Litoria myola, Hoskin, 2007

Hoskin, Conrad J., 2007, Description, biology and conservation of a new species of Australian tree frog (Amphibia: Anura: Hylidae: Litoria) and an assessment of the remaining populations of Litoria genimaculata Horst, 1883: systematic and conservation implications of an unusual speciation event, Biological Journal of the Linnean Society 91, pp. 549-563 : 552-558

LITORIA MYOLA SP. NOV. (KURANDA TREE FROG) ( FIG. 2 )

This species is assigned to Litoria on the basis of external morphology and mitochondrial DNA sequence data. Species of the genus Litoria are easily distinguished from the only other hylid genus in Australia with large finger and toe pads, Nyctimystes , by having a horizontal pupil and lacking venation on the lower eyelids ( Cogger, 2000).

Holotype: Deposited in the Queensland Museum, QMJ82420 , male (calling when captured), Jumrum Ck. (16°49.3′S, 145°38.3′E), north-east Queensland, 330 m elevation, C. J. Hoskin and K. R. McDonald, 30 October 2001; measurements of preserved specimen (mm): SVL 36.1; TL 19.8; HW 12.4; HL 11.4; BW 8.9; ED 3.8; EN 3.0; IN 2.6; 3DW 2.0; 3FL 7.7; 4DW 1.8; 4TL 11.8; proportions: TL/SVL 0.55; HW/SVL 0.34; HL/SVL 0.32; HW/HL 1.09; BW/SVL 0.25; ED/SVL 0.11; EN/IN 1.15; 3DW/4DW 1.11; measurements in life: SVL 37.4 mm; TL 20.2 mm; HW 12.1 mm; 3DW 1.9 mm; weight 2.45 g .

Paratypes: QMJ82422 -25, males (calling when captured), collection details as for holotype ; QMJ82426 , male (calling when captured) location details as for holotype, C. J. Hoskin and K. R. McDonald, 9 March 2001 ; QMJ82421 , female (gravid and in amplexus when captured), location details as for holotype, C. J. Hoskin and K. R. McDonald, 6 November 2001 ; QMJ82427 , male (calling when captured), Kowrowa (16°48.2′S, 145°35.4′E), north-east Queensland, 335 m elevation, C. J. Hoskin and K. R. McDonald, 9 March 2001 .

DIAGNOSIS

Litoria myola sp. nov. can be distinguished from all Australian congeners by its distinctive short, fasttapping call ( Fig. 3 ). Litoria myola sp. nov. is restricted to the Kuranda area ( Fig. 1 ), where it co-occurs with L. genimaculata , the species to which it is most similar. Although there is some overlap in the range of the individual call characters between L. myola sp. nov. and L. genimaculata ( Table 1 ), multivariate analyses clearly distinguish the two species, particularly in the Kuranda area ( Fig. 4 ; Hoskin et al., 2005). In this area, the courtship call of Litoria myola sp. nov. is relatively easily diagnosed from that of L. genimaculata by its shorter duration (no overlap), faster note rate (no overlap), and generally higher dominant frequency ( Table 1 ). Litoria myola sp. nov. can be further distinguished from L. genimaculata (from both the Kuranda area and the rest its range) because males of L. myola sp. nov. are smaller in all aspects of body size ( Table 1 ; Fig. 5 ; Hoskin et al., 2005). In the Kuranda area, an SVL cut-off of 42 mm, or a weight cut-off of 3.75 g, sep- arates 90% of the males of these two species (weight is the more reliable measure because it is not reliant on measuring technique). Although L. myola sp. nov. females are generally smaller than those of L. genimaculata ( Table 1 ), there is no significant difference in size between L. myola sp. nov. and L. genimaculata where the two species co-occur in the Kuranda area ( Hoskin et al., 2005). Litoria myola sp. nov. and L. genimaculata cannot be distinguished on the basis of coloration, pattern, body shape, distribution or habitat.

DESCRIPTION

Variation across type series (seven males and one female, all adult): Data are range followed by mean in brackets. Male measurements in spirit (mm): SVL 32.8–38.6 (36.3); TL 18.7–21.3 (20.3); HW 11.9–13.8 (12.7); HL 11.4–12.0 (11.7); BW 8.0–10.0 (9.1); ED 3.8– 4.4 (4.1); EN 3.0–3.5 (3.3); IN 2.6–3.1 (2.9); 3DW 1.9– 2.1 (2.0); 3FL 6.5–7.9 (7.4); 4DW 1.6–1.9 (1.7); 4TL 9.1–11.8 (10.5). Male proportions: TL/SVL 0.53–0.58 (0.56); HW/SVL 0.33–0.36 (0.35); HL/SVL 0.31–0.35 (0.32); HW/HL 1.03–1.15 (1.09); BW/SVL 0.24–0.26 (0.25); ED/SVL 0.10–0.12 (0.11); EN/IN 1.06–1.17 (1.13); 3DW/4DW 1.11–1.19 (1.15); Male measurements in life: SVL (mm) 36.0–41.1 (38.9); TL (mm) 18.6–21.4 (20.5); HW (mm) 12.1–13.9 (13.1); 3DW (mm) 1.9–2.1 (2.0); weight (g) 2.0–3.3 (2.8); Female measurements in spirit (mm): SVL 53.5; TL 30.6; HW 18.4; HL 15.4; BW 15.7; ED 5.5; EN 5.0; IN 4.1; 3DW 2.8; 3FL 9.8; 4DW 2.4; 4TL 12.9; Female proportions: TL/SVL 0.57; HW/SVL 0.34; HL/SVL 0.29; HW/HL 1.19; BW/SVL 0.29; ED/SVL 0.10; EN/IN 1.22; 3DW/ 4DW 1.17; Female measurements in life: SVL 59.6 mm; TL 31.2 mm; HW 18.6 mm; 3DW 3.4 mm; weight 10.0 g (8.5 g after laying 1.5 g clutch).

Field measurements: These are presented in Table 1 . (N = 117 males and 18 females, all adult).

Sexual dimorphism: Body size dimorphism is pronounced, with females being an average of 1.6-times the SVL and 4.5-times the weight of males. In contrast, there appears to be little sexual dimorphism in external morphological character states, pattern or coloration; thus, the sexes are dealt with together below.

Head: Broad and flattened, slightly wider than body; snout bluntly rounded to truncate in dorsal view and distinctly projecting in profile, canthus rostralis slightly angular, loreal region steep, nostrils much closer to tip of snout than to eye, nostrils directed dorso-laterally; eye very large, diameter greater than eye to naris distance, horizontal pupil; internarial distance less than distance from eye to naris; tympanum large and obvious.

Body: Slender; urostyle moderately prominent to indistinct; cloaca positioned immediately below urostyle, orientated posteriorly and dorsally, no ornamentation.

Limbs: Hindlimbs long and slender; two pale, distinct pointed tubercles or several indistinct tubercles on the heel; forearms broad, particularly in males; pale serrated ridge along the trailing edge of the forearm (distinct) and along the trailing edge of the foot (moderately distinct); fingers half webbed (webbing formula ( Savage & Heyer, 1997): I 2–2 1 / 2 II 1 1 / 2 –2 1 / 2 III 2–2 IV); toes nearly fully webbed (webbing formula: I 1 1 / 2 –2 II 1–2 III 1–2 IV 2–1 V); fingers long, relative length 3> 4> 2> 1, finger discs fleshy and rounded, obviously expanded from penultimate phalanx, first finger short with disc obviously expanded; in males a broad, black or grey nuptial pad covers the base of the first finger; indistinct tubercle present in the centre of the base of the palm; relative length of toes 4> 3> 5> 2> 1, rounded toe discs expanded from penultimate phalanx, disc on first toe marginally expanded; small yet obvious oval inner metatarsal tubercle; discs on fingers larger than discs on toes.

Skin: Ventral surface coarsely granular; dorsal surface of body, head and limbs smooth or finely granular with scattered low tubercles in some specimens; distinct postorbital skin fold extending along dorsolateral surface to mid-body.

Pattern and colour. In preservative: Dorsal pattern and colour highly variable ranging from blotched brown (N = 6), speckled brown (N = 1), to evenly grey (N = 1). Blotched individuals are generally darker down the centre of the body with paler areas on the shoulders, lower back and forehead. This results in a roughly hourglass pattern. Several individuals are marked with a distinct pale triangle between the eyes and nares. Pigmentation is often darker in the canthal region. Hindlimbs often marked with several, irregular, broad, dark bars. Evenly cream over the majority of the ventral surface. The chin and throat region has a light brown wash and some degree of fine dark speckling. Speckling is obvious over the entire throat and chin in some individuals (N = 5) and restricted to the chin area in others (N = 3). Fine brown mottling covers the lateral surfaces, underside of lower hindlimbs and feet, posterior thigh, and groin. In some males, the fine mottling gives way to immaculate cream on the latter half of the flanks. Underside of discs cream and underside of hands ranging from cream to mottled brown. Eye has fine venation throughout, a dark horizontal bar passing through the pupil, and a pale upper iris. In life: Dorsal colour and pattern highly variable, ranging from even brown or tan ( Fig. 2A ), through mottled grey, fawn or brown ( Fig. 2B ), to blotched tan, brown and green. Ventral surfaces cream to white with a faint to obvious light brown or grey wash on the throat and chin (particularly in males). Dark speckling on the chin, and on the entire throat in some individuals (particularly in males). Nuptial pads on males usually black but occasionally grey. Pupil edged by a horizontal dark brown strip and iris cream or grey with a distinctly green upper crescent, and covered in fine brown venation.

COMPARISON

Litoria myola sp. nov. is only likely to be confused with other Australian members of the ‘ L. eucnemis ’ species group ( L. genimaculata and L. eucnemis Lönnberg, 1900 ). Members of this group can be distinguished from other Australian frogs by having a serrated ridge along the trailing edge of the feet and forearms ( Barker, Grigg & Tyler, 1995). Litoria myola sp. nov., L. genimaculata , and L. eucnemis also have green coloration in the upper iris (in life) and males lack a vocal sac ( Tyler, 1971), which is present in most other Australian Litoria . Amongst these three species, L. eucnemis is restricted to northern Cape York Peninsula and has a distinctive call of a series of short growls (vs. a series of soft ‘tocs’) ( Richards et al., 1993). Litoria myola sp. nov. is most likely to be confused with L. genimaculata , with which it co-occurs in the Kuranda area. From this species, L. myola sp. nov. differs in having a distinctive call of faster note (‘toc’) rate, shorter duration and higher dominant frequency, and also in being smaller in all aspects of male body size (see Diagnosis). These differences are evident between L. myola sp. nov. and Australian L. genimaculata in general, and are particularly pronounced between L. myola sp. nov. and the northern lineage of L. genimaculata with which it co-occurs (see Diagnosis). Although L. myola sp. nov. was not compared with New Guinean populations of L. genimaculata in the present study, the New Guinean populations appear to be similar in morphology and call to the Australian populations of L. genimaculata ( Richards et al., 1993; Cunningham, 2001). Therefore, it would appear that the characters outlined above would distinguish L. myola sp. nov. from both Australian and New Guinean populations of L. genimaculata . Additionally, the taxonomy of L. genimaculata is currently under revision to reflect the distant (and deeply paraphyletic) genetic relationship between the Australian and New Guinean populations of L. genimaculata across the ‘ L. eucnemis ’ species group ( Moritz et al., 1997; Cunningham, 2001).

ETYMOLOGY

The specific epithet is in recognition of Myola, a locality where this species occurs. This name is believed to be of aboriginal origin, although the language and dialect are not recorded (http://www.nrm.qld.gov.au/ property/placenames). The common name ‘Kuranda tree frog’ refers to the township around which the distribution is centred.

GENETICS

Litoria myola sp. nov. co-occurs with the northern lineage of L. genimaculata ( Fig. 1 ), from which it is genetically highly distinct at both mitochondrial (13% COI mtDNA sequence divergence) and nuclear loci ( Hoskin et al., 2005). Litoria myola sp. nov. is genetically similar (0.1% net COI mtDNA sequence divergence) to, and nested within, the southern lineage of L. genimaculata ( Hoskin et al., 2005) .

CALL

The courtship call of L. myola sp. nov. is an excited, short call consisting of rapidly uttered notes ( Fig. 3 ). Each note consists of two pulses heard as a single ‘toc’. The call has the following characteristics (mean followed by range in brackets, N = 19): duration 0.85 s (0.57–1.35), number of notes 7 (5–11), note rate 8.8 notes s −1 (6.7–11.4), and dominant frequency 1.79 kHz (1.53–2.07). There is no frequency modulation in the call (i.e. all notes of each call are of similar dominant frequency). The call of L. myola sp. nov. is significantly different to that of L. genimaculata , being of faster note rate, shorter duration, and generally higher dominant frequency ( Table 1 , Fig. 4 ; Hoskin et al., 2005). Litoria myola sp. nov. also occasionally utters a longer call of slower note rate, which is used during aggressive encounters between males. This call appears similar to the aggressive call of L. genimaculata , although too few recordings were made to assess this in detail. Like other members of the ‘ L. eucnemis ’ species group ( Tyler, 1971), L. myola sp. nov. lacks a vocal sac and the call is of relatively low volume.

REPRODUCTIVE ISOLATION FROM L. GENIMACULATA

Hoskin et al. (2005) conducted experimental crosses between the northern and southern lineages of L. genimaculata . These crosses included males and females of L. myola sp. nov., which was termed the ‘iS’ population of the southern lineage. The trials suggest that successful breeding in L. genimaculata is only possible between northern lineage females and southern lineage (including L. myola sp. nov.) males. Reciprocal crosses, southern lineage (including L. myola sp. nov.) females with northern lineage males, resulted in fertilized clutches that hatched out but died at an early larval stage. Litoria myola sp. nov. only co-occurs with the northern lineage of L. genimaculata ; there- fore, the potential for hybridization between L. myola sp. nov. and L. genimaculata appears to be limited to L. myola sp. nov. males mating with L. genimaculata females.

Hybridization appears to occur very rarely, due to call divergence and associated mate choice. Mate choice trials have been conducted in which gravid L. genimaculata (then including L. myola sp. nov. as the ‘iS’ population of the southern lineage) females were given a choice between alternative male calls in a laboratory mate choice chamber ( Hoskin et al., 2005). These trials revealed significant premating isolation between L. myola sp. nov. and the co-occurring northern lineage of L. genimaculata . This is supported by: (1) a preliminary genetic analysis of the contact zone that suggested hybridization between L. myola sp. nov. and L. genimaculata is very low, with an estimated 0–1.4% of individuals being hybrids at mixed sites ( Hoskin et al., 2005), and (2) field observations of pairs in amplexus at mixed sites in the Kuranda area, with no mixed pairings in the 10 pairs observed (six L. myola sp. nov. pairs and four L. genimaculata pairs). The laboratory-based call trials also revealed highly significant premating isolation between L. myola sp. nov. and the genetically similar southern lineage of L. genimaculata .

DISTRIBUTION

Litoria myola sp. nov. has a very small distribution, being known from short sections of 13 streams draining into the Barron River in the Kuranda area (between the localities of Kuranda, Fairyland, Myola, Mantaka, Kowrowa, and Oak Forest ) in north-eastern Queensland ( Fig. 6 ). The distribution is bound to the north, east and south by the northern lineage of L. genimaculata (which also occurs at most L. myola sp. nov. sites), and to the west by the limit of rainforest distribution ( Figs 1 , 6 ; see also ‘iS’ population in Hoskin et al., 2005). All sites are between 320 and 360 m a.s.l.

HABITAT AND HABITS

The habitat and habits of L. myola sp. nov. appear to be similar to those of L. genimaculata . All records of L. myola sp. nov. are from rainforest along permanent and ephemeral streams ( Fig. 7 ). Rainforest along the streams ranges from mesophyll vine forest to rainforest regrowth dominated by Acacia and Calamus . Litoria myola sp. nov. is a stream breeder. Stream substrate at the sites ranges from rock and gravel ( Fig. 7A ) through to coarse sand ( Fig. 7B ), and stream gradient at all sites is low. Males were only encountered along the streams, at high density at some sites (discussed below). Females were rarely encountered, with most found on the streams, primarily as gravid individuals (17 out of 21), and always where L. myola sp. nov. males were calling. Occasionally, females were sighted perched high in trees, and several females were encountered a considerable distance from the streams. Metamorphs were rarely encountered (on streamside vegetation) and juveniles were not observed. Therefore, as is in L. genimaculata ( Richards & Alford, 2005; K. R. McDonald, unpubl. data; C. J. Hoskin, unpubl. data), L. myola sp. nov. utilizes streams as breeding habitat, primarily in spring and summer, and nonbreeding adults and subadults utilize the surrounding rainforest. Given the apparent rarity of nonbreeding individuals on the streams, and how infrequently the species is encountered in the surrounding rainforest, it would appear that L. myola sp. nov. utilizes the mid and upper forest levels when not breeding.

Breeding was observed during the summer wet season between October and March. Breeding activity (male calling intensity and female presence on the streams) is greatest in the nights prior to and following heavy rain. Males call throughout the night from elevated perches, primarily around riffle zones and small waterfalls. Perch height is usually between 30 cm and 1.5 m, but sometimes up to 5 m above the ground. Courtship calling is occasionally interrupted by periods of aggressive calling between neighbouring males. Aggressive calling intensifies as the males approach each other and on several occasions aggressive calling bouts were observed to result in wrestling. Such wrestling has been previously reported in L. genimaculata ( Richards & James, 1992; Richards & Alford, 2005). At mixed sites wrestling was observed between L. myola sp. nov. males, between L. genimaculata males, and on one occasion between a male L. myola sp. nov. and a male L. genimaculata . Males display dark, fleshy nuptial pads on the thumbs during the breeding season and amplexus is axillary. Sexual dimorphism is pronounced, with males being on average 63% of the SVL and 22% of the weight of females. This is equivalent to the sexual size dimorphism seen in L. genimaculata ( Table 1 ; McDonald et al., 1999; Richards & Alford, 2005). The only clutch data for L. myola sp. nov. is a clutch of 509 pigmented eggs (15% of the females body weight) that was laid as a cohesive gelatinous clump. The diameter of each egg was approximately 2 mm. This is similar to clutch data for L. genimaculata (described under the name L. eucnemis in Davies, 1989; C. J. Hoskin & K. R. McDonald, unpublished data).

Sixteen species of frog were recorded in rainforest in the Kuranda area during fieldwork between 2001 and 2007 but only six of these species were observed sharing breeding habitat (rainforest streams) with L. myola sp. nov. These species were: L. genimaculata , Litoria jungguy Donnellan & Mahony, 2004 / Litoria wilcoxii Günther, 1864 (these two species could not be diagnosed in the field), Rana daemeli Steindachner, 1868 , Mixophyes coggeri Mahony, Donnellan, Richards & McDonald, 2006, Litoria rheocola Liem, 1974 and Nyctimystes dayi Günther, 1897 . The first four species were those most often recorded calling alongside L. myola sp. nov., while L. rheocola and N. dayi were very rarely recorded in the Kuranda area.

Figures

Figure 1. Distribution of Litoria myola sp. nov. (formerly termed iS) and the northern (N) and southern (S) lineages of Litoria genimaculata in the Wet Tropics, northeast Queensland. CT, Carbine Tableland; BMC, Black Mountain Corridor; LR, Lamb Range; BK, Bellenden Ker Range; AT, Atherton Tableland; MT, Malbon Thompson Range; GR, Graham Range.

Figure 2. Litoria myola, sp. nov., males (A, a pale individual; B, a heavily marked individual), Kuranda, north-east Queensland.

Figure 3. Spectrogram of the courtship call of Litoria myola sp. nov. (recorded at an air temperature of 25 °C). The spectrogram displays a single courtship call consisting of six notes (‘tocs’). The degree of shading displays call intensity.

Figure 4. A representation of the difference in call between Litoria genimaculata and Litoria myola sp. nov. The box plots compare calls of L. genimaculata from across the Wet Tropics, L. genimaculata from the Kuranda area, and L. myola sp. nov. PC1 accounts for 67.2% of the variation in call (duration, dominant frequency and note rate) across L. genimaculata and L. myola sp. nov. PC1 is loaded heavily by inverse call duration (0.95) and note rate (0.93), and moderately by dominant frequency (0.51). The box plots show the median, 25th and 75th quartiles, and minimum and maximum data of PC1.

Figure 5. A representation of the difference in morphology between male Litoria genimaculata and Litoria myola sp. nov. The box plots compare morphology of L. genimaculata from across the Wet Tropics, L. genimaculata from the Kuranda area, and L. myola sp. nov. PC1 accounts for 87.8% of the variation in morphology (SVL, TL, HW, and weight) across L. genimaculata and L. myola sp. nov. PC1 is loaded equally and positively by all four characters (approximately 0.94 for each) and therefore represents body size. The box plots show the median, 25th and 75th quartiles, and minimum and maximum data of PC1.

Figure 6. The distribution of Litoria myola sp. nov. and Litoria genimaculata in the Kuranda area. The pie charts show the proportion of L. myola sp. nov. (black) and L. genimaculata (grey) individuals on five streams. Nearby sites on the streams are grouped together to provide a consistent scale. Sample size for the pie charts averages 20 individuals. The squares show additional sites where L. myola sp. nov. (black squares) and L. genimaculata (grey squares) have been recorded but relative proportions of each have not been determined. All L. genimaculata are northern lineage individuals. Major stream catchments flowing into the Barron River are marked. The sections of stream without records of either species are either unsuitable habitat or have not yet been surveyed. Sites surrounding this area are occupied by northern lineage L. genimaculata or are unsuitable habitat (Fig. 1).

Figure 7. Rainforest stream habitat, with rocky (A) and sandy (B) substrate, Kuranda, north-east Queensland. Litoria myola sp. nov. and Litoria genimaculata are present at both sites.