Zearaja Whitley 1939

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 : 48-52

Zearaja Whitley 1939 : 254

Type species: Raja nasuta Mueller and Henle 1841 . Type by original designation.

Diagnosis

Rajin genus defined by the following combination of characters: clasper greatly enlarged in mature males, its length 24-32% TL; apex of clasper broadly spatulate, its width 6-8 in clasper length; shield relatively broad and short (its length 3.7-4.7 in length of glans), capable of extended ventro-lateral rotation, situated proximally (its length shorter than post-shield length of glans); glans oblique to clasper shaft when closed, extremely everted when open; dorsal lobe of clasper greatly thickened at dorsal terminal 2; ventral terminal cartilage with deep proximal groove enabling loose articulation with ventral marginal cartilage; rhipidion reduced, not stout and fleshy; sentinel and funnel absent; spike concealed by integument, its tip producing a triangular skin fold; distal inner portion of dorsal marginal cartilage forming a twisted, flag-shaped pseudorhipidion.

Definition

Long-snouted skates with a quadrangular disc; tail short, 37-46% TL, not expanded at its midlength; dorsal fins close together near tail apex; orbit large, 3-7 in snout length; exhibiting varying levels of sexual dimorphism in some shape characters, in particular of the snout length, head width and length, and elements of the pelvic fin; no terminal process at tip of snout; dorsal surface largely naked or with weak covering of fine denticles; orbital thorns present; thorns present along dorsal midline of tail, additional row along each lateral in males, additional 2 rows along lateral margin in large females; alar thorns depressible into grooves; malar thorns lateral to eyes; rostral shaft firm, robust, stout proximally, uncalcified apically; anterior fontanelle of neurocranium without distinct anterior margin; rostral appendices not evident from radiographs; scapulocoracoid moderately expanded, lacking anterior bridge, postdorsal fenestra greatly expanded, one postventral fenestra; propterygium of pectoral girdle extending into about a third of pre-nasobasal space; clasper glans of relatively simple construction (Figure 2); no external pseudosiphon; components cleft, pseudorhipidion, and sentina present; clasper skeleton with 3 dorsal terminal cartilages (Figure 3); one accessory terminal cartilage, and ventral terminal of group-typical shape; shield with pleated lamellae in its inner surface, with a strong mesial ridge forming a dike distally; median ridge of ventral terminal clasper cartilage weakly developed, not extended to form a funnel; ventral surface of disc light or dark, pores dark-edged; pectoral radiais 78-86; trunk centra 29-34; predorsal caudal centra 52-61.

Composition

The genus Zearaja presently consists of at least three valid species: Z. nasuta from throughout New Zealand (Ayling & Cox 1982; Paul 2000), including the Challenger, Bounty and Campbell Plateaus, and the Chatham Fuse (Francis et al. 2001); Z. chilensis in the SE Pacific from 30°15'-55°13’S, and the SW Atlantic from 34°35'-55°S (Licandeo et al. impress); and the undescribed species treated herein from cold temperate, southern Australia (formerly Raja sp. L sensu Last and Stevens 1994). Raja stabuliforis Garman , from the western North Atlantic, was erroneously placed in synonymy with Zearaja chilensis (Guichenot 1848) by Lloris and Rucabado (1991). Based on clasper morphology, it is more likely to belong to Dipturus than Zearaja and D. laevis (Mitchill) is a likely senior synonym.

Nevertheless, other Zearaja species may exist. Zearaja nasuta varies greatly in colour, from uniformly brownish or grey, to having strong patterns of spots and blotches. This species also appears to be unusually eurybathic for an Australasian skate, occurring from coastal habitats to at least 658 m depth on the continental slope (NMNZ data). Forms from the New Zealand continental shelf and slope were compared morphometrically. Females and males from each region differed from each other in several characters but there were some interregional differences in tail morphology. The tail of the slope form appears to be more robust (tail width at mid-length 3.4-3.9 vs. 2.9-3.4% TL; tail width to height ratio at midlength 2.06-2.40 vs. 1.76-2.05; and tail width at first dorsal-fin origin 2.7-3.2 vs. 2.2-2.9% TL), and the disc is relatively broader (disc width to length ratio 1.17-1.22 vs. 1.15-1.17). Populations of Z. nasuta from New Zealand need to be more carefully examined and this work should be coupled with the use of a molecular study. Similarly, the South American skate, Z. chilensis , is thought to occur in the SE Pacific, as well as the SW Atlantic where it was formerly known as D. flavirostris (Philippi) . However, there are regional differences in Z. chilensis populations, even in Chile. Leible (1987) examined females with abnormal spination, and some male specimens that were late maturing (890-940 mm TL rather than 750-850 mm TL), so these populations also need further investigation.

Remarks

Zearaja was nominated by Whitley (1939) for Z. nasuta based on its produced snout, elevated pectoral girdle, rough disc, thorn distribution, and dark-edged ventral pores. These characters also apply to members of several other rajin genera so it is unsurprising that members of the genus Zearaja have been consistently placed in synonymy with Dipturus (for example Stehmann 1990; Compagno 2005), the members of which they superficially resemble. Last and Yearsley (2002) resurrected Zearaja to subgeneric status within Dipturus but suggested that the group is more likely to be generically distinct. The external morphology, neurocranium, pelvic girdle and scapulocoracoid of these taxa are of a similar type but major differences exist in clasper morphology (Table 1). Dipturus species, including the type species D. batis (Linnaeus) based on data from Stehmann (1970), typically have both a spike (formed from the distal tip of the accessory terminal 2 cartilage) and a sentinel (formed from the distal tip of the accessory terminal 1 cartilage). In Zearaja , the clasper skeleton is comparatively simple and one accessory terminal cartilage is missing. Leible’s (1987) figure of the clasper of Z. chilensis (as Raja (Dipturus) flavirostris ) shows an accessory terminal 1 cartilage present with the accessory terminal 2 cartilage missing. However, in our material of the three Zearaja species, the accessory terminal cartilage arises on the inner lateral margin of the ventral marginal cartilage, in the position equivalent to the accessory terminal 2 cartilage in both Dipturus gudgeri and D. sp A . The accessory terminal 1 cartilage arises in the median ventral axis in all Dipturus species examined, as well as D. batis (Stehmann 1970). No equivalent structure exists at this location in Zearaja . Hence, based on the position of this cartilage, we consider this element to be more akin to the accessory terminal 2 cartilage than the accessory terminal 1 cartilage, and its terminal element should be called the spike rather than the sentinel (sensu Leible 1987).

Zearaja claspers also are capable of much greater rotation of the shield than Dipturus . In Zearaja , this structure is relatively more robust and shorter (clasper length more than 3.5 vs. less than 2.5 times shield length in Dipturus ), and is situated well away from the clasper apex (length of clasper distal to shield exceeding length of shield vs. than less than half length of shield). The long post-shield section of the glans in Zearaja , which terminates in a broadly spatulate distal lobe, is unlike any Dipturus species. The claspers of other rajin genera are more complex and all have an accessory terminal 1 cartilage and sentinel.

The internal morphology of the clasper glans in Zearaja changes dramatically when the clasper is fully everted. The shield and posterior glans are capable of a rotation in excess of 90° to the axis of the clasper shaft. In this condition, the cleft between the dorsal marginal cartilage and dorsal terminal cartilages has a greatly increased volume. The spike, which is covered in integument, becomes visible against a thickened portion of the axial cartilage. The truncate distal ventral marginal cartilage extends more posteriorly into the central glans. Union of the dorsal terminal 2 and dorsal terminal 3 cartilages at the disto-lateral margin of clasper are broken with the proximal limb of the dorsal terminal 3 cartilage projecting beneath external integument as a spatulate limb. Similarly, the proximal half of the dorsal terminal 1 cartilage forms a cover that projects prominently through the external integument.

Sexual dimorphism in shape is evident in Z. nasuta in material from both the continental shelf and slope (Table 2). The disc is relatively wider (70.4-77.2 vs. 66.5-70.3% TL) and longer (61.6-63.4 vs. 56.8-60.3% TL), and the preorbital (20.1-22.0 vs. 14.7-17.0% TL), prespiracular (25.3-27.2 vs. 20.2-22.5% TL), and head (27.6-29.2 vs. 22.3-25.0% TL) lengths longer, in adult females than in mature males. Similar trends exist for ventral head, intergill, pre upper jaw, and prenasal lengths. The pelvic-fin base is relatively broader in females (9.9-10.4 vs. 8.3-9.0% TL) but the posterior pelvic-fin lobe is shorter (16.1-18.3 vs. 20.5-23.2% TL). There also appears to be a minor difference in the first dorsal-fin height being slightly shorter in females (2.7-3.4 vs. 3.4-3.9% TL).