Taxonomic review of Metaxia ( Gastropoda : Triphoridae ) from Brazil , with description of a new species

The taxonomy of the species of Metaxia Monterosato, 1884 from Brazil is revised. Previously to this study, only Metaxia excelsa Faber & Moolenbeek, 1991 was known to occur in this country, under its older name Metaxia exilis (C.B. Adams, 1850). The review of a large amount of shells in malacological collections in Brazil revealed the occurrence of three known additional species: Metaxia taeniolata (Dall, 1889); Metaxia rugulosa (C.B. Adams, 1850); Metaxia prompta Rolan & Fernandez-Garces, 2008, all previously reported from the southeast coast of the USA and/or the Caribbean region. Metaxia gongyloskymnus sp. nov., diagnosed by the globose protoconch with continuous axial ribs and microscopic spiral sculpture, is restricted to localities off the Southeastern Brazilian coast.

Triphoridae is a large group of marine microgastropods that includes both sinistral (left-coiled) and dextral (rightcoiled) species.The morphology of the protoconch is crucial to the proper classification of the species in the family (WELLS 1998).The dextral species are all included in Metaxiinae, that is characterized by the presence of crisp, zigzag spiral cords on the first embryonic whorls and subsequent whorls axially ribbed (MARSHALL 1977) and includes the 28 described species of Metaxia Monterosato, 1884 (APPELTANS et al. 2011), which are distributed worldwide in tropical and temperate seas (MARSHALL 1983).
The taxonomic knowledge on species of Metaxia in the western Atlantic was expanded considerably by several recent contributions, especially from the Caribbean region (ROLÁN & FERNÁNDEZ-GARCÉS 1992, 2007, 2008, ROLÁN & REDFERN 1996).A total of eleven species are presently known from the western Atlantic (ROSENBERG 2009, FABER 2010).There are two cases of amphi-Atlantic species (FERNANDES & ROLÁN 1994), although BOUCHET (1985) considered the occurrence of Metaxia abrupta (Watson, 1880) in the western Atlantic to be doubtful.Most of these eleven species are restricted to the northwestern Atlantic (Caribbean and the south-southeast coast of the USA).The only species recorded from the southwestern Atlantic is Metaxia excelsa Faber & Moolenbeek, 1991, which was recorded from Pará to Rio de Janeiro states, under its older name Metaxia exilis (C.B.Adams, 1850) (e.g., LEAL 1991, RIOS 1994, 2009).

Metaxia excelsa
Type material: Holotype MCZ 177150 (examined).Type locality: Jamaica.Characterization.Shell elongated, slender and conical, reaching 4.4 mm in length, 1.1 mm in width.Protoconch narrow, elongated and subcylindrical, with around 4.5 whorls; first whorl dome shaped with fine and numerous zigzag spiral lines, closely spaced; remaining whorls with keeled outline, with predominant axial riblets somewhat discontinuous at middle of whorl and irregular, tiny, somewhat spiral growth lines.Teleoconch with up to nine convex whorls; sculpture by ~13 equally strong axial ribs on eighth whorl crossed by four spiral cords per whorl, the first, adapical one located just below suture is very thin on initial whorls, becoming stronger towards last whorls; prominent nodules formed at the crossing of axial ribs and the three abapical spiral cords, slightly larger on the median one; one or two additional very thin cordlets at suture; last whorl with strong, smooth, slightly wavy subperipheral cord; base short and concave; aperture almost round, with short siphonal notch; outer lip thin; columellar lip sinusoidal.Color light brown to somewhat dark brown, nodules usually whitish.(JONG & COOMANS 1988); BRAZIL: Amapá (this study), Pará to Rio de Janeiro (RIOS 1985), Fernando de Noronha and Abrolhos Archipelago (RIOS 1985, LEAL 1991, GOMES et al. 2006), Vitória-Trindade Chain (LEAL 1991).
Remarks.Although FABER & MOOLENBEEK (1991) created the new name Metaxia excelsa for M. exilis (C.B.Adams, 1850) non Cerithium exile Eichholtz, 1829, all literature on Brazilian mollusks continued to use the older, preoccupied name, M. exilis (e.g., RIOS 1994, 2009, ABSALÃO et al. 2006, GOMES et al. 2006, SANTOS et al. 2007).This was the only species of Metaxia reported from Brazil, but previous records proved to be mixed with the other four taxa studied in this paper.
Type locality: off Carolina coast, USA.Characterization.Shell elongated, slender and conical, reaching 5.0 mm in length, 1.1 mm in width.Protoconch narrow and elongated, with 3.5 to four whorls; first whorl dome shaped, with fine and numerous zigzag spiral cords, closely spaced; remainder whorls with convex, somewhat keeled outline, with predominant axial ribs and irregular, fine, somewhat spiral growth lines at the middle of the whorl.Teleoconch with up to eight convex whorls, and ~13 strong axial ribs on seventh whorl, crossed by four spiral cords per whorl, the first, adapical one, located just below suture is very thin on initial whorls, becoming stronger towards last whorls; prominent nodules formed at the crossing of axial ribs and the three abapical spiral cords, slightly larger on the median one; an additional very thin cord just above suture; last whorl with smooth or slightly wavy subperipheral cord; base short and concave; aperture almost round; outer lip thin; columellar lip sinusoidal.Teleoconch white and even translucent in well preserved shells, with light brown spots at the inter-nodular spaces of second spiral cord around the fifth to the sixth whorl, becoming well sharp in the seventh whorl; light brown protoconch.Remarks.Metaxia taeniolata (Figs 7-11) shares some similar characteristics with M. excelsa (Figs 1-6), including the narrow and elongated form of the protoconch (Figs 2, 4 and 8) and the presence of relatively large nodules on the teleoconch.However, the smaller number of whorls on the protoconch and, especially, the abrupt change in the coloration from the brown protoconch to the white teleoconch, clearly distinguish M. taeniolata (Fig. 11) from M. excelsa (Fig. 6).
Characterization.Shell elongated, slender and conical, reaching 6.0 mm in length, 1.1 mm in width.Protoconch wide, with 2.5 to three whorls, first whorl dome shaped or somewhat pointed; first whorl with zigzag spiral cords of varying expression, sometimes smooth or irregular, but threads always well spaced; second whorl with axial riblets varying in number and arrangement (curved or straight), and with one or two spirals cords.Teleoconch with up to 10 convex whorls; four spiral cords, with the adapical one being smaller and less nodulose; ~13 axial ribs on the eighth whorl, forming nodules of me- taeniolata without a protoconch may be similar to white shells of M. rugulosa that also lack a protoconch, but M. taeniolata has well pronounced nodules on the intersections of the spi-rals with the axial lines.In addition, M. rugulosa never develops a light brown spot in the inter-nodular spaces of the second spiral row.Metaxia prompta Rolán & Fernández-Garcés, 2008 Figs 20-24 Metaxia sp.1: Leal (1991: 125, pl. 17 Type material: Holotype and two paratypes: FLMNH 359136 (not examined).
Type locality: Bermuda, Hamilton Parish, Shelly Bay.Characterization.Shell elongated, slender and conical, reaching 5.7 mm in length and 1.1 mm in width.Protoconch narrow, with truncated apex, three whorls, outline slightly angled; first whorl with tapered apex, sculptured by strong, short pustules of irregular shape and size; remaining whorls with numerous axial ribs discontinuous at middle of the whorl, with summits partially covering the suture.Teleoconch with up to nine whorls and weakly developed shoulder; ~21 axial ribs on fourth whorl, crossed by four spiral cords, the subsutural one being less prominent; a very small cord appears just above the shallow suture; axial sculpture very weak, sometimes disappearing even in the first whorls, and there are very small nodules or only slight elevations on the intersections with the spiral cords; smooth subperipheral cord on last whorl, base short and concave; aperture almost round, with an open siphonal canal; outer lip thin; columellar lip sinusoidal.Shell entirely white, sometimes golden-white.
Material Remarks.ROLÁN & REDFERN (1996) considered that Metaxia sp. 1, recorded by LEAL (1991) from southeast Brazil, actually referred to M. rugulosa.Examination of the shell illustrated by LEAL (1991), as well as shells from other localities of southeast Brazil, furnished sufficient evidences to determine that material as M. prompta.The main difference between M. rugulosa  and M. prompta is in the protoconch, since M. prompta has a narrower protoconch, with a unique distribution of axial ribs (Figs 21 and 23).The weaker axial sculpture and the less shouldered shell of M. prompta (Fig. 20) are other important aspects to distinguish the two species.
Diagnosis.Protoconch globular and broad with slightly sinusoidal axial ribs continuous between adjacent whorls, and a single spiral cord; adapical region on second and third protoconch whorls with several microscopic spiral threads.
Description.Shell elongated, conical, reaching 4.1 mm in length, 1.1 mm in width.Protoconch globular and broad, with almost three whorls of convex to somewhat angled outline; first whorl with very thin, somewhat zigzag spiral threads; remaining whorls with 19 to 22 slightly sinusoidal axial ribs, almost continuous between adjacent whorls, and one spiral cord above suture, more evident at third whorl, with distance between spiral cord and suture around 1/3 of total height of whorl; several microscopic spiral threads on adapical region of second and third whorls.Teleoconch with up to six whorls of convex outline; axial sculpture formed by 16 to18 weak ribs on third whorl, almost disappearing on last whorls; spiral sculpture formed by four cords, the adapical one less prominent, forming small nodules or only slight elevations on the intersections with the axial ribs; one additional small cord just above the impressed suture; smooth subperipheral cord; base short and concave; aperture almost round; open siphonal canal; outer lip thin; columellar lip sinusoidal.Shell white.
Geographic distribution.Southeast of Brazil: off Espírito Santo and Rio de Janeiro coasts.
Remarks.Metaxia gongyloskymnus  is unique among the species studied herein by having microscopic spiral threads on the protoconch (Fig. 30), between the adapical suture and the spiral cord.Metaxia gongyloskymnus has some variation in the protoconch, such as the number of axial ribs and the strength and position of the spiral cord.The holotype and most of paratypes have a larger number of axial ribs and a less marked spiral cord (Fig. 26), almost absent on the third whorl, whereas others have a well marked spiral cord (Fig. 28), forming a more angled outline of protoconch whorls.The most similar western Atlantic species to M. gongyloskymnus is Metaxia propria Rolán and Fernández-Garcés, 2008, from Florida, especially by the sharing of the continuous axial ribs on the whorls of the protoconch and some features of the teleoconch.Metaxia gongyloskymnus has a larger number of axial ribs on the protoconch and teleoconch, in addition to the slightly globular outline of the protoconch.While M. propria has two well defined spiral cords on the protoconch, all shells of M. gongyloskymnus here described bear only a weak (sometimes almost absent) lower spiral cord.Also, M. propria lacks microscopic spiral threads on the protoconch whorls.Another similar species to M. gongyloskymnus is Metaxia hapax Linden, 1998, restricted to the Cape Verde Archipelago (Linden, 1998;Rolán, 2005), with the same pattern of continuous axial ribs between the whorls of the protoconch.Metaxia hapax presents fewer axial ribs on protoconch than M. gongyloskymnus, and it lacks the spiral cord above suture and the microscopic spiral threads.Metaxia gongyloskymnus can be distinguished from M. rugulosa by its broader protoconch with a somewhat globular aspect and by the presence of continuous axial ribs along the entire length of the protoconch.Secondarily, the teleoconch of Metaxia gongyloskymnus has a weaker axial sculpture (Fig. 25).Metaxia gongyloskymnus shares with M. prompta the presence of weak nodules on the teleoconch, forming almost smooth spiral cords in some shells.However, axial ribs are visible in some shells (Figs 29 and 31).The aspects of the protoconch serve to separate them, including the broader shape and the typical distribution of the axial ribs in Metaxia gongyloskymnus (Figs 26 and 28).
DISCUSSION MARSHALL (1977), while proposing Metaxiinae based on two species of Metaxia from the southwest Pacific, recognized two patterns of zigzag threads on the first whorl of the protoconch: numerous and closely spaced threads, almost in contact with each other, in Metaxia exaltata (Powell, 1930); or few and more widely spaced threads in Metaxia kermadecensis Marshall, 1977.Among the species studied here, M. excelsa (Figs 2-4) and M. taeniolata (Figs 8 and 9) have a similar pattern to M. exaltata, while that of M. rugulosa ) is similar to the pattern of Metaxia kermadecensis.Metaxia prompta , in contrast, has a distinct sculpture consisting of short pustules of irregular shape and size.
In Metaxia gongyloskymnus , the zigzag threads are most similar to the pattern of M. rugulosa, but they are less visible and restricted to the upper portion of the first whorl.The protoconch of M. gongyloskymnus is very similar to that of Metaxia maoria (Finlay, 1930), a species from New Zealand redescribed and illustrated by MARSHALL (1979), especially in the presence of continuous axial ribs between two adjacent whorls.However, the protoconch of M. maoria lacks a spiral cord, and the microscopic spiral lines are more numerous and distinct.
According to ROSENBERG (2009) and FABER (2010), Metaxia is represented in the Western Atlantic by eleven species, all of them restricted to northern localities except for M. excelsa, the only species known from the southwestern Atlantic prior to this study.
The results obtained here show that other Metaxia species are widespread in the western Atlantic, as far south as around 21°S.Metaxia rugulosa, M. taeniolata and M. excelsa have similar geographical distributions, from the southeast coast of the USA to the southeast coast of Brazil, with several records in the Caribbean region.Metaxia prompta, previously recorded only from its type locality (Bermuda), was found in Brazil, from off the Bahia to Rio de Janeiro coast, and it is also relatively widely distributed on the seamounts of the Vitória-Trindade Chain.Beside M. prompta, only M. excelsa occurs off the oceanic is-lands of Brazil, including the Vitória-Trindade Chain and the Fernando de Noronha Archipelago.
Metaxia gongyloskymnus is, up to now, known only from the Espírito Santo coast and the Campos Basin off the north coast of Rio de Janeiro.

Figs 25- 31
Figs 25-31 Type material: holotype MORG 51299.Paratypes (one shell in each lot): IBUFRJ 18873 off Piúma, Espírito Santo state, 1993; MNRJ 30293; MORG 48456; MORG 48568; MORG 48642 type locality.Type locality: Bacia de Campos, off Rio de Janeiro state, southeast Brazil (22°43´S, 40°40´W, 120 m).Diagnosis.Protoconch globular and broad with slightly sinusoidal axial ribs continuous between adjacent whorls, and a single spiral cord; adapical region on second and third protoconch whorls with several microscopic spiral threads.Description.Shell elongated, conical, reaching 4.1 mm in length, 1.1 mm in width.Protoconch globular and broad, with almost three whorls of convex to somewhat angled outline; first whorl with very thin, somewhat zigzag spiral threads; remaining whorls with 19 to 22 slightly sinusoidal axial ribs, almost continuous between adjacent whorls, and one spiral cord above suture, more evident at third whorl, with distance between spiral cord and suture around 1/3 of total height of whorl; several microscopic spiral threads on adapical region of second and third whorls.Teleoconch with up to six whorls of convex outline; axial sculpture formed by 16 to18 weak ribs on third whorl, almost disappearing on last whorls; spiral sculpture formed by four cords, the adapical one less prominent, forming small nodules or only slight elevations on the intersections with the axial ribs; one additional small cord just above the impressed suture; smooth subperipheral cord; base short and concave; aperture almost round; open siphonal canal; outer lip thin; columellar lip sinusoidal.Shell white.Etymology.gongylos, Gr. = round, spherical; skymnos, Gr. = young animal.This species is named after its globular protoconch (larval shell).Geographic distribution.Southeast of Brazil: off Espírito Santo and Rio de Janeiro coasts.Remarks.Metaxia gongyloskymnus (Figs 25-31) is unique among the species studied herein by having microscopic spiral threads on the protoconch (Fig.30), between the adapical suture and the spiral cord.Metaxia gongyloskymnus has some variation in the protoconch, such as the number of axial ribs and the strength and position of the spiral cord.The holotype and most of paratypes have a larger number of axial ribs and a less marked spiral cord (Fig.26), almost absent on the third whorl, whereas others have a well marked spiral cord (Fig.28), forming a more angled outline of protoconch whorls.The most similar western Atlantic species to M. gongyloskymnus is Metaxia propriaRolán and Fernández-Garcés, 2008, from Florida, especially by the sharing of the continuous axial ribs on the whorls of the protoconch