Metamorphosis of the «Non-aquatic Frog" of thePalau Islands, Western Carolines

KEN]I ATODA 1

INTRODUCTION

THE FROGS in the Palau Islands seldom ifever, leave their refuges during the daytime,but after sunset they are found easily in weedyplaces. There are two types of frogs, the largeand the small, of which the larger is thefemale and the smaller the male. The malehas strong jumping powers. Neither type hasany webs between toes or fingers .

The adult frogs are well known to thePalau natives , who call them "gedegedug,'"hedehedh,' or "dechedech," but it is inter­esting to know that no one has seen their

- radpoies. I made a search for the tadpoles insome pools and ponds, but without success,and the question soon arose as to where andhow these frogs pass through their larvalstages. During my long stay in the PalauTropical Biological Station on Koror IslandI was fortunate several times in discoverin~the spawning place of the frogs, and wasthus given the opportunity to study theirmetamorphosis.

Here I wish to express my thanks to Pro­fessor Shinkishi Hatai, then Director of thePalau Tropical Biological Station, for hiskind guidance throughout the observations.

METAMORPHOSIS

The eggs collected from the natural spawn­ing place were placed in a petri dish, the bot­tom of which was covered with a wet , filterpaper to maintain a humidity of about '100per cent. The room temperature ranged from25.10 to 31.0 0 C. throughout the period ofobservation.

' The Second Higher School Sendai Japan.Manuscript received July 20, 19,49. '

The observations were made on one hatcht hi h ' 'o. w rc the youngest larva is shown in Figure

1a. ~he egg is covered with a rather tough?ela~mous membrane and just after spawn­mg It measures about 6 mm. in diameter. Alarge mass of yellowish-white yolk is attachedto the abdomen of the larva. The three partsof the body-head, trunk, and tail-are dis­tinguishable. The central part of the 'head issomewhat hollow, and rudimentary eyes ap­pear on both sides. The mouth is a shallowinvagination not yet open. The end of thetail is bent toward the abdomen. Each pairof limbs appears as rounded and protuberantswellings, the hind limbs being slightly largerthan the forelimbs. Fine blood vessels runalong the forelimbs, then toward the abdo ­men and finally branch into networks. Themelanophores are not yet visible.

On the second day (Fig. 1b), a pair ofwell-developed eyes appears and the mouthopens, the tail becomes much longer, 'and thevascular system is conspicuously developed.The melanophores appear scattered over thehead and trunk.

On the fourth day (Fig. I e) , the mouth iswide open and a pair of external nares isseen on its upper part. Numerous blood ves­sels are distributed over the abdominal re­gion; ~he heart, which is perceptible throughthe skin , beats at an average rate of 120 timesper minute (at a temperature of 30.8 0 C.).

On the seventh day (Fig. 1d, e) , theyolk decreases and becomes flattened dorso­ventrally. Five toes can be distinguished inthe hind limbs but the forelimbs are hiddenby the yolk mass. The melanophores increasein number on both the head and trunk.

On the tenth day (Figs. If; 2a), both the

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hind limbs and tail become more developed,and the latter reaches about 4 mm. in length.The body length, except for the tail, is about5 mm.

On the fourteenth day (Fig. 2b) , the eggreaches about 9.5 mm. in diameter. The eggseems to expand day by day as the larvagrows in ir. ' The skin of the abdomen, whichbegins to expand from about the seventh dayof development, swells conspicuously andforms a pair of large , balloon-like sacs witha great number of blood vessels on the walls.

On the seventeenth day (Fig. 2c, d, e ), thegreater part of both tail and yolk are ab­sorbed, and, except for the extreme expansionof the abdominal skin, the larva closely re­sembles an adult frog.

On the nineteenth day ( Fig. 2f), nearlyall of the yolkhas been consumed and thetail is now rudimentary. When the egg iscarefully removed from the vessel, the gela ­tinousmembrane readily slips off and a mini- 'ature frog jumps free.

After a few more days all the larvae com­plete their metamorphosis, and as soon asthey leave their eggs, the expanded abdom­inal skin contracts to adhere closely to theabdomen, showing the same appearance asin the adult frog. These young frogs measureabout 6 mm. in length from the tip of snoutto the anus.

ADULT FROG

Some of the external features of the maleand female frogs are shown in Figure 3a-e.

Both types of frogs are dark brown butthe male is the darker. The inside of thethighs is yellowish-red. In some frogs a fineline arises from the tip of the rostral andextends over the ridge of the back to the toe.

The tip of the rostral is somewhat roundedand the' outer nostrils open close to its ex­tremity. The distance between the rostrum andthe point where the upper and lower jawsunite is slightly longer than the rostrum re-

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gion. The distance between the extremity ofthe rostrum to the joint of each jaw is approx­imately equal to the width of head. Thepupil is ellipsoid. The circular tympanum isconspicuous. Its diameter is about one anda half times the length between the hindmargin of the eye and the fore margin of thetympanum, and is slightly shorter than thedistance between each outer nostril. Close tothe upper margin of the tympanum there isa narrow skin fold . .

In the buccal cavity a pair of internal nos­trils , a pair of triangular vomerine toothplates, and a tongue are found. The tip ofthe tongue is bifurcated. The two types offrogs have tongues of different shapes; thatof the large (female) frog is slenderer thanthat of the small (male ) frog ; and the dis­tance between each small process is less inthe large frog than in the small frog . Thereare minute teeth in the upper jaw but notin the lower jaw. The vocal sac is not veryevident.

Some skin folds are scattered .over the dor­sal part of the trunk. Of the fingers of theforelimb, the third is longest, then the firstis next longest; the other two fingers arenearly equal in length. Of the toes of thehind limb, the fourth is much more prom­inent than the others, and the first is shortest.The webs are completely degenerated. Manyrounded protuberances of skin are found onthe insides of the fingers , toes, palm, andmetatarsus. The tips of both fingers and toesare rounded. The length from the base ofthe .tibia to the tip of the longest toe is twoand a half times that from the base of thearm to the tip of the longest finger , and is,1.6 times the length from the tip of therostrum to the anus .

DISCUSSION

The eggs upon which the observ ationswere made were collected in the months ofMay, October, and December. They were

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a

PACIFIC SCIENCE, Vol. IV, July, 1950

d

e

y

cf

FIG. 1. Progressive stages of metamorphosis of the frog from the Palau Islands , X 7. a, Dorsalview of the youngest larva ( the outer gelatinous egg membrane is omitted ) ; b, 2 days later, dorsal view( only the innermost egg membrane is drawn ) ; c, 4 days later, ventral view ; d, 7 days later ( all mem­branes are removed from the egg); frontal view; e, same as d, ventral view ; i, 10 days later, dorsalview.( am, Abdominal membrane; bv, blood vessel; e, eye; f1, forelimb; h, heart; hl, hind limb; m, mouth ;t, tail . )

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a ' ,

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b

fFIG. 2. Further stages in metamorphosis of the frog, X 7. a, Same as 11, lateral view, only the

innermost egg membrane is drawn; b, 14 days later, dorsal view; e, 17 days later, dorsal view; d, sameas e, lateral view; e, same as e, ventral view; t, 19 days later, egg membranes removed, dorsal view.

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spawned in a weedy place on the ground.The number of eggs in one hatch was usuallyabout 30. At the time of each collection itwas noticed that a considerable number ofeggs with large yellow -yolks could be seenthrough the skin in the abdomens of largefrogs. These observations lead to the con­elusion that the frogs have no definite spawn­ing time, but spawn every month throughoutthe year.

Metamorphosis is completed so rapidlythat the first stage observed was probablyclose to the earliest stage. Apparently, thetime required to complete the metamorphosisis about 3 weeks.

The characteristic features of the meta­morphosis of this frog are the extreme swell­ing of the abdominal -skin and the absence

. of both external gills and gill clefts. Thelarva does not pass a free-swimming stage inwater; therefore external gills for respirationare unnecessary. The larva grows rapidlywithin the egg, and metabolism takes placeactively. The expansion of abdominal skinor the increase of body surface and the abun­dant blood vessels in the skin may be helpfulin increasing respiration through the skin.In addition to these characteristics, the larvadepends wholly upon the yolk for nutriment

. throughout metamorphosis, and its yolk islarge , like that of a fish egg. Observations onthe cleavage of the egg were not made.

Hitherto only two species of frogs havebeen known to exhibit behavior more or lesssimilar to that of the Palau frog : H ylodes(= Eleutherodactylus ) martinicensis ( Peters1876; Lynn 1940) and Rana opistbodon( Boulenger 1886 ) . Both of them undergotheir whole metamorphosis within their eggs,and their embryos possess neither gills norgill openings.

Hylodes martinicensis lays its eggs on a ·broad leaf of a plant, and the eggs are thenglued to the leaf. The egg measures 4- 5 mm.in diameter. The ' metamorphosis is com­pleted during 21 days, approximately the

PACIFIC SCIENCE, Vo!' IV, July, 1.950

decFIG. 3. Some external featur es of the adult

frog. a, Lateral view of the head (approx. X 1.6 ;0, outer nostril ; r, rostrum; s, skin fold ; t, rympa­num ) ; b, buccal caviry of the large frog; c, same ofthe small frog ( b and c, approx, X 2; i, int ernal"nostril ; 1, lower jaw; t, tongue; v, vomerine tooth"plate; u, upper jaw) ; d, palm and fingers (approx.X 2.6; 1, 2, etc., the first, second, etc., fingers ) ; .e, metatarsu s and toes (approx, X 2.6; 1, 2; etc.,the first, second , etc., toes ) .

same time as was observed for the Palau frog.Its most noteworthy character istic is the de­velopment of a large well-vascularized tail ,the function of which is to serve as a respira­tory organ, just as the extreme expansion ofthe abdominal skin of the Palau frog 'doesfor it.

Rana opisthodon deposits its eggs in moist .crevices of rock close to water. Its egg meas­ures 6-10 mm. in diameter. Its embryo de­velops without any evidence of a tail, andhas as breathing organs several regular trans­verse folds on each side of the abdomenwhich function in a "manner quite similar to

Hylodes'tail. Thus the Palau frog is ent irelydifferent from these two species.

At present it seems that the data are notsufficient to determine the genus to which th ePalau frog belongs. Although both hand andfoot well resemble those of Hylodes, theother general features rather resemb le thoseof the Ran idae, and the frog probably is anew species of Rana. The two types of frogs

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found, the small and the large, seem to indi­cate the occurrence of sexual dimorphism inthe species.

SUMMARY

1. The frog described in this article iscommonly found in the Palau Islands. Thereare two types, large and small , apparently aneffect of sexual dimorphism. Neither typepossesses webs between toes or fingers. Thisfrog is probably a new species of Rana.

2. The number of eggs in one hatch isabout 30 and they are spawned on the groundin weedy' or bushy places. Metamorphosisis completed in the egg, and the frogs do notexperience the free-swimming stage of a tao­pole . About 3 weeks are required for com­plete metamorphosis.

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3. During metamorphosis neither exter­nal gills nor gill clefts appear, but the ab­

. domin al skin expands to an extreme degree.

REFERENCES

BOULENGER, G. A. 1886. Zool. Soc. Lon­don, Trans. 12 ( fide Brehms, T ier/eben 4:287, 1920 ). .

LYNN, W. GARDNER. 1940. The develop ­ment of the skull in the non-aquatic l arvaof the tree-toad Eleutherodactylus nubicola.BioI. Bull. 79: 375.

PETERS, W. 1876. Akad. Wiss. Berlin,Monats. (fide Brehms, T ierleben 4: 259,1920; Fr. N ieden, Das Tierreich 46(1):4 17-418, 1923; and H. Gadow in Cam-

. bridge N at. Hist., 214- 215, 19( 1) . .

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