Zearaja maugeana, Peter R. Last & Daniel C. Gledhill, 2007

Peter R. Last & Daniel C. Gledhill, 2007, The Maugean Skate, Zearaja maugeana sp. nov. (Rajiformes: Rajidae) - a micro-endemic, Gondwanan relict from Tasmanian estuaries., Zootaxa 1494, pp. 45-65 : 52-63

Zearaja maugeana sp. nov.

Figs 4-12; Table 3

Raja sp. L Last and Stevens, 1994: 339, fig 34.27, pl. 55.

Holotype

CSIRO H 3975-01 , 659 mm TL, mature male, Macquarie Harbour , Australia, 42°21' S, 145°32' E, 25 June 1995, D. Haslam.

Paratypes

CSIRO H 1987-01 , 706 mm TL, mature male, Bathurst Harbour , Australia, 43°21' S, 146°08' E, 12 February1989, G. Edgar ; AMS I 40748-001 , 682 mm TL, mature male, Bathurst Harbour , Australia, 43°21' S, 146°08′ E, 27 May 1989, G. Edgar ; CSIRO H 5544-01 , 725 mm TL, female, Bathurst Harbour , Australia, 43°22′ S, 146°09′ E, 29 March 1992, J. Stevens & P. Mooney ; CSIRO H 3747-01 , 578 mm TL, female, Macquarie Harbour , Australia, 42°23′ S, 145°27′ E, 30 July 1994, M. Lewis ; CSIRO H 3976-01 , 571 mm TL, female , CSIRO H 3976-02 , 770 mm TL, female, Macquarie Harbour , Australia, 42°24′ S, 145°30′ E, 17 June 1995, M. Lewis & D. Haslam ; CSIRO H 4486-01 , 685 mm TL, adolescent male, Macquarie Harbour , Australia, 42°12′ S, 145°18′ E, 3 September 1995, B. Zampatti ; CSIRO H 4487-01 , 744 mm TL, female, Macquarie Harbour , Australia, 42°16′ S, 145°15′ E, 3 September 1995, B. Zampatti .

Diagnosis

A relatively small species of Zearaja with the following combination of characters: granular denticles absent or poorly developed on dorsal surface of pectoral disc, disc width 67-71% TL, 1.09-1.12 times its length; pre upper jaw length 20.6-24.7% TL; ventral head length 36.1-38.8% TL; adult clasper relatively short, 23.9-26.8% TL; tail evenly tapering, not robust, height at midlength 1.3-1.4% TL; snout length 3.75- 4.38 times interorbital width; pre upper jaw length 2.20-2.58 times internasal width; orbit diameter 0.58-0.69 of interorbital width; total pectoral radials 78-85; predorsal vertebrae 85-93; teeth in upper jaw 38-43.

DESCRIPTION.- Disc quadrangular (Figures 4, 5), 1.12 times as broad as long in holotype (1.09-1.12 times in paratypes); maximum angle in front of spiracles 71° (68-72°); axis of greatest width 62% (60-69%) of disc length; anterior margin weakly double concave (slightly less pronounced in females), strongly concave anteriorly, straight to weakly convex beside eyes, slightly concave behind level of spiracles in holotype; apex narrowly rounded; free rear tip broadly rounded. Head elongate, preorbital snout length 5.80 (5.71-6.69) times orbit length, 3.75 (3.87-4.38) times interorbit; preoral snout length 2.20 (2.30-2.58) times internarial distance. Snout tip produced, narrowly pointed; no fleshy process at apex. Orbit well developed, diameter 0.65 (0.58- 0.69) times interorbital distance; eye lateral; deep subocular furrow extending from origin of orbit to spiracle (not evident in some paratypes). Spiracle large, length 1.70 (1.61-2.01) in orbit diameter; opening broadly suboval. Nostril subcircular (Figure 6), often distorted or oval in some paratypes; anterior nasal flap expanded slightly; lateral margin of nostril with a well-defined lobe forming a semi-circular tube, anterior margin also lobe-like, concealed beneath nasal curtain; inner margin bordered by nasal curtain; posterior lobes forming nasal curtain, well developed, produced, narrowly rounded to angular distally, with well developed fringe along posterior margin; internarial distance 1.73 (1.65-1.76) in distance between first gill slits, 0.97 (0.96- 1.14) in distance between fifth gill slits. Upper jaw arched slightly on either side of symphysis in holotype, not indented in females; lower jaw more uniformly convex in both sexes; lateral teeth concealed by lobe of nasal curtain. Teeth unicuspid with subcircular bases; arranged in distinct longitudinal rows rather than quincunx; in mature males, with long, upright medial cusps at symphysis, cusps becoming shorter, more oblique laterally (variable, but more robust and generally more upright in female paratypes); teeth at angle with very short cusps.

Pelvic fins strongly forked; anterior lobe short, slender, narrowly rounded distally, lateral margin entire, inner margin moderately incised; posterior lobe very elongate, relatively longer in males (length 20.3% TL) than in females (length 13.9-16.0% TL), lateral margins moderately convex, inner margin almost straight; anterior lobe 0.65 in mature male holotype (0.69 in mature male paratype, CSIRO H 4486-01; 0.73-0.83 in female paratypes) times posterior lobe. Tail depressed, moderately broad at base, barely tapering to first dorsal-fin origin (female paratype, CSIRO H 3747-01, with badly shrivelled tail); tapering strongly to tip behind first dorsal fin; width at insertions of pelvic fins 1.30 (1.32-1.53) times width at midlength of tail and 1.57 (1.34-1.72) times width at first dorsal-fin origin respectively; length from rear of cloaca 0.69 (0.60-0.72) times distance from tip of snout to rear of cloaca; anteriorly cross-section weakly convex dorsally and ventrally, becoming more strongly convex on dorsal surface than ventral surface posteriorly, almost flat ventrally toward tail apex; width 1.60 (1.26-1.69) times height at insertion of pelvic fin, 2.25 (1.89-2.27) times height at midlength, 1.99 (1.93-2.10) times height at first dorsal fin origin; lateral skin fold well developed, originating about an orbit diameter behind pelvic-fin insertion, extending almost to tail tip, not becoming appreciably broader distally. Dorsal fins large (Figure 7), of similar shape and size, first dorsal fin slightly more upright than second; first dorsal-fin height 1.47 (1.32-1.70) in base length; fins strongly raked, low, elongate, with long bases; anterior margins weakly convex, apices broadly rounded, posterior margins convex, free rear tip narrowly rounded to acute; inner margin of first dorsal longer than second; interdorsal distance short, 3.58 (3.3-8.2) in length of first dorsal-fin base. Epichordal caudal-fin lobe well developed, height about equal to half tail width at fin origin; usually taller posteriorly than anteriorly; usually truncate distally, dorsal margin straight or weakly convex; connected sub-basally to second dorsal fin; hypochordal caudal lobe vestigial.

Dorsal surface with well-developed nuchal, orbital, tail, malar and alar thorns in adult males; no thorns along most of mid-disc or on scapular region; denticles concentrated on tail, around orbits, on snout tip, and along anterior margin and middle of disc; pectoral fin mostly naked in males, scattered minute denticles present in largest female (CSIRO H 3976-02). Orbital thorns forming a rosette, variable in size, often barely larger than surrounding denticles; 7-8 in holotype (2-3 on preorbit, 2 on midorbit and 2-3 on postorbit), 5-13 in paratypes; low, slender, directed posteriorly, recurved. Nuchal thorns 2 (0-3 in paratypes, mainly 2), widely spaced; somewhat laterally compressed, otherwise subequal in size and similar in shape to largest orbital thorns. Malar thorns 4-7 (about 8-15 in adult male paratypes), all lateral to orbit; forming a single, weakly curved series (two thorns paired on left side of holotype), in 2-4 irregular series in paratypes; mostly subequal in size and similar in shape to each other and largest orbital thorn; directed posterolaterally. Alar thorns 18-20 (including missing thorns), 16-28 in paratypes, well developed, longer than other thorns; embedded, tips pungent, directed postero-medially in 2-3 irregular rows. Tail thorns well developed, in three regular rows in males (Figure 8), 5 regular rows in largest females. Median row best developed of these, commencing on dorsal disc just forward of cloaca and extending along length of tail in a single series; with 27 (up to 45 in paratypes) pungent, recurved or upright thorns with large bases, slightly larger than largest orbital thorn; 10 much smaller interstitial thorns interspersed with these larger thorns, interstitial thorns present on both sexes; interdorsal thorn 1 (1-3 in paratypes). Lateral rows in males short, with 8-9 (6-13) thorns (interstitial thorns absent); located closer to lateral skin fold than to thorns of median thorn row; commencing from above pelvic fins and extending along tail to between tail midlength and dorsal fins; thorns similar in size and shape to largest median thorns; in females, additional lateral row bordering lateral skin fold, upper lateral row shortest, originating and terminating more anteriorly than lower lateral row, lower lateral row terminating under dorsal fins; largest females with a few isolated mid dorsal thorns.

Denticles generally not well developed; patches on head confined to snout tip, preorbit, interorbit and interspiracular regions (very weakly developed in smallest paratype); narrow band of minute, widely spaced denticles extending along dorsal midline from nuchal region to tail tip. Anterior mid-dorsal band triangular in shape anteriorly, extending from anterior nuchal thorn across scapular region, its maximum width equivalent to interspiracular width; band tapering gradually posteriorly, its width subequal to tail width at pelvic-fin insertions, sharply defined from naked lateral disc adjacent; band covering most of dorsal tail to its tip. Denticle band on margin of disc well developed, originating at about level of front of eye and extending along disc margin almost to level of anterior alar thorns in males (less well developed in females, absent in smallest specimen CSIRO H 3976-01); band broadest near posterior malar thorns; denticles much larger and denser than those of mid-disc. Denticles of mid-disc and unpaired fins very small, granular, widely spaced on mid-disc; densest near dorsal-fin outer margins, at snout tip, adjacent orbit, and along disc margin; those on snout tip and around orbit slightly enlarged; a few small denticles along margin of rostral cartilage; mid-disc preceding first nuchal thorn, suborbit, orbital membrane, pelvic fins, claspers and most of pectoral fins naked (largest females with scant covering of denticles on suborbit, and pelvic and pectoral fins). Ventral surface mostly naked; denticle patches confined to head forward of mouth and along margin of disc; posterior extension and width of marginal band similar to that of dorsal marginal band; a few isolated denticles present on tail.

Rostral cartilage of neurocranium (Figure 9, based on radiograph) elongate, slender; rostral appendix indistinct from radiographs; nasal capsule elliptical in shape and set at about 20º angle to transverse axis of neurocranium; anterior fontanelle relatively long and broad, with distinct anterior margin preceded by gentle groove in rostral shaft; posterior fontanelle moderately long, not constricted posteriorly; jugal arch robust. Scapulocoracoid (Figure 10, based on mature male paratype CSIRO H 1987-01) with moderately expanded lateral face, particularly at procondyle; anterior bridge absent; mesocondyle horizontally expanded; distance between pro- and meso- much less than distance between meso- and metacondyles; anterior fenestra oval, expanded vertically; postdorsal fenestra subcircular; postventral fenestra oval, expanded horizontally, much smaller than postdorsal fenestra. Propterygium of pectoral girdle with anterior tip well short of rostral tip. Pelvic girdle with slightly arched puboischiadic bar with moderately long lateral prepelvic processes (Figure 11); 2-3 orbturatorial foramina, iliac process well developed.

Clasper long, rather robust, not depressed or tapering distally, without dermal denticles or pseudosiphon; strongly oblique to main axis of cartilage, directed postero-mesially (most pronounced with clasper closed); distal part of clasper beyond glans broad, flattened, spatulate (Figure 12); glans greatly expanded along proximo-lateral margin, dorsal lobe bulbous adjacent end of hypopyle, ventral lobe even more significantly expanded immediately beyond dorsal expansion (visible when viewed dorsally). Shield relatively small, short, its length only slightly more than twice its width; capable of extended ventro-lateral rotation; lateral margin of cartilage exposed, sharp edged, blade-like, strongly convex; ventral surface covered in thick skin fold; fold attached to secondary, less regularly shaped ridge; secondary ridge situated subparallel and ventral to lateral margin of shield; strong dorsal ridge (component dike) situated on proximal portion, forming deep concavity with lateral margin (also forming right angle), concavity filled with thick pleated epithelia. Rhipidion weak, very slender, not wavy, located along inner margin of shield, well removed from lateral margin of glans, terminating as a short point near midpoint of shield. Spike slender, digitiform, obscure, fully concealed by thick skin; its apex forming triangular skin fold in disto-medial part of glans; fold extending laterally to envelope dike, forming deep, blind, posteriorly directed sac (component sentina). Sentinel absent. Hypopyle with an enlarged, truncate extension of the dorsal marginal cartilage (component pseudorhipidion); deep, open-ended cleft located on dorsal lobe distal to beside dorsal margin of pseudorhipidion. Clasper skeleton simple (Figure 13). Terminal bridge weak, knob-like; second, enlarged cleft located beside distal tip of terminal bridge. Dorsal terminal 1 cartilage enveloping proximal region of glans, not riding freely on dorsal surface; connected proximally at inner margin of dorsal terminal 2 cartilage; abutting axial cartilage and distal extension of ventral terminal cartilage on inner lateral margin of glans. Dorsal marginal cartilage expanded distally, extending distally beyond its junction with dorsal terminal 2 cartilage as a flag-shaped tongue; disto-lateral extension digitiform, overlapping ventro-lateral margin of dorsal terminal 2 cartilage; dorsal terminal 2 cartilage short, very robust, arched laterally, producing proximo-lateral bulge on glans; dorsal terminal 3 cartilage elongate, longer than ventral terminal cartilage and dorsal terminal 2 cartilage, broadest near terminal bridge. Accessory terminal 2 cartilage digitiform, curving evenly dorso-posteriorly, arising from distal end of ventral marginal cartilage on its inner medial margin, its tip proximal to ventral terminal cartilage tip. Ventral marginal cartilage expanded distally, ventro-distal part subtriangular, without accessory terminal cartilage (normal position of accessory terminal 1 cartilage). Axial cartilage tip disc shaped, united distally with dorsal terminal 3 cartilage to form spatulate clasper tip. Ventral terminal cartilage crudely s-shaped in ventral profile; proximal portion dove-tailed into ventral marginal cartilage, articulating at lateral margin of ventral marginal cartilage mesially, articulating by a long, narrow extension of secondary ridge ventrally; connected to axial cartilage distally by narrow, hooked arm, its apex extending slightly beyond distal tip of lateral margin.

Teeth in upper jaw 43 (38-42); lower jaw 41 (39-42). Pectoral-fin propterygial radials 31 (27-31); mesopterygial radials 15 (14-17), anterior portion 12 (9-13), posterior portion 3 (3-6); metapterygial radials 37 (35-39); total radials 83 (78-85). Monospondylous centra 32 (31-34); predorsal caudal centra 55 (52-61); predorsal centra 87 (85-93); total centra about 148 (144-153).

Colour (In Preservative)

Dorsal surface of disc and tail almost uniformly dark greyish brown (some paratypes more uniformly darker); darkest at pre-orbit and over supraorbit; slightly darker medially on back and along dorsal tail, outer margin of disc paler than central part; lateral snout whitish (less pronounced in some paratypes), rostral cartilage strongly demarcated from white area adjacent; anterior and posterior parts of orbital membrane pale; inner portion of spiracle whitish; pelvic fin and claspers similar to dorsal disc; dorsal and caudal fins greyish to black; thorns distinct, paler than soft tissue adjacent. Ventral surface with variable markings, with irregular whitish and brownish areas, mostly paler than dorsal surface; most of disc, pelvic fins and tail pale to medium brown, darkest on abdomen, tail, tips of anterior lobe of pelvic fin and in perianal region (sometimes with extensive white areas that extend laterally over pelvic fin bases); prominent black-edged pores over snout, and anterior and central portions of disc, densest on internasal flap and along lower lip, absent from most of belly, posterior portion of disc and pelvic fins; snout mostly whitish, grading evenly into darker posterior portion of disc (sharply demarcated in some paratypes); granular anterior margin of disc pale, usually sharply demarcated from rest of disc; median snout dusky to almost black; mouth and teeth white; internasal-flap fringe, anterior margin of gill slits, cloaca and origin of pelvic fins whitish.

When fresh: Similar to preserved colour but somewhat darker, almost black dorsally; light and dark areas forming a blotched pattern ventrally, markings variable, strongly contrasted.

Size

Largest female type was 770 mm TL but known to reach 840 mm TL (Treloar pers. comm.); one male paratype (CSIRO H 4486-01) adolescent at 685 mm TL, but male holotype fully mature at 659 mm TL; mature males ranged between 659-706 mm TL; no information on juveniles or egg cases.

Distribution

Endemic to Bathurst and Macquarie Harbours, in southwestern Tasmania. Occurs primarily in the upper, oligo-mesohaline regions of these estuaries. The total available habitat is less than 300 km2 and initial surveys suggest that the population is likely to be less than 1000 individuals (Treloar pers. comm.).

Etymology

The epithet ‘maugeana’ was derived to draw a link with the Australian cool temperate biogeographic region, the Maugean Province, in which this skate is a keystone species. Its common name, the Maugean Skate, follows this theme.

Ecology and Life History

Unique among extant skates in that it occurs primarily in brackish water. The two estuarine systems in which it lives are high in tannin content with poor light penetration and have silty bottoms. Several deepwater invertebrate taxa normally found in deep habitats of the continental slope encroach into relatively shallow depths. The snout morphology of this skate, which resembles that of Dipturus species found in silty habitats on the continental slope, may be a feeding adaptation. Little is known of other biological or ecological requirements of this recently discovered species. The estuaries are well separated and given that this skate has never been taken in the sea, may form genetically distinct populations. Specimens have been caught in a broad range of brackish salinities to almost fresh water.

Small populations and the geographically restricted distribution of this species have significant conservation implications. The isolation of Bathurst Harbour, which is situated in the heart of large terrestrial and marine parks in southwestern Tasmania, affords partial protection for this species. However, the other remnant population lies in an estuary heavily polluted by prolonged mining operations and anthropogenic interference. Both populations are in otherwise scenic and important recreational areas facing increasing pressure from ecotourism. Zearaja maugeana is also caught occasionally by recreational gill netting in Macquarie Harbour.

Remarks

The three Zearaja species can be distinguished based on size, morphometrics and squamation. Zearaja maugeana is physically smaller than both Z. nasuta and Z. chilensis and has a smaller size at maturity. The three adult male types of Z. maugeana were mature at 659-706 mm TL and females attain 840 mm TL. Age and growth in Z. chilensis has been comprehensively investigated and reviewed (Licandeo et al. 2006; Licandeo et al. in press). Zearaja chilensis exhibits geographic variation in size, but the onset of maturity varies from 750-850 mm TL in males (Licandeo et al. in press), with the largest recorded female 1680 mm TL (Lloris & Rucabado 1991). Zearaja nasuta is reported to occasionally reach 1000 mm TL (Ayling & Cox 1982), and 1084 mm TL based on museum specimens (NMNZ P 4783, not viewed).

Zearaja maugeana differs from Z. nasuta in the following morphometrics. Apart from attaining a smaller maximum size, Z. maugeana has a longer pre upper jaw length (20.6-24.7 vs. 15.1-21.7% TL), prenasal length (18.1-21.2 vs. 13.6-19.7% TL) and ventral head length (36.1-38.8 vs. 31.1-36.9% TL), shorter adult clasper (23.9-26.8 vs. 25.9-29.5% TL), and a less robust tail (width at axil of pelvic fin 3.6-4.4 vs. 4.4-5.8% TL; at midlength width 2.7-3.2 vs. 2.9-3.9% TL, height 1.3-1.4 vs. 1.5-1.8% TL). Its disc is proportionally narrower (width 1.09-1.12 times length) than either Z. nasuta (1.12-1.22 times) or Z. chilensis (1.19-1.29 times), and its snout to interorbit ratio (3.75-4.38 vs. 2.89-3.85 in Z. nasuta and 2.92-3.27 in Z. chilensis ) and pre upper jaw to internasal ratio (2.20-2.58 vs. 1.66-2.26 in Z. nasuta and 1.98-2.20 in Z. chilensis ) reflect its relatively narrow head and long snout. In addition, Z. maugeana differs markedly from Z. chilensis in disc width (66.5-70.5 vs. 73.9-76.9% TL), has a more evenly tapered tail (width at axil of pelvic fin 1.30-1.53 vs. 1.57-1.59 times width at midlength, 1.34-1.72 vs. 1.72-1.82 times width at first dorsal-fin origin), and a higher orbit to interorbit ratio (0.58-0.69 vs. 0.48-0.55). Males available to us of Z. chilensis were immature but based on Leible (1987), Z. chilensis attains an even larger size than its congeners and appears to have relatively large claspers as an adult. Leible examined male specimens exceeding 1050 mm TL and their claspers varied from 27-32% TL.

Zearaja nasuta , known as the rough skate (Ayling & Cox 1982), has a well-developed covering of fine denticles over its dorsal surface. In comparison, the dorsal surface of the pectoral fins of Zearaja maugeana is largely naked. There are also minor differences between the Zearaja species in the clasper configuration but their mode of operation is presumably identical. Differences relate primarily to the strength of various cartilages and to the position of the rhipidion. In Z. maugeana and Z. nasuta , the rhipidion is slender, rather than wavy, and is dispersed centrally along the dorsal margin of the ventral terminal cartilage, well away from lateral margin of the glans. The wavy rhipidion of Z. chilensis lies submarginal to the sharp edge of the shield.

Zoogeography

The genus Zearaja is a small group of skates whose ancestral form probably occurred on the continental shelves of Gondwana in the late Mesozoic (Last & Yearsley 2002). Continental drift and subsequent isolation of mainland Australia, New Zealand and South America within the last 80 my, has resulted in speciation leading to the existence of endemic extant skates in each region. The absence of the group in Antarctica could be due to Oligocene cooling, coinciding with a major loss in biodiversity of the Paleogene ichthyofauna (Long 1994). Zearaja are primarily inshore skates, unlike Bathyraja and Dipturus which are more diverse on deep continental slopes, and thus less affected by paleoclimatic upheavals. The extant distribution of Zearaja is congruent with a vicariance-based hypothesis rather than a cross-ocean dispersal based origin. Skates are very poor cross ocean dispersers (Springer 1982) and appear to move within narrow bathymetric pathways along continental shelves and slopes. Zearaja maugeana was presumably more widely distributed in southern Australian seas but it is unclear whether its present relic distribution is a product of past climate change, a reduction in habitat availability, interspecific competition, a change in habitat preference, or a combination of these factors. Intermittent oceanic warming and cooling in the Pleistocene is thought to be responsible for the derivation of much Australia’s recent temperate marine biota. It is also thought to have created extinctions. The benthic habitats of Port Davey are unique within Australia, with the substrate more typical of the silts and terrigenous oozes found on the continental slope. Deepwater species of Dipturus and Zearaja usually have a long snout and this condition could be an adaptation to feeding on the infauna of very soft substrates.

Knowledge of evolutionary history Zearaja may be important in understanding the origin of skates. If the group is ancestral to other rajins, then the accessory terminal 2 cartilage probably preceded the accessory terminal1 cartilage as a skeletal element in skates. Conversely if the group is advanced, it is likely that the accessory terminal 1 cartilage has been lost. Hulley (1972) noted that only one accessory terminal component is present in Rhinobatos , a presumed ancestral group of skates. This state is also present in Zearaja but species of Dipturus have two accessory terminal cartilages. The most likely scenario, based on a Gondwanan lineage of Zearaja , is that the accessory terminal 2 is plesiomorphic and the presence of an accessory terminal 1 cartilage is more derived. Dipturus is probably the more advanced of the two genera. A molecular study (Holmes pers. comm.), based on sequences of the CO1 gene, supports the hypothesis that Zearaja is ancestral to Dipturus but confirmation from other morphological and genetic analysis is needed.