Mixophyes balbus Straughan, 1968

Bertozzi, Terry & Guzinski, Jaro, 2023, A new species of barred frog, Mixophyes (Anura: Myobatrachidae) from south-eastern Australia identified by molecular genetic analyses, Zootaxa 5297 (3), pp. 301-336 : 321-325

Mixophyes balbus Straughan, 1968 .

Northern Stuttering Frog

( Figs 6 , 7 , 11 )

Holotype. AMS R.25922, a male collected from Point Lookout , New England National Park , New South Wales, between 4,250 and 4,750 feet altitude on 15 October 1965.

Dimensions of holotype (in mm). SVL, 70; TL, 43; HL, 27; HW, 27; EN, 6; IND, 6; ED, 8; IOD, 17; TD, 7; FLL, 19; Fin3L, 14; MT, 6. Straughan (1968) recorded the SVL of the holotype as 75 mm. We are unable to explain this discrepancy other than measurement error by Straughan and note that none of the males that we measured had an SVL > 70.6 mm (Supplementary Table S2). Straughan (1968) did not provide any other measurements of the holotype.

Material Examined. Vouchers examined for morphological analyses are listed in Supplementary Table S2.

Diagnosis. Mixophyes balbus is distinguished from M. hihihorlo by a interrupted vertebral stripe (vs. uninterrupted) and presence of dark triangular patch on upper lip in front of nostril (vs. absent); from M. iteratu s and M. carbinensis by absence of black colouration with numerous small rounded pale spots on posterior surface of thigh (vs. light brown without any pale spots); from M. coggeri by having posterior of thigh uniform (vs. with irregular pale blotches); from M. schevilli by having a discontinuous (vs. continuous) mid-dorsal stripe; from M. fleayi and M. fasciolatus by lack of prominent black spots on flanks and further from M. fasciolatus by occurrence of pigmented patches on maxilla; from M. australis sp. nov. by having five wider and more diffusely marked cross bands on thigh (vs. up to ten thin dark cross band in M. australis sp. nov.); male advertisement call comprises one to four notes with mean number of pulses in first and second notes being more than in M. australis sp. nov. (mean 12.24 vs. 7.24) with more double pulses in these notes.

In the field M. balbus is most likely to be confused with M. fasciolatus and M. iteratus which occur sympatrically. In addition to the features listed above, M. balbus can be distinguished from M. iteratus by the webbing on the toes reaching only to the second joint on the third toe, compared to distal joint in M. iteratus ; from M. fasciolatus by the occurrence of a metallic silver-blue crescent in the top quarter of the iris (vs. absent in M. fasciolatus ), and the absence of sharply demarcated cross bands on the limbs that form distinct triangular markings on the posterior surfaces of the arms and legs (vs. presence in M. fasciolatus ).

Description including variation. Straughan’s (1966, 1968) description of variation in M. balbus included material from the southern group which we show is a new species. Here we provide detailed images of the type ( Fig. 11 ) and a revised description of variation in M. balbus that excludes material from the range of the new species.

A moderate sized frog, sexually dimorphic, females larger (up to 83 mm SVL) than males (up to 71 mm SVL). Females are an average of 1.2 times the SVL of males ( Table 5), but they are otherwise similar in body proportions ( Table 5), colour in preservative and patterning.

Tympanum large, conspicuous (TD/ED mean = 0.79±0.07, 0.62–0.96), oval shaped with longer axis vertical. Eyes prominent, pupil vertical, iris in life silver-blue or brown above, brown below. Head flattened, length slightly shorter than or equal to width (HL/HW mean = 0.93±0.05, 0.70–1.03); snout prominent (ED/EN mean = 1.4±0.1, 1.2–1.7), gently rounded when viewed from above and in profile; nostrils more lateral than superior; canthus rostralis well defined, slightly concave; loreal region nearly flat, inclined laterally; eye to naris distance slightly smaller than or approximately equal to internarial span (EN/IND mean = 0.88±0.1, 0.71–1.13). Vomerine teeth relatively long, oblique rows angled from anterior margin of choanae to midline between choanae. Tongue approximately rectangular, not notched posteriorly. Dorsum smooth or finely granular, belly smooth.

Hind limbs long and robust (TL/SVL mean = 0.6±0.02, 0.55–0.65).

Fingers moderately long, robust, unwebbed, relative length 3>2=4=1. Subarticular and palmar tubercles prominent. Finely granular brown nuptial pad on first and second finger and extends ventrally to the inner palmar tubercle in breeding males.

Toes moderately webbed; on outer side of toes webbing reaches penultimate subarticular tubercle on toes 1 and 2, penultimate tubercle on toe 3, and second subarticular tubercle on toe 4. Tips of toes slightly expanded but lacking obvious discs ( Fig. 11 ). Relative lengths of toes 4>5>3>2>1. Prominent round or oval conical sub-articular tubercles present; outer metatarsal tubercle absent; inner metatarsal tubercle prominent relatively straight, with well-developed outer edge, approximately equal to length of toe 1, not sharp or keratinized, pigmentation same as plantar surface.

Colour and Pattern in Life. Dorsal surfaces rich copper to tan or dark brown, sometimes with burnt orange or pink wash towards groin; a posteriorly oriented v-shaped inter-orbital bar that extends as a mid-dorsal stripe, finely edged in black and highly variable in length and shape with uneven edges terminating in mid-sacral region; remainder of dorsal surface either sparsely flecked with small black spots or a few larger blotches but not both ( Fig. 7 ). A dark triangular patch on either side of snout with apex at external nostril does not incorporate area immediately surrounding nostril. A dark lateral head-stripe extends from slightly posterior of nostril, through eye, curves over tympanum, and extends posteriorly down anterior margin of tympanum, sometimes including upper third of tympanum, terminating above axilla; width variable but in all cases posterior margin is widest. Loreal region light-brown coloured with darker patches along upper jaw (maxilla).

Lateral surfaces and groin flushed with reddish tan or peach, without spots or blotches, these surfaces noticeably lighter than dorsum, with colour diffusing gradually into ventral colouration. Venter including throat and ventral surface of thighs immaculate white, cream or with light lemon-yellow wash; lower jaw edged with diffuse brown wash.

In adults, iris dark brown merging into golden brown above pupil with brown to iridescent pale blue dorsal crescent. Upper third of iris in juveniles may be as above or flame orange.

Upper surfaces of legs, feet, arms and hands with dark transverse cross bars, not extending on to posterior thighs; posterior surface of thighs uniform brown to burnt orange brown and slightly mottled; anterior lateral edge of shank, posterior lateral edge of arms, and plantar surface of feet and of tarsal surface dark brown; lateral palmar surfaces of outer fingers dark brown to black.

Reproductive call. The male advertisement call comprises one to four short notes repeated in rapid succession. Because of the variation in the number of notes in the call we provide means and ranges for measurements of call traits for the first two notes. Mean duration for a two note call is 0.45 s (range 0.45–0.48 s). The first note is of longer duration than the second (means 0.68 and 0.41 s, ranges 0.39–0.96 s, and 0.28–0.63 s respectively), with a short inter-note interval (mean 0.36 s), and with more pulses in the first than the second note (mean 12.8, and 9.1, ranges 9–17 and 3–12 respectively). Pulse duration is much shorter (mean 0.015 s, range 0.014–0.016 s) than inter-pulse interval (0.083 s, range 0.081–0.084 s). Pulse structure varies within notes, with single pulses produced initially, followed by a series of pulses produced in couplets ( Table 2, Fig. 9 ). The second note has more couplets than the first note ( Fig. 9 ). Pulse repetition rate in the first note is lower than the second note (16.1 pps vs 21.0 pps) ( Table 2). Mean inter-call interval is 4 s (range 3–8 s) and males usually produce a series of calls before a period in which all calling ceases. Calling occurs during the spring and summer seasons when climatic conditions are suitable (MM pers. obs. Details of suitable conditions not reported here). Within the calling season, calling is initiated by a male, and this stimulates calling in nearby males and a chorus of calls with a short lag time or small overlap between the call of one male and a nearby male occurs. The male encounter call which is a single note of moderate duration (mean 0.23 s, range 0.24–0.48 s) is often uttered among advertisement calls and males may produce several encounter calls in succession ( Fig. 9C ). The dominant frequency of the notes in the advertisement call differs slightly with the first note having a higher frequency than the second (817 Hz and 758 Hz, ranges 709–782, 659–961 Hz respectively). The dominant frequency of the encounter call (788 Hz, range 709 and 792Hz) is higher than the advertisement call and shows an initial narrow band frequency that expands to a broadband pattern. Advertisement and encounter calls are not frequency modulated. Representative call recordings have been deposited with the Australian Museum as multimedia record AMS R.188750.

Distribution. Mixophyes balbus occurs in drainages that flow to the east of the Great Dividing Range from the upper reaches of the Timbarra River catchment in north-eastern NSW, south to the northern catchments of the Macleay River ( Fig. 1 ). The majority of records occur between 600–1500 m asl, and the species occurs entirely within the upper North Coast Bioregion ( Thackway & Cresswell 1995).

Habitat and Ecology. Statistical modelling using presence and absence data and 24 environmental predictors sampled throughout the range of M. balbus and for northern populations of M. australis sp. nov., showed a preference for the interiors of large forest tracts in areas with relatively cool mean annual temperatures, at sites that were typically free from any disturbance with a thick canopy and relatively simple understorey ( NSW NEFBS 1994). Tracking studies indicate that the adults spend the majority of their life in the riparian zone ( Lemckert & Morse 1999), and adults are observed rarely in terrestrial survey transects away from the riparian zone ( Knowles et al. 2015) but are sometimes observed on roads near riparian zones.

Eggs and larvae. Embryonic development and larval morphology were described by Anstis (2013) from specimens collected at five locations that include two within the range of M. balbus (near Point Lookout, New England Range, Coombadjha Creek, Gibraltar Range) and three from within the range of M. australis sp. nov. (Tirrill Creek, Bulga State Forest, Sharpes Creek, Barrington Range, Gap Creek, Central Coast Range). No mention was made of morphological variation among the sites sampled that would indicate a difference between those from what is now known as M. balbus and M. australis sp. nov. ( Anstis 2013), but reanalysis of these data based on our refined taxonomy is required. Larval morphology was described for specimens from Point Lookout NSW (type location) by Watson & Martin (1973) and are the same as described by Anstis (2013). Knowles et al. (2015) did not observe any differences in egg deposition mode at four locations for M. australis sp. nov. and one for M. balbus .

Assessment of IUCN threat category for Mixophyes balbus . Following our systematic findings and taxonomic actions herein, it is necessary to assess the IUCN threat categories for M. balbus and M. australis sp. nov. because previous conservation assessments were based on the presumption of a single species (Supplementary Text).

Based on the IUCN Red List criteria ( IUCN 2019) and with information presented in other recent assessments ( Gillespie et al. 2020, Geyle et al. 2021), we assess M. balbus to be Endangered, under category B1(a)(c) (Supplementary Text). The measured EOO (alpha-hull) is 1,477 km 2, which is well below the threshold score of <5000 km 2, and the distribution is fragmented (occurring in <5 locations). The Area of Occupancy (AOO) in the decade 2010–2020 is 880 km 2, which is well below the threshold level of 2000 km 2 for Endangered but above Critically Endangered (500 km 2) (Supplementary Text). While observed occupancy (EOO and AOO) place this species in the Endangered category, mapping of observation records for the past two decades shows no evidence of a continued decline in occupancy that can be inferred or projected, rather the occupancy has apparently stabilised during this time period, and the EOO and AOO have remained constant in the past two decades (Supplementary Text Fig. S4 ). Field work indicates that the species can be found in each of the five locations, and there is no evidence of population reduction at these locations based on mapping of records over three generations (Supplementary Text Fig. S1 ). There is evidence that declines occurred at many sites prior to 2000 ( NSW NEFBS 1994), but after this time there is evidence of a gradual recovery at many sites apart from previously occupied high elevation sites, and the species occurs across its historic latitudinal range ( Mahony 2007, 2013).

A direct threat to M. balbus is the amphibian disease chytridiomycosis ( Scheele et al. 2021). The available evidence is that M. balbus is an example of a species that is susceptible to chytridiomycosis, and that population declines can be precipitous at times when climatic conditions are suitable for disease transmission and progression. The observation of gradual recovery in population abundance and distribution in the past two decades indicates that some process of selective adaptation to combat the disease may be occurring and this provides several options for conservation recovery actions.

The indirect effects of climate change such as drought and wildfires may also impact on the species. The extent of fragmentation and isolation may be increased by landscape scale catastrophic wildfires ( Legge et al. 2022), which may exacerbate local and regional declines. However, an assessment of vulnerabilities to wildfires based on biological traits indicated some level of resilience because of the species’ burrowing behaviour and restriction to moist riparian habitats ( Mahony et al. 2022). To understand and mitigate these threats we recommend that systematic monitoring be undertaken since there is good evidence of rapid declines in this species in the past.

Figures

FIGURE 1. Map showing collection locations sampled for molecular genetic analysis of Mixophyes in eastern Australia. Alphanumeric codes refer to collection location details in Table 1. Small black dots are voucher records from the Atlas of Living Australia, yellow triangles—M. fleayi, blue circles—M. balbus northern group, red squares—M. balbus southern group.

FIGURE 4. Single Nucleotide Polymorphism Principal Component Analysis of Mixophyes fleayi and M. balbus. A) PC1 v PC2, B) PC3 v PC4, C) STRUCTURE barplot of all ingroup samples, D) STRUCTURE barplot of southern group samples only.

FIGURE 6. Cross bar patterns on hindlimbs of Mixophyes. Side by side comparison of A) thigh and B) tibia-fibula in life of left Mixophyes balbus from Coombadjha Creek Washpool NP and right M. australis sp. nov. from Congewai Creek, Watagan NP. Thighs of Mixophyes balbus from C) Little Styx River, New England NP, D) Thungutti Creek, New England NP, E) Coombadjha Creek, Washpool NP, and Mixophyes australis sp. nov. from F) Congewai Creek, Watagan NP, G) Frying Pan Creek, Chichester SF and H) Sharpes Creek, Gloucester Tops NP.

FIGURE 7. Mixophyes balbus in life. A) Rockadooie Ck, Curramore SF, Michael Mahony, B) Coombadjha Ck, Washpool NP, Stephen Mahony, C) Washpool NP, Gibraltar Range, Stephen Mahony, D) Coombadjha Ck, Washpool NP, Stephen Mahony, E) Forestland SF, Stephen Mahony, F) Forestland SF, Stephen Mahony.

FIGURE 9. Reproductive calls of Mixophyes balbus and Mixophyes australis sp. nov. A) Oscillogram and spectrogram of a series of advertisement calls and encounter calls of Mixophyes australis sp. nov. over 40 seconds duration, B) a single call with the time axis expanded (1.5 seconds duration), C) oscillogram and spectrogram of a series of advertisement calls and encounter calls of Mixophyes balbus over 40 seconds duration, D) a single call with the time axis expanded (1.5 seconds duration), E) oscillogram and spectrogram of an encounter call of Mixophyes australis sp. nov., (0.4 second time axis), and F) oscillogram and spectrogram of an encounter call Mixophyes balbus (0.4 second time axis). a = advertisement call, e = encounter call, and kHz is kilohertz.

FIGURE 11. Images of Mixophyes balbus holotype, scale bar 10 mm (AMS R25922).