The anatomy of the amphibians. Larval Amphibians

The anatomy of the amphibians will be presented with emphasis on clinically salient features. There are several excellent texts that have been published on the anatomy of amphibians, but their availability is limited. The interested clinician is advised to use book search services and inquire with used book dealers with an interest in herpetological texts. An excellent comprehensive text of the anatomy of the European fire salamander, Salamandra salamandra, has been published, but it is rare and somewhat expensive (Francis, 1934).

There exists a similar comprehensive text of the anatomy of two European ranid frogs, the edible frog, Rana esculenta, and European common frog, R. temporaria (Haslam, 1971). These two texts are highly recommended and should be consulted for anatomic details not found in this chapter or in the chapters on pathology. The more easily obtainable texts concern the anatomy of the mudpuppy, Necturus maculosus (Gilbert, 1973) and the European common frog, Rana temporaria (Wells, 1968). An ex- haustive text concerning the amphibian ear exists (Wever, 1985). Standard herpetological references also contain anatomic information of use to the clinician (Duellman & Trueb, 1986a, 1986b; Goin et ai., 1978; Porter, 1972; Stebbins & Cohen, 1995; Zug, 1993a, 1993b, 1993c), as do some of the popular works on amphibians (e.g., Obst et aI., 1988). The clinician is advised to continually review herpetological journals, including, but not limited to, Copeia, Journal of Herpetology, Herpetological Review, and Amphibia-Reptilia, to build a reprint library of relevant anatomic and physiologic information.

Larval Amphibians. Reviews of amphibian larval anatomy and metamorphosis are available (Duellman & Trueb, 1986a; White & Nicoll, 1981; Zug, 1993c). Metamorphosis is a complex and metabolically demanding process, and the clinician with a particular interest in amphibian propagation and pediatrics is encouraged to seek out additional sources of information.

The larval Caecilian. Late stage larval caecilians strongly resemble adult caecilians except for the presence of gills for a few hours after birth (Plate 3.1). The posthatching metamorphosis that is typical of salamanders and anurans is relatively undocumented in caecilians, and all neonates thus studied appear to resemble the adults. Branchial respiration occurs in larval oviparous caecilians within the egg and within the oviduct of viviparous species. Prior to hatching or birth, or within a few hours of birth, the gills are lost. Gill slits disappear during these final stages, as does the larval fins of the terrestrial forms.

The kidney of the larval caecilian functions to excrete nitrogenous waste in the form of ammonia and maintain water balance by excreting excess body water. Paired pronephric kidneys are present in the larva, and the pronephric tubules occur in a metameric (segmental) fashion along the dorsal aspect of the coelom. Depending on the species, 8-12 tubules may be present, which is more than the number of tubules found in species in the other two orders of amphibians. The pronephric kidney filters coelomic fluid via the nephrostome that accesses the coelomic cavity. Glomeruli in the form of outpocketings of the dorsal aorta also serve as an access to the pronephric kidney.

The nephrostome drains into a short tubule, which in turn is serviced by the renal portal vein. The short tubule empties into the common (pronephric) duct whereupon the wastes may be excreted into the external environment. This system only allows for filtration, not reabsorption (and thus concentration of the solute level), so the osmolality of the urine of a larval caecilian is equal to the osmolality of its plasma.

During metamorphosis, the pronephros degenerates and is phagocytized while the mesonephros is beginning to function. This transition is to a large degree controlled by fluctuations in the circulating levels of the thyroid hormones triiodothyronine (T₃ ) and tetraiodothyronine (T₄ ).

The larval Salamander. A staging system for the larval development of the spotted salamander, Ambystoma maculatum, has been described (Rugh, 1962). Immediately after hatching the larva possesses no external processes except for gills. The larva begins life with a large head bearing three pairs of plumate gills and a streamlined body and tail. Forelimbs erupt a variable length of time later, followed by hind limbs. In species with a terrestrial stage, and in some aquatic species, the gills start to recede shortly before metamorphosis is complete. Salamanders are carnivorous throughout life, and the digestive system changes little with metamorphosis.

There may be morphological differences between larvae which suggest either habitat adaptations (a small morph of the tiger salamander, Ambystoma tigrinum, adapted to ephemeral ponds and a large morph adapted to permanent ponds) or cannibalism (a morph of the tiger salamander, A. tigrinum, with an enlarged head and proportionately larger gape) (Rose & Armentrout, 1976). Some salamander species, such as the axolotl, Ambystoma mexicanum, and the mudpuppy, Necturus maculosus, continue to mature internally and become reproductive despite the retention of the external appearance of the larva, a process known as neoteny.

Not all salamanders undergo metamorphosis. Some races of the European fire salamander (e.g., Salamandra salamandra bernardezi, S. salamandra fastuosa) and the alpine salamander, Salamandra atra, give birth to fully metamorphosed young that resemble the adults (Griffiths, 1996). Some plethodontids, such as the green salamander, Aneides aeneus, develop directly into the adult form from the egg and thereby by- pass the free-living larval stages.

The pronephric kidney of the larval salamander is similar to that of the caecilian, however the number of tubules is much reduced. In certain primitive salamanders (e.g., Cryptobranchidae), there is a total of five paired tubules. The number of tubules is reduced to two pairs in more advanced families of salamanders.

The larval Anuran. Anuran larvae are commonly referred to as tadpoles. Tadpoles are distinctively shaped with a round to oval body and a laterally compressed tail (Plate 3.2). The structure of the tadpole changes dramatically throughout its growth, and various staging systems have been described in an effort to standardize the approach to anuran developmental anatomy. One of the better known staging systems describes the development of the Gulf Coast toad, Bufo valliceps (Limbaugh & Volpe, 1957). The oral groove is variably shaped. The shape and structure of the mouth are tailored to the diet of the tadpole and may be taxonomically significant. Anuran tadpoles have a varied diet, and there is considerable variation in diet.

The tadpoles within some species may vary in morphology, as some individuals may be programmed for cannibalism. The digestive tract is typically long and coiled in the filter feeding and herbivorous tadpoles and much shorter in carnivorous tadpoles. Paired external nares are present. Gills are visible upon hatching, but eventually are covered by an operculum until a pair of branchial spiracles are present. The cloaca may be located subjugular or more terminally located toward the tail base. There are no known neotenic anurans.