CARTILAGINOUS FISHES

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Chapter  5 - Chondrichthyes

  GNATHOSTOMATA

Learn  more about gnathostomes from the Tree of Life

CLASS CHONDRICHTHYES

Characteristics

  1. internal skeleton is composed of cartilage, which may be prismaticly calcified
  2. placoid scales ( a bone-like tissue is present at base of placoid scales)
  3. second gill arch (hyoid) involved in jaw suspension
  4. swim bladder or lung absent; have oil filled liver to provide neutral buoyancy [sharks are slightly negatively buoyant-will sink]
  5. claspers (modified pelvic fins) present in males [internal fertilization]
  6. persistent notochord
  7. intestine with spiral valve
  8. ventral mouth
  9. replacement teeth rows [Fig. 5-2]

Taxa

Subclass Elasmobranchii

Superorder NEOSELACHII 

  • the dominant Mesozoic and Cenozoic elasmobranchs
  • sharks, skates, rays
  • carnivores and scavengers; largest forms are filter feeders

 

 

Characteristics

  1. advanced forms with hyostylic jaw support (hyomandibular is a mobile connection between braincase and palatoquadrate; postorbital connection lost). Mouth frequently opens ventrally.
  2. placoid scales are modified to form teeth
  3. 5-7 pairs of gill slits with separate external openings and a spiracle
  4. vertebral centra strongly calcified; notochord constricted except intervertebrally
  5. enlarged nasal capsules: chemoreception. sensitive to one part in 10 billion; widely spaced nostrils in hammerheads may increase sensitivity for locating prey
  6. neuromast organs: mechanoreceptors
  7. Ampullae of Lorenzini [Fig. 4-7] electroreception for detection of prey, possibly for navigation
  8. heterocercal tail (may or may not provide lift--see p. 63)

General information on elasmobranch classification from ReefQuest by R. Aidan Martin.  This web site is a an important source of information about many aspects of elasmobranch biology.

ORDER SQUALOMORPHA

ORDER GALEOMORPHA
  • includes nurse, tiger, blue, white sharks, etc., and basking shark, megamouth, and 
  • active sharks, fusiform bodies
  • spiracle small or absent; mainly predaceous
  • high brain size to body ratio

 

ORDER BATOIDEA - skates and rays
  • most diverse elasmobranchs: 476 extant species
  • dorsoventrally-flattened bottom dwellers  [Fig. 5-8]
  • large pectoral fins; dorsal fins reduced to absent; caudal fin reduced
  • large dorsal spiracle
  • external gill openings are on the ventral side of the body; water (for breathing) is taken through the large spiracle on the dorsal side
  • teeth usually flattened and united to form a pavement for crushing mollusks; also feed on crustaceans, and occasionally, fish.

 

DIET in NEOSELACHIANS

predators

The largest chondrichthyans are filter-feeders

REPRODUCTION [fig. 5-6]

  1. claspers present in males for internal fertilization; internal fertilization
  2. mostly sharks and rays live birth
    1. ovoviviparous
      1. eggs retained in reproductive tract
    2. viviparous: nourishment not limited to egg
      1. extensions of the oviduct wall penetrate mouth and gills and secrete a milky substance
      2. young feed on additional eggs produced by mother
      3. yolk sac placenta (most common) attaches to uterine wall and obtains nutrients from mother's blood supply
  3. oviparous:
    1. some sharks, skates
    2. large yolked eggs; skate egg case
  4. slow rate of reproduction and commercial and recreational overfishing is threatening many species [Fig. 5-7]

  shark claspers BIODIDAC. used with permission


SUBCLASS HOLOCEPHALI

Characteristics

  1. gills close behind braincase (primitive)
  2. jaw support autostylic (holostylic)
  3. dentition a small number of large, continuously growing tooth plates
  4. fin structures similar to sharks
  5.  CHIMAERIFORMES

Characteristics

  1. Fleshy operculum covers four gill slits
  2. Skin naked; placoid denticles present primitively, absent in Recent forms
  3. No spiracle
  4. flattened, grinding teeth; eat seaweed, mollusks, echinoderms, crustaceans, and other small fish; rabbit fish has nipping teeth
  5. tail a narrow whip in recent genera
  6. claspers on pelvic fins and forehead
  7. recent forms marine

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