3Systematics and Diversity of Extant Amphibians

Systematics and Diversity of 3 Extant Amphibians he three extant lissamphibian lineages (hereafter amples of classic systematics papers. We present widely referred to by the more common term amphibians) used common names of groups in addition to scientifi c Tare descendants of a common ancestor that lived names, noting also that herpetologists colloquially refer during (or soon after) the Late Carboniferous. Since the to most clades by their scientifi c name (e.g., ranids, am- three lineages diverged, each has evolved unique fea- bystomatids, typhlonectids). tures that defi ne the group; however, salamanders, frogs, A total of 7,303 species of amphibians are recognized and caecelians also share many traits that are evidence and new species—primarily tropical frogs and salaman- of their common ancestry. Two of the most defi nitive of ders—continue to be described. Frogs are far more di- these traits are: verse than salamanders and caecelians combined; more than 6,400 (~88%) of extant amphibian species are frogs, 1. Nearly all amphibians have complex life histories. almost 25% of which have been described in the past Most species undergo metamorphosis from an 15 years. Salamanders comprise more than 660 species, aquatic larva to a terrestrial adult, and even spe- and there are 200 species of caecilians. Amphibian diver- cies that lay terrestrial eggs require moist nest sity is not evenly distributed within families. For example, sites to prevent desiccation. Thus, regardless of more than 65% of extant salamanders are in the family the habitat of the adult, all species of amphibians Plethodontidae, and more than 50% of all frogs are in just are fundamentally tied to water. six families (Hylidae, Craugastoridae, Bufonidae, Micro- 2. The permeable skin of amphibians acts as a re- hylidae, Ranidae, and Rhacophoridae). Amphibian popu- spiratory organ that, in addition to the lungs, lations worldwide have been devastated by the introduc- exchanges oxygen and carbon dioxide with the tion of the chytrid fungus, as well as by habitat loss, pol- environment. This exchange can occur only when lution, and poaching or harvesting for folk remedies (see the skin is moist, and the thin, moist skins of Chapter 17). nearly all amphibians render them susceptible to evaporative water loss. 3.1 What Is an Amphibian? Because of their reliance on water for respiration and reproduction, amphibians are particularly sensitive both Herpetologists use two different measurements of am- to water pollution and to increasing aridity caused by cli- phibian body length (Figure 3.1), and most amphibians mate change. are small in terms of both length and weight. Amphibians In this chapter, we fi rst discuss several important mor- share numerous other traits, including ectothermy (see phological and physiological traits that characterize Chapter 1), but their life histories and skin structure are amphibians. We then explore amphibian diversity on a central in defining the ways these animals interact with fi ner scale by discussing general biological features of their environments. each family, as well as its geographic distribution and conservation status. We provide detailed discussions of Amphibian life histories taxonomy when appropriate, and provide references for The ancestral reproductive mode of amphibians is aquatic, the most recent systematics studies and sometimes ex- a trait inherited from their tetrapod forebears. Eggs are laid uncorrected page proofs–1st Pages © 2015 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher. 42 Chapter 3 Systematics and Diversity of Extant Amphibians (A) Total length hatching and emerges from the egg as a miniature of the adult. Some direct-developing species are viviparous, re- taining the embryos within the oviducts and giving birth to fully formed young. Paedomorphic species retain larval characters (such as external gills) throughout life and repro- duce without metamorphosing to a fully adult form. Amphibian skin The basic structure of the skin is similar among the three Snout–vent length (SVL) major clades of amphibians. The permeable, glandular na- ture of amphibian skin plays crucial roles in respiration, defense, courtship, locomotion, and reproduction. These (B) SVL and total length are the same characteristics of the skin also render amphibians susceptible to pollution in aquatic habitats and to dehydration in terrestrial habitats. As in other vertebrates, amphibian skin is composed of an outer epidermis and an underlying dermis. The glands, nerves, muscles, scales (in caecilians), and pigment cells associated with the skin are located in the dermis, although their processes and ducts may extend to the skin surface. Functionally, developmentally, and anatomically, the epidermis and dermis are highly integrated. The thin, moist skin of amphibians allows exchange of carbon dioxide and oxygen with the atmosphere. Indeed, cutaneous gas ex- Figure 3.1 Herpetologists measure length in two ways. change is the only mode of respiration for plethodontid sal- (A) The total length reptiles and amphibians is measured from amanders and a few other species that lack lungs entirely. the tip of the snout to the tip of the tail. However, loss of a por- Cutaneous mucus and granular glands are synapomor- tion of the tail (caudal autotomy) is common in some groups phies of Lissamphibia. Amphibian skin contains two types (notably salamanders and lizards), so snout–vent length, or mucus glands SVL—the distance from the tip of the snout to the poste- of that secrete mucoproteins (see Figure 6.4). rior margin of the cloaca—is often used in preference to total Ordinary mucus glands provide a moist coating over the length. (B) Almost all adult anurans lack tails, so total length body surface that is critical for cutaneous gas exchange and and SVL are generally the same, measured from the tip of the for limiting water loss. Some frog species use this mucus snout to the end of the urostyle (the fused caudal vertebrae; and shed epidermis to form a cocoon that allows the frog see Figure 3.18). (Photographs: A, © Francesco de Marco/ to estivate, a prolonged state of of dormancy, during dry Shutterstock; B, © Eric Isselee/Shutterstock.) periods. The skin overlying the skull of some frogs becomes inseparably fused with the underlying bone. This condition, called co-ossification, also inhibits evaporative water loss (Seibert et al. 1974), and is often associated with the pres- in water and hatch into aquatic larvae that grow for some ence of bony crests in the skull. period and ultimately metamorphose into terrestrial adults. Sexually dimorphic mucus glands, or breeding glands, This complex life history allows an individual to harvest differ structurally and chemically from ordinary mucus energy and nutrients from two different habitats, and am- glands (Thomas et al. 1993; de Perez and Ruiz 1996). In phibiansPough 4e therefore play an important ecological role in response to increased levels of androgen hormones dur- transportingSinauer Associates nutrients from aquatic to terrestrial habitats. ing the breeding season, males of many frog species de- BecauseIn house the larva and adult have different lifestyles, selec- velop clusters of mucus glands (Emerson et al. 1999). When tionPough4e_03.01.ai acts independently 04-15-15 on these two life stages. The coun- present on the hands and/or forearms, breeding glands are teracting forces of selection are evident in tadpoles, which called nuptial pads, and they often develop a dark, highly are very different from adult frogs. During metamorpho- keratinized (even spiny) overlying epidermis (Figure 3.2). sis, practically every body structure of a tadpole is broken Nuptial pads help males grasp females securely during am- down and reconstituted into the adult form. The body forms plexus. In some frogs, they are used in male–male combat of salamander and caecilian larvae are similar to those of and may bear enlarged bony spines projecting from the adults, and the changes that occur during metamorphosis base of the thumb (prepollical spines). These spines can are correspondingly less dramatic. inflict lethal wounds during encounters between males Variations on the ancestral life-history pattern are wide- (Kluge 1981). spread. For example, among direct-developing species, the Granular glands produce defensive secretions in the embryo passes through an abbreviated larval period before form of toxic amines, peptides, proteins, steroids, or alka- uncorrected page proofs–1st Pages © 2015 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher. 3.1 What Is an Amphibian? 43 Figure 3.2 Nuptial pad of a male frog. During the breeding season, males of some frog species develop thick, rough, or even spiny areas of skin on the prepollex and forearm. These nuptial pads assist them in grasping a female during amplexus (mating). Shown is a nuptial pad of Perez’s frog (Pelophylax per- ezi, Ranidae). (Photograph © Wildlife GmbH/Alamy.) loids (Daly 1995; Conlon 2011a). Like mucus glands, the granular glands of amphibian skin are often concentrated into macroscopic clusters. The most obvious examples are the parotoid glands (and dorsal warts) of many frogs, es- pecially bufonid toads and some salamanders (e.g., Sala- mandra), that may secrete copious quantities of toxin when disturbed. In addition,

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