Discrepancies in Observations of Hymenochirus boettgeri, Rabb vs. Sokol

Copyright © 1998

Works compared in this article:

On the behavior and breeding biology of the African pipid frog Hymenochirus boettgeri
Copyright © 1963 Zeitschrift für Tierpsychologie (Ethology, Blackwell Wissenschaft, Berlin)

The Tadpole of Hymenochirus boettgeri
Copyright © 1962 Copeia No. 2


Though Sokol and Rabb agree on the existence of two species under the genus Hymenochirus (boettgeri and curtipes) they differ widely on the physical characteristics and the environmental requirements that define them.  Sokol speculates that the difference in the tadpoles of H. boettgeri and H. curtipes is based on ecology.  He refers to Noble (1924) who reports that H. curtipes comes from the waters of open regions and H. boettgeri comes from the waters of forested areas.  My opinion is that a hybrid of H. boettgeri  and H. curtipes has developed over the decades probably due to the intermixing of the species in the aquarium trade and that this hybrid is now common in the United States.  Neither author has observed the species in the wild.  Both relied on US distributors for their specimens and took the word of the supplier concerning the origin of the animals (purportedly the Congo region of Africa).

To be fair to both authors it should be stated that their papers were devoted to other aspects than the definitive definition of the species.  Both papers where in depth and represented a good deal of time and effort.  My fault finding is based on my frustration in finding information on the specific needs of the species for the purpose of raising the tadpoles.  I present the following comparison in the hope that others will find it helpful in identifying the species and serve as a helpful starting point for further study.

The Species

Regarding the existing descriptions of  the Hymenochirus species (boettgeri and curtipes) Rabb stated, "The species mentioned appear to be valid, although some of the descriptions in the literature are confusing and variation has not been adequately studied."1  Sokol is less kind when he states, "Since the appearance of my preliminary description of the peculiar tadpoles of two Hymenochirus species (Sokol, 1959), two other short papers dealing with the breeding behavior, eggs, and tadpoles of H. boettgeri have appeared (Olsson and Österdahl, 1960; Yuhas, 1961).  The former mentioned the peculiar movements performed by the spawning pair, provided very brief description of the eggs and larvae, and presented two rather uninformative figures of the last.  The other paper, which misidentifies the species as H. boulengeri, has good photographs of adults in amplexus, eggs, and rather starved larvae in various stages; the text is very brief.  My own paper (Sokol, 1959) dealt almost entirely with H. curtipes larva [sic], and only briefly mentioned that of H. boettgeri which is quite different in general appearance and behavior."2  Before the confusion increases lets assume that Sokol is using the word "larva" as defined in Merriam Webster's 1997 edition, 2 : the early form of an animal (as a frog or sea urchin) that at birth or hatching is fundamentally unlike its parent and must metamorphose before assuming the adult characters.

Sokol flatly denies the validity of H. boulengeri and Rabb agrees that based on his comparisons the species identified as H. boulengeri is actually H. curtipes.  Rabb goes on to speculate that perhaps H. boulengeri is a northern race of H. curtipes.  I wonder if it was an early hybrid.

Eggs and Development

The authors differ in their opinions on the factors that determine the egg's viability and make inaccurate statements about the buoyancy of the eggs.  Rabb states, "The eggs that are suspended at the water [sic] surface appear to be held there by surface tension alone, with only a slight deformation of the outer jelly layer at the top.  In one laying we scooped away 25 eggs left at the surface within a minute after they were laid.  Only one of these surface eggs developed, in contrast to about 20 per cent for the 100 eggs that had sunk to the bottom.  Both sets were kept in the same container.  Such differential fertility of surface and bottom eggs has not prevailed in other layings of which we have accurate records."3  Sokol says, "The eggs spread out under the water's surface very rapidly in a manner suggesting that they travel along with a concurrently discharged oily substance. They are actually attached to the surface tension layer and will sink if they are detached."  Where Rabb seems to hedge his bets, Sokol is willing to commit himself to the means by which the eggs stay at the surface.  Sokol goes on to say, "Olsson's and Österdahl's (1960) and Yuhas' (1961) statements that the eggs float are incorrect.  Presumably this method of depositing the eggs assures an adequate oxygen supply.  Eggs that fall to the bottom of the aquarium almost always fail to develop.  Even those that stick to objects below the surface are less viable unless the aquarium is well aerated."4

We should examine the previous statements more closely as they illustrate well the pattern of assumptions and inaccuracies that dominate both papers.  On the question of whether or not the eggs are buoyant I can confidently say that in my experience a percentage of every batch that I've handled is buoyant. What Sokol omits in his paper is how he came to the conclusion that three other researchers were wrong.  Did he observe 100 % of the eggs from every laying of his specimens sink?  If he didn't test for buoyancy (or lack the of it) in every egg from every laying then he was irresponsible in accusing another researcher of being in error.  My assertions as to buoyancy are based on  my experiences while collecting eggs over a period of months.  I used a device consisting of a Monoject 412 syringe with 10 inches of clear tubing attached to the end.  This device served to gently suction up the eggs floating on the surface and from where they had become attached to objects in the aquarium.  By pressing the plunger on the syringe I could deposit the eggs into a nursery area out of reach of the adult frogs.  The advantage of this method of egg collecting is that the jelly layer does not sustain damage by contact with an abrasive surface.  I routinely observed a percentage of eggs from a given laying demonstrating positive buoyancy after being deposited in the nursery area.  The pressure of the water/egg mixture being expelled from the tubing was always sufficient to overcome the surface tension.  The descent of the buoyant eggs would halt, then reverse direction where they would come to rest at the surface.  In other words, some of the eggs would float.  Rabb mentions scooping eggs but doesn't explain how.  Did he use a net?  Did he collect the eggs in a spoon or similar implement?  What  precautions did he take to prevent damage to the egg's jelly layer?

Rabb confuses the issue of fertility with viability.  If an egg fails to develop is it necessarily infertile?  I have seen eggs fail to develop after laying and I have seen them stop in their development after varying lengths of time (24 to 72 hours).  Neither Rabb nor Sokol give us enough information to know when the development of the eggs halted or if development ever began.  Rabb and Sokol had vastly different results in egg development within the tank.  Sokol postulates that the amount of oxygen in the water is a determining factor.  Although this is a reasonable supposition, he fails to elaborate on his method (if any) of oxygenating the water.  Rabb's description of his tanks leads me to believe that he did not use any form of aeration5.  In my experience there is little difference in viability based on where the eggs develop within my aerated breeding tank.

Sokol includes an illustration that compares the larvae of H. boettgeri and H. curtipes.  My observations are at variance with his in that I have never seen the larvae hatch at the stage he shows in example "A".   My larvae appear like those in example "A" within 24 hours after laying.  With 10X magnification they can be seen to be enclosed in the jelly layer.  My larvae always hatch after their eye spot has developed, typically within 36 to 48 hours at 80° F.

According to Sokol, "In a group of well-cared-for tadpoles, the first ones show hind limb buds after about 12 days of free swimming life (about 10 mm total length).  About one week later the arms appear and in another 8 days metamorphosis is complete."6  In Rabb's experience, "Limb buds appear in the third week, and transformation  takes place as early as the sixth week at temperatures ranging from 20° C. to 24° C." 7 My experience most closely matches Rabb's. Perhaps neither of us cared well for our tadpoles.


Sokol states, " Hymenochirus boettgeri tadpoles are fairly active midwater hunters. … [they] will swallow anything that can be overpowered including Daphnia, Tubifex, Corethra, and Culex larvae and much smaller members of their own species."6  A bit later he states, "In addition to the physical differences, the active hunting behavior of the Hymenochirus boettgeri tadpoles differs from that of H. curtipes larvae, which tend to wait passively at the water's surface for prey to come their way."7  In typical contrast Rabb states, "Young tadpoles cruise slowly near the surface in search of prey."8 My own observations confirm Rabb's in the feeding behavior of the tadpoles from the point when they begin feeding on infusoria.  Sokol's description applies at the point that the tadpoles begin feeding on crustaceans through to the beginning of metamorphosis as illustrated here.  Once their limbs appear (as illustrated here) their feeding behavior is much like that of the adults—feeding from various places in the tank with a lot of attention given to thoroughly searching the gravel.


I agree with Rabb that the species has not been adequately defined.  The task is further complicated by the passage of time and a shrinking geography.   I received email recently from a breeder in Indonesia who is supplying Hymenochirus to the U.S. market.  It's possible that specimens equivalent to those observed by Tornier and Noble are no longer available from the Congo due to environmental destruction.  It proves to me that we can't afford to take even the most humble of aquatic frogs for granted.  We can't assume that what is common today will even exist tomorrow.

Researchers who decide to publish their work owe it to their readers to be accurate in their facts and fair to others researchers. There is no room in science for arrogance, especially when it leads to incorrect and misleading assumptions. Even the humble African dwarf frog deserves to be accurately and honestly documented if for no other reason than to improve their quality of life in captivity.

1 Ibid. page 216
2 Ibid. page 273
Ibid. page 229
4 Ibid. page 273
5 Ibid. page 216
6 Ibid. page 274
Ibid. page 275
Ibid. page 229
NOBLE, G. K.  1924.  Contributions to the herpetology of the Belgian Congo based on the collection of the American Museum Congo expedition, 1909-1915.  Bulletin of the American Museum of Natural History 49:147-347.
OLLSON, R. AND L. ÖSTERDAHL.  1960.  Aquarium behavior and breeding of Hymenochirus.  Nature 188:869.
SOKOL, O. M.  1959.  Studien an pipiden Fröshen.  I. Die Kaulquappe von Hymenochirus curtipes Noble.  Zool. Anz. 162:154-160.
TORNIER,G.  1897.  Die Kriechthiere Deutsch-Ost-Afrikas.  Beiträge zur Systematik und Descendenzlehre.  Berlin, 164 pp.
YUHAS, P. J. 1961.  Breeding of the African Water Frog.  Tropical Fish Hobbyist 9:10-14.