Chapter 10

SUBCLASS LISSAMPHIBIA

Pseudotriton montanus, the Gulf Coast mud salamander, photograph by Catherine Nordfelt, Department of Ichthyology, Auburn University and TAMUK graduate.

Derived Characteristics [see Table 10.1 for more]

1. single element vertebral centrum
2. bicusped, pedicellate teeth (uncalcified zone at base)
3. skull table bones reduced
4. moist, scaleless (liss = smooth), vascular skin for respiration
5. mucous glands keep the skin moist
6. poison glands are present in the skin

other Characteristics

8. Ectotherms
9. Amphibians shed eggs in water for external fertilization (not fully adapted to land)
10. Eggs first evolve as aquatic larvae with external gills, then undergo metamorphosis to emerge from water as adults
11. They have thin skin (needed gas exchange), thus in danger of desiccation if removed from a moist/wet environment
12. Amphibians gave rise to amniotes during the Pennsylvanian Period
13. All living amphibians are carnivorous; will eat anything they catch; no morphological specializations related to diet.
14. Amphibian populations are undergoing dramatic declines, which has been attributed to ultraviolet radiation, environmental toxins, etc.  Parasites have been identified as the cause of deformities in frogs.

The 5500 species of amphibians are grouped into 3 orders:

1. salamanders (order Caudata or Urodela)
2. frogs and toads (order Anura or Salientia)
3. the secretive, earthworm-like tropical caecilians (order Gymnophiona or Apoda) - limbless amphibians.

ORDER URODELA [CAUDATA] 

Salamanders and Newts

• 10 families [table 10.2] and 500 species of tailed amphibians [Fig. 10-1]
• middle Jurassic - Recent
• found in mainly in Northern Hemisphere (North America, Eurasia)
• Most species occur in North America.
• southernmost occur in tropical South America

Ambystoma tigrinum
photograph © 2001by Dr. Allan H. Chaney,  used with permission

Characters

1. least specialized of all living amphibians
2. elongate body
3. simple primitive limbs, set at right angles to body; most have 4 limbs but a few aquatic species have only 2 limbs.
4. locomotion similar to that of primitive tetrapods--lateral bending (fishlike) plus leg movement
5. aquatic, semi-aquatic, terrestrial
6. paedomorphosis is common in aquatic salamanders
7. terrestrial salamanders live in moist places under stones and rotten logs, usually not far from water.

Diet

1. Most salamanders are carnivorous preying on worms, small arthropods, and small mollusks; most eat only things that are moving.
2. food is rich in proteins; therefore, they do not store much fat or glycogen.

Size:

• Most of them are small. The common North America salamanders are less than 15 cm long. Some aquatic species are rather large. Cryptobranchus (hell bender) is 60 cm; The Japanese giant salamander reaches lengths > 1.5 meters.

Respiration

• all salamanders hatch with external gills, but during development, they are lost in all except aquatic forms or in some species which do not undergo complete metamorphosis.
• most terrestrial salamanders have well-developed, but simple lungs.
• salamanders lack ribs
• amphibians use positive pressure from hypobranchial apparatus of buccal pump to force air into lungs

Cutaneous respiration

• The skin contains an extensive vascular network of capillaries which allow for gas exchange to take place just below the epidermis. Cutaneous respiration is facilitated by pumping air in and out of the mouth where further respiratory gas exchange can take place. The buccal (mouth) cavity has a highly vascularized membrane system that supplements cutaneous respiration in lungless, gill-less salamanders.

Family Plethodontidae

• 265 species of lungless aquatic and terrestrial salamanders
• lungs were lost in forms inhabiting cool, swift running streams; disadvantage of too much buoyancy
• Small size of terrestrial species high surface/volume ratio provides adequate surface for gaseous exchange to take place across the skin.
• absence of lung and hypobranchial apparatus leaves room for a large protrusible tongue; especially well-developed in the bolitoglossines [fig 10-3]

Plethodon glutinosus, Northern Slimy Salamander, photograph © 1999 John White, CalPhotos, Digital Library Project

Paedomorphosis is common

• retention of larval characteristics such as lateral line
• some species are permanently aquatic
  • Cryptobranchidae: do not retain external gills
  • some never produce true lungs: the gills are the major respiratory organ for the entire life cycle. e.g., Necturus. these aquatic salamanders have all other amphibian characteristics. They can exist for short periods out of the water
  • axolotls are perennibranchiate forms of Ambystoma tigrinum that can be transformed using thyroxin.

cave salamanders

• fully metamorphosed adults to paedomorphic forms (Typhlomolge--Texas blind salamander-blind, white, external gills)

Ambystoma mexicanum, Axolotl, © 1999 Henk Wallays, CalPhotos

Breeding

1. some aquatic salamanders (Cryptobranchidae, Hynobiidae, possibly Sirenidae) use external fertilization
2. Most salamanders use internal fertilization. The female picks up a packet of sperm (spermatophore [fig 10-141]) usually deposited by the male on the substrate.
3. aquatic species lay their fertilized eggs in small, group-like clusters in the water. Larvae have external gills and a fin-like tail
4. Terrestrial species lay masses of eggs under logs or in holes of soft dirt. Many species remain near eggs to guard them. direct development--bypass the aquatic larval stage and hatch as miniature adults.
5. most complex life cycle in some American newts. Often have a terrestrial stage (toxic red eft stage) interposed between the aquatic larvae and the secondarily aquatic, breeding adults (e.g., Notophthalmus viridescens: Family Salamandridae).

ORDER ANURA  

Frogs and toads

• 27 families [table 10.3] and 4900 species of tailless amphibians
• cosmopolitan (except Antarctica)
• Jurassic - Recent

• the most well-known North American families are

1. Ranidae - frogs
   1. Most abundant and successful of the frogs are of the genus Rana (Gr. for frog). Found all over the temperate and tropical regions of the world except New Zealand, many islands, and southern regions of South America.
   2. Usually found near water. Some, such as the wood frog spend most of their time in on damp forest floors and often some distance from water. It returns to water only to breed in early spring. Bullfrogs and green frogs are usually found in or near permanent water or swampy regions.
   3. Ranidae from the USDA Utah

Rana berlandieri photograph © 2001by Dr. Allan H. Chaney,  used with permission 

2. Hylidae - the tree frogs
   • Toe pads for climbing [fig 10-10]: mucous glands produce sticky, long chain polymers--wet adhesion enables frogs to stick to surfaces
   • Hylidae from the USDA Utah
   • Hyla cinerea photograph © 2001by Dr. Allan H. Chaney,  used with permission 

3. Bufonidae - toads
   1. paratoid gland present
   2. Poisonous skin secretion, some species can kill dogs--e.g., Bufo alvarius--Colorado River toad
   3. Bufonidae from the USDA Utah
   4. Bufo marinus photograph © 2001by Dr. Allan H. Chaney,  used with permission 

4. Pelobatidae
   1. spadefoot toads
   2. Pelobatidae from the USDA Utah
   3. Scaphiopus  photograph © 2001by Dr. Allan H. Chaney,  used with permission 

• other families include

5. Rhinophrynidae
   1. Rhynophrynus dorsalis Mexican burrowing frog
   2. Weak hind legs, doesn't hop
   3. Burrows in termite nests, rotten logs
   4. Usually collected at ponds when breeding.
   5. Male calls while floating in middle of pond

6. Pipidae
   1. Africa and S. America, Aquatic
   2. examples: Suriname toad Pipa pipa, African clawed frog, Xenopus

7. Dendrobatidae
   1. Arrow-poison frog--Neotropical
   2. Ex. Dendrobates tinctorius Dye frog

Order Gymnophiona (Apoda)

• caecilians

  • members of an obscure order called Gymnophiona (naked snake). 160 species of worm-like amphibians.
  • Found in tropical forests of South America (primarily), Africa, and south-east Asia.
   

Ichthyophis bannanicus, photograph by Nikolai Orlov. Digital Library Project, University of California, Berkeley

Characters

1. limbless
   1. an early Jurassic caecilian possesses limbs
2. mostly burrowing species, some aquatic
3. solidly built skull
4. long, slender--body up to 200 vertebra
5. small dermal scales
6. no postanal tail
7. many species are blind as adults, have the name caecilians (caecus, blind). Because they are burrowers, the eyes are replaced by specialized sensory tentacles on the snout. Very rarely seen due to their burrowing nature.

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