A new species of Maldivea Gerlach, 1962 (Nematoda, Oxystominidae) from Felidhoo atoll (Maldives, Indian Ocean) and an emended diagnosis of the sub-family and genus

Maldivea Gerlach, 1962 is a possible endemic genus of the Maldivian archipelago for which only M. xarifae Gerlach, 1962 has been described so far. A new species of this genus, M. complexa n.sp., was recently found in Felidhoo atoll. It reveals a more complex structure of gubernaculum than in type species which appears to be divided into two pieces: one is a sort of long wing in the ventral part of the spicule and the other one, more complex, is characterized by several curved stripes which envelop the dorsal side of the spicule distal part. According to the present considerations, diagnoses of Paroxystomininae and Maldivea are emended.

A new species of Maldivea genus was collected during a scientific cruise carried out in 2005 in the Felidhoo Atoll, a location south to the type locality of M. xarifae . It was found in coarse carbonate sediments of a shallow subtidal habitat same as for the type species.
Maldivea belongs to the family Oxystominidae Chitwood, 1935 and the sub-family Paroxystomininae De Coninck, 1965, in which only two genera have been described: Paroxystomina Micoletzky, 1924, the type genus, and Maldivea Gerlach, 1962. Maldivea xarifae Gerlach, 1962 was described from Fadiffolu atoll in the Maldivian archipelago after the "Xarifa-Expedition 1957/1958" in the Indian Ocean. Since then, this species was no longer found and no other species has been assigned to this genus. Furthermore, only two species are included in the genus Paroxystomina suggesting that the subfamily Paroxystomininae may be a very rare taxon in the phylum Nematoda.
Given the rarity of the representatives of this sub-family, even this one specimen was used to describe the new species in order to get new morphological details of the genus which will be useful to integrate the sub-family and genus diagnoses.

Material and Methods
Sampling was carried out at Felidhoo atoll in the Central part of the Maldivian Archipelago (Indian Ocean) (Fig. 1). Sediment was directly collected by a Scuba diver using a plexiglass corer (Ø: 2 cm). Samples were previously treated with 7% MgCl 2 aqueous solution and then fixed with a 4% formaldehyde solution . First, meiofaunal organisms were separated from sediments following decantation through a 42 µm mesh sieve (see Semprucci et al. 2010 for details). Retrieved organisms were then counted and sorted by taxon level using a stereo-microscope. Nematodes were isolated and studied with a Nikon Optiphot 2 interferential contrast (Nomarski) microscope under a 100x oil immersion objective. All measurements were obtained using the NIS Elements software on a Nikon Optiphot 2 microscope and reported in micrometers, while drawings were made using a camera lucida on a Zeiss Universal microscope.
The abbreviations used in the text are as follows: L = total body length; hd = head diameter; PL = pharynx length; cbd = corresponding body diameter; mbd = maximum body diameter; abd = anal body diameter; a = total body length divided by maximum body diameter; b = total body length divided by pharynx length; c = total body length divided by tail length; s' = spicule length divided by anal body diameter.

Order Enoplida Filipjev, 1929
Superfamily Ironoidea de Man, 1876 Family Oxystominidae Chitwood, 1935 Diagnosis (after Smol & Coomans 2006). Body elongated and very thin at the anterior end. Anterior sensilla in three separate circles, the second and third circle clearly separated; the inner labial sensilla papilliform or setiform, outer labial and cephalic setae very slender. Buccal cavity narrow, tubular or funnel-shaped and without teeth. Among species, amphids are unusually polymorphic. Only orthometanemes with very short caudal filament present. Pharynx inserts into the body cuticle in the region of the buccal cavity; however, the cephalic capsule is not well developed. The posterior section of the pharynx has an undulating outline. Females didelphic-amphidelphic with antidromously reflexed ovaries or monodelphic-opisthodelphic. Males diorchic with opposed testes or only one anterior testis. Position of caudal glands variable.
Material studied: one male collected by Giuseppe Baldelli on May 2005 and mounted on glycerin slide. The studied specimen is held in the author's collection at the Department of Biomolecular Sciences (DiSB), University of Urbino, Italy.
Description of the male: Body very long (L = 4654 µm, a = 78) and thin from the anterior end to the nerve ring region (Fig. 2). Cuticle smooth over the entire body. Six inner labial sensilla, six outer labial sensilla 3 µm long and four cephalic sensilla 6 µm long. Somatic setae about 5 µm long along the pharynx region, shorter in the remaining part of the body (~3 µm long). Anterior end truncate, buccal cavity cup-shaped and surrounded by the pharynx. Buccal wall characterized by 6 folds that are completely cuticularized (Fig. 3a,  4a,b). Distance from anterior edge to base of buccal cavity 7 µm. Amphidial aperture as a large transverse slit. Fovea large and pocket-shaped, located just after the cephalic sensilla at 15 µm from the anterior end (Fig. 3a, 4c). Diameter of the amphidial fovea 6 µm, i.e. ~36% of the cbd. Pharynx cylindro-conical very long (1166 µm long, b = 4) widening slightly as from the nerve ring region to the posterior end where the musculature appeared more developed than in the first region (Fig. 2). Nerve ring at ~ 23-43% of the PL. Cardia well-developed (~20 µm long) and embedded in the intestine. No metanemes observed. Only anterior male gonad observed, situated ventrally to the intestine (Fig. 2). Spicules equal, strongly cuticularized and arcuate (84 µm long, i.e. 1.6 abd) (Fig. 3c, 4d). Gubernaculum without apophysis. It consists of two pieces: a wing lateral to the distal part of the spicule and a dorsal part with a complex structure of stripes (Fig. 2b, 3d). Gubernaculum lateral wing envelops the spicule for about the 60% of its length. In the precloacal region, a longer pair of setae (~7 µm long) at about 11 µm from the cloacal opening and two sub-ventral pairs of short and stout setae (~4 and 5 µm long, respectively) at about 23 and 40 µm from cloaca (Fig. 3c, 4e,f). At about 123 µm from the cloacal opening towards the anterior end, a sub-ventral pair of very short and stout setae (~3 µm long) located in an elevation of the cuticle (Fig. 3c, 4f). Five setae arranged in two sub-ventral rows in the post-cloacal region at about 58 µm from the cloaca (Fig. 3c, 4g). The first three pairs of setae are 8 µm long and the last two are shorter and stouter (~4 µm long). Tail short, 164 µm long corresponding to 29 of the ratio c and 3 of c'. Caudal glands observed. Terminal pore and canal well developed (Fig. 3c). Subterminal seta observed.  Remarks: M. complexa n.sp. is very similar to M. xarifae in the general morphology, but differs in a and c de Man ratios which are lower in the new specimens compared to those of M. xarifae (a= 78 vs. 90-106; c = 29 vs. 34-39). Also the spicule length appears greater in the new species than in the type species (84 µm long vs. 60 µm) along with the s' ratio (1.6 vs. 1.2). However, the most evident difference between M. complexa n.sp. and M. xarifae is the more complex structure of the gubernaculum in M. complexa in which it consists of two pieces: one is a sort of long wing in the ventral part of the spicule and the other, more complex, is characterized by several curved stripes enveloping the dorsal side of the spicule distal part (Fig. 3b).
It is problematic to define a meiofaunal species as endemic, because the punctual distribution of a species could be related to its small body size, to the low number of specialists and to the scarce availability of the literature especially on the species described in the first 19 th century (see Semprucci 2013 for review). However, the few world records of the sub-family Paroxystomininae and the remote areas in which P. micoletzkyi Wieser, 1953 andM. xarifae Gerlach, 1962 were found could support that hypothesis.
The new finding in the Felidhoo atoll is fundamental since it gives additional data on the Maldivea genus. One of the differences detected between the new species and M. xarifae Gerlach, 1962 is the gubernaculum structure that appears more complex in M. complexa n.sp.. Note of worthy is that the gubernaculum of the new species seems rather similar to that documented in P. asymmetrica Micoletzky, 1924 andP. micoletzkyi Wieser, 1953 in showing lateral wings even if they appear much more developed then in the new species.
As Gerlach (1962) underlined, Paroxystomina and Maldivea show completely different pre-cloacal supplements, which appear as winged precloacal supplements in Paroxystomina and short and stout setae in Maldivea. However, a common feature of both genera is the arrangement of the supplements in two sub-ventral rows. According to the present considerations, the sub-family and genus diagnoses are emended as follows.

Subfamily Paroxystomininae De Coninck, 1965
Emended diagnosis. Buccal cavity small, conical. Males with winged or short stout setae; precloacal supplements arranged in two sub-ventral rows. Females with supplements anterior and posterior to the vulva. Two marine genera.

Genus Maldivea Gerlach, 1962
Emended diagnosis. Similar to Paroxystomina (after Smol et al. 2014), but winged precloacal supplements absent and replaced by sub-ventral pairs of short stout setae located on a raised papilla.