Th e dentition of many benthic rays is sexually dimor-phic; males have sharper teeth in the front of the jaw than females. Diff erent dentitions coupled with the generally larger size of females might reduce competi-tion for food resources between the sexes, but no dif-ference in stomach contents has been found. It seems more likely that sexual selection is the basis for the diff erence in dentition of males and females because a male uses its teeth to hold a female before and during copulation. Males of the stingray Dasyatis sabina have blunt teeth like those of females for most of the year;
but, during the breeding season, males grow sharp-cusped teeth. Th e precopulatory bites of males of this species, as well as other species of rays, can leave last-ing scars on females that are used for courtship.
Skates are oviparous, laying eggs enclosed in horny shells popularly called “mermaid’s purses.” Fetal nutri-tion is entirely lecithotrophic, and embryonic develop-ment can extend over many months. Rays, in contrast, are viviparous. In the early stages of development the egg yolk nourishes the embryos, and when the yolk is ex-hausted the embryos consume a nutrient secretion pro-duced by the walls of the oviduct; thus, they shift from lecithotrophy to matrotrophy during their development.
(a)
(b) 10 cm
10 cm
Figure 5–13 Holocephalans. (a) A chimaera, the spotted ratfi sh ( Hydrolagus colliei ), is an extant holocephalan. (b) Rep-resentatives of two other extant groups: ( upper ) Pacifi c long-nose chimaeran ( Harriotta raleighana ) and ( lower ) plowlong-nose chimaeran ( Callorhinchus callorynchus ).
Summary
Fossil remains of chondrichthyans fi rst appear in the fossil record in the Ordovician period, although de-fi nitive dental remains are not known until the Early Silurian. Chondrichthyans are distinguished by an en-tirely cartilaginous endoskeleton with a unique form of prismatic mineralization, and by the presence of pelvic claspers in the males, indicating reproduction via inter-nal fertilization. Chondrichthyans are subdivided into the holocephalans (chimaerans and fossil relatives) and the elasmobranchs (sharks, rays, and fossil relatives).
Th ree radiations of elasmobranchs—Paleozoic, Mesozoic, and Cenozoic—can be traced through in-creasingly derived characters of the jaws and fi ns. Mod-ern sharks and rays have modifi cations of the skull that allow the jaws to be protruded, enabling the fi shes to bite chunks of fl esh from prey too large to be swal-lowed whole. Sharks are mainly midwater predators.
Skates and rays are adapted for life on the sea bot-tom. Th ey are dorsoventrally fl attened, with eyes and
spiracles on the tops of their heads. Many skates and rays lie buried in sand and ambush their prey, but the largest species—the manta rays—are plankton feeders.
Th e holocephalans were much more diverse in the Paleozoic than they are today. Th e paraselachians are a separate clade from the one that gave rise to the extant chimaerans, which are a small group of bizarre fi shes (also known as ratfi shes). Chimaerans generally occur in deep water, and little is known of their natural his-tory and behavior.
Th e life histories of most species of elasmobranchs are based on producing a few relatively large young at a time. Th is reproductive strategy depends on a high survival rate for the young and long life expectan-cies for adults. It worked well for more than 350 mil-lion years, but loss of coastal habitat and outrageous overfi shing within the past 50 years have now brought many species of sharks to the edge of extinction.
Discussion Questions
1. “Chondrichthyes” means “cartilaginous fi shes.” Biolo-gists used to assume that bone was not evolved until the Osteichthyes, or bony fi shes. What clues do we
have that shark ancestors, at some level, had bone?
How can we infer this from (a) evidence from the fossil record and (b) evidence from comparative anatomy?
Chimaerans have long been grouped with elasmo-branchs as Chondrichthyes because of the shared spe-cializations described in section 5.1, but they have a bizarre suite of unique features as well. Th ey have only four gill openings, which are covered by a soft tissue fl ap so that only one opening is visible to the outside;
they have no spiracle; the branchial skeleton is below the cranium (rather than behind it, as in sharks); and their teeth have been reduced to tooth plates.
Several species of chimaerans have elaborate ros-tral extensions that are densely studded with lateral line mechanoreceptors and ampullary electrorecep-tors. Th ese receptors are especially dense around the mouth, which is relatively small and faces downward.
Holocephalans prefer soft substrates, and they feed on invertebrates and small fi shes that live on the seafl oor.
Th ey consume soft-bodied organisms, such as anemo-nes and jellyfi shes, but their tooth plates can crush hard-bodied prey, such as crabs. Like other fi shes that have crushing tooth plates (e.g., lungfi shes) chimaerans
have a derived type of jaw suspension, where the upper jaw is completely fused to the cranium (see Figure 5–
7 f). Th is type of jaw suspension is called holostyly in chimaerans, and they retain an unmodifi ed hyoid arch.
(Th e similar condition in lungfi shes and tetrapods evolved independently in a diff erent fashion from a condition where the hyoid arch was incorporated into the jaw suspension; this is called autostyly .)
Some species of holocephalans have a poison gland that is associated with a mobile dorsal spine. Th e spine can be erected when the chimaera is attacked, and a predator that was stabbed in the mouth as it tried to swallow a chimaeran might well decide to release the fi sh and seek less noxious prey. Males of some species of chimaeran have a spine-encrusted cephalic clasper that is used in courtship. Oddly, although the clasper is on the top of the head, it is controlled by muscles that also move the lower jaw. A male uses the cephalic clasper to pin the female’s pectoral fi n against his fore-head while he is attempting copulation.
ends of the hammer and collect water via a pre-narial groove that probably increases the volume of water fl owing across the olfactory epithelium.
•
Th e surface area of the olfactory epithelium of the scalloped hammerhead is no larger than that of non-hammerhead sharks.It appears that observations have taken us as far as they can and it’s time to try a diff erent approach to testing hypotheses about the functional signifi -cance of the hammerhead morphology.
a. What experimental tests can you propose to evaluate the two hypotheses? (Don’t worry too much about how you would carry out a manipulation—if a test involves putting sharks into a fl ow tank, for example, assume that you have access to a tank and sharks of the appropri-ate species and sizes.)
b. Is there another possibility that you miss if you consider the hydrodynamic and sensory func-tions to be mutually exclusive hypotheses?
4. John Musick and Julia Ellis have proposed that the ancestral reproductive mode for elasmobranchs was
“yolk-sac viviparity.” Th at is, embryos were retained throughout their development in the oviducts of the female and emerged as miniatures of the adults (i.e., viviparity), but nutrition was provided by yolk that was deposited at the time the egg was formed, not from the mother during development (i.e., lecithotrophy, not matrotrophy). Th ey suggest that oviparity (depositing eggs that develop outside the body of the mother) was associated with the evolu-tion of small body size because it increased the fe-cundity of small species of elasmobranchs.
What is their reasoning? Th at is, why would oviparity provide greater fecundity than viviparity for small species of elasmobranchs? What other fac-tors might make one mode superior to the other?
5. A hyostylic jaw suspension in chondrichthyans allows protrusion of the upper jaw independent of the lower jaw. Evolutionary biologists have as-sumed that the ability to protrude the upper jaw is advantageous for feeding—in other words, that it is an adaptive derived character. An alternative hypothesis is that a hyostylic jaw suspension is a neutral ancestral character (i.e., one that is neither advantageous nor disadvantageous).
Th ese two hypotheses (i.e., hyostylic jaw is advantageous/hyostylic jaw is neutral) generate diff erent predictions about the phylogenetic distri-bution of hyostyly. What are those predictions, and which one is supported by the phylogenetic distri-bution of hyostyly?
2. Why do we call Paleozoic chondrichthyans such as Cladoselache “sharks,” and why isn’t that terminol-ogy taxonomically correct?
3. Th e peculiar heads of hammerhead sharks are called “cephalofoils,” a term that refl ects an inter-pretation of the head as a hydrodynamic structure that helps to direct the shark’s head upward while it is swimming or to make the shark more maneu-verable. An alternative hypothesis is that the head increases the shark’s chemosensitivity or electro-sensitivity. Proponents of the two interpretations have used anatomical, behavioral, and evolutionary information to support their views and to discredit the alternative hypothesis.
Hydrodynamic Function
•
Th e cross section of the cephalofoil is shaped like a wing, and the angle of its trailing edge can be altered in a manner reminiscent of changing the angle of the fl aps on the trailing surface of an airplane’s wing to control the amount of lift.
•
Th e sizes of the two anterior planing surfaces, the cephalofoil and the pectoral fi ns, are inversely related. Th at is, species with wide cephalofoils have relatively small pectoral fi ns, and vice versa.As a result of that relationship, the total surface area of those surfaces is fairly constant.
•
Hammerheads are among the least buoyant spe-cies of sharks.Sensory Function
•
Hammerheads swing the head from side to side when they are searching for prey buried in the sediment. Th is is the same way a person searches for buried objects with a metal detector.
•
Th e ampullae of Lorenzini are distributed over the entire ventral surface of the hammer.Additional Observations
•
Th e least-derived extant species of hammer-head, the winghead shark ( Eusphyra blochii ), has the broadest hammer; its width is nearly 50 per-cent of the total length of the shark.
•
Th e size of the hammer in more derived spe-cies of hammerheads does not change unidi-rectionally according to current phylogenetic interpretations of the lineage (that is, there ap-pears to be no tendency for the size of the ham-mer to have increased or decreased during the evolution of the extant hammerheads).
•
Th e nostrils of the winghead shark are near the midline of the hammer, but they lie toward the ends of the hammer in more derived species.
•
Th e nostrils of the scalloped hammerhead ( Sphyrna lewini ), which is a well-studied species, lie near theAdditional Information
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Websites
General information about cartilaginous fi shes American Elasmobranch Society http://elasmo.org Florida Museum of Natural History, Sharks http://www
.fl mnh.ufl .edu/fi sh/Sharks/ISAF/ISAF.htm ReefQuest Centre for Shark Research
http://elasmo-research.org/index.html
Shark Research Institute http://www.sharks.org/
Fossil chondrichthyans
Elasmo.com—Th e Life and Times of Long Dead Sharks http://www.elasmo.com/
Fossil Fishes of Bear Gulch: information about stem chon-drichthyans, including the photograph of the fossilized pair of Falcatus falcatus , with the male’s spine inserted through the gills of the female, that is shown in Figure 5–3 d http://
www.sju.edu/research/bear_gulch/pages_fi sh_species/
Falcatus_falcatus.php
Conservation of chondrichthyans
European Elasmobranch Association: a coalition of national organizations that promote sustainable management of shark and ray fi sheries on a regional basis http://www .sharkalliance.org/
Shark Alliance: a coalition of nongovernmental organiza-tions dedicated to the conservation of sharks http://
www.sharkalliance.org/