Arthroleptis, Smith, 1849

Schweiger, Susan, Naumann, Benjamin, Larson, Joanna G., Möckel, Lars & Müller, Hendrik, 2017, Direct development in African squeaker frogs (Anura: Arthroleptidae: Arthroleptis) reveals a mosaic of derived and plesiomorphic characters, Organisms Diversity & Evolution (New York, N. Y.) 17 (3), pp. 693-707 : 697-700

Reproduction in Arthroleptis

The males of many species of Arthroleptis are characterized by moderately to extremely elongated third fingers ( Blackburn 2009). The function of this sexually dimorphic trait is unclear but is thought to perhaps be used in male to male combat or during amplexus (see Blackburn 2009 for details). We observed axillary amplexus in A. wahlbergii ( Fig. 2 ) and the palms of the hands of the male were facing outward. We did not observe any special motion or posturing of the third finger and it does not seem to play a particular role in amplexus. However, given the large size difference between male and female, the elongated lower arm and third finger might overall improve the hold on the female.

The first observations on reproduction in Arthroleptis were made by Barbour and Loveridge (1928), who described three clutches of A. stenodactylus eggs (33, 40 and 54 eggs) collected from shallow burrows with some adults, plus an additional clutch of 80 eggs that they suspected to be a mix of at least two clutches. They pointed out the disproportionately large ova in gravid A. xenodactylus but did not explicitly infer a direct mode of development. The first explicit

R Fig. 4 External development in Arthroleptis wahlbergii . All embryos have been removed from the egg capsules. a TS 1. Formation of blastopore. b TS 3, dorsal view. Neural folds formed. c TS 4, dorsal and frontal view. Early limb bud stage, neural tube formed, gill arches apparent. d TS 5. Limb buds attached to trunk. e TS 6, left: lateral view (arrow indicates ECD), middle: frontal view, right: dorsal view. Opercular fold (op) begins to grow over developing forelimb. f TS 8, left: lateral view, right: close-up of head region, with edge of opercular fold and outline of limb bud indicated by dotted lines. Opercular fold covers two thirds of the forelimb bud. g TS 9, left: lateral view, right: ventral view. Opercular fold completely covers forelimbs. h TS 10, both forelimbs erupted (arrow), tail fin begins to decrease, pigmentation denser. i TS 11/12, left: lateral view, right: ventral view. Toes are much longer, tail begins to decrease in size. j TS 13, fingers longer, tubercles visible. k TS 14, left: lateral view, right: dorsal view. Distinct eyelids visible, toes full length, banding patterns on legs evident. l TS 15, hatchling. Scale bars = 1 mm

statement of direct development in Arthroleptis seems to have been made by Loveridge (1953), who reported that A. stenodactylus lay their eggs in a burrow and that metamorphosis is completed inside the egg. Guibé and Lamotte (1958) reported direct development for the West African A. crusculum and a clutch size of 15 eggs. The eggs were deposited in a spherical chamber of about 1–2 cm in diameter, a few centimetre below the surface of the soil. The wall of the chamber was reported to be rather regular and seemingly lined with mucus ( Guibé and Lamotte 1958). Channing (2001) reported a clutch of A. xenodactyloides consisting of 20 eggs that he found 20 cm beneath leaf litter accumulated next to a wet rock face on Mt. Zomba, Malawi. Channing and Howell (2006) reported clutches of A. xenochirus to consist of about nine eggs that were laid in shallow nests in leaf litter and provided a photograph of a clutch. They also published a photograph of A. adolfifriederici eggs at approximately TS 6, and Altig and McDiarmid (2007) published a photograph of a clump of eggs of A. schubotzi found in leaf litter, confirming terrestrial reproduction for two additional species.

Lamotte and Perret (1963) provided a brief description of reproduction and development of the West African A. poecilonotus from material collected in Cameroon, where clutches were found in the soil in gardens planted with peanut and cassava. Clutch size averaged 20 to 25 eggs and the large eggs were surrounded by a thick gelatinous capsule that consisted of two to three concentric, spherical layers plus an outer and an inner membrane ( Lamotte and Perret 1963). They illustrated five different developmental stages and provided some information on embryonic development, including that the tail was always bent to the left and the forelimbs were hidden under a membrane (presumably the opercular fold) for some unspecified time during development. Interestingly, Lamotte and Perret (1963) reported hatching to occur after 15 to 20 days at a seemingly less advanced stage compared to A. wahlbergii and A. xenodactyloides . Upon hatching, A. poecilonotus still possessed a tail remnant of about half the snout-vent length and eyelids were not yet differentiated. Lamotte and Perret (1963) reported raising their clutches inside a crystallizer at constant temperature (27 °C) and humidity. A constant 27 °C appears to be higher than normal soil temperatures and might explain the comparatively short developmental time reported. Whether hatching generally occurs at an earlier stage in A. poecilonotus or might be an artefact of higher-than-average incubation temperatures remains unclear at present. Tapley (2009) reported hatching to occur after about 1 month in captive-breeding A. stenodactylus . Some data on reproduction and development in A. wahlbergii were provided by Wager (1965), who described discovering eggs in a large pile of leaves, about 3 cm or more below the surface. He reported five clutches that consisted of 11, 18, 20, 26 and 30 eggs, respectively, with the eggs in the individual clutches being somewhat scattered over an area of about 8 cm in diameter. Wager (1965) very briefly summarized embryonic development and also illustrated six stages, but his drawings are very schematic and depict proportions incorrectly. However, he reported that the forelimbs are covered by skin and that hatching occurs at the end of the fourth week of development when the young are fully formed and about 6 mm long, which match our own observations presented here.

Although data on aspects of egg laying or development are only available for nine species of Arthroleptis (out of 47), these species do show a similar pattern of reproduction in that eggs are laid terrestrially in a moist, secluded place and develop directly into fully developed young. Egg deposition varies slightly in that eggs are either laid among leaf litter or buried in the soil beneath leaf litter. This is likely a function of the ability to burrow—better in species with a well-developed metatarsal tubercle like A. stenodactylus and A. wahlbergii and less so in small species with weakly developed metatarsal tubercles like A. xenodactyloides —and the local conditions (wetness, shade, thickness of leaf litter, etc.). The spherical breeding chamber described by Guibé and Lamotte (1958) for A. crusculum seems unusual and resembles the nesting chamber of Anhydrophryne rattrayi as described by Wager (1965). The number of eggs per clutch and size at hatching vary among species and seem to correlate with adult size, with smaller species having fewer eggs and smaller hatchlings and larger species having more eggs and larger hatchlings. The average hatching size of 3.7 mm in A. xenodactyloides is among the smallest hatchling sizes reported for any direct-developing anuran, except Sooglossus ( Callery et al. 2001) . Hatching at a somewhat earlier stage in A. poecilonotus , as reported by Lamotte and Perret (1963), might possibly be a result of suboptimal rearing and would require confirmation. In captive A. stenodactylus, Tapley (2009) observed occasional male egg-guarding behaviour but did not provide further details. In all other investigated species, no adults were found attending the clutches and it seems that dedicated egg-guarding does not occur in Arthroleptis . Interestingly, the eggs we raised seemed resilient and we lost none to fungal infections. Although little information is available for anurans, in direct-developing salamanders, eggs are more likely to succumb to fungal infections if the attending parent is removed ( Wells 2007). Arthroleptis eggs might possess some antimycotic properties.

Both species of Arthroleptis investigated here showed a remarkably similar embryonic development. The few details reported in earlier studies ( Lamotte and Perret 1963; Wager 1965) are also in agreement with the observations reported here (with the exception of A. poecilonotus discussed earlier).

of toes and digits. h TS 8, dorsal aspect, nubs of three digits and four toes visible. i TS 10, dorsal aspect of hand, ventral aspect of foot. Tips of toes and digits enlarged. At this stage, forelimbs are fully erupted and no longer covered by the opercular fold. j TS 12. Lateral aspect of hand, ventral aspect of foot. Digits enlarged, tubercles visible. k TS 15, ventrolateral aspect. Limbs fully developed. Hatching occurs at any time. Scale bars: a–g 0.5 mm; h–k 0.25 mm

Arthroleptis wahlbergii and A. xenodactyloides are not closely related ( Blackburn 2008), and the near identical embryonic development observed in both species and available data on other species suggest that similar patterns of development might likely be characteristic for Arthroleptis in general.

Figures

Fig. 2 Pair of Arthroleptis wahlbergii in amplexus. The male on top is slightly displaced to the side in a, showing the outward facing palm and elongated third finger. b Pair in full amplexus; note the disproportionately long forearm of the male