A new species of Mysidopsis ( Crustacea , Mysida , Mysidae ) from coastal waters of Catalonia ( north – western Mediterranean )

A new species of Mysidopsis (Crustacea, Mysida, Mysidae) from coastal waters of Catalonia (north–western Mediterranean).— A new species of the genus Mysidopsis (Crustacea, Mysida, Mysidae, Leptomysinae) is described based on specimens sampled with a suprabenthic sled in the littoral located near the coastal city of Mataró (north–western Mediterranean). The new species lives in the soft–bottom suprabenthic habitat near a Posidonia oceanica meadow, at depths between 17 and 21 m. The main distinguishing features of Mysidopsis iluroensis n. sp. are the small body size, a prominent rostrum, the absence of carapace dorsal nodules, and the armature of the antennule, telson and uropod. The morphology of the new species is compared with other species of Mysidopsis in the Mediterranean Sea.


Introduction
The Mediterranean Sea is one of the most well studied geographical areas in the world, but many regions and habitats remain insufficiently studied and little is known about several taxonomic groups (Coll et al., 2010).Additionally, the Mediterranean Sea is a complex region where ecological and human interaction has a strong impact on marine biodiversity.It is therefore of high priority to improve descriptions of species and to expand our knowledge of their geographical distribution.
Mysids are an important component of the mobile fauna associated with seagrass meadows and algal communities (Ledoyer, 1962;Mazzella et al., 1989;Scipione et al., 1996;Sánchez-Jerez et al., 1999;Wittmann, 2001;Gan et al., 2010).Their diversity and abundance in the Mediterranean Posidonia oceanica meadows has been related to several environmental factors, including the influence of edge effects (Barberá-Cebrián et al., 2002).These edges could be considered a supplementary habitat for mobile fauna because they increase the spatial heterogeneity of seagrass ecosystems (Sánchez-Jerez et al., 1999).
During a monitoring program of the Posidonia oceanica meadow off the Mataró coast (Barcelona, NE Iberian peninsula), many suprabenthic Mysida were sampled in the neighbouring soft bottoms in 2001 and 2011.The following species were collected: the Erythropinae Erythrops erythrophthalma (Goës, 1864), the Gastrosaccinae Anchialina agilis (Sars, 1877), the Leptomysinae Leptomysis gracilis (Sars, 1864) and Paraleptomysis banyulensis (Bacescu, 1966), as well as some individuals ascribed to a new species within the genus Mysidopsis (Leptomysinae).This paper deals with the morphological description of this new taxon and provides an updated identification key to the known European Mysidopsis.

Material and methods
The specimens examined in the present study were recorded at Mataró II and Mataró III stations located at the edges of a Posidonia oceanica meadow at 20.6 and 17 m depth, respectively (fig.1).This well conserved Posidonia oceanica meadow covers more than 730 hectares of substrate and the distribution of specimens has been studied in detail by Manzanera & Cardell (2002).
In 2001 and 2011, the mysid fauna was qualitatively sampled using a suprabenthic sled with a rectangular opening of 50 cm wide and 25 cm high, equipped with a 0.5 mm mesh size net, designed to skim over the surface of the sediment to collect the fauna swimming within the near bottom water layer.The sled was towed by a scuba diver who opened and closed the net at the beginning and end of the bottom sampling.On board, samples were fixed with a solution of 4% formalin in sea water.At the laboratory, the Mysidopsis specimens were sorted and conserved separately in 70% ethanol for later examination.
The total body length (TL) of individuals was measured from the apex of the rostrum to the posterior end of the telson, excluding setae.Specimens selected for the species description were dissected and temporarily mounted on slides.Dissected appendages were drawn with the aid of a camera lucida mounted on a Zeiss Axioscop 20 microscope.The terminology for cuticle projections (spines and setae) follows Watling (1989) and Garm (2004).Type material is deposited at the Institut de Ciències del Mar (CSIC) of Barcelona, Spain.The de scription below refers to both sexes, unless otherwise stated.Nomenclature of higher Mysida taxa follows Mees & Meland (2012).

Etymology
This species is dedicated to the archaeological remains of the ancient Roman city of Iluro preserved around the present village of Mataró.

Diagnosis
Small-size mysid characterized by a prominent triangular rostrum; carapace without dorsal nodules; eyestalk with a finger-like papilla on dorsal anterior margin; third article of antennular peduncle armed with one and three small cuspidate setae on the dorsal medium and posterior margins, respectively; linguiform telson with a rounded apex armed with two median cuspidate setae; endopod of uropod armed on its inner medium margin with 6-8 cuspidate setae.

Description
General form robust and compact.Carapace with anterior margin produced in the middle line into a large triangular rostrum which extends to the middle of the first segment of antennular peduncle; without dorsal nodules; posterior margin emarginated dorsally, leaving the last two, and part of the sixth thoracic somites uncovered; posterolateral lobe not covering anterior abdominal somite (fig.2A).Abdomen moderately robust, as wide as the middle portion of the carapace and not tapering posteriorly; first five somites subequal in length; last somite two times as long as the fifth.
Eyes (fig.2B) large, globular, broader than the eyestalk, laterally extending beyond carapace limits; eyestalk with dorsal finger-like papilla; ommatidial pigment orange (in preserved specimens).Antennular peduncle (fig.2C) shorter than antennal scale.First article longer than wide; second article short, half as long as broad, third article broader than long, armed with one and three small cuspidate setae on the dorsal medium and posterior margins, respectively; male lobe large and hirsute.
Antennal sympod (fig.2D) with a pointed spine on the ventral outer posterior angle.Peduncle slightly shorter than the scale length; first article short as long as broad; second article three times as long as broad, inner posterior margin armed with two simple setae; third article half the length of the second article, posterior inner margin armed with three simple setae.Antennal scale lanceolate, four times as long as maximum width, extending beyond the antennular peduncle; outer margin straight, inner margin convex; a small suture present in posterior one-eleventh.
Mandibles (fig.2F-H) with a three-segmented palp, second article about twice as long as the third, with simple setae on both margins; third article armed on the posterior third of the inner margin with 10 ventral simple setae, five distally ventral serrate setae and one posterior large conspicuous pappose seta.Setal row and molar process reduced.
Maxillule (fig.2I) apex of outer lobe armed with eight strong cuspidate setae; inner lobe with two apical and one median simple seta.
Maxilla (fig.3A) with posterior article of endopod oval, longer than wide, inner posterior two-thirds margins armed with 18 pappose setae; exopod relatively narrow, extending to the third length of the posterior margin endopod article, with 10 pappose setae, the eight short simple setae on its anterior inner margin; inner margin of coxal and bilobulate basal endites armed with simple setae (fig.2A).
First thoracopod (fig.3B-C) short and robust, with unarmed epipodite; inconspicuous coxa and articulation between basis and preischium; endopod with preischium and ischium fused, carpopropodus subequal in length to fused preischium and ischium; dactylus with one strong posterior spine and nine simple setae.
Third to eighth thoracopods (fig.3E-I) much longer than first and second, with endopod longer than exopod; isquium, merus and carpoprodus subequal in length; carpopropodus three-segmented; dactylus with a posterior simple curved seta; exopods 8-9 segmented.Sixth to eighth female thoracopods with a pair of developed oostegites, first pair smaller than posterior pairs.Genital appendage of male short and cylindrical, armed distally with six simple setae (fig.3J).
Telson (fig.4K-L) linguiform, one and half times as long as broad at base; lateral margins armed with 8-10 short cuspidate setae similar in size; apex rounded-truncate, armed with two median cuspidate setae.
Uropod (fig.4K, M) short and robust; endopod extending beyond apex of telson for 1/5 of its length, armed on the medium inner ventral margin with 6-8 short cuspidate setae (seven in adult males, 6-8 in adult females and immature individuals); exopod longer than endopod, having entire margin setose, outer margin straight, inner margin convex.
Colour (in preserved specimens): almost opaque white tegument with brown pigmentation irregularly distributed on the abdomen and some appendages; two black spots on the dorsal anterior part of the telson (fig.5).

Distribution and habitat
The known distributional area of the new Mysidopsis species is at present restricted to the sandy edges of the Posidonia oceanica meadow from the coast of Mataró (north-western Mediterranean).Although the genus Mysidopsis probably represents a polyphyletic melange of taxa (Price et al., 1994), the new species described in this paper undoubtedly shares the morphological diagnosis of the genus sensu Sars (1872), Tattersall & Tattersall (1951) and Tattersall (1969).The most distinctive characters whereby members of this genus may be recognized is the 4-articulate endopod of first thoracic appendages which was due to the fusion of the preischium and ischium, the fused carpopropodus divided into three subsegments in thoracopods 3-8, only setae along the inner margin of the uropodal endopod, not modified mandibular palps and the presence of an expodod in the maxilla.The pair of dark spots at the base of the telson is also a characteristic feature of the genus (Tattersall, 1969).All these characters are present in the new described species.In addition, the overall appearance of the individuals examined is very close to the type species of the genus: Mysidopsis didelphys (Norman, 1863).

Remarks
The new species can be distinguished from M. didelphys and M. angusta by the telson shape and the uropod armature.The rounded apex of the telson      M. iluroensis n. sp.can be easily distinguished from M. cachuchoensis by the ornamentation of its eyestalk (with dorsal finger-like papilla in the new species, absent in M. cachuchoensis), the structure of its antennal scale (without posterior article in M. cachuchoensis), the armature of the inner margin of its uropodal endopods (more than 30 graduated cuspidate setae from statocyst to sub-apex in M. cachuchoensis) and the armature of the lateral margins of its telson (anterior half naked in M. cachuchoensis).
M. iluroensis sp.nov.shows the closest morphological similarity to M. gibbosa.The new species can be distinguished from M. gibbosa by its largest rostrum and the armature of the antennules and the uropod endopod.The dorsal surface of the carapace (without dorsal nodules in the new species versus two quite conspicuous and large nodules in M. gibbosa) is ano-ther morphological difference.However, as mentioned by Bacescu (1941) and Ariani (1967), these nodules are sometimes poorly visible or even absent in some Mediterranean specimens.The armature of the dorsal surface of the third article of the antennular peduncle, with one and three small setae on the dorsal medium and posterior margin, respectively in M. iluroensis n. sp.distinguished also the new species from M. gibbosa (without any setae on the dorsal surface of the antennule).M. iluroensis sp.nov.differs from M. gibbosa in the inner endopod armature of the uropod (with 6-8 cuspidate setae in M. iluroensis versus only three-five small setae in M. gibbosa).
After Tattersall (1909), Mauchline (1971) and Lagardère & Nouvel (1980) two forms of M. gibbosa may be found; one living in shallow bays, usually near the outflow of a freshwater stream, the other living at depths of 40-70 m.The general body shape of the shallow water form is as described by Tattersall & Tattersall (1951) for this species.The deep water form usually resembles young M. didelphys, the abdomen not being sigmoid to any great degree in lateral view but examination of the telson immediately distinguishes it from young M. didelphys.Neither of these two forms is morphologically close to M. iluroensis sp.nov.
None of the morphological peculiarities described for the new species have previously been described in M. gibbosa even in the Mediterranean specimens (Sars, 1877;Carus, 1885;Lo Bianco, 1903;Tattersall, 1909;Colosi, 1929;Bacescu, 1941;Ariani, 1967;Barberá Cebrián et al., 2001).Moreover, re-examination of specimens of M. gibbosa sampled on sandy bottoms near the coast of Mataró (Masnou beach between 5-10 m depth) by San Vicente & Munilla (2000) confirms the differences between the two species, especially in the armature of the antennule (without any setae on the dorsal surface) and the uropod endopod (with four setae on all M. gibbosa specimens analyzed).
Adapted from Tattersall & Tattersall (1951), Tattersall (1969) and San Vicente et al. (2012), an identification key is proposed to include the new Mediterranean species herein described.

M
. iluroensis n. sp.represents the fifth species of the genus currently known from NE-Atlantic and Mediterranean.Previously known species of the genus are M. didelphys (Norman, 1863), M. angusta Sars, 1864 and M. gibbosa Sars, 1864, reported from northern Europe and Iceland to the Mediterranean, between coastal waters (known depth range: 1-220 m) and the recently described M. cachuchoensis San Vicente et al., 2012, reported from the 'Le Danois' Bank (Bay of Biscay) at 828 m depth.
of M. iluroensis n. sp. is armed with two median cuspidate setae whereas in M. didelphys the apex of the telson is truncate and armed at the outer corners with two cuspidate setae, and in M. angusta the apex of the telson has a median cleft.M. iluroensis sp.nov.differs from both latter species in the inner endopod armature of the uropod (with 6-8 cuspidate setae in M. iluroensis n. sp.versus only one in M. didelphys and M. angusta).