A new species of Lynceus Müller, 1776 from New Caledonia (Crustacea: Branchiopoda: Laevicaudata) from dolines, with remarks on zoogeography

. A new species of laevicaudatan branchiopod, Lynceus insularis sp. nov., is described. It is reported from five sinkholes (dolines) in the southern part of New Caledonia. Lynceus insularis sp. nov. is closest to Lynceus species from Australia, but can be separated from these on the basis of clasper morphology and the form of the lamina abdominalis. Lynceus insularis sp. nov. is the first record of a remote insular endemic laevicaudatan. The habitats of the species may be threatened due to hydrological changes, reduction of water supply, acidification of fresh of dorsal extension digitiform, of anterior extension. lamina abdominalis and exopods, to distal setae of modified exopods of thoracopods

We present a new species of Lynceus that represents the first record from New Caledonia and the first of a remote insular endemic laevicaudatan. New Caledonia (NC) and New Zealand (NZ) were part of "Zealandia", a Gondwana-derived microcontinent that sank (except NC and NZ) after having separated from the northeastern margin of Australia ca 80 Ma. It has been discussed whether New Caledonia's biodiversity is that of an ancient continental island, which has retained ancient groups since its separation from the northeastern margin of Australia, or whether it is an oceanic island with a composite biota dominated by more recent colonization and neo-endemism, a so-called 'Darwinian' island (Gillespie & Roderick 2002;Grandcolas et al. 2008). The discovery of a new endemic, doline-inhabiting Lynceus is interesting in this context. The new species was found during a biological survey undertaken in connection with a nickel and cobalt mining project in the southern part of New Caledonia.

Collecting and description
Material was collected from southern New Caledonia (Fig. 1). The specimens were collected with a hand net and preserved in 90% alcohol. Specimens were photographed with an Olympus DP73 fitted to an Olympus SZX10 dissecting microscope. To increase the depth of focus of each final illustration, several photographs (about 10) were taken at different focal planes and combined in Zerene Stacker 1.04 (Figs 2,6,7). Several males and females were prepared for scanning electron microscopy (SEM), which involved dehydration in a graded alcohol series, critical point drying, mounting on stubs, and coating with a mixture of palladium and platinum; the SEM used was a JEOL JSM-6335-F (FE) housed at the Natural History Museum of Denmark.

Comparative material
We compared our material to other species in the genus from Asia and Australia, either by direct comparisons or to original descriptions in the literature. The following material was studied (

Etymology
The species epithet 'insularis' is the genitive form of the Latin word for 'island' (insula), literally 'of an island' in reference to the insular distribution of this species.
head. 0.75 to 0.80 of body length. Occipital condyle rounded, longitudinal. Fornices broad posteriorly, rounded above second antennae, folded anteriorly over sides of rostrum base. Fornices project anteriorly as sharp ridges on each side of rostral constriction and extend to ends of rostral carina bifurcation. Ocular tubercle somewhat prominent, with shallow concavity posterolaterally between tubercle and occipital condyle. Frontal setal fields subcircular, separated by rostral carina, about ⅔ size of compound eye. Dorsal organ narrowly oval, elongate. Rostrum constricted basally, bearing pronounced medial carina. Greatest rostral width 0.8 times rostral length. Rostral carina simple, projecting and narrow along margin. Rostral carina bifurcated distally, with each branch continuing to fornices. Rostrum distad of carina bifurcation bent nearly 90° posteriorly. Carina bifurcations and rostral anterior surface to apex densely setose. Rostrum apex subequal in width to distance between compound eyes and rostral constriction. Apex margin projecting slightly, lacking setae and arcing between fornix margins.
First antenna.With two antennomeres. Proximal antennomere cylindrical, twice as long as broad. Distal antennomere cylindrical, 5 times as long as broad, with apex rounded and bearing numerous short sensory setae (olfactory papillae sensu Martin et al. 1986) in a fringe around anterior surface.
Labrum. Large, smooth, apically tapering to elongated spine. Labrum apex with ventral surface bearing fine setae. Mandible broadly spatulate, molar surface with 18 transverse ridges becoming larger in size posteriorly. Posteriormost three ridges more broadly spaced than other ridges, with penultimate ridge separated from previous ridge by its basal width and projecting as two spines, posteriormost ridge separated by twice its basal width and prolonged into a single spine; semicircularly arranged row of 7 bent spines present anterior to anteriormost small ridge. Possible paragnaths posterior to mandibles: lobiform, with dense setae. maxiLLa i. Typical for the genus (Martin et al. 1986;Martin & Belk 1989;Fryer & Boxshall 2009), elongate, broad-margined, distally with 10-15 stout setae, each with double pectinate row in proximal half and densely plumose in distal half. With three posterior, intermediate-length, robust setae lacking plumose portion, but with two rows of spiniform denticles, and one short robust seta terminating in 4-5 spines. Posterior margin fringed with fine setae. maxiLLa ii. Absent.
carapace. 8-10 mm in length, globose, subspherical, smooth and without ornamentation, subtriangular in lateral outline, with anterior margin being less arcuate than other margins. Maxillary gland ducts arranged transversely, posterior to adductor muscle scar.
teLson. Broad, smooth, lacking denticles. Telson terminating in a pair of triangular setule-covered protrusions. Dorsoposterior angles with a posterior, transverse fringe of setae. Posterior surface of telson pilose, with pelage short and posteriorly directed. Telsonal setae each born on low conical mound. Setal base set in shallow, circular recesses. Telsonal setae filiform, elongate, longer than telson, slightly tumid at base. head. Occipital condyle rounded, longitudinal. Fornices broad posteriorly, rounded above second antennae, folded anteriorly over sides of rostrum base. Fornices project anteriorly as sharp ridges on each side of rostral constriction, extend just short of distolateral rostral corners. Distal apices of fornix rounded. Ocular tubercle less pronounced than in male. Setal fields subcircular, separated by rostral carina, about ⅔ size of compound eye. Dorsal organ as in male. Rostrum constricted basally, bearing pronounced medial carina. Greatest width of rostrum 0.8 times rostral length. Rostral carina simple, projecting, narrow along margin. Rostral carina not bifurcated distally. Apex margin rounded, projecting as a ridge, lacking setae.
antennae and mouthparts. As in males.
thoracopods. Twelve pairs, claspers absent. General shape of many thoracopods and setal patterns of endites, endopod and exopod as in male. Epipod becoming gradually longer from thoracopod 1 through 4, in thoracopod 4 of maximum size, from thoracopod 5 through 7 decreasing in size, absent on thoracopods 8 through 12. Exopod increases in size posteriad from thoracopod 1 until thoracopod 3 or 4, after which it becomes smaller, entirely absent in thoracopods 11-12. Proximolateral part of exopod broadly oval in thoracopods 1-2, from thoracopod 3 gradually more narrow and curved distally; proximolateral part of exopod in throacopods 9-10 distinctly modified into curved lobe with distal setation to which egg clusters are attached. teLson. As in male.

Type locality
Temporary body of water (doline or sinkhole DOL-03; see Table 1, Fig. 1) with a perimeter at about 290 m, a surface area of about 3800 m² and a maximum depth at about 1.2 to 2.6 m. Melaleuca quinquenervia (Cav.) S.T. Blake trees grow both along the margin and in the deeper parts of the sink hole, and there are some scattered spots of Eleocharis spiralis (Rottb.) Roem. and Schult. and Lepironia articulata (Retz.) (both Cyperaceae); the muddy bottom is covered with smaller, submersed macrophytes like the New Caledonian endemic Eriocaulon neocaledonicum Schltr. Lynceus insularis sp. nov. occurs among vegetation or is free swimming.

Habitat
All sites where this species occurs are dolines (sinkholes) in Pliocene/Quarternary laterite deposits (Lillie & Brothers 1970) (Fig. 1). All localities have Cyperaceae growing on the bottom, sometimes being more than 2 m in length. The soil of the region is nutrient poor but rich in heavy metals (Mg, Fe, Cr, Co and Ni) and is ultrabasic.

Distributional range
This species is only known from New Caledonia. To date it is known from five sites, all in the Mont-Dore and Yaté communities of Southern Province. The species was first recognized in April 2000 (DOL-03). However, since it was not found in 2004, a larger survey of 17 additional, similar localities was undertaken in 2009, of which 5 had either empty carapaces or living Lynceus specimens. All localities are within a distance of 2 km from each other (in mining area), except one (DOL-16) which is situated approximately 9 km northeast of DOL-03 (the type locality).

Activity Period
The entire life cycle takes 3-4 months (from February to May/June). Larvae usually hatch in February, which is in the middle of the rainy season (from December to April). February is the warmest month, with average water temperatures ranging from 24-26°C. The highest population density and largest   mating intensity was observed at the end of April. From about June, Lynceus is absent from the ponds even if water still remains. The ponds are dry from about September to November.

IUCN Red List status
Lynceus insularis sp. nov. currently meets the red list definition (IUCN 2001) as a critically endangered species, with its distribution area being only about 1.7 ha. Furthermore, the distribution area of L. insularis sp. nov. is severely fragmented into small subpopulations, each of which shows extreme fluctuations in population size (IUCN Red List Criteria B2a, c). The probability of extinction is estimated to be at least 50% within 50 years due to a high risk of degradation of the biotope of the species which might result as a cumulative effect of a number of factors: hydrological changes, reduction of water supply, acidification of fresh water, invasive species (Cervus timorensis rusa Müller & Schlegel, 1845) and mining activities in the vicinity (less than 1 km).

Discussion
Lynceus insularis sp. nov. appears to be most closely related to members of the Australian fauna, and appears extraordinarily similar to the widespread L. macleayanus (King, 1855) and to L. tatei (Brady, 1886) in the general form of the head, rostrum, thoracopods and telson. In fact, using the key in Timms (2013), L. insularis sp. nov. would be identified as L. macleayanus. We compared L. insularis sp. nov. to the tropical Asian Lynceus taxa, but the similarities were limited to typical genus level characteristics.
The tropical Asian species generally have a serrated distal margin on the rostrum and fine denticles covering the telson. These characters are absent in the Australian fauna and in L. insularis sp. nov. The occurrence of a Lynceus species in New Caledonia, apparently having its closest relatives in Australia, can be explained in two fundamentally different ways. Either the distributional pattern is an ancient 'Gondwana pattern' from before the Gondwana-derived microcontinent 'Zealandia' (incl. New Caledonia and New Zealand) broke off from Australia (appr. 80 Ma, see Introduction), or the distribution is the result of more recent dispersal. New Caledonia has classically been considered a Gondwanan refuge with a fauna largely dating back to Gondwanan times, but this view was challenged by Grandcolas et al. (2007), who found evidence for a more recent colonization since 37 Ma. It has even been suggested that Zealandia has been fully submerged at various intervals, in which case a recolonization of all terrestrial and limnic fauna would have been required. The discovery of a new species of Lynceus does not in itself provide support to either of these two hypotheses. For this a more comprehensive phylogenetic study of a broad range of laevicaudatan species from, e.g., Australia and other nearby regions would be required. However, if dispersal indeed is the explanation for the occurrence of Lynceus in New Caledonia, it could have been from resting eggs dispersed from mainland Australia (Rogers 2014(Rogers , 2015.