Petalophthalmus papuaensis Vicente & Corbari, 2015, sp. nov.

Petalophthalmus papuaensis sp. nov.

(Figs. 2–7)

Material examined. Holotype: 1 empty female, 6.5 mm CL and 33.8 mm TL, MNHN-IU-2013-11965, Basamuk Bay, Bismarck Sea, RV “Alis”, Madang 2012 cruise, PAPUA NIUGINI expedition, 26 December 2012, beam trawl, Station CP 4082, 05 º27’S 146º09’E, 800-1065 m depth; dissected, one vial.

Etymology. This species is named with the adjective papuaensis for its present known distribution (Papua New Guinea).

Diagnosis. Anterior margin of the carapace rectilinear, without rostrum. Eye with definite stalks and globular cornea with an ocular papilla on mesial margin. Antennal scale with an apical lobe. First article of mandibular palp with a small lobe on the inner distal margin. Distal outer margin of the basal segment of the uropod exopod with three cuspidate setae that increase in length from outer to inner one. Posterior half of lateral margins of telson armed with 20–22 cuspidate setae. Telson apex with three pairs of serrate setae decreasing in length towards medial point, one medial serrate seta and six small spines.

Description. The following morphological characteristics refer to the adult female (male unknown).

Carapace short and membranous, leaving the last three or four thoracic somites uncovered dorsally (Fig. 2 A); anterior margin rectilinear and without rostral projection, lateral corners anteriorly rounded; one median acute tooth in front of cervical sulcus; posterior margin emarginated dorsally and with posterolateral lobes (Fig. 2 A–B).

Eye with definite stalks (Fig. 2 B–C), cornea not depressed, globular in shape, possessing some distinctly retinular cells irregularly distributed, and a definite ocular papilla on mesial margin. Cornea about four times as long as the stalk.

Antennule peduncle very long and slender, longer than carapace (Fig. 2 A–B). First article longest, armed on distal margin with one cuspidate seta and three pappose setae; second article armed with two simple setae and one cuspidate seta on distal margin; third article shorter than second, armed with two dorsal setae on distal margin; outer flagellum thinner than inner one.

Antenna peduncle 3-articulate, extending to 5/6 scale length (Fig. 2 D); first article short, longer than broad, inner margin produced proximally into triangular lobe; second article five times as long as broad, distal margin armed with three simple setae; third article longer than second one, distal margin armed with four simple setae; flagellum 7-articulate. Antennal scale about six times as long as maximum width, extending slightly beyond anterior end of first article of antennular peduncle; margins setose all round; apical lobe short, nearly 1/20 scale length.

Labrum quadrangular in shape, more or less symmetrical, posterior distal margin with short irregularly distributed thin simple setae (Fig. 2 E).

Mandibles with elongated and prehensile 3-articulate palp (Fig. 3 A, C); first article very small, armed with a small lobe on the inner distal margin; second article about twice as long as third one with 8–9 strong simple setae and about 14–21 short simple setae on inner margin; third article armed with five strong simple setae and three large conspicuous simple setae on distal margin. Right mandible with incisor process composed of a single chitinous ridge with one broad terminal bifid tooth, lacinia mobilis absent, setal row reduced to single spine, molar process with one chitinous ridge (Fig. 3 D). Left mandible with incisor process composed of two chitinous ridges with rounded distal tip, lacinia mobilis developed, setal row with three short spines, molar process similar to that of right mandible (Fig. 3 B).

Maxillule comparatively small (Fig. 3 E), outer lobe distally armed with seven strong cuspidate-serrate setae, each one with one row of denticles (Fig. 3 F); inner lobe with six pappose setae.

Maxilla with elongate and narrow exopod, extending to half length of endopod distal article, outer margin armed with long pappose setae, inner margin with eight distal pappose setae (Fig. 3 G). Endopod with the distal article long and narrow, about three times as long as broad, densely setose on inner margin. Two coxal endites armed with pappose setae on inner margins, basal endite with proximal longer pappose setae.

First thoracopod with long narrow epipodite, without exopod (Fig. 4 A). Endopod powerful and robust; basis without lobe; preischium very short; ischium produced into inner triangular lobe, tipped with three pappose long setae; carpopropodus about twice as long as its greatest width, tapering somewhat distally; dactylus more or less fused with nail to form a long curved claw armed with a few pappose setae.

Second thoracopod longer and broader than first thoracopod, without epipodite and exopodite (Fig. 4 B). Endopod with preischium armed with one distal simple seta; ischium inner margin produced into a large quadrangular lobe overreaching distal margin of merus, about three times as long as broad and bearing short cuspidate setae on inner and outer margins; merus longest, three times as long as broad, inner margin armed with row of simple long and short cuspidate setae, outer margin with six cuspidate setae increasing distally in length and one distal-most seta shorter; carpopropodus about twice as long as greatest width, tapering somewhat distally, outer margin armed with three long cuspidate setae and six simple setae increasing distally in length, inner margin armed with an irregular row of simple long setae and short cuspidate setae; dactylus more or less fused with nail to form long curved claw, armed with two long and robust simple setae and four short simple setae on proximal and medial margins.

Third and fourth thoracopods (Figs. 4 C–D) with endopods reduced to 1-articulate naked articles; exopod 13–15-articulate.

Fifth thoracopod endopod longer and larger than all other thoracopods (Fig. 5 A). Preischium, ischium and merus subequal in length; carpopropodus divided into 2 segments, the distal segment about one and half times as long as the proximal one and armed on its inner distal margin with one row of simple setae; dactylus very small and densely setose, armed with two short cuspidate seta and two larger serrate setae (Figs. 5 B–C). Exopod shorter than endopod, 16-articulate.

Sixth to eighth thoracopod endopods bearing simple setae (Figs. 5 D–E, F). Preischium shorter than ischium; merus about twice as long as carpopropodus; carpopropodus divided into three (sixth), two (seventh) and one (eighth) articles; dactylus short, terminating in one simple seta; a short distal nail with rounded tip (Fig. 5 G). Exopod subequal in length with endopod, 13–16-articulate.

Marsupium composed of seven pairs of oostegites.

Pleopods uniramous, increasing in length towards posterior pairs; first pleopod not articulate, remaining pleopods 3-articulate (Figs. 6 A–E).

Uropod endopod slender, without statocyst, extending to telson apex, fully setose, inner margin straight (Fig. 6 F). Uropod exopod 2-articulate, longer and broader than endopod, extending slightly beyond telson apex; distal article about one third of the basal article length; outer margin of the basal article naked, ending with three cuspidate setae lengthening from outer to inner one (Fig. 6 G).

Telson quadrangular, 2.7 times as long as broad, nearly as long as the sixth abdominal somite; posterior half of lateral margins armed with 20–22 cuspidate setae increasing in size distally (Fig. 6 H). Apex with three pairs of serrate setae decreasing in length towards medial point, one medial serrate seta and six small spines, one of them bifid (Figs. 6 I, J).

Colour (in the preserved specimen): almost transparent tegument with some brown pigmentation irregularly distributed on the carapace and abdomen (Fig. 7).

Remarks. The main diagnostic features of the genus Petalophthalmus are: the long and slender antennular peduncle; the powerful, long and prehensile mandibular palp; the prominent lobe on the ischium of thoracopods 1–2; the 2-articulated uropodal exopod with outer proximal margin entire, ending with three cuspidate setae at distal angle; and the quadrangular shape of the telson with a slightly emarginate apex, armed with serrate setae (Tattersall 1968, Bravo and Murano 1997). In accordance with this definition, the placement in the genus Petalophthalmus of the specimen herein described seems beyond doubt.

P. papuaensis sp. nov. is the seventh species to be discovered in the genus Petalophthalmus. It can be easily distinguished from the other known species by the structure of the eyes and the armature of the telson.

P. papuaensis sp. nov. shows some similarity to P. armiger and P. papilloculatus (similar morphology of maxilule, maxilla, thoracopods, pleopods and uropods) but can be distinguished by the structure of its eyes, with definite eyestalks and cornea globular in shape with some functional visual elements (versus eyes leaf-like shaped, without eyestalks and without visual elements). The new species can also easily be distinguished from these two species by the telson armature. The telson of P. papuaensis sp. nov. being armed with 20–22 cuspidate setae located on posterior half of the lateral margins, whereas in P papilloculatus and P. armiger the distal 2/3 of the lateral margins of the telson is armed with 25–37 and 40–50 setae, respectively.

P. papuaensis sp. nov. can be distinguished from P. oculatus, P. caribbeanus, P. m a c rop s and P. l i u i by the following features:

(1) Eye cornea globular, with ocular papilla (versus eye cornea hemispherical or reniform, without papilla in P. oculatus, P. caribbeanus, P. macrops and P. liui).

(2) Eyestalk shorter than the cornea (versus eyestalk subequal or slightly longer than the cornea in P. oculatus, P. caribbeanus, P. macrops and P. l iui).

(3) Telson apex without pappose setae described in P. oculatus, P. caribbeanus, P. macrops and P. liui.

The endopod of the third and fourth thoracopods of P. papuaensis sp. nov. shows a rudimentary structure, reduced to one article. Such a peculiarity was previously mentioned for P. macrops (Tchindonova and Vereshchaka, 1991), P. liui (Wang, 1998) and P. papilloculatus (San Vicente et al., 2014). On the contrary, these thoracopod endopods are not rudimentary in P. caribbeanus, as shown by Tattersall (1968) and were not described in the case of P. armiger (see Willemoës-Suhm 1875, Sars 1885, Tatttersall and Tattersall 1951, broken appendages) and P. oculatus (see Pillai 1968: figure 15) ‘fourth thoracopod 4’ is in fact the fifth one, illustrated in figure 1 as the habitus of this species.

Distribution. The known distributional area of P. papuaensis sp. nov. is at the moment limited to Basamuk Bay (off New Guinea, Bismarck Sea, W Pacific Ocean), between 800 and 1065 m depths.

The distribution of all known Petalophthalmus species is shown in Figure 8. With the exceptions of the cosmopolitan species P. armiger occurring in the Atlantic, Pacific, Indian and Southern Oceans (Willemoës-Suhm 1875; Sars 1885; Faxon 1893, 1885; Alcock and Anderson 1894; Ortmann 1905; Holt and Tattersall 1906; Tattersall 1925, 1939, 1951; Hansen 1927; Bartsch 1933; Tattersall and Tattersall 1951; Birstein and Tchindonova 1958; Pillai 1965; Kathman et al. 1986; Escobar Briones and Soto 1991; Casanova 1993; Ledoyer, 1995; San Vicente 2010) and P. oculatus occurring in the Arabian Sea, Indian Ocean and Japan (Illig 1906, 1930; Tattersall 1939, 1955; Pillai 1968, 1973; Murano 1970; Vereshchaka 1995), the geographical distributions of all remaining species of Petalophthalmus are respectively confined to one ocean (Tattersall 1937, 1968; Mauchline and Murano 1977; Tchindonova and Vereshchaka 1991; Vereshchaka 1995; Wang 1998; San Vicente et al. 2014).

Their known latitudinal distribution ranges from 56º N (Bering Sea: P. armiger: Tattersall 1951) to 46ºS (Crozet Islands, P. ar m i g e r: Ledoyer 1995) (Fig. 9A). Petalophthalmus species have not been reported from Mediterranean waters, suggesting that the Gibraltar Strait constitutes a biogeographical barrier in the distribution of these mysids (Coll et al., 2010; San Vicente, 2010b). The distribution of P. papuaensis sp. nov. is at the moment limited to the Bismarck Sea and is accordingly considered a Tropical Western Pacific Ocean endemic. In consequence, the discovery of the new taxon increases the high degree of endemicity of the genus Petalophthalmus.

Petalophthalmus is a eurybenthic shelf, mesopelagic and bathypelagic genus (sensu Mauchline 1980), with a bathymetric distribution ranging from about 200 m in the case of P. liui from the northern South China Sea (Wang 1998) and P. oculatus from the north-west Arabian Sea (Pillai 1968) to 4572 m in P. ar m i g e r from the Tropical Atlantic (Willemoës-Suhm 1875, Sars 1885) (Fig. 9B).

It is worth mentioning that P. armiger is characterized by both the widest latitudinal (56°N–46°S) and bathymetric (900–4572 m) distributions within the genus Petalophthalmus (San Vicente et al. 2014), possibly due to more stabilty in deeper water masses than in shallower habitats. The size of Petalophthalmus species is significantly related to their known maximum depth distribution (Fig. 9C). Such depth variations in the maximum size of individuals are related to water temperature, a general biological phenomenon known as the Bergmann rule described in some mysids (Birstein and Tchindonova 1958, San Vicente and Sorbe 2013), other crustaceans (Khmeleva and Gouloubev 1986) and also other taxa (Margalef 1980).

Despite a potentially high species diversity of Mysida in Oceanian waters (Hanamura and de Grave 2004), information on mysids, especially in waters surrounding New Guinea and its satellite islands, is very scarce. Species records retrieved from the published literature and databases, particularly from WoRMS (Mees and Meland 2012) indicate a poorly understood fauna. As an example, only three species were collected in Indonesian waters during the Dutch Siboga expedition (1899–1900): Euchaetomera oculata Hansen, 1910 and Hypererythrops spinifera (Hansen, 1910) in the Ceram Sea and Meterythrops pictus Holt & Tattersall, 1905 in the Banda Sea (Hansen 1910, Mees and Meland 2012). Also, Murano (1977) reported Arachnomysis megalops Zimmer, 1914 from the north of the Bismarck Sea and Murano and Fukuoka (2008) listed three Sirella species from the Arafura Sea.

Papua New Guinea is located in the diffuse frontier of two West Pacific geographical regions, often used in defining the distribution of the mysid species (Mauchline & Murano 1977, Mauchline 1980): Region 6 (West Pacific Ocean) and Region 7 (Japan, South China Sea, Philippines, and southern Australia), both between about 40ºN and 40ºS. These regions supposedly conform to biogeographical limits applicable to the Mysida. The sea areas of southeast Asian and Australia has received considerable attention in recent years; many new species have been described and distributional records have been produced (e.g. Murano 1974, 1975 1976, 1977, 1981, 1983; Bacescu and Iliffe 1986; Wang and Liu 1997; Hanamura 1998; Hanamura and de Grave 2004; Sawamoto and Fukuoka 2005; Yerman and Lowry 2007; Gan et al. 2010). We expect that detailed studies of mysid fauna off the coasts off Papua New Guinea will produce species lists of equal length.

In the family Petalophthalmidae, there are six species (15.4 %) that live in the tropical zone of the West Pacific Ocean: Petalophthalmus armiger and Hansenomysis carinata Casanova, 1993 from New Caledonia (Casanova 1993); Ceratomysis egregia Hansen, 1910 from Indonesia (Hansen 1910); Parapetalophthalmus suluensis Murano & Bravo, 1998 from the Sulu Sea (Murano and Bravo 1998); Petalophthalmus liui Wang, 1998 from South China Sea (Wang 1998) and Pseudopetalophthalmus australis (Panampunnayil, 1982) from the southwestern coast of Australia (Panampunnayil 1982). The discovery of P. papuaensis sp. nov. increases the number to seven (18%) Petalophthalmidae species found in the tropical zone of the West Pacific.