves that emerge directly from the brain. Sharks and other fishes have 10 used for sensory, motor and mixed sensory and motor functions, plus 6 associated with the lateral line system. Familiarize yourself with the major parts of the vertebrate eye using the diagrams on display, and the demonstration dissection. The retina is an outgrowth of the brain, and the photoreceptive rods and cones embedded in it absorb and refract light waves. Rods are responsible for black and white vision and function well in low-light conditions; cones are responsible for color vision. In sharks, rods outnumber cones in the retina by a large margin, making their vision more attuned to low-light conditions. Sharks also have a tapetum lucidum, a layer of guanine crystals, behind the retina. As light shines through the retina it reflects off of the guanine crystals and back to the rods and cones of the retina, causing the light coming into the sharks eye two separate
instances to be absorbed by the retinal pigments. This further refines sharks acuity of night vision. Sharks have an extremely well-developed sense of smell. Some species can detect the equivalent of a single drop of fish extract in a volume of water equivalent to that of an Olympic-sized swimming pool! This is made possible by tremendous sensitivity in receptors in the olfactory sacs and enlargement of the olfactory lobes and bulbs in the sharks forebrain, where the signals from the olfactory sacs are transmitted for integration. Examine the olfactory sacs in the dogfish demonstration dissection and using the diagrams on display. Be sure you have also located the olfactory bulbs and lobes in the brain. The inner ear allows a vertebrate to detect its position in space and plays a major role in hearing, although the precise mechanisms of hearing in different groups of vertebrates vary. The semicircular canals are part of the vestibular apparatus in the inner ear, which is located in the otic capsule of the chondrocranium. Remember from station #1 and last weeks lab that having three semicircular canals on each side of the head is a synapomorphy of the Gnathostomata, and that having at least two semicircular canals on each side of the head is a synapomorphy of the Craniata. The semicircular canals are filled with a substance called endolymph. Expansions on the canals known as ampullae contain hair cells that detect the displacement of endolymph during motion. The hair cells relay these signals to the nervous system and the brain. Each endolymphatic duct connects from a pore on the animals head to the vestibular apparatus. In the chondrichthyes, chambers within the inner ear contain calcium carbonate crystals that vibrate in response to sounds, allowing sharks to hear. The endolymphatic duct facilitates the movement of sound waves from the external environment into the inner ear. Be sure that you have located the external opening to the endolymphatic duct on a dogfish, and familiarize yourself with the anatomy of the inner ear using the demonstration dissection and the diagrams below and on display. The vestibular apparatus in a lamprey (a) and a gnathostome (b). The presence of three semicircular canals is a synapomorphy of the Gnathostomata. Figure from Pough et al 2009.
Identify and closely examine the lateral line in the dogfish on display. The lateral line is a series of pores running in a line from anterior to posterior on the side of the animal. All aquatic vertebrates have them, but they have been lost in terrestrial vertebrates. The pores lead to neuromast organs, whose main components are hair cells connected to the nervous system. Water movement displaces kinocilia, which excites or inhibits the associated nerves discharge depending on the direction of movement and the orientation of the kinocilia. A) Structure of a neuromast organ. The kinocilia are embedded in the gelatinous cupula, and it is displacement of the cupula which causes the kinocilia, and their associated steriocilia, to bend. B) The neuromasts are associated with pores in the skin and scales (if present). Figure from Liem et al 2001.
Many sharks and rays have the ability to sense electric fields, and they do this using the ampullae of Lorenzini. The ampullae are electroreceptors with connections to the nervous system. During your dissection at station #7 you identified these pores, and you may have noticed that when you squeezed them that a gel-like substance emerged. The canal leading from the surface pore to the ampulla is filled with electrically conductive gel (see figure below). Because all muscle activity generates electrical signals, sharks and other fishes with the ability to sense electric fields can use this sensory ability to detect prey. The electrical sensitivities achieved through the ampullae of Lorenzini are incredibly sensitive, being able to detect charges of less than 0.01 microvolts per centimeter. The receptor cells in the ampullae are structurally similar to the hair cells of the lateral line, leading scientists to believe that the electroreceptive ability in sharks is derived from the lateral line system.
The ampullae of Lorenzini are what give sharks, rays, and some other fishes the ability to sense electric fields. Note the nervous connection to the ampulla and the canal connection the ampulla to the surface pore in (b). (a) shows the distribution of ampullae on a sharks head. Figure from Pough et al. 2009.
For more information: Much of the information presented in this lab follows two major sources: 1) Gergus E.W.A. and G.W. Schuett, Labs for Vertebrate Zoology, an evolutionary approach. Biological Sciences Press, 2000; and 2) Pough, F.H., C.M. Janis, and J.B. Heiser, Vertebrate Life, 8th edition, 2009. Helfman G.S., B.B. Collette and D.E. Facey. The Diversity of Fishes. Blackwell Sciences, 1997, is an excellent source for further information about fish evolution and functional anatomy. Many figures are from Liem, Bemis, Walker and Grande, Functional Anatomy of the Vertebrates, 3rd edition, 2001. If you are interested in learning more about fish diversity, functional morphology and physiology, take BioEE 4760 Biology of Fishes! Several Shoals Marine Lab courses also focus on fishes. For more information see: http://www.sml.cornell.edu/sml_students_creditcourses.html . If youd like a virtual repeat of your dogfish dissection, or to see the dissection of a ray, see this great online seminar from the American Museum of Natural History: http://www.amnh.org/learn/pd/sharks_rays/rfl_dissection/