Echiodon prionodon , a new species of Carapidae ( Pisces , Ophidiiformes ) from New Zealand

A new species of pearlfi sh, Echiodon prionodon, is described from three specimens. This species is diagnosed by having a serrated margin on the posterior edge of the fangs, expanded thoracic plates on some abdominal vertebrae and ventral swimbladder tunic ridges. This species was only found in coastal waters around the North Island of New Zealand. The diagnosis of Eurypleuron is revised.


Introduction
The Carapidae, which include Pyramodontinae and Carapinae, are eel-like fishes.They range from shallow water to moderately deep waters of the continental slope (Nielsen et al. 1999).Several species belonging to the genera Onuxodon, Carapus and Encheliophis are well known for their unusual behavior of entering and living inside invertebrate hosts such as sea cucumbers, sea stars, or bivalves (Trott 1981).Some Echiodon species could be commensals with sponges or polychaete worm tubes (Nielsen et al. 1999), but it is not proven.
The Echiodontini tribe (Carapinae, Carapidae) comprises three genera (Markle & Olney 1990).1) Onuxodon Smith, 1955, with three species, diagnosed by the presence of a rocker bone in front of the swimbladder and a commensal relationships with species of Mollusca (Tyler 1970;Parmentier et al. 2000;Amaoka & Yoseda 2005).2) Echiodon Thompson, 1837, comprising a group of 11 valid species (and 1 unnamed larva) found over a depth range of 18-2000 m (Williams 1984a, b;Markle & Olney 1990;Williams & Machida 1992;Anderson 2005).This taxon is supported by one synapomorphy: a ventral patch of tunic ridges on the posterior swimbladder (Markle & Olney 1990).3) Eurypleuron Markle & Olney, 1990, characterized by having thoracic plates formed by expanded parapophyses (transverse processes) on the fifth through 18-20th vertebrae (males only) and an exterilium gut supported by elongate, cartilaginous, ventral processes of the coracoid in vexillifer larvae.Markle & Olney (1990) considered the genus monospecific, Eurypleuron owasianum (Matsubara, 1953), with two major disjunct populations.However, these two populations have differences in D 30 counts, precaudal vertebral numbers and stomach color.On the basis of these characters and the antitropical distributions (the Northern population is found above 20° North and Southern population occurs below 20° South), Williams & Machida (1992) proposed that these populations correspond to two different species: Eurypleuron owasianum in the north and Eurypleuron cinereum (Smith, 1955) in the south.According to the authors, additional specimens are required to better resolve this issue.
Examination of the collection of carapids held at the Museum of New Zealand Te Papa Tongarewa (Te Papa) revealed three specimens with characters shared by Echiodon and Eurypleuron.The three specimens are here described as a new species and comments on the diagnosis of these genera are given.

Material and methods
Measurements were made with digital calipers to the nearest 0.1 mm.Information of the axial skeleton was obtained from X-ray digital photographs.The following abbreviations for meristic characters are from Markle & Olney (1990):  (FAKU 34517,34518,34519).

Etymology
From the Greek priôn meaning saw, and odous (odon) meaning tooth, in reference to the unique morphology of the fang at the tip of the jaw.Two enlarged caniniform teeth near symphysis in premaxilla and in dentary (one tooth missing on left lower jaw in holotype).Each of these caniniform teeth has a serrated margin posteriorly (Fig. 2).Upper and lower jaw fangs are separated from the posterior teeth rows by a pronounced diastema.Eight to nine rows of small, minute, straight, conical teeth on dentary, 3-4 outer rows with smaller teeth.Four to five rows of small conical teeth on the upper jaw, teeth on inner rows somewhat longer and curved inwardly, teeth of outer rows similar to lower jaw teeth.Palatine with 4-5 rows of villiform teeth.Vomer a small oblong bump with irregular disposition of small conical teeth, posterior one being somewhat bigger.In the holotype (Fig. 3), vertebral centra 6 to 24 with parapophyses expanded to form lateral plates (vertebral centra 6 to 14 in NMNZ P.052493, 6 to 17 in NMNZ P.003281).Holotype swimbladder extends to 30 th vertebral centra (57.9 mm from the snout tip), slight central constriction separating anterior part, brown with dark spots, and shorter posterior part, unpigmented and with ventral tunic ridges.

Color pattern
After eight years in alcohol, the holotype has cream-colored body and head.Melanophores highly concentrated at the level of the geniohyoideus (throat), but not on the lower jaws, at the oro-branchial cavity on the tongue, the palate, branchial arches, on the inner face of the opercula, on the parietals and on the posterior parts of the frontal.Stomach and peritoneal cavity black, anus unpigmented.Melanophores concentrated at the base of dorsal and anal fins and forming lines on the lateral lines and on different myosepta.Pterygiophores of anal and dorsal fins black from the tip to approximately 1/5 of the body length.

Differential diagnosis
Echiodon prionodon sp.nov. is unique in having serrated posterior margin on the fangs.Moreover, it differs from all others Echiodon species by the thoracic plates on some abdominal vertebrae and it differs from Eurypleuron species by having tunic ridges on the swimbladder.

Distribution
Endemic to coastal waters around the North Island of New Zealand (Fig. 4).The species seems to be benthic from 30 to 315 m depth.The holotype was caught along with other fish species which usually are associated with shelly-gravel to sandy bottoms with patch reefs as Gnathophis umbrellabius (Whitley, 1948) (Congridae), Hoplostethus mediterraneus Cuvier, 1829 (Trachichthyidae) and Paraulopus nigripinnis (Günther, 1878) (Paraulopidae).

Discussion
Echiodon and Eurypleuron are sister genera with similar morphologies (Markle & Olney 1990).Both have eel-like bodies, one to several large symphyseal fangs on dentary and premaxilla.They lack cardiform teeth, pelvic bones and rocker bone (Nielsen et al. 1999) from both hemispheres, the only autapomorphic character (ventral tunic ridges on the swimbladder) that supports Echiodon was not found.
Echiodon prionodon sp.nov.shows characters of both genera: ventral tunic ridges on the swimbladder and expanded parapophyses.The number of plate-like parapophyses is, however, more variable than in Eurypleuron owasianum (from 8 to 18), which seems related to the size of the fish.I was unable to confirm the sex of the three specimens and therefore cannot confirm if it is a sex-related character.In addition, radiographs do not show the parapophyses expanded in the same way as in Eurypleuron and I conclude that this character results from either convergence or is a homoplastic character.
Many Ophidiiformes use their swimbladder and associated bones to produce sounds (Courtenay & McKittrick 1970;Parmentier et al. 2002;Fine et al. 2007;Parmentier et al. 2010).Previous anatomical studies showed all the carapids have sound-producing muscles and therefore are able to produce sounds (Parmentier et al. 2002).It is noteworthy that all the characters separating adult Echiodon from Eurypleuron are related to the sound-producing mechanism (Parmentier & Diogo 2006).This system could play an important role in the speciation process of the group.
Because swimbladder ventral tunic ridges are common to all the Echiodon species, it is more parsimonious to place Ec. prionodon sp.nov. in this genus; otherwise, the genus would be deprived of autapomorphic characters.Moreover, Ec. prionodon sp.nov. shares additional characters with other Echiodon species.It has in common with Ec. drummondi Thompson, 1837, Ec. dentatus (Cuvier, 1829) and Ec.rendahli a central constriction of the swimbladder, with Ec. rendahli a high number of PCV, the same kind of dentition (minute teeth in several rows on both lower and upper jaws) and the same geographic range (Williams 1984a;Markle & Olney 1990).
More research is required to define the diagnosis of Eurypleuron.The former diagnosis was: thoracic plates formed by expanded parapophyses (transverse processes) on the fifth through 18-20th vertebrae (males only); an exterilium gut supported by elongate cartilaginous ventral processes of the coracoid in vexillifer larvae; lack of ventral tunic ridges on the posterior portion of the swim bladder; dorsal-fin origin over anal-fin origin resulting in equivalent (or almost so) A 30 and D 30 values (Markle & Olney 1990).Two characters are now invalid: expanded parapophysis (this study) and the equal D 30 and A 30 counts, recently also found in the newly described species Echiodon atopus Anderson, 2005 from the South Atlantic (Anderson 2005).
Fig. 4. Distribution of Echiodon prionodon.The red circle depicts the locality where the holotype was caught.

Table 1 .
Morphometric (mm)and meristic data for Echiodon prionodon sp.nov.Numbers in brackets refer to measurements in % head length.Selected counts and measurements are given in Table1and the holotype is shown in Figure1.Body slender, much higher than wide, tapering into a pointed tail; greatest body depth (at anus) approximately 5% of total length; no caudal fin; dorsal fin origin posterior to anal fin origin, anus ends in a tube at half length of the pectoral fin.Dorsal profile of head slightly convex.Snout rounded in lateral view, slightly projecting beyond upper jaw.Olfactory lobe approximately 70% of snout length, closer to the eye than to the snout tip; anterior nostril developed in a small tube; posterior nostril directly in front of anterior margin of eye, elliptical, being higher than long.Eye elongate, longer than high.Mouth oblique, upper jaw extending beyond posterior margin of eye; posterior portion of maxilla unsheathed.Anterior tip of lower jaw behind tip of snout, lower jaw occlusion with palatine bones.Short opercular spine exposed through an elliptical slit in skin.Gill opening extends from upper end of pectoral fin base to below rear end of maxillary.Seven branchiostegal rays.Ceratobranchials 1 with three slender gill-rakers, with tooth pads on upper limb; other gill-rakers tubercular.